Absorption Design Using Radfrac Model in Aspen

Introduction

In the past decade, electrochemical reduction of CO₂ (CO2R) to C₁ products (e.g., CO and formic acid) has been studied extensively. (1−3) The outcome of all these efforts is that CO and formic acid/formate can be produced with high Faraday efficiencies (FEs > 90%) and industrial scale current densities (CDs > 150 mA/cm²), but only in near-neutral to alkaline pH conditions. Recent studies show that CO2R to C₂₊ products such as ethylene and acetic acid/acetate are also favored in alkaline media. The selectivity of the existing (copper-based) catalysts for C₂₊ products is significantly lower than that for C₁ products, which results in a mixture of several (by)products. Although CO2R in alkaline media seems to be promising in terms of FEs, it has some major drawbacks, which significantly affects the economics and scale-up of CO₂ electrolyzers. For example, CO2R to ethylene in alkaline media can be represented by the following reaction: (1)

Note that eq 1 is often written in acidic form (i.e., a proton (H⁺) instead of water is used as a hydrogen source), even though the reaction is performed in alkaline media. In this way, the formation of hydroxide ions, which forms the basis for all the problems in alkaline CO₂ electrolysis, is eliminated from the reaction. The reaction should be written in alkaline form, not only to be consistent with the pH conditions, but also due to the fact that water and not H⁺ is involved in the CO2R mechanism, as ascertained by Hori. (4) The drawbacks of CO2R in alkaline conditions are related to the formed hydroxide ions, which react with fresh CO₂ supplied to the cathode resulting in (bi)carbonate precipitation in gas diffusion electrodes (GDEs). A large fraction of the supplied CO₂ is converted to (bi)carbonate, which has a dramatic effect on the CO₂ utilization efficiency. (5) In the best case, 12 mol of CO₂ is converted to (bi)carbonate for every mole of ethylene obtained. In practice, more CO₂ will be converted to (bi)carbonate, because part of the CO₂ also reacts with the alkaline electrolyte. It is difficult to avoid CO₂ losses in alkaline solutions, because the absorption rate of CO₂ in concentrated potassium or sodium hydroxide (KOH or NaOH) is much higher than the electrochemical conversion rate of CO₂. For example, the initial absorption rate of CO₂ in a 7 M KOH solution is around 6 sccm/cm², (6) which is a factor of 10 faster than the electrochemical conversion rate of CO₂ to ethylene at 250 mA/cm². Furthermore, in alkaline solutions, weak acids such as formic acid or acetic acid almost completely dissociate into the ionic form, which is not the desired product from a market perspective and complicates the downstream processing. (7) A simple solution would be to perform the CO2R in (slightly) acidic conditions, but the Faraday efficiency tends to be lower, because the competing hydrogen evolution reaction (HER) is dominant in low pH solutions. (8,9) Recently, Huang et al. (10) achieved promising results for CO₂ electrolysis to multicarbon products in very acidic solutions, but this approach is still in its infancy and needs to be developed further. An alternative solution is to use a three-compartment cell, where CO₂ is reduced in the cathode compartment, water is oxidized in the anode compartment, and protons from the anode and the conjugate bases (e.g., formate, acetate, bicarbonate) from the cathode are combined in the center compartment to produce acids. (11) However, the combination of protons and bicarbonate ions will cause CO₂ evolution in the center compartment, which might result in a potential drop. In addition, the center compartment needs to be filled up with an ion conducting material, because the conductivity of undissociated acids is poor. Therefore, the capital (CAPEX) and operating (OPEX) costs of a three-compartment CO₂ electrolyzer will be higher due to the increased complexity and higher potential requirement. Another option is to convert (bi)carbonate to chemicals using bipolar membrane (BPM) based electrochemical cells, but the potential of this route has yet to be explored. (12−14) Recently, Lee et al. (15) showed that CO₂ bound to an amine could be electrochemically converted to CO with an FE of 72% at 50 mA/cm². These integrated CO₂ capture and conversion methods are promising, but more research and optimization is required to assess their potential for scale-up.

As a possible solution, a two-step process has been proposed to overcome the limitations posed by the direct electrochemical reduction of CO₂ in alkaline media. (16−18) In the first step, CO₂ is converted to CO in neutral to slightly acidic conditions to prevent bicarbonate formation. In a subsequent step, CO is electrochemically reduced (COR) in alkaline media to desired C₂₊ products such as ethylene. The advantage of the two-step process is that (1) the parasitic loss of CO₂ and bicarbonate precipitation in the GDE are avoided, because CO does not react with the electrolyte; (2) the FEs for C₂₊ products in the second step are higher, because COR requires fewer electrons than CO2R; and (3) higher reaction rates and reactant conversion are observed for COR. Furthermore, it is important to note that (1) the product distribution of COR can be different than that for CO2R, (2) COR in alkaline media also results in the dissociation of carboxylic acids to carboxylates (e.g., acetate), and (3) CO can react with water, nonaqueous solvents, and alkaline electrolytes, but typically high temperature and pressure conditions are required. In the worst case, the two-step conversion will require two electrolyzers, which will significantly affect the capital cost of the process. In the best case, the two electrolyzers can be integrated into a single electrolyzer stacked alternately with two different types of catalyst. (19) For example, by using silver catalysts in the first stack CO₂ can be converted to CO, which is further reduced in a second stack of copper-based catalysts to C₂₊ products. The concept of such an integrated electrolyzer is interesting, but might be difficult to implement in practice due to the increased complexity of the process, which requires management of different reaction conditions (pH, temperature, pressure), product and recycle streams, and lifetime of catalysts. However, the increased CAPEX of the two-step process, whether integrated into a single electrolyzer or not, relative to the direct CO2R process might be offset by the higher FEs, CDs, single-pass conversion, and CO₂ utilization (i.e., lower OPEX). Therefore, the choice between direct CO2R and the two-step CO2R/COR conversion to C₂₊ products will be governed by the economics and scalability of both processes. Several studies reported the techno-economics of CO₂ reduction to ethylene, but none of these considered a realistic downstream processing of the CO2R or COR to C₂ products. (20−28)

Here, we will perform a detailed process design and techno-economic analysis of the direct CO2R process and the two-step conversion of CO₂/CO to C₂ products including ethylene, ethanol, and acetic acid. The design and economic analysis include CO₂ capture, electrochemical CO₂ and CO conversion, reactant recycling, and downstream product separation. An extensive literature review is performed, and the currently best available technologies (BATs) for CO₂ separation, electrochemical CO₂/CO conversion, and product separation are selected for the process design. It is very unlikely that CO₂ or CO electrolyzers will operate on a standalone basis due to the requirement of different feedstocks and the challenges related to the condensation, storage, transportation, and distribution of a range of difficult to handle products. Therefore, to improve the economics, we investigate different strategies to integrate the CO₂/CO electrolyzer into the existing chemical industry infrastructure. The best integration options are selected on the basis of the product distribution and process conditions for CO2R and COR. We present guidelines for the design, scale-up, integration, and implementation of CO₂/CO electrolyzers on industrial relevant scales.

In the following, we will start with a literature review of technologies and methods for the different processing steps in the value chain. On the basis of this review, the best available technologies/methods will be selected for the process design modeling in the next section. Aspen Plus will be used for detailed flowsheeting, optimization, and sizing of process units, and to estimate capital and operating costs of the downstream process. In a subsequent section, an economic analysis of the full value chain for producing chemicals from CO₂ will be presented. Next, strategies for integrating the CO₂ electrolysis process into the existing infrastructure are outlined. In a follow-up section, the main barriers that impede successful implementation of CO₂ electrolyzers on a commercial scale are discussed. Finally, we will summarize the outcome of this study and present the main conclusions.

State of the Art of CO2R and COR to C₂₊ Products

The research on CO2R and COR to hydrocarbons started in the 1980s with the pioneering work of Hori. (29,30) At that time, both reactions were performed in the liquid phase, which caused significant mass transfer limitations due to the poor solubility of CO₂ and CO in aqueous electrolytes. It is now generally recognized that gas diffusion electrodes are indispensable for CO2R or COR at industrial scale current densities. So far, only copper-based catalysts with varying morphologies have been demonstrated to reduce CO₂ or CO with a reasonable selectivity and reaction rates to C₂₊ products. In Table S1, we have compiled a list of landmark studies that reported current densities higher than 100 mA/cm² for CO2R to C₂₊ products. (31−44) In Table S2, a compilation of interesting studies on CO reduction to C₂₊ products is provided. (45−54) We note that several studies reported high FEs for C₂₊ products but at much lower CDs, which is less interesting from an economic point of view and have been excluded from the list. As noted by Romero Cuellar et al. (45) and Xia et al., (55) COR has a few advantages compared to CO2R: (1) the FEs for C₂₊ products are higher, because COR typically requires a lower number of electrons for a specific product; (2) the current densities are higher due to the higher reactivity of CO, which results in a higher single-pass conversion of CO; (3) the cell potential is lower for COR and (4) the CO₂ utilization efficiency is higher for COR because of the parasitic loss of CO₂ in CO2R due to reactions with the electrolyte. Furthermore, it is clear that the main CO₂/CO electroreduction products on copper-based catalysts are ethylene, acetic acid/acetate, ethanol, propanol, and hydrogen. All three liquid products (i.e., acetic acid, ethanol, and propanol) exhibit an azeotropic behavior with water, which will add significant expenses in the downstream process. Strictly speaking, the acetic acid/water system shows a pinch point, which is like an azeotropic point difficult/impossible to overcome by ordinary distillation. Therefore, in practice, azeotropic distillation is used to obtain pure acetic acid. It is important to note that the reaction pathway can be steered to some extent to yield higher fractions for one of these products by controlling the composition, size, morphology, grain boundaries, oxidation states, type of dopants, facets, fragmentation, dealloying, confinement, and porosity of the catalyst. (56−59) Even cofeeding of CO₂/CO mixtures on Cu catalysts seems to have a significant effect on the product distribution. (60) Many of these selectivity controlling measures (especially morphologies, facets, and grain boundaries) are affected at high current densities and results in performance degradation over time. However, the key characteristic of CO2R or COR on copper catalysts is that a multicomponent mixture is obtained as product, which requires purification to meet customer specifications.

Furthermore, a very concerning experimental observation is that for C₂₊ products a relatively pure CO₂ or CO stream is required. A dilute CO₂ stream results in a low CO₂ coverage of the catalyst surface, which affects the C–C coupling process and shifts the mechanism from C₂₊ products to methane. (61) The main consequence of this observation is that typical industrial CO₂ or CO streams cannot directly be used in the electrochemical process, but will require a purification step to increase the concentration. Therefore, upstream and downstream separation, and smart system integration, will play a crucial role in reducing the cost of CO₂ electroreduction products. Recently, a tandem catalysis approach has been used to demonstrate efficient CO₂/CO electroreduction to C₂₊ products for some specific CO₂/CO ratios. (60) In this case, a separation step will also be required, because industrial CO₂/CO streams often contain nitrogen, methane, hydrogen, and other impurities. We note that neither the liquid products nor the involved gas mixtures from a CO₂/CO electrolyzer are easy to separate. CO₂ forms an azeotrope with ethylene, which means that cryogenic distillation cannot be used for product purification. Similarly, the separation of CO from ethylene is also not straightforward due to their similar kinetic diameters and adsorption behavior. In Process Design and Modeling, we will present some guidelines to separate such a multicomponent mixture, which is not a trivial task due to the presence of several gases, liquids, recycle streams, and azeotropes.

In the two-step process, CO₂ is first converted to CO, which is further reduced in a subsequent step to C₂₊ products. For this reason, in Table S3, we have compiled a list of ground-breaking studies on CO₂ reduction to CO. (11,62−79) The main goal of the two-step process is to minimize the loss of CO₂ due to (bi)carbonate formation, which can only be achieved when the reaction is performed in acidic or neutral conditions. However, most of the studies were performed in alkaline conditions, but it is possible to obtain relatively high FEs for CO in slightly acidic or near-neutral conditions and in membrane electrode assembly (MEA) based cells. (79−81) An alternative technology, based on a solid oxide electrochemical cell (SOEC), has been developed and commercialized by Haldor Topsoe to convert CO₂ to CO, which has a claimed energy requirement of 6–8 kWh/Nm³ CO. (82) Furthermore, many industrial (purge) streams already contain substantial amounts of CO, which can be utilized (after purification) in the second step of the process. Note that it is crucial to have a high conversion of CO₂ in the first step. Otherwise, a mixture of CO₂ and CO is obtained, which will cause CO₂ loss in the second step and compromise the benefits of the two-step process. Often, the FE is not 100% and a mixture of CO and hydrogen (i.e., syngas) in a variety of ratios is produced. If both the conversion and FE are <100%, then a mixture containing CO₂, CO, and hydrogen is obtained. In the second step, which is performed in alkaline conditions, part of the CO₂ will be converted to bicarbonates, while the presence of hydrogen might result in the hydrogenation of ethylene. An option is to purify the reaction mixture from the first step before feeding to the second step, but this will increase the costs of the two-step process.

It is clear from the foregoing discussion that both processes, i.e., the direct CO2R process and the two-step CO2R/COR process, need to be designed carefully for optimal functioning. In Process Design and Modeling, we will present a detailed process modeling of both processes, including CO₂ capture, CO₂ conversion, reactant recycling, and downstream separation of products. A detailed discussion on downstream separation is presented with the aim to help electrochemists in making catalyst and process design decisions. For this reason, a relatively complex (gaseous and liquid) mixture is chosen for the downstream separation.

Process Design and Modeling

In this section, we will present the process design and modeling of the direct CO2R to ethylene (i.e., the single-step process) and the indirect CO2R/COR to ethylene (i.e., the two-step process). The modeling of both processes includes CO₂ capture from a point source, electrochemical conversion of CO₂, recycling of reactants, and downstream separation of the multicomponent product mixture. As we will show later, it is better to integrate the CO₂ electrolysis process into the existing (oil and gas) infrastructure to minimize costs for purification, transportation, storage, and distribution of reactants and products. However, here we will design an autonomous decentralized power-to-ethylene process, which excludes any integration. This is done on purpose to have a system independent benchmark case and to demonstrate the importance of process integration. For the single-step CO2R to ethylene process, only low temperature (<100 °C) electrolysis will be considered, since high temperature electrolysis of CO₂ has only been demonstrated for CO or syngas as the main products. For the two-step CO2R/COR process, low and high temperature electrolysis (e.g., the solid oxide electrochemical process of Haldor Topsoe) will be considered.

We will design a process that can convert 10 ton/h CO₂ to C₂₊ products with the assumption that only ethylene, ethanol, and acetic acid are formed in the CO2R and COR processes. In addition, we assume that hydrogen is the only gaseous byproduct that is formed in both processes. These compounds typically account for the majority of the C₂ products (>90%), with the remainder being a mix of C₁ and C₃ products. We implicitly assume that with proper catalyst and process design the formation of C₁ and C₃ products can be suppressed. If the development of such a selective catalyst remains elusive, much more complicated downstream processing will be required than presented here. We assume that the COR process has a slightly higher conversion than CO2R (75% vs 50%), which can be justified on the basis of recent experimental results. The CO₂/CO electrolyzers will be operated at elevated pressures (10 bar) to achieve a higher single pass conversion. We assume that the concentrations of ethanol and acetic acid are 10 and 20% (w/w), respectively. These numbers depend on the reaction conditions (e.g., flow rate of reactants and catholyte, FE, and conversion), which cannot be changed independently in a real process. The concentration of ethylene cannot be chosen independently if the conversion is fixed, but the concentration of liquid products can be varied by changing the supply rate of water to the cathode or center compartment of a three-compartment cell. In the Supporting Information (section S6), we have calculated the concentrations of ethanol and acetic acid as a function of the water supply rate for different cell configurations (zero-gap and flow cells). It is important to note that much higher ethanol concentrations will likely require new membranes, because Nafion membranes can only tolerate small amounts of organics (<10 wt %). The concentration of acetic acid is based on the current status of electrochemical CO₂ conversion to formic acid, which produces around 20 wt % formic acid. In the following, a detailed process modeling of both processes is presented.

Process Design for CO2R to C₂ Products

An overview of the CO₂ to ethylene process is provided in Figure 1. We capture CO₂ from a relatively high partial pressure stream (e.g., biogas) using absorption with amines. The costs of CO₂ capture from biogas using membranes, pressure swing adsorption (PSA), and scrubbers are very similar for large scale processes and are in the range $25–50/ton CO₂. (7) The cost of CO₂ capture from air is a factor of 5–10 higher and will not be considered here. (83) The captured CO₂ is fed to a high pressure (10 bar) GDE-based electrolyzer, which converts CO₂ to ethylene, acetate, and ethanol. Note that the CO₂ feed to the electrolyzer does not necessarily need additional pressurization, because CO₂ from a biogas plant is often available at elevated pressures. The electrolyzer is operated in alkaline media using a three-compartment configuration, which converts acetate to acetic acid in the center compartment. For the base case of the CO2R process, it is assumed that ethylene, ethanol, acetic acid, and hydrogen are produced at a total CD of 500 mA/cm² with FEs of 50, 20, 20, and 10%, respectively. It is difficult to choose a distribution for the products, since it depends on many factors such as temperature, pressure, catalyst type and morphology, cell potential, current density, pH, and type of reactant (CO₂/CO). We have fixed the Faraday efficiency of ethylene and that of hydrogen to 50 and 10%, respectively, which is realistic as can be seen in Tables S1 and S2. The Faraday efficiencies of ethanol and acetic acid are highly condition dependent, but CO2R tends to produce more ethanol than acetic acid while this seems to be the opposite for COR. For simplicity, we have decided to use an FE of 20% for both components. Later, we will show that the distribution of the C₂ products does not matter much for the economics.

Figure 1. Overview of the single-step process for CO2R to C₂ products. CO₂ is captured from biogas (40% CO₂ and 60% methane) and fed to the electrolyzer, which converts CO₂ to ethylene, ethanol, and acetate. The electrolyzer is operated in alkaline conditions in a three-compartment configuration, which converts the acetate to acetic acid in the center compartment. An amine absorber is used to separate the unconverted CO₂, which is recycled back to the electrolyzer. The remaining ethylene/H₂ mixture is separated in an adsorber using activated carbon. The acetic acid stream from the center compartment is flashed to separate dissolved CO₂, which is recycled to the electrolyzer. The liquid stream from the flash is fed to the liquid–liquid extractor, which uses ethyl acetate to extract acetic acid. The extract is sent to the azeotropic distillation column, where pure acetic acid is obtained as bottoms, while an azeotropic mixture of water and ethyl acetate is distilled and condensed in two liquid phases in a decanter. The ethyl acetate rich stream from the decanter can be recycled to the extractor. The water-rich stream from the decanter and the raffinate stream from the extractor are typically combined and sent to the water treatment (not shown). The ethanol stream from the cathode compartment is sent to an ordinary distillation column, which can purify ethanol up to the azeotropic point. This ethanol stream is dehydrated in an azeotropic distillation column using cyclohexane as entrainer. Almost pure ethanol is obtained in the bottom of the azeotropic distillation column. The distillate, which is a ternary azeotropic mixture, is sent to a decanter to condense two liquid phases. The cyclohexane-rich phase is recycled to the azeotropic distillation column, while the water-rich phase is sent to a stripper (not shown).

At the assumed conditions and a CO₂ conversion of 50%, the outlet concentrations of ethylene, CO₂, and hydrogen are 16, 65, and 19 mol %, respectively. The gaseous ethylene, hydrogen, and unconverted CO₂, and the liquid containing around 10 wt % ethanol from the cathode compartment are separated in a flash tank. The gas stream from the flash mostly contains ethylene, hydrogen, and CO₂, which is sent to the gas purification section (GPS). The aim of the GPS is to provide a nearly pure ethylene stream, recycle the unconverted CO₂, and recover as much as possible hydrogen with a high purity. Such a separation cannot be achieved in a single unit but will require multiple (at least two) steps to obtain the desired products. The technologies available for separating hydrogen/CO₂/ethylene mixtures include absorption, adsorption, membranes, and cryogenic distillation. By using an elimination procedure, one can select the most suited technology for the separation. The starting point is that CO₂/ethylene selectivities of existing membranes and adsorbents are relatively low. Several recent techno-economic studies have used pressure swing adsorption to separate CO₂/ethylene mixtures without specifying the adsorbent. (20,22,27,84) To the best of our knowledge, currently available industrial adsorbents cannot be used for efficient CO₂/ethylene separation due to their similar adsorption behaviors. In principle, hydrogen selective membranes and adsorbents could be used, but these processes typically require much higher hydrogen concentrations (>40 mol %) to justify the economics. Cryogenic distillation cannot be used, because CO₂ and ethylene form an azeotrope and CO₂ will cause dry ice formation in the column. (85) From this elimination procedure, absorption appears to be the most interesting option for the first separation step.

In the absorber, a physical solvent (e.g., Selexol) could be used to remove CO₂, because the partial pressure of CO₂ is relatively high (∼6.5 bar). However, the CO₂/ethylene selectivity of classical solvents (e.g., Selexol, NMP, Purisol, and Rectisol) is very low (around 2–3), (86) which will result in a high ethylene concentration in the CO₂ recycle stream. In principle, the ethylene in the recycle is not lost but will dilute the CO₂ feed to the electrolyzer, which might affect the CO2R process. The CO₂/ethylene selectivity in water is around 10, (87) but the feed stream needs to be pressurized, because the solubility of CO₂ in water is relatively low. For this reason, we have decided to use a chemical solvent (e.g., a monoethanolamine (MEA) solution) to selectively remove CO₂ from the ethylene and hydrogen mixture. The absorption of CO₂ is performed at the high feed pressure (∼10 bar), which is not necessary for chemical solvents but is beneficial as repressurization of the ethylene/hydrogen stream is avoided. On the other hand, the CO₂ recycle stream needs to be pressurized, because the CO₂ desorption step is performed at low pressures. The gas stream after the CO₂ capture step will likely be saturated with water, which is not desired for downstream processes (e.g., membranes, adsorbents, and ethylene reactions). In the process design and economics, the drying step to remove water is neglected. After removal of all the CO₂, the concentrations of ethylene and hydrogen are increased from 16 to 45 mol % and from 19 to 55 mol %, respectively. Such a mixture is often present in industrial streams (e.g., ethylene off-gas or refinery off-gas) and can be separated by membranes, PSA, or cryogenic distillation. The selection between these technologies depends on the operating conditions and requirements (feed pressure, feed composition, flow rate, desired purity, (by)product recovery, process flexibility, turndown ratio, reliability, and scale-up considerations). Guidelines for selecting a hydrogen separation process are provided by Benson et al. (88) and Miller et al. (89) We have considered membranes and adsorption to separate hydrogen from ethylene. Note that for membranes ethylene will be obtained approximately at feed pressure, since hydrogen will selectively permeate through the membrane. For adsorption, hydrogen will be obtained at feed pressures, since ethylene is selectively adsorbed on the adsorbent. This means that, in the case of membranes, the hydrogen stream needs to be compressed for storage or transportation, but at low pressures it could be used on-site as fuel. We have neglected these details in the process design, but they are important to consider in a real process. The selectivity and permeability data of hydrogen and ethylene in polyamide membranes of UBE were taken from Al-Rabiah et al. (90)

The countercurrent hollow fiber membrane model of Pettersen and Lien (91) was used for the design calculations. In this algebraic model, the permeate mole fraction of component i is calculated from known feed concentrations and design variables such as the molar stage cut, pressure ratio, and a dimensionless permeation factor, which is related to the membrane area. The simplified model of Pettersen and Lien (91) is suitable for multicomponent mixtures and can easily be implemented in flow sheet calculations. In the Supporting Information (section S2), we show that it is hard to achieve 99% purity for ethylene using commercial membranes. A purity of 85–90% can be achieved with a single-stage membrane process using a stage cut of around 0.5 and a pressure ratio of 10. The purity can be increased by using a cascade of membranes, but this will significantly increase the separation costs. Therefore, we have decided to use adsorption for the separation of ethylene from hydrogen with activated carbon as adsorbent. A five-bed vacuum pressure swing adsorption (VPSA) process was designed to recover ethylene with a purity of >99%. The adsorption process was modeled at 25 °C and 10 bar feed pressure. No feed pressurization was required, since the pressure at the electrolyzer outlet is 10 bar. VPSA processes include the following four basic steps: (1) adsorption, where the feed enters the bed at the bottom and nonadsorbed components leave at the top; (2) blow down, where the bed is partly regenerated by releasing the pressure to the atmosphere; (3) evacuation, where the bed pressure is reduced further with a vacuum pump to achieve higher regeneration levels; (4) and repressurization, where the bed pressure is increased to a level similar to that in the adsorption step. Often, one or more of these basic steps are included to increase the performance of the process (i.e., increase the purity and/or recovery, decrease the energy costs, etc.). In our process, three pressure equalization steps were used for the separation of H₂ and ethylene. More details of the VPSA process can be found in the Supporting Information (section S3). The purities of ethylene and hydrogen were 99.5 and 97.5% at recoveries of 97 and 99%, respectively. Note that the purity specifications for ethylene depend on the application. For example, for polymerization processes at least 99.9% ethylene is required, while other processes (e.g., vinyl acetate) can tolerate higher concentrations of impurities. Therefore, the ethylene stream from the adsorption unit might require some polishing steps to remove traces of H₂ and other impurities. These polishing steps are not included in the process design and techno-economic evaluation.

The acetic acid stream from the center compartment is flashed to separate CO₂, which results from the protonation of bicarbonate. Due to the operation in alkaline media, (bi)carbonate is formed and transported through the anion exchange membrane to react with the protons from the anode to give water and CO₂ in the center compartment. We have assumed that all hydroxide ions generated in the CO2R process will be converted to (bi)carbonate; see the Supporting Information (section S7) for more details. The liquid stream from the flash contains around 20% acetic acid, which is further purified in a hybrid liquid–liquid extraction followed by an azeotropic distillation process. It is well-known that liquid–liquid extraction is the most economic method to separate acetic acid from dilute streams (i.e., concentrations of <30%). (92) We have used ethyl acetate as the extracting solvent, which is the industrial standard for acetic acid separation. The extract containing acetic acid, ethyl acetate, and coextracted water is fed to the azeotropic distillation column. In this column, an azeotropic mixture of water and ethyl acetate is obtained as distillate, while almost pure acetic acid is obtained as bottoms. Water and ethyl acetate form a heterogeneous low boiling azeotrope, which can be separated in a decanter into an ethyl acetate rich stream (which is recycled to the extraction column) and a water-rich stream, which is sent to the raffinate treatment process (not shown). The liquid–liquid extraction process was designed and modeled in Aspen Plus according to the procedures outlined by Shah et al. (93) The extractor was modeled with the EXTRACT unit block in Aspen Plus and operated at 25 °C and 1 bar. The number of stages and the solvent flow in the extractor were optimized for an acetic acid recovery of 99.0 wt %. The optimization was performed with the constraint that the extraction factor should be between 1.5 and 2. For the design, the number of stages was set to 15 and a solvent flow of 25 000 kg/h was chosen. For more details on the liquid–liquid extraction process, the reader is referred to the Supporting Information (section S5).

The ethanol stream from the flash tank can be purified further in an ordinary distillation column up to the azeotropic point (95.6 wt % ethanol). If anhydrous ethanol is desired, an additional step will be required to break the low boiling azeotrope by, for example, azeotropic distillation, extractive distillation, membranes, or adsorption. We will concentrate the ethanol stream up to 99.9% using azeotropic distillation with cyclohexane as the entrainer. The distillation column was modeled in Aspen Plus using the RADFRAC unit block. The distillation columns were optimized using two design specifications: (1) the purity of the ethanol stream and (2) the ethanol mass recovery. The reflux ratio and the bottoms rate were varied to meet the design specifications. The Model Analysis tool in Aspen Plus was used to optimize the number of stages and the feed stage by reducing the reboiler duty. See the Supporting Information (section S4) for the optimized parameters of the distillation column.

The proposed process in Figure 1 was simulated, from which the capital and operating costs of all the units (electrolyzers, absorbers/adsorbers, membranes, extraction and distillation columns) were derived. More details are provided under Economic Analysis of Value Chain.

Process Design for CO2R/COR to C₂ Products

The design of the two-step (CO2R/COR) process is very similar to the single-step CO2R process explained in the previous section. The only difference is that the CO₂ electrolyzer in the single-step process is replaced by a couple of CO₂ and CO electrolyzers in the two-step process, as shown in Figure 2. In the first electrolyzer, CO₂ is converted to CO, which is further reduced in the second electrolyzer to C₂ products. Two cases are considered for the conversion of CO₂ to CO: (1) low temperature electrolysis and (2) high temperature electrolysis using a SOEC. Recently, Küngas et al. (94) reviewed the advantages and disadvantages of both technologies. The high temperature SOEC process for CO production has a few advantages over the low temperature process; i.e., the electric power consumption of the SOEC is much lower, the Faraday efficiency is higher (near 100%), the conversion of CO₂ to CO is higher, the stability of the cell is higher and the degradation rate is lower, the overpotentials are lower, and the technology readinesss level (TRL) is higher (SOEC is nearly commercial). It is important to note that the conversion of CO₂ in both (high and low temperature) processes is less than 100%, which means that a mixture of CO and unconverted CO₂ will be obtained as product in the first electrolyzer. In the low temperature process, the first electrolyzer is operated at high pressures but in nonalkaline conditions to minimize the loss of CO₂ due to bicarbonate formation. In the first electrolyzer, we assume Faraday efficiencies of 95% for CO and 5% for hydrogen at a current density of 300 mA/cm² and a cell voltage of 2.5 V. Furthermore, we assume a CO₂ conversion of 50%. (95) The small amount of hydrogen is neglected in the process design (i.e., no downstream processing is designed for the separation of hydrogen from CO and unconverted CO₂). The CO₂/CO mixture from the first electrolyzer can in principle directly be fed to the second electrolyzer, but initial experimental results show that the presence of large amounts of CO₂ in the mixture has a detrimental effect on the product distribution. (46)

Figure 2. Two-step (tandem) CO₂/CO electrolysis to value-added products. CO₂ is first converted to CO in a high temperature (e.g., SOEC) or low temperature CO₂ electrolyzer. The unconverted CO₂ is removed from the product mixture using an amine absorber. The nearly pure CO is converted to ethylene, ethanol, and acetic acid in a CO electrolyzer operated in a three-compartment configuration. The downstream separation of the gases and liquids is similar to the single-step CO2R process. More details are provided in the text.

Since the second electrolyzer is operated in alkaline conditions, part of the CO₂ from the outlet of the first electrolyzer would be converted to (bi)carbonate, compromising the usefulness of the two-step process. Therefore, in the process design, we have decided to separate the CO₂ from the CO₂/CO mixture using amines. The captured CO₂ is recycled to the first electrolyzer, while the almost pure CO is fed to the second electrolyzer, which is operated at high pressure (10 bar) in a three-compartment configuration. We again assume that only ethylene, ethanol, acetic acid, and hydrogen are produced in the second electrolyzer. As explained earlier, the FE, CD, concentration, and conversion of the COR process is slightly higher than that of the single-step CO2R process. For the base case of the COR process, we have assumed that ethylene, ethanol, acetic acid, and hydrogen are produced at a total CD of 750 mA/cm² with FEs of 50, 20, and 20, and 10%, respectively. Clearly, the partial CD of the products in the COR process is assumed to be higher than that in the CO2R process. At these conditions and a conversion of 75%, the outlet concentrations of ethylene, CO, and hydrogen are 31, 45, and 24 mol %, respectively. Furthermore, the concentrations of ethanol and acetic acid are 10 and 20% (w/w), respectively. The concentrations of ethanol and acetic acid are kept the same as in the CO2R process to reduce the (possibly dominating) effect of the liquid separations on the overall cost. Note that the concentration of acetic acid can be controlled independently by the flow rate of water in the center compartment. The concentration of ethanol depends on the water supply rate at the cathode.

The purification steps for acetic acid and ethanol are the same as in the single-step process. The separation of ethylene from CO/H₂ is far more challenging than that from CO₂/H₂. The reason for this is that CO and ethylene have very similar kinetic diameters, diffusion properties, and adsorption behaviors. Methods for CO separation, but not necessarily in the presence of ethylene, can be found in the paper of Dutta and Patil. (96) Commercial membranes are not suitable for the separation of CO and ethylene mixtures, because the CO/ethylene selectivity is very low. Cryogenic separation is not selected due to the high operating costs. Since the pressure is relatively high, physical solvents such as Selexol and NMP, which show relatively high ethylene solubilities and ethylene/CO selectivities (∼10), could be used. We will use adsorption to separate ethylene from a CO/H₂ mixture. Many different types of adsorbents have been reported for ethylene/ethane separation, but adsorption studies on CO/ethylene separation are scarce. Bachman et al. (97) studied the adsorption of ethylene from different gases including CO using metal–organic frameworks (MOFs) and a commercial zeolite CaX, which exhibited a relatively high ethylene/CO selectivity. However, these adsorbents are expensive, in particular the MOFs, which also have some stability issues in the presence of water. We have selected activated carbon for the separation of ethylene from the CO/H₂ mixture. A five-bed VPSA process was designed to recover ethylene with a purity of at least 99%. The adsorption process was modeled at 25 °C and 10 bar feed pressure. The basic steps in the VPSA cycle are similar to the one discussed for H₂/ethylene separation in the previous section. Here, we have used two pressure equalization steps and a purge step to purify the ethylene stream. In the purge step, partial ethylene product is pumped back into the adsorption bed from the bottom before the blow down step moving impurities up from adsorbents or void spaces for obtaining a clean product in the following desorption step. The purge gas amount is 63% of total ethylene desorption gas amount. Note that in this case an additional compressor is needed to pump ethylene from 1 bar (after vacuum pump) to 10 bar for purging the bed. The technical details of the VPSA process can be found in the Supporting Information (section S3). The five-bed VPSA system is able to recover 76% of the ethylene with a purity of 99% (the remaining 1% is mainly CO). It is not possible to obtain higher recoveries with the current VPSA process with activated carbon as adsorbent. Therefore, it is highly desired to develop better adsorbents for CO/ethylene separation. The syngas-rich stream leaving the adsorber contains around 10% ethylene, 31% hydrogen, and 59% CO. This ethylene containing syngas mixture can be utilized on-site as a fuel, but it is better to recover the hydrogen and to recycle the valuable reactant (CO) and product (C₂H₄) to the electrolyzer. We have separated the C₂H₄/CO/H₂ mixture with a polyimide membrane into a CO-rich stream (including ethylene), which is recycled to the electrolyzer, and a H₂-rich stream, which can be used as fuel or purified further for storage and transportation. The model of Pettersen and Lien (91) and the C₂H₄/CO/H₂ permeability/selectivity data from Al-Rabiah et al. (90) were used to design the membrane process. The details of these calculations can be found in the Supporting Information (section S2).

In the high temperature SOEC process, CO₂ is electrochemically converted at 700–850 °C to CO. In the absence of water in the feed, the SOEC process does not produce hydrogen as a byproduct. For the SOEC, we do not assume Faraday efficiencies, current densities, and cell voltages, but we compute the required power to convert 10 tons/h of CO₂ directly from the energy consumption reported by Haldor Topsoe (6 kWh/Nm³ CO). (82) A high degree of conversion is avoided in the SOEC process to limit carbon formation from the Boudouard reaction. The concentration of CO at the exit of the SOEC is typically between 20 and 80 wt %, which corresponds to conversions of approximately 30 and 85%, respectively. In the Haldor Topsoe process, the CO₂ is captured from the CO₂/CO mixture using PSA and recycled back to the SOEC. In the process design, we will assume a CO₂ to CO conversion of 75%, which is higher than that of the low temperature CO2R process. As mentioned earlier, a mix of CO₂ and CO has a possibly negative effect on the product distribution, FEs of C₂ products, and CO₂ utilization efficiency. For this reason, the CO₂/CO mixture from the SOEC will be purified before feeding to the COR process. We have used absorption with amines to remove the CO₂ from the CO₂/CO mixture, because the CO₂ partial pressure is relatively low as the SOEC is operated at atmospheric pressures. The captured CO₂ is recycled back to the SOEC, while the pure CO is reduced in the second (low temperature) electrolyzer to C₂ products. This electrolyzer is operated at high pressure and alkaline conditions in a three-compartment configuration. The remaining steps and assumptions are the same as in the low temperature electrolysis process. An advantage of the high temperature SOEC process is that the excess heat can be integrated with the ethanol and/or acetic acid distillation columns.

Economic Analysis of Value Chain

To assess the potential of CO2R and COR to ethylene, a detailed economic analysis of the full value chain, including CO₂ capture, electrochemical conversion, reactant recycling, and product separation has been performed. Two cases have been considered for the conversion of CO₂ to ethylene. In the first case, CO₂ is directly converted to ethylene in alkaline media (i.e., the single-step process). In the second case, CO₂ is first converted in acidic or neutral conditions to CO, which is subsequently converted to ethylene (i.e., the two-step (tandem) process). The estimation of the capital and operating costs of all components in the value chain involve some degree of uncertainty. To take this variability into account, a sensitivity analysis will be performed to investigate the effects of different parameters on the process economics. For the base case, we will use the currently best available estimates for the cost components. In case of lacking data, we will estimate the costs based on closely related processes (e.g., water electrolysis). The base case will be supplemented with two additional (worst and best case) scenarios. In the following, we will shortly discuss some of the parameters (CO₂ price, electricity price, CAPEX and OPEX of CO₂ electrolyzers, and product selling price) that significantly effect the cost analysis.

Base Case Assumptions

For the price of CO₂, we have used the Sherwood (cost versus concentration) correlation of Bains et al.: (98) (2)

This correlation is based on cost data for different gas capture technologies (NO_x, SO_x, and CO₂) calculated with the Integrated Environmental Control Model (IECM) by Rubin. The correlation of Bains et al. (98) accounts for CO₂ capture costs including CAPEX and OPEX, but it excludes costs related to compression, transportation, and storage. To decouple the CAPEX and OPEX costs, we have assumed a CAPEX to OPEX ratio of 25% to 75% (i.e., 25% of the cost ($/kg) is due to CAPEX and 75% is due to OPEX). The cost of CO₂ capture can be calculated once the CO₂ concentration in the feed is known (the higher the concentration the lower the capture cost). For CO₂ capture from flue gas with 10% CO₂, the correlation predicts a cost of around $50/ton, which is in good agreement with costs reported for commercial scale processes (e.g., Boundary Dam and Petra Nova (99)). In our process design, CO₂ is captured from a biogas plant with a concentration of 40% CO₂, which results in a CO₂ capture cost of ∼$25/ton. The concentration of CO₂ in the product mixture, hence the cost of recycling, depends on the conversion in the electrolyzer. We assumed that all CO₂ reacted to (bi)carbonate is recovered in the three-compartment cell and recycled to the process. Finally, we note that the effects of carbon taxes or credits, and other climate change policies on the CO₂ price, were not considered in the techno-economic analysis.

The electricity price has a huge influence on the cost of power-to-X processes, including CO₂ electrolysis to chemicals and fuels. It is crucial to use electricity from renewable energy sources to have a significant impact on the CO₂ emissions. Using electricity generated from an energy mix with a high carbon intensity will compromise the usefulness of power-to-X concepts. Before the COVID-19 pandemic, the wholesale prices of electricity in Europe were between $40/MWh and $50/MWh, which decreased to $20/MWh just after the COVID-19 outbreak, but the prices are now bouncing back to the old level. (100) For most European countries the share of renewable energy is still relatively low, but it is expected to increase rapidly. However, the cost of electricity (COE) in countries that do have a high share of renewables in the energy mix (e.g., Scandinavian countries) is similar to the COE in countries with a low degree of renewable energy sources. A few conclusions can be derived from this observation: (1) renewable energy sources such as solar and wind are already competitive with conventional (fossil-based) electricity generation technologies; (2) the high share of renewables does not necessarily lead to lower electricity prices, because the cost is also determined by other factors (e.g., taxes and levies, market competition, environmental policies and regulation, supply and demand, etc.); and (3) in the short term it will be very challenging to have an electricity price lower than $20/MWh. Recently, the U.S. Energy Information Administration (EIA) (101) and Lazard (102) estimated the levelized cost of electricity (LCOE) from renewable sources (wind and solar) to be around $30/MWh. It is important to realize that electricity prices have a huge impact on the economics of power-to-X concepts, because the operating cost is typically dominant. In the techno-economic analysis, we do not consider operating the process in an intermittent mode (e.g., running the process only during off-peak hours when the electricity price is low or negative). It is very unlikely that large scale CO₂ electrolyzers will be operated on a discontinuous basis due to the very high capital cost of these processes, which will result in an extremely high payback time. For the base case of the techno-economic analysis, we will assume an electricity price of $25/MWh. The operating costs of the low temperature CO₂ or CO electrolyzers were computed from the power consumption: (3) where P _j is the power required to produce component j, i _j is the partial current density for component i, A is the electrode area, and V is the cell voltage. The electrode area (A) required to convert 10 tons/h of CO₂ was estimated from (4) where N _CO₂ is the mole flow of CO₂, i _t is the total current density, F is the Faraday constant, FE_j is the Faraday efficiency for component j, n _j is the number of electrons involved in the CO2R (12, 12, and 8 for ethylene, ethanol, and acetic acid, respectively), and ν_j is the stoichiometric number of CO₂ in the respective CO2R (−2 for ethylene, ethanol, and acetic acid), where the convention is used that reactants have a negative stoichiometric number.

The operating cost of the high temperature SOEC unit was derived from the total energy consumption (6–8 kWh/Nm³ CO) reported by Haldor Topsoe for CO₂ electrolysis to CO. A value of 6 kWh/Nm³ CO was used in the economic analysis. The operating cost can then be determined from the required amount of CO, corresponding to the conversion target of 10 tons/h CO₂, and the electricity price. We have assumed that the total energy consumption includes the electrical and thermal energy demands of the SOEC but excludes the energy required for the downstream separation. The energy/cost required for CO₂ separation from the CO product was obtained from the correlation of Bains et al. (98)

It is difficult to estimate the capital cost of CO₂/CO electrolyzers, because there are currently no large scale CO₂/CO electrolyzers available on the market. For this reason, we have estimated the capital cost by comparison with related electrolysis processes. In Table 1, we estimated the capital costs of water electrolyzers (alkaline and SOEC), the chlor-alkali process, and aluminum smelters. For the water electrolyzers, we have used target current densities and capital costs per kilowatt reported by Hydrogen Europe. (103) The capital cost of the chlor-alkali process was estimated in our previous work. (7) Data for aluminum electrolyzers have been taken from the literature. (104−108) Using typical values for the current density and operational voltage of the processes, we have converted the capital cost per unit of power ($/kW) to a capital cost per unit of electrolyzer area ($/m²). For low temperature CO₂ or CO electrolyzers, we have assumed a capital cost of $20,000/m², which lies between the SOEC and chlor-alkali capital costs. In the absence of commercial scale units, we feel that this cost of merit is justifiable considering the similar complexities and operating conditions of these processes. For the SOEC, we have used a projected cost of €1250/kW reported by Hydrogen Europe. (103)

Table 1. Capital Costs of Water Electrolyzers, Chlor-Alkali Process, and Aluminum Smelters

The capital and operating costs of the ethanol and acetic acid distillation columns were calculated by Aspen Plus. As utilities, cooling water, low pressure steam, and medium pressure steam were used at a cost of $1.5/GJ, $6.0/GJ, and $8.0/GJ, respectively. The capital cost of the extractor was estimated with the correlations from Woods. (109) The operating cost of the extractor was neglected, because this is typically very small compared to the solvent recovery (acetic acid distillation) column. The capital cost of the five-bed VPSA process was estimated according to the guidelines provided by Woods. (109) The operating cost of the VPSA process is mainly determined by the power consumption of the vacuum pumps and/or compressors. The power input (W) for adiabatic vacuum pumps and compressors for ideal gas can be estimated from (110) (5) where n _f is the mole flow, η is the compressor/pump efficiency assumed to be 0.7, γ = C _P/C _V is the adiabatic expansion coefficient, R is the ideal gas constant, T ₁ is the inlet temperature, and P ₂/P ₁ is the pressure ratio. The capital costs of the vacuum pump and the compressor were estimated from the correlation of Luyben. (111)

The capital costs of the membrane units were estimated using a skid price of $500/m² membrane area. This cost is based on the works of Baker et al. (112) and includes the cost of membrane modules, module housing, valves, instrumentation, piping, and frame structures. The cost of compressors is not included in the turnkey skid price, but in our process design compressors are not required, since the electrolyzer is operated at high pressure. The required membrane area for the different gas separations was calculated from the countercurrent hollow fiber model of Pettersen and Lien. (91) The details of all these calculations are provided in the Supporting Information (section S2).

The selling prices of products can have a huge effect on the economic analysis. The prices assumed here are based on the European market, which can be very different from U.S. or Middle East prices. For example, the price of ethylene in Europe ($1,200/ton) is almost twice the U.S. price of ethylene. The same holds for the prices of other products such as ethanol and acetic acid, which can differ strongly depending on the region. Therefore, the competitiveness of the electrochemical process will highly depend on the region and market conditions. Also, the grade of the products can have a significant influence on the price. Here, we have designed the downstream process to produce absolute ethanol (>99.5%) and glacial acetic acid (>99.5%), which have much higher market prices compared to the lower grades of the products. Note that the byproduct hydrogen is purified up to 99%, which can be sold to conform to the market price ($1,000/ton). The value of oxygen produced at the anode in the electrolyzers is not taken into account in the economic analysis. However, in the system integration section, we provide guidelines how the produced oxygen can be utilized. Furthermore, we do not consider any premium pricing for the carbon-neutral products. It is obvious that any carbon credits will have a positive impact on the economics of CO₂ utilization processes.

Financial Assumptions

The profitability of a process is often judged on the basis of the payback time (PBT), the return on investment (ROI), or the discounted cash flow, also referred to as the net present value (NPV) approach. Here, we will employ the NPV criteria to evaluate the economic feasibility of the single-step or two-step CO2R/COR processes. The NPV is calculated by taking the sum of the discounted cash flows over the lifetime of the process: (6) where C ₀ is the initial investment, C _n is the cash flow, n is the year, and ir is the interest rate. We assumed a nominal interest rate of 5% and an income tax rate of 25%. The straight line depreciation method was applied over a depreciation period of 10 years using a salvage value of 10% of the total capital investment at the end of plant life. The working capital was assumed to be 5% of the capital investment, which was recovered at the end of the project. The total CAPEX was calculated as the sum of the capital cost of all units. The yearly profit was calculated from the revenues generated by selling the products minus the annual OPEX of the process. In the economic analysis we have assumed that 1% of all products are lost in the downstream separation process. The lifetime of the process was assumed to be 20 years with 8000 h/year of operation.

Economic Analysis for CO2R to C₂ Products

In this section, we will present the results of the economic analysis for the single-step CO2R process to C₂ products. In Table 2, the capital and operating costs of all the major units are presented. The total capital cost and the operating cost of the CO2R process are around $180M and $30M/year, respectively. A breakdown of the CAPEX and OPEX is also shown in Table 2. It is interesting to see that the share of the CO₂ electrolyzer in the CAPEX and OPEX is >80%. Despite the difficult separations, the downstream processing costs are relatively low compared to the electrolyzer costs. The revenues generated from selling the products is approximately $36M/year. The NPV of the CO2R process is negative, and the payback time is higher than the operational lifetime of the plant. Therefore, the CO2R process is not profitable under the base case conditions considered here. It is clear that the CAPEX and OPEX of the CO₂ electrolyzer need to be reduced drastically to make the process profitable. For the CAPEX this means that a higher current density is required or the capital cost per electrolyzer area ($/m²) needs to be reduced. To reduce the OPEX, the power requirement (i.e., the cell voltage) should be reduced or the electricity price should drop significantly.

Table 2. Capital and Operating Costs of the Single-Step CO2R Process

In Figure 3, a sensitivity analysis is performed to show the effects of cell voltage, electricity price, product price, current density, and electrolyzer capital cost on the economics. It is clear that the product price and the electricity price have a strong influence on the economics. A positive NPV can be obtained by reducing the cell voltage to 2.0 V, or by lowering the capital cost of the electrolyzer to <$3,000/m², or by using an electricity price of <$15/MWh, or by increasing the selling price of all the products by 35%. All these individual targets are very hard to achieve, but the economics can be improved significantly if progress is made on all fronts. For example, the NPV of the process increases to $38M and a payback time of 13 years is achieved for a cell voltage of 3.0 V, an electricity price of $20/MWh, and a capital cost of $10,000/m². For the economics, it is important to have a high C₂ selectivity, not necessarily a high ethylene selectivity, because all the CO2R products are valuable and can be sold (after separation) for a relatively high price. To understand this, in Table 3, we have computed the value of 1 mol of supplied electrons (V _e) based on the required number of electrons and the market price of the products: (7) where V _e is in ($/mol of electrons), P _p is the market price of the product in ($/g), M _w is the molecular weight in (g/mol), and n is the mole of electrons required to produce 1 mol of product.

Table 3. Value of 1 mol of Electron Input Based on Market Price of the Components

Figure 3. Sensitivity analysis of NPV for the single-step CO₂ reduction to C₂ products. Base case parameters: electrolyzer capital cost, $20,000/m²; electricity price, $25/MWh; cell voltage, 3.5 V; product price $1,200/ton, $800/ton, $800/ton, and $1,000/ton for ethylene, ethanol, acetic acid, and hydrogen; current density, 500 mA/cm²; and base case NPV, −$97M. The best case and worst case scenarios represent an increase or decrease of the base case parameters by 25%.

The values of V _e for ethylene, ethanol, acetic acid and hydrogen are $2.8 × 10^–3/mol of electrons, $3.1 × 10^–3/mol of electrons, $6.0 × 10^–3/mol of electrons, and $1.0 × 10^–3/mol of electrons, respectively. From this we can conclude that both ethanol and acetic acid are more valuable than ethylene per electron input. On the other hand, hydrogen is almost 3 times less valuable than ethylene. Hence, the coproduction of ethanol and acetic acid will not have a negative impact on the economics of the ethylene process, but hydrogen production should be minimized. In other words, the economics of the ethylene process will not be affected if the sum of the FEs for the C₂ products are high (i.e., a relatively low FE for hydrogen). However, we note that an increase in the FE for ethylene will likely cause a decrease in the FEs of acetic acid and/or ethanol and vice versa. In general, a high FE toward a single product will reduce the separation costs, but the cost reduction will be marginal, because the contribution of the downstream processing to the overall cost is relatively low. This conclusion is somewhat different from those of previous studies, (20,22) which showed a strong dependence of the ethylene price on the FE of ethylene. The main reason for the apparently conflicting conclusion is due to the underlying assumption for the product distribution. Most of these studies assumed ethylene as the only CO2R product with hydrogen as the byproduct. In this case, a decrease in the FE of ethylene automatically results in an increase in the FE of hydrogen. This will affect the economics, because (1) hydrogen is less valuable than ethylene per electron input and (2) often no value is given to the produced hydrogen. In our case, a decrease in the FE of ethylene can be compensated by the increase in the FEs for ethanol and/or acetic acid, while keeping the FE of hydrogen constant. Finally, we note that it is currently not possible to only produce ethylene, since ethanol and acetic acid are coproduced on Cu catalysts. Current research is mainly dedicated to optimizing the catalyst, process conditions, and reactor design for a better selectivity, but there is much to be gained from an optimized separation train. Given the limited number of catalysts that can produce hydrocarbons and the complex multielectron transfer reactions involved, we feel that CO2R or COR to multicarbon products will always yield a mixture of different components. For this reason, it is important to develop efficient downstream processes tailored for the separation of CO2R or COR products.

Economic Analysis for CO2R/COR to C₂ Products

In this section, we will present the results of the economic analysis for the two-step CO2R/COR process to C₂ products. The low temperature CO₂ to CO process will be discussed first and then the high temperature SOEC process. In Table S4, the capital and operating costs of the low temperature process for CO₂ reduction to CO followed by CO electrolysis to C₂ products are presented. The total CAPEX and OPEX of the low temperature two-step process are around $181M and $25M/year, respectively. The electrolyzers contribute approximately >75% to the total CAPEX and OPEX. Revenues generated from selling the products are similar to those in the single-step CO2R process ($36M). The NPV of the low temperature two-step process is negative, which means that the process is not profitable under the base case scenario. However, a positive NPV can be obtained by setting the cell voltage of both electrolyzers to 2.0 V, or by using a capital cost of $10,000/m² for both electrolyzers, or by using an electricity price of $15/MWh. Simultaneously reducing the cell voltage of the COR process (2.5 V), the electricity price ($20/MWh), and the capital cost of both electrolyzers ($10,000/m²) yields a NPV of $67M and a PBT of 10 years. These results show that only slight improvements, but at all fronts, are required to have an economically feasible process.

In Table S5, the capital and operating costs of the high temperature CO2R to CO followed by the low temperature COR process are presented. The total CAPEX and OPEX of the process are around $130M and $24M/year, respectively. Again, the CAPEX and OPEX of the CO₂ and CO electrolyzers have a high share in the total costs. The income from selling the products is approximately $36M. The process has a positive NPV under the base case scenario, but the payback time is 20 years. The NPV increases to $46M (PBT of 13 years), $41M (PBT of 14 years), and $28M (PBT of 15 years) by individually changing the cell voltage to 2 V, using an electricity price of $20/MWh, and lowering the capital cost of the CO electrolyzer to $10,000/m², respectively. A NPV of $79M and a PBT of 9 years are obtained by simultaneously reducing the cell voltage (2.5 V), the electricity price ($20/MWh), and the capital cost of the CO electrolyzer ($10,000/m²). In Figure 4, a sensitivity analysis is performed to show the effects of different parameters on the economics. Again, the product price and electricity price seem to have a huge effect on the economics. It is clear that the high temperature two-step process is more profitable than the low temperature two-step process and the single-step CO2R process. The two-step process, in particular the high temperature route, has better technical and economic feasibility compared to the single-step route due to a higher TRL, lower capital cost and operating cost, and higher conversion efficiency and selectivity for C₂ products.

Figure 4. Sensitivity analysis of NPV for the two-step CO₂/CO reduction to C₂ products. Base case parameters: electrolyzer capital cost, $20,000/m² for CO electrolyzer and $1,250/kW for SOEC; electricity price, $25/MWh; cell voltage, 3.0 V; product price $1,200/ton, $800/ton, $800/ton, and $1,000/ton for ethylene, ethanol, acetic acid, and hydrogen; current density, 750 mA/cm²; and base case NPV, $4.5M. The best case and worst case scenarios represent an increase or decrease of the base case parameters by 25%.

In summary, neither the single-step nor the two-step process is profitable under the base case scenario considered here, but the economics can be improved significantly by reducing the cell voltage, the capital cost of the electrolyzers, and the electricity price. A cell voltage of 2.5 V, a capital cost of $10,000/m², and an electricity price of $20/MWh will yield a positive NPV and a payback time of less than 15 years for all three conversion processes studied here. Therefore, improvements at all fronts are required to have an economic feasible process that can be scaled up. Future studies should focus on the reduction of the CAPEX and OPEX of the electrolyzers, because these account for >75% of the total cost. Furthermore, we have provided guidelines to separate the complex gaseous and liquid products using currently best available technologies. We have shown that it is not necessary to have a high FE for a single CO2R product (e.g., ethylene), since the coproduced chemicals are also valuable and can be recovered at a relatively low cost. Our analysis shows that the high temperature two-step tandem process is currently the best technology to produce C₂ products. This is in agreement with the conclusions of a number of recent studies. (18,22,51,84)

We have already discussed a couple of options to improve the economics of CO2R or COR to C₂ products. Most of these options require significant technological and/or manufacturing advancements in terms of catalyst/materials development to improve FEs and CDs, reduce cell voltages, reduce power requirements (lower electricity prices), and reduce capital costs of electrolyzers. An interesting way to improve the economics of the overall process is to couple the CO2R/COR at the cathode with an oxidation reaction at the anode that produces a more valuable product than oxygen. Verma et al. (113) showed that the coelectrolysis of CO₂ and glycerol can reduce the electricity consumption by 53%. Recently, Khan et al. (114) demonstrated that the cost of CO2R to ethylene can be reduced by 80% when combined with glycerol oxidation at the anode to produce glycolic acid. These coelectrolysis concepts are very promising, but they will require simultaneous optimization of both reactions, and strategies to prevent product crossover and recovery of products. A more appealing approach to improve the economics is by smart system integration where the CO2R/COR electrolyzer is embedded into an existing manufacturing process. System integration can significantly reduce the CAPEX and OPEX costs of upstream and downstream processes and does not require any additional technological advancement other than catalyst stability. Recently, Barecka et al. (110) showed that it is economically viable to integrate the CO2R unit into an existing ethylene oxide (EO) plant, which had a payback time of 1–2 years in regions with low electricity prices and high carbon taxes. We believe that system integration will play a crucial role in the acceptance and scale-up of CO₂/CO electrolyzers. An example of such an integration was recently presented by van Bavel et al., (115) who discussed the integration of CO₂ electrolyzers into gas-to-liquid (GTL) and power-to-liquid (PTL) processes.

In the following, guidelines and strategies are presented to smartly integrate CO₂/CO electrolyzers into the existing oil and gas infrastructure. Such an integration will be beneficial in the transition period to avoid the high cost associated with stranded assets.

Integration of CO₂/CO Electrolyzers

As explained earlier, it is very unlikely that CO₂/CO electrolyzers will operate on a standalone basis, because (1) the required feedstocks (e.g., CO₂ and electricity) should be available from nearby sources to minimize logistics costs and (2) a range of difficult to handle (gaseous and liquid) products are obtained which requires a costly infrastructure for further processing, storage, transportation, and distribution. Note that difficult to condense or toxic molecules are often directly used on-site at a chemical plant to minimize storage/transportation costs and environmental and safety issues. For these reasons, CO₂/CO electrolyzers should be integrated into the existing infrastructure, which has been unrolled in the past century for the oil and gas industry around the globe. In the following, the best strategies for system integration are analyzed on the basis of feedstock requirements, distribution of products, and process conditions. Considering the feedstocks, CO₂, clean water, and renewable electricity, it would be beneficial to integrate the electrolyzer with readily available CO₂ streams and renewable energy sources (e.g., solar or wind). The products of CO₂/CO electrolysis to C₂₊ products are typically ethylene, acetic acid or ethanol, and oxygen. The aim is to avoid storage and transportation of ethylene by directly converting it to desired easy to handle (liquid) products. Therefore, one option is to integrate the CO₂ electrolyzer into processes that use ethylene as feedstock. Ethylene is mainly used to produce a range of intermediates for the polymer industry, e.g., polyethylene (59%), ethylene oxide (13%), ethylene dichloride (13%), ethylbenzene (7%), and others (8%).

In Table 4, a selection of ethylene-based processes and their operating conditions are reported. The most obvious solution would be to integrate the CO₂/CO electrolyzer into an existing ethylene plant which already has an infrastructure for reactant and product handling. For example, most ethylene plants have a gas removal (CO₂ capture) section and a downstream section to purify ethylene. Additionally, the byproduct hydrogen could easily be used on-site in a refinery, reducing the costs of compression, storage, and transportation. Eliminating the CO₂ capture step and some downstream units will significantly improve the economics of the electrolysis process. However, based on the product distribution of CO₂/CO electrolyzers and the required reactants and conditions for the processes in Table 4, it is probably better to integrate the electrolysis process within a vinyl acetate (VA) plant. To understand why this is the ideal integration, it is important to first discuss the VA process. VA is produced via the exothermic reaction of ethylene, acetic acid and oxygen over a palladium catalyst: (8)

Table 4. Typical Reaction Conditions of Ethylene-Based Processesa

In Figure 5, a typical process flow diagram of a vinyl acetate plant integrated with a CO₂ electrolyzer is shown. (116) The VA process involves the following steps: feed preparation, reaction, phase separation, gas washing and recycling, and product distillation. In the feed preparation step, fresh ethylene, acetic acid, and recycled feed materials are mixed in a 2–3:1 mole ratio of ethylene to acetic acid and preheated to a temperature of 120–180 °C. This mixture is diluted with (recycled) CO₂ (10–30%) to control the exothermicity and explosive limits in the reactor. In a next step, up to 0.5 mol equivalent of oxygen relative to acetic acid and some catalyst promoter (potassium acetate) are added in the stream just before the high pressure reactor. The reactor is operated between 5 and 12 bar, but the conversion of reactants is relatively low due to the low residence time in the reactor to prevent overoxidation. In a subsequent step, the reaction mixture is phase-separated into a gaseous stream mostly containing the unconverted reactants, an organic-rich phase containing the liquid (by)products, and a water-rich phase. The gas stream is treated in a washing column (not shown in Figure 5) to remove traces of acetic acid and (by)products. After the washing step, part of the ethylene and CO₂ mixture is recycled to the feed preparation unit and another part is sent to a CO₂ scrubber to remove excess CO₂ formed due to side reactions in the reactor. The organic-rich phase, containing 20–40% vinyl acetate, >50% acetic acid, 6–10% water, and small amounts of byproducts (e.g., ethyl acetate), is sent to an azeotropic distillation column. VA and water form a low-boiling heterogeneous azeotrope and leave the column as distillate, while acetic acid is recovered as bottoms and recycled back to the feed preparation step. The distillate is condensed into a water-rich stream and a vinyl acetate rich stream, which is further purified in a product distillation column (not shown in Figure 5).

Figure 5. Integration of a CO₂ electrolyzer into the vinyl acetate (VA) process. CO₂ produced in the VA process or from other sources are fed to the electrolyzer, which produces ethylene and acetic acid at the cathode and oxygen at the anode. These electrolysis products, together with the unconverted CO₂, are mixed in the vaporizer and fed to the high pressure reactor, which operates at 120–180 °C and 5–12 bar. After the reaction mixture is cooled, the gaseous and liquid streams are separated. The gaseous stream is washed to remove traces of liquid products (washing step not shown) and sent to a CO₂ capture unit, which removes additional CO₂ produced in the reactor due to overoxidation of ethylene. The liquid products from the separator is fed to the azeotropic distillation column, where acetic acid is recovered as bottoms and recycled back to the vaporizer. The azeotropic mixture of vinyl acetate and water azeotrope leaves the column as tops and is condensed in a decanter into a VA-rich stream and an aqueous stream. Both streams might be purified further, but this is not shown in the diagram.

The integration of the CO₂-to-ethylene electrolyzer into the VA process is ideal, because (1) the electrolyzer produces ethylene and acetic acid in a ratio similar to that desired in the VA process; (2) the gas stream from the electrolyzer contains ethylene and unconverted CO₂, which can directly be fed to the VA process; (3) the pure oxygen produced at the anode can be used in the VA reactor, which eliminates the need for an air separation unit; (4) the excess CO₂ and water produced in the VA process can be utilized in the CO₂ electrolyzer; (5) the VA process already has CO₂ capture and distillation units, which will simplify retrofitting of the CO₂ electrolyzer; and (6) multiple gaseous feedstocks are converted to a single relatively easy to handle liquid product, which simplifies storage and transportation. Furthermore, it is beneficial to operate the electrolyzer at slightly elevated temperatures and pressures to match the conditions of the VA process. In addition, the current density and CO₂ conversion in the electrolyzer are higher for elevated temperature and pressure conditions. Furthermore, it is beneficial to use a three-compartment alkaline CO₂ electrolyzer, since acetic acid and not acetate is required as feedstock in the VA process.

In the previous integration example, we have assumed that acetic acid is the main byproduct of CO2R. However, it is clear that the system integration will be different for ethanol as the main byproduct, but the strategy is again to convert ethylene to some liquid products. In Table 4, different options for integrating the CO₂/CO electrolyzer into ethylene- or ethanol-based processes are provided. The first option is to convert ethylene to ethylene oxide, which can subsequently be reacted with ethanol to produce 2-ethoxyethanol. The second option is to react ethylene and ethanol to produce diethyl ether. The third option is to convert ethylene to ethanol, but this route seems to be economically less attractive compared to the fermentation process. Of course, ethylene can be transformed to any other products mentioned in Table 4 but not involving ethanol in the reaction. In the latter case, two liquid products will be produced in the integrated process, which is not an issue as long as the existing infrastructure can be used.

The above proposed integration is based on the assumption that CO₂ is the reactant (i.e., single-step CO2R), which results in a CO₂ and ethylene mixture for conversions lower than 100%. However, in the two-step process (i.e., CO2R/COR), CO is the reactant, which will yield a mixture of CO and ethylene for incomplete conversions. An option in this case is to integrate the CO electrolyzer into a propionic acid plant for the hydrocarboxylation of ethylene according to the reaction (117) (9) One of the main byproducts in COR is ethanol, which can be converted with ethylene and CO to ethyl propionate by essentially replacing water by ethanol in eq 9. Clearly, the system integration will be affected by the choice of the conversion process (i.e., CO2R or COR) and the formed byproducts.

For the system integration, we have selected processes on the basis of the typical product distributions of CO2R and COR electrolyzers, the reaction temperature and pressure conditions, and the required reactants. However, the integration might also depend on the location, the availability of feedstocks, the desired purity of products, the operational flexibility and reliability of renewable energy based processes, and the costs of retrofits. Since there are a couple of options for system integration, the ultimate decision can only be made after a detailed techno-economic analysis of the fully integrated system, which is beyond the scope of the current work. Nevertheless, we hope that the basic strategies presented here will help to accelerate the commercialization of CO₂/CO electrolyzers. Furthermore, the strategies presented here are also applicable to other gaseous CO₂ electroreduction products such as CO and methane. It is clear that decentralized production of gaseous products will bring additional expenses for transport and storage, which can be avoided when CO₂ electrolyzers are smartly integrated into the existing infrastructure. Such an integration will be crucial for the large scale implementation of power-to-X concepts including CO₂ electrolysis to value-added products. In the next section, we present a list of current barriers that impede scale-up and commercialization of CO₂ electrolyzers. These barriers were partly derived from the carbon capture and storage (CCS) field, (118−122) but they apply equally well to carbon capture and utilization (CCU) (123−125) and were partly identified during the Energy-X workshop "research needs: toward sustainable production of fuels and chemicals" in Brussels (Belgium). (126)

Barriers for Industrialization of CO₂ Electrolyzers

Often, it takes a lot of effort, time, and persistence to replace well-established (fossil fuel based) processes with new (renewable energy based) technologies. To accelerate the implementation of CO₂ electrolyzers in the chemical industry, the following barriers need to be addressed.

1. Lack of upstream and downstream processing studies: The effect of impurities in the reactants and products has rarely been investigated, but it is well-known that upstream and downstream purification steps can account for >30% of the total costs. It is obvious that the feedstock costs will be significantly higher if ultrapure CO₂ and water are required in the electrolysis process.

2. Lack of system integration studies: As explained earlier, it is very unlikely that CO₂ electrolyzers will operate on a standalone basis due to the lack of infrastructure. It is better to integrate CO₂ electrolyzers into the existing fossil fuel based infrastructure. However, it is currently unclear how to retrofit CO₂ electrolyzers into chemical processes.

3. Lack of process design and techno-economic feasibility studies: It is important to consider the economics of a new technology in an early stage of the development process to assess the competitiveness, select the most promising alternatives, and identify research and development gaps.

4. Lack of scale-up studies: For new technologies, it is common practice to first run long-term pilot scale experiments before implementing on a commercial scale. To assess the feasibility of CO₂ electrolysis at an industrial scale, it is important to move from lab to pilot scale.

5. Lack of infrastructure: Power-to-X concepts such as CO₂ electrolysis require an infrastructure for the feedstocks (e.g., renewable energy and CO₂) and products. The lack of such an infrastructure is a significant barrier for scale-up and industrialization of CO₂ electrolyzers.

6. Lack of funding opportunities: Development of new technologies requires a high up-front investment, which is one of the main hurdles for start-up companies to bringing the product on the market. This initial investment should come from fund-raising, because major companies are typically reluctant to invest in low TRL technologies.

7. Lack of regulations and policy incentives for large scale CCU projects: Currently, it is extremely difficult for CCU processes to economically compete with the fossil fuel based counterparts. In the absence of direct economic drivers, a clear regulatory framework and policy incentives are crucial for successful implementation of CCU projects on a large scale.

8. Lack of environmental, health and safety, and societal impact studies: A large number of CCS projects have been canceled due to underestimation of ecological and societal factors such as public acceptance. To avoid similar issues with CCU, it is important to thoroughly assess the impact of new technologies on the environment and society and to involve all stakeholders at an early stage of the development.

9. Lack of education and training: A large amount of manpower with skills in power-to-X technologies will be required for the envisioned large scale deployment of CO₂ electrolyzers. The new generation of operators, technicians, and engineers needs to be educated and trained for the operation of renewable energy based processes.

By adequately addressing these barriers, we might be able to accelerate the implementation of power-to-X technologies including CO₂ electrolyzers on an industrial scale. However, experience from the CCS field shows that a significant effort from all stakeholders (i.e., energy companies, industry, policy makers, technology suppliers, environmental agencies, local public, nongovernmental organizations, and academia) will be required to make a success of CCU.

Conclusions

Direct electrochemical reduction of CO₂ to value-added products (i.e., the single-step process) is more efficient is alkaline conditions, but it has a negative impact on the carbon utilization due to (bi)carbonate formation. A two-step (tandem) process, where CO₂ is first converted to CO which is then further reduced to the desired products (indirect route), has been proposed to overcome the problems associated with direct CO₂ conversion in alkaline media. Here, we performed a detailed process design and techno-economic analysis for direct and indirect CO₂ conversion to C₂ products (ethylene, acetic acid, and ethanol). For the two-step tandem process, CO production by high temperature (i.e., SOEC) or low temperature CO₂ electrolysis has been considered in the design. For both (CO2R and CO2R/COR) processes, guidelines are provided for the downstream processing of the complex gas and liquid mixtures containing CO₂, ethylene, CO, H₂, acetic acid, and ethanol. Process modeling and economic analysis of both (single-step and two-step) routes have been performed. Capital and operating costs of CO₂ capture, CO₂/CO reduction, CO₂ recycling, and product separation have been calculated for both routes. Our economic analysis shows that with the current electrolyzer performance, electricity prices, and electrolyzer capital costs both routes are economically not compelling. However, the economics of both processes can be improved significantly by reducing the CAPEX and OPEX of the electrolyzers, which have a high share (>75%) in the total cost. For both routes, a cell voltage of <2.5 V, an electricity price of <$20/MWh, and a capital cost of <$10,000/m² for the electrolyzers will result in a significantly improved economics (NPV of >$60M and payback times between 9 and 11 years). We demonstrate that the coproduction of ethanol and acetic acid does not have a negative impact on the economics of the process, because the downstream separation costs are relatively low and both products can be sold for a high market price. For this reason, it is not necessary to have a high FE for a single product, but it is crucial to keep the sum of the FEs for the C₂ products high. Overall, the high temperature two-step tandem process has a better technical and economic viability than the single-step CO2R process and the low temperature two-step process. Guidelines are provided to integrate CO₂/CO electrolyzers into the existing oil and gas infrastructure, which will be crucial to increasing the acceptance of these technologies, to reducing upstream and downstream processing costs, and to avoiding problems with logistics, storage, transportation, and distribution of difficult to handle gaseous and liquid products. Finally, we provide an overview of the current barriers that impede commercialization of CO₂/CO electrolyzers.

Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.iecr.1c03592.

Data compilation for CO₂ electrolysis to CO and C₂ products; data compilation of CO electrolysis to C₂ products; capital and operating cost estimations for low and high temperature two-step tandem processes; details of modeling of the membrane, VPSA, and extraction/distillation processes; estimation of concentrations of liquid products; and estimation of CO₂ loss to (bi)carbonates (PDF)

ie1c03592_si_001.pdf (403.06 kb)

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Author Information

- Mahinder Ramdin - Engineering Thermodynamics, Process & Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands; https://orcid.org/0000-0002-8476-7035; Email: [email protected]
- Bert De Mot - Applied Electrochemistry & Catalysis, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium; https://orcid.org/0000-0002-8510-7960
- Andrew R. T. Morrison - Large-Scale Energy Storage, Process & Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands
- Tom Breugelmans - Applied Electrochemistry & Catalysis, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium; https://orcid.org/0000-0001-5538-0408
- Leo J. P. van den Broeke - Engineering Thermodynamics, Process & Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands
- Ruud Kortlever - Large-Scale Energy Storage, Process & Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands; https://orcid.org/0000-0001-9412-7480
- Wiebren de Jong - Large-Scale Energy Storage, Process & Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands
- Othonas A. Moultos - Engineering Thermodynamics, Process & Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands; https://orcid.org/0000-0001-7477-9684
- Penny Xiao - Department of Chemical Engineering, The University of Melbourne, Victoria 3010, Australia
- Thijs J. H. Vlugt - Engineering Thermodynamics, Process & Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands; https://orcid.org/0000-0003-3059-8712
The authors declare no competing financial interest.

Acknowledgments

T.J.H.V. acknowledges NWO-CW (Chemical Sciences) for a VICI grant.

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Competition between Hydrogen Evolution and Carbon Dioxide Reduction on Copper Electrodes in Mildly Acidic Media

Ooka, Hideshi; Figueiredo, Marta C.; Koper, Marc T. M.

Langmuir (2017), 33 (37), 9307-9313CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)

Understanding the competition between H evolution and CO2 redn. is of fundamental importance to increase the faradaic efficiency for electrocatalytic CO2 redn. in aq. electrolytes. Here, by using a Cu rotating disk electrode, the major H evolution pathway competing with CO2 redn. is H2O redn., even in a relatively acidic electrolyte (pH 2.5). The mass-transport-limited redn. of protons takes place at potentials for which there is no significant competition with CO2 redn. This selective inhibitory effect of CO2 on H2O redn., as well as the difference in onset potential even after correction for local pH changes, highlights the importance of differentiating between H2O redn. and proton redn. pathways for H evolution. In-situ FTIR spectroscopy indicates that the adsorbed CO formed during CO2 redn. is the primary intermediate responsible for inhibiting the H2O redn. process, which may be one of the main mechanisms by which Cu maintains a high faradaic efficiency for CO2 redn. in neutral media.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXmvVGntL4%253D&md5=ad2663ea95e8f1c53a702dacbbae4060
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Bondue, C. J. ; Graf, M. ; Goyal, A. ; Koper, M. T. M. Suppression of Hydrogen Evolution in Acidic Electrolytes by Electrochemical CO₂ Reduction. J. Am. Chem. Soc. 2021, 143 , 279– 285, DOI: 10.1021/jacs.0c10397

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Suppression of Hydrogen Evolution in Acidic Electrolytes by Electrochemical CO2 Reduction

Bondue, Christoph J.; Graf, Matthias; Goyal, Akansha; Koper, Marc T. M.

Journal of the American Chemical Society (2021), 143 (1), 279-285CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

In this article the electrochem. redn. is investigated of CO2 at gold electrodes under mildly acidic conditions. Differential electrochem. mass spectroscopy (DEMS) is used to quantify the amts. of formed hydrogen and carbon monoxide as well as the consumed amt. of CO2. How the Faradaic efficiency of CO formation is affected is investigated by the CO2 partial pressure (0.1-0.5 bar) and the proton concn. (1-0.25 mM). Increasing the former enhances the rate of CO2 redn. and suppresses hydrogen evolution from proton redn., leading to Faradaic efficiencies close to 100%. Hydrogen evolution is suppressed by CO2 redn. as all protons at the electrode surfaces are used to support the formation of water (CO2 + 2H+ + 2e- → CO + H2O). Under conditions of slow mass transport, this leaves no protons to support hydrogen evolution. On the basis of the results, a general design principle is derived for acid CO2 electrolyzers to suppress hydrogen evolution from proton redn.: the rate of CO/OH- formation must be high enough to match/compensate the mass transfer of protons to the electrode surface.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXis1KrsrrN&md5=78fe7891f0cb31b68c9f5522b8bc2214
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Huang, J. E. ; Li, F. ; Ozden, A. ; Sedighian Rasouli, A. ; García de Arquer, F. P. ; Liu, S. ; Zhang, S. ; Luo, M. ; Wang, X. ; Lum, Y. ; Xu, Y. ; Bertens, K. ; Miao, R. K. ; Dinh, C.-T. ; Sinton, D. ; Sargent, E. H. CO₂ electrolysis to multicarbon products in strong acid. Science 2021, 372 , 1074– 1078, DOI: 10.1126/science.abg6582

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10

CO2 electrolysis to multicarbon products in strong acid

Huang, Jianan Erick; Li, Fengwang; Ozden, Adnan; Sedighian Rasouli, Armin; Garcia de Arquer, F. Pelayo; Liu, Shijie; Zhang, Shuzhen; Luo, Mingchuan; Wang, Xue; Lum, Yanwei; Xu, Yi; Bertens, Koen; Miao, Rui Kai; Dinh, Cao-Thang; Sinton, David; Sargent, Edward H.

Science (Washington, DC, United States) (2021), 372 (6546), 1074-1078CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)

Carbon dioxide electroredn. (CO2R) is being actively studied as a promising route to convert carbon emissions to valuable chems. and fuels. However, the fraction of input CO2 that is productively reduced has typically been very low, <2% for multicarbon products; the balance reacts with hydroxide to form carbonate in both alk. and neutral reactors. Acidic electrolytes would overcome this limitation, but hydrogen evolution has hitherto dominated under those conditions. We report that concg. potassium cations in the vicinity of electrochem. active sites accelerates CO2 activation to enable efficient CO2R in acid. We achieve CO2R on copper at pH <1 with a single-pass CO2 utilization of 77%, including a conversion efficiency of 50% toward multicarbon products (ethylene, ethanol, and 1-propanol) at a c.d. of 1.2 A per square centimeter and a full-cell voltage of 4.2 V.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXht1GmsLnN&md5=1b392dc3956d2b753e5a689e15c4fb13
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Kaczur, J. J. ; Yang, H. ; Liu, Z. ; Sajjad, S. D. ; Masel, R. I. Carbon Dioxide and Water Electrolysis Using New Alkaline Stable Anion Membranes. Front. Chem. 2018, 6 , 263, DOI: 10.3389/fchem.2018.00263

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11

Carbon dioxide and water electrolysis using new alkaline stable anion membranes

Kaczur, Jerry J.; Yang, Hongzhou; Liu, Zengcai; Sajjad, Syed D.; Masel, Richard I.

Frontiers in Chemistry (Lausanne, Switzerland) (2018), 6 (), 263/1-263/16CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)

The recent development and market introduction of a new type of alk. stable imidazole-based anion exchange membrane and related ionomers by Dioxide Materials is enabling the advancement of new and improved electrochem. processes which can operate at com. viable operating voltages, current efficiencies, and current densities. These processes include the electrochem. conversion of CO2 to formic acid (HCOOH), CO2 to carbon monoxide (CO), and alk. water electrolysis, generating hydrogen at high current densities at low voltages without the need for any precious metal electrocatalysts. The first process is the direct electrochem. generation of pure formic acid in a three-compartment cell configuration using the alk. stable anion exchange membrane and a cation exchange membrane. The cell operates at a c.d. of 140 mA/cm2 at a cell voltage of 3.5 V. The power consumption for prodn. of formic acid (FA) is about 4.3-4.7 kWh/kg of FA. The second process is the electrochem. conversion of CO2 to CO, a key focus product in the generation of renewable fuels and chems. The CO2 cell consists of a two-compartment design utilizing the alk. stable anion exchange membrane to sep. the anode and cathode compartments. A nanoparticle IrO2 catalyst on a GDE structure is used as the anode and a GDE utilizing a nanoparticle Ag/imidazolium-based ionomer catalyst combination is used as a cathode. The CO2 cell has been operated at current densities of 200 to 600 mA/cm2 at voltages of 3.0 to 3.2 resp. with CO2 to CO conversion selectivities of 95-99%. The third process is an alk. water electrolysis cell process, where the alk. stable anion exchange membrane allows stable cell operation in 1M KOH electrolyte solns. at current densities of 1 A/cm2 at about 1.90 V. The cell has demonstrated operation for thousands of hours, showing a voltage increase in time of only 5μV/h. The alk. electrolysis technol. does not require any precious metal catalysts as compared to polymer electrolyte membrane (PEM) design water electrolyzers. In this paper, we discuss the detailed tech. aspects of these three technologies utilizing this unique anion exchange membrane.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXisFaisb%252FO&md5=f28282e1392079e9672ce1c4a13bbf37
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Lees, E. W. ; Goldman, M. ; Fink, A. G. ; Dvorak, D. J. ; Salvatore, D. A. ; Zhang, Z. ; Loo, N. W. X. ; Berlinguette, C. P. Electrodes Designed for Converting Bicarbonate into CO. ACS Energy Lett. 2020, 5 , 2165– 2173, DOI: 10.1021/acsenergylett.0c00898

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12

Electrodes Designed for Converting Bicarbonate into CO

Lees, Eric W.; Goldman, Maxwell; Fink, Arthur G.; Dvorak, David J.; Salvatore, Danielle A.; Zhang, Zishuai; Loo, Nicholas W. X.; Berlinguette, Curtis P.

ACS Energy Letters (2020), 5 (7), 2165-2173CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

Deployment of electrolyzers which convert CO2 into chems. and fuels requires appropriate integration with upstream C capture processes. The electrolytic conversion of aq. (bi)carbonate offers the opportunity to avoid energy-intensive steps currently used to ext. pressurized CO2 from C capture solns. This work demonstrated an optimized Ag gas diffusion electrode (GDE) architecture enabled conversion of model C capture solns. (3 M KHCO3) to CO at partial current densities (JCO) >100 mA/cm2 with CO2 utilization rates of ∼70%. Results exceeded the performance of any previously reported liq.-fed CO2 electrolyzer and rival gas-fed devices. The authors hit these metrics by systematic design of gas diffusion layer (GDL) components (e.g., polytetrafluoroethylene) and catalyst layer constituents (i.e., Nafion, Ag) for CO prodn. A key result is that hydrophobic GDE components (common to gas-fed CO2 RR electrolyzers) decreased in-situ CO2 generation; hence, formation of the final CO product. A clear path toward industrially relevant reactors which couple electrolytic CO2 conversion with C capture was demonstrated.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhtVyrt7jK&md5=7db70ff9fc398467aad2ecc61391fae3
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Li, Y. C. ; Lee, G. ; Yuan, T. ; Wang, Y. ; Nam, D.-H. ; Wang, Z. ; García de Arquer, F. P. ; Lum, Y. ; Dinh, C.-T. ; Voznyy, O. ; Sargent, E. H. CO₂ Electroreduction from Carbonate Electrolyte. ACS Energy Lett. 2019, 4 , 1427– 1431, DOI: 10.1021/acsenergylett.9b00975

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13

CO2 Electroreduction from Carbonate Electrolyte

Li, Yuguang C.; Lee, Geonhui; Yuan, Tiange; Wang, Ying; Nam, Dae-Hyun; Wang, Ziyun; Garcia de Arquer, F. Pelayo; Lum, Yanwei; Dinh, Cao-Thang; Voznyy, Oleksandr; Sargent, Edward H.

ACS Energy Letters (2019), 4 (6), 1427-1431CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

The process of CO2 valorization-from capture of CO2 to its electrochem. upgrade-requires significant inputs in each of the capture, upgrade, and sepn. steps. Here the authors report an electrolyzer that upgrades carbonate electrolyte from CO2 capture soln. to syngas, achieving 100% C use across the system. A bipolar membrane was used to produce proton in situ to facilitate CO2 release at the membrane:catalyst interface from the carbonate soln. Using a Ag catalyst, the authors generate syngas at a 3:1 H2:CO ratio, and the product is not dild. by CO2 at the gas outlet; the authors generate this pure syngas product stream at a c.d. of 150 mA/cm2 and an energy efficiency of 35%. The carbonate-to-syngas system is stable under a continuous 145 h of catalytic operation. The work demonstrates the benefits of coupling CO2 electrolysis with a CO2 capture electrolyte on the path to practicable CO2 conversion technologies.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtVejtrfF&md5=54aa3a6b76f98e7735a2748b2eb9de57
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Li, T. ; Lees, E. W. ; Goldman, M. ; Salvatore, D. A. ; Weekes, D. M. ; Berlinguette, C. P. Electrolytic Conversion of Bicarbonate into CO in a Flow Cell. Joule 2019, 3 , 1487– 1497, DOI: 10.1016/j.joule.2019.05.021

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14

Electrolytic Conversion of Bicarbonate into CO in a Flow Cell

Li, Tengfei; Lees, Eric W.; Goldman, Maxwell; Salvatore, Danielle A.; Weekes, David M.; Berlinguette, Curtis P.

Joule (2019), 3 (6), 1487-1497CODEN: JOULBR; ISSN:2542-4351. (Cell Press)

Electrolyzers designed to convert CO2 into carbon products typically rely on a gaseous CO2 feedstock or CO2-satd. electrolyte. We show herein that aq. HCO-3 solns. can also be electrochem. converted into CO gas at meaningful rates in a flow cell contg. a bipolar membrane (BPM). Electrolysis upon a N2-satd. 3.0 M KHCO3 soln. yields CO with a faradic efficiency of 81% at 25 mA cm-2 and 37% at 100 mA cm-2, outputs that are comparable to the analogous expt. where the bicarbonate soln. is satd. with gaseous CO2. This electrolytic process is made possible by the membrane delivering protons for reaction with the bicarbonate feed to form electrocatalytically active CO2. This reaction pathway offers the potential to use electrolysis to bypass the thermally intensive step of extg. CO2 from HCO-3 solns. generated in carbon-capture schemes.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXht1WqsL%252FL&md5=5b860953a3f3891582ec6eae746ec061
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Lee, G. ; Li, Y. C. ; Kim, J.-Y. ; Peng, T. ; Nam, D.-H. ; Sedighian Rasouli, A. ; Li, F. ; Luo, M. ; Ip, A. H. ; Joo, Y.-C. ; Sargent, E. H. Electrochemical upgrade of CO₂ from amine capture solution. Nat. Energy 2021, 6 , 46– 53, DOI: 10.1038/s41560-020-00735-z

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15

Electrochemical upgrade of CO2 from amine capture solution

Lee, Geonhui; Li, Yuguang C.; Kim, Ji-Yong; Peng, Tao; Nam, Dae-Hyun; Sedighian Rasouli, Armin; Li, Fengwang; Luo, Mingchuan; Ip, Alexander H.; Joo, Young-Chang; Sargent, Edward H.

Nature Energy (2021), 6 (1), 46-53CODEN: NEANFD; ISSN:2058-7546. (Nature Research)

CO2 capture technologies based on chemisorption present the potential to lower net emissions of CO2 into the atm. The electrochem. upgrade of captured CO2 to value-added products would be particularly convenient. Here we find that this goal is curtailed when the adduct of the capture mol. with CO2 fails to place the CO2 sufficiently close to the site of the heterogeneous reaction. We investigate tailoring the electrochem. double layer to achieve the valorization of chemisorbed CO2 in an aq. monoethanolamine electrolyte. We reveal, using electrochem. studies and in situ surface-enhanced Raman spectroscopy, that a smaller double layer distance correlates with improved activity for CO2 to CO from amine solns. With the aid of an alkali cation and accelerated mass transport by system design-temp. and concn.-we demonstrate amine-CO2 conversion to CO with 72% Faradaic efficiency at 50 mA cm-2.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXosFekurg%253D&md5=2ce866646d5c54f39c242468c37f8d4c
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Jouny, M. ; Hutchings, G. S. ; Jiao, F. Carbon monoxide electroreduction as an emerging platform for carbon utilization. Nat. Catal. 2019, 2 , 1062– 1070, DOI: 10.1038/s41929-019-0388-2

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16

Carbon monoxide electroreduction as an emerging platform for carbon utilization

Jouny, Matthew; Hutchings, Gregory S.; Jiao, Feng

Nature Catalysis (2019), 2 (12), 1062-1070CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

A review. The electrochem. conversion of carbon dioxide to value-added chem. products has been heavily explored as a promising strategy for carbon utilization. However, the direct synthesis of multi-carbon (C2+) products suffers from undesired side reactions and relatively low selectivity. Electrochem. converting CO2 to single-carbon products is much more effective and being com. deployed. Recent studies have shown that CO can be electrochem. transformed further to C2+ at high reaction rates, high C2+ selectivity and inherently improved electrolyte stability, raising the prospect of a two-step pathway to transform CO2. In this Perspective, the progress towards high-rate CO conversion is shown alongside mechanistic insights and device designs that can improve performance even further. A techno-economic anal. of the two-step conversion process and cradle-to-gate lifecycle assessment shows the economic feasibility and improved environmental impact of a high-vol. com. process generating acetic acid and ethylene compared to the current state of the art.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXisVWqurbO&md5=da59e3e06b3426f9011b9955718a8c55
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Fu, X. ; Zhang, J. ; Kang, Y. Electrochemical reduction of CO₂ towards multi-carbon products via a two-step process. React. Chem. Eng. 2021, 6 , 612– 628, DOI: 10.1039/D1RE00001B

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17

Electrochemical reduction of CO2 towards multi-carbon products via a two-step process

Fu, Xianbiao; Zhang, Jiahao; Kang, Yijin

Reaction Chemistry & Engineering (2021), 6 (4), 612-628CODEN: RCEEBW; ISSN:2058-9883. (Royal Society of Chemistry)

A review. The electrochem. conversion of carbon dioxide (CO2) towards clean fuels and chems. powered by renewable energy is a promising strategy to realize the closing of the loop of carbon footprint. However, the direct redn. of CO2 to multi-carbon (C2+) products suffers from low activity in non-alk. electrolyte or electrolyte degrdn. problem caused by carbonate formation in alk. electrolyte. The two-step process for CO2 electrocution can circumvent such problems by converting CO2 to CO (the first step) in the non-alk. electrolyte and promote the rate of carbon-carbon coupling for CO-to-C2+ conversion (the second step) in alk. electrolytes. We summarize the recent progress of CO-selective catalysts, C2+-selective catalysts, tandem catalysts, and tandem reaction systems, which aim to achieve the efficient prodn. of C2+ products with high selectivity. The two-step route of CO2 redn. pushes the chem. prodn. from environmentally abundant mols. closer to the practical application, offering a promising replacement in the petrochem. industry for chem. prodn. under hydrogen economy in the future.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXjtlKntbc%253D&md5=9e5a344571b48c5e06ae05d249b9527f
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Overa, S. ; Feric, T. G. ; Park, A.-H. A. ; Jiao, F. Tandem and Hybrid Processes for Carbon Dioxide Utilization. Joule 2021, 5 , 8– 13, DOI: 10.1016/j.joule.2020.12.004
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Gurudayal ; Perone, D. ; Malani, S. ; Lum, Y. ; Haussener, S. ; Ager, J. W. Sequential Cascade Electrocatalytic Conversion of Carbon Dioxide to C–C Coupled Products. ACS Appl. Energy Mater. 2019, 2 , 4551– 4559, DOI: 10.1021/acsaem.9b00791

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Sequential Cascade Electrocatalytic Conversion of Carbon Dioxide to C-C Coupled Products

Gurudayal; Perone, David; Malani, Saurabh; Lum, Yanwei; Haussener, Sophia; Ager, Joel W.

ACS Applied Energy Materials (2019), 2 (6), 4551-4559CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)

Cascade catalytic processes perform multistep chem. transformations without isolating the intermediates. Here, the authors demonstrate a sequential cascade pathway to convert CO2 to C2+ hydrocarbons and oxygenates in a two-step electrocatalytic process using CO as the intermediate. CO2 to CO conversion was performed by using Ag, and further conversion of CO to C-C coupled products was performed with Cu. Temporal sepn. between the two reaction steps is accomplished by situating the Ag electrode upstream of the Cu electrode in a continuous flow reactor. Convection-diffusion simulations and exptl. evaluation of the electrodes individually were performed to identify optimal conditions. With the upstream Ag electrode poised at -1 V vs. reversible H electrode in a flow of CO2-satd. H2O in aq. carbonate buffer, over 80% of the CO can be converted on the downstream Cu electrode. When the Ag electrode is on, a supersatn. of CO is achieved near the Cu electrode, which leads to a relative increase in the formation rate of C2 and C3 oxygenates as compared to ethylene.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtVGit7zI&md5=e8170bff212658e0042e8585485e9975
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Jouny, M. ; Luc, W. ; Jiao, F. General Techno-Economic Analysis of CO₂ Electrolysis Systems. Ind. Eng. Chem. Res. 2018, 57 , 2165– 2177, DOI: 10.1021/acs.iecr.7b03514

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General Techno-Economic Analysis of CO2 Electrolysis Systems

Jouny, Matthew; Luc, Wesley; Jiao, Feng

Industrial & Engineering Chemistry Research (2018), 57 (6), 2165-2177CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)

The electrochem. redn. of carbon dioxide (CO2) has received significant attention in academic research, although the techno-economic prospects of the technol. for the large-scale prodn. of chems. are unclear. In this work, we briefly reviewed the current state-of-the-art CO2 redn. figures of merit, and performed an economic anal. to calc. the end-of-life net present value (NPV) of a generalized CO2 electrolyzer system for the prodn. of 100 tons/day of various CO2 redn. products. Under current techno-economic conditions, carbon monoxide and formic acid were the only economically viable products with NPVs of $13.5 million and $39.4 million, resp. However, higher-order alcs., such as ethanol and n-propanol, could be highly promising under future conditions if reasonable electrocatalytic performance benchmarks are achieved (e.g., 300 mA/cm2 and 0.5 V overpotential at 70% Faradaic efficiency). Herein, we established performance targets such that if these targets are achieved, electrochem. CO2 redn. for fuels and chems. prodn. can become a profitable option as part of the growing renewable energy infrastructure.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhtlyqtLo%253D&md5=95c1c8ed22cc0b13b8bbfc6cd3eb8376
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Spurgeon, J. M. ; Kumar, B. A comparative technoeconomic analysis of pathways for commercial electrochemical CO₂ reduction to liquid products. Energy Environ. Sci. 2018, 11 , 1536– 1551, DOI: 10.1039/C8EE00097B

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A comparative technoeconomic analysis of pathways for commercial electrochemical CO2 reduction to liquid products

Spurgeon, Joshua M.; Kumar, Bijandra

Energy & Environmental Science (2018), 11 (6), 1536-1551CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)

Electrochem. redn. of CO2 to fuels and chems. is currently a focus of significant research effort as a technol. that can simultaneously mitigate greenhouse gas emissions while storing renewable electricity for use on demand. Liq. products are particularly desirable as an easily storable and portable energy-dense form. To be widely implemented, CO2 electroredn. technologies must be able to produce chems. at costs that are economically competitive with existing com. prices. In this work, four possible routes to the electrochem. synthesis of liq. products from CO2 derived from post-combustion flue gas were compared with one consistent approach to technoeconomic anal. In the first case, diesel fuel was produced from electrosynthesized CO plus H2 to make syngas which was subsequently converted through the Fischer-Tropsch process. Liq. ethanol was modeled through two comparable approaches, a one-step electrolysis and a two-step cascade electrolysis. Lastly, the direct electrosynthesis of formic acid from CO2 was considered. In the base case scenarios established on current state-of-the-art CO2 redn. research, none of the processes were modeled to be competitive with present fuel prices. High capital expense for the electrolyzer units was the primary limiting factor. With conceivable improvements in an optimistic scenario, the diesel process was projected to have the best pathway to making cost-effective fuels, while ethanol would be prohibitively expensive without major improvements to the present electrosynthesis performance. Formic acid, though projected to be expensive relative to its stored energy content, was projected to have perhaps the simplest pathway to prodn. at costs competitive with its com. bulk price. In each case, the levelized cost of the liq. product was most strongly influenced by parameters that affect the electrolyzer capital cost (i.e., c.d., faradaic efficiency, and cost per electrode area).

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntV2lsbg%253D&md5=38200748159f3323adcb95570a98a86d
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Sisler, J. ; Khan, S. ; Ip, A. H. ; Schreiber, M. W. ; Jaffer, S. A. ; Bobicki, E. R. ; Dinh, C.-T. ; Sargent, E. H. Ethylene Electrosynthesis: A Comparative Techno-economic Analysis of Alkaline vs Membrane Electrode Assembly vs CO₂-CO-C₂H₄ Tandems. ACS Energy Lett. 2021, 6 , 997– 1002, DOI: 10.1021/acsenergylett.0c02633

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Ethylene Electrosynthesis: A Comparative Techno-economic Analysis of Alkaline vs. Membrane Electrode Assembly vs. CO2-CO-C2H4 Tandems

Sisler, Jared; Khan, Shaihroz; Ip, Alexander H.; Schreiber, Moritz W.; Jaffer, Shaffiq A.; Bobicki, Erin R.; Dinh, Cao-Thang; Sargent, Edward H.

ACS Energy Letters (2021), 6 (3), 997-1002CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

This work has demonstrated an anal. of the cost of producing ethylene in a neutral MEA, an alk. flow cell electrolyzer, and a high-temp. SOEC. By first comparing single-step redn. of CO2 to ethylene, using a comparison of cell metrics from literature, it was detd. that low eEE and loss of CO2 to carbonate and crossover contribute the most cost to each system. As a final study, we calcd. the cost of producing ethylene in a two-step tandem process that has been discussed in the literature as a technique to avoid carbonate formation. After comparing this system to the previous single-step processes, we found that it has more promise for producing low-cost ethylene due to its efficient use of energy and CO2.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXktVCgu78%253D&md5=1e721c5c7b632b52c4b67fad87fd31a8
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Pappijn, C. A. R. ; Ruitenbeek, M. ; Reyniers, M.-F. ; Van Geem, K. M. Challenges and Opportunities of Carbon Capture and Utilization: Electrochemical Conversion of CO₂ to Ethylene. Front. Energy Res. 2020, 8 , 557466, DOI: 10.3389/fenrg.2020.557466
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Somoza-Tornos, A. ; Guerra, O. J. ; Crow, A. M. ; Smith, W. A. ; Hodge, B.-M. Process modeling, techno-economic assessment, and life cycle assessment of the electrochemical reduction of CO₂: a review. iScience 2021, 24 , 102813, DOI: 10.1016/j.isci.2021.102813

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Process modeling, techno-economic assessment, and life cycle assessment of the electrochemical reduction of CO2: a review

Somoza-Tornos Ana; Crow Allison M; Smith Wilson A; Hodge Bri-Mathias; Guerra Omar J; Crow Allison M; Smith Wilson A; Hodge Bri-Mathias

iScience (2021), 24 (7), 102813 ISSN:.

The electrochemical reduction of CO2 has emerged as a promising alternative to traditional fossil-based technologies for the synthesis of chemicals. Its industrial implementation could lead to a reduction in the carbon footprint of chemicals and the mitigation of climate change impacts caused by hard-to-decarbonize industrial applications, among other benefits. However, the current low technology readiness levels of such emerging technologies make it hard to predict their performance at industrial scales. During the past few years, researchers have developed diverse techniques to model and assess the electrochemical reduction of CO2 toward its industrial implementation. The aim of this literature review is to provide a comprehensive overview of techno-economic and life cycle assessment methods and pave the way for future assessment approaches. First, we identify which modeling approaches have been conducted to extend analysis to the production scale. Next, we explore the metrics used to evaluate such systems, regarding technical, environmental, and economic aspects. Finally, we assess the challenges and research opportunities for the industrial implementation of CO2 reduction via electrolysis.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BB2czptFWmug%253D%253D&md5=27540c594b7522a2457d31d8fab91ddd
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Na, J. ; Seo, B. ; Kim, J. ; Lee, C. W. ; Lee, H. ; Hwang, Y. J. ; Min, B. K. ; Lee, D. K. ; Oh, H.-S. ; Lee, U. General technoeconomic analysis for electrochemical coproduction coupling carbon dioxide reduction with organic oxidation. Nat. Commun. 2019, 10 , 5193, DOI: 10.1038/s41467-019-12744-y

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General technoeconomic analysis for electrochemical coproduction coupling carbon dioxide reduction with organic oxidation

Na Jonggeol; Seo Bora; Kim Jeongnam; Lee Hyunjoo; Hwang Yun Jeong; Min Byoung Koun; Lee Dong Ki; Oh Hyung-Suk; Lee Ung; Na Jonggeol; Kim Jeongnam; Lee Chan Woo; Lee Hyunjoo; Hwang Yun Jeong; Oh Hyung-Suk; Lee Ung; Hwang Yun Jeong; Min Byoung Koun; Lee Ung

Nature communications (2019), 10 (1), 5193 ISSN:.

Electrochemical processes coupling carbon dioxide reduction reactions with organic oxidation reactions are promising techniques for producing clean chemicals and utilizing renewable energy. However, assessments of the economics of the coupling technology remain questionable due to diverse product combinations and significant process design variability. Here, we report a technoeconomic analysis of electrochemical carbon dioxide reduction reaction-organic oxidation reaction coproduction via conceptual process design and thereby propose potential economic combinations. We first develop a fully automated process synthesis framework to guide process simulations, which are then employed to predict the levelized costs of chemicals. We then identify the global sensitivity of current density, Faraday efficiency, and overpotential across 295 electrochemical coproduction processes to both understand and predict the levelized costs of chemicals at various technology levels. The analysis highlights the promise that coupling the carbon dioxide reduction reaction with the value-added organic oxidation reaction can secure significant economic feasibility.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BB3Mjptl2ltA%253D%253D&md5=da5bfd44648874a14b4cd1de29522c51
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De Luna, P. ; Hahn, C. ; Higgins, D. ; Jaffer, S. A. ; Jaramillo, T. F. ; Sargent, E. H. What would it take for renewably powered electrosynthesis to displace petrochemical processes?. Science 2019, 364 , eaav3506 DOI: 10.1126/science.aav3506
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Shin, H. ; Hansen, K. U. ; Jiao, F. Techno-economic assessment of low-temperature carbon dioxide electrolysis. Nat. Sustain. 2021, 4 , 911– 919, DOI: 10.1038/s41893-021-00739-x
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Orella, M. J. ; Brown, S. M. ; Leonard, M. E. ; Román-Leshkov, Y. ; Brushett, F. R. A General Technoeconomic Model for Evaluating Emerging Electrolytic Processes. Energy Technol. 2020, 8 , 1900994, DOI: 10.1002/ente.201900994
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Hori, Y. ; Kikuchi, K. ; Murata, A. ; Suzuki, S. Production of methane and ethylene in electrochemical reduction of carbon dioxide at copper electrode in aqueous hydrogencarbonate solution. Chem. Lett. 1986, 15 , 897– 898, DOI: 10.1246/cl.1986.897
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Hori, Y. ; Murata, A. ; Takahashi, R. ; Suzuki, S. Electroreduction of carbon monoxide to methane and ethylene at a copper electrode in aqueous solutions at ambient temperature and pressure. J. Am. Chem. Soc. 1987, 109 , 5022– 5023, DOI: 10.1021/ja00250a044

[ACS Full Text ], [CAS], Google Scholar

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Electroreduction of carbon monoxide to methane and ethylene at a copper electrode in aqueous solutions at ambient temperature and pressure

Hori, Yoshio; Murata, Akira; Takahashi, Ryutaro; Suzuki, Shin

Journal of the American Chemical Society (1987), 109 (16), 5022-3CODEN: JACSAT; ISSN:0002-7863.

The electrochem. redn. of CO in aq. soln. using phosphate, carbonate, and hydroxide electrolytes is reported. The highest total current efficiency was obtained at a current of 2.5 mA cm-2 with the KHCO3 electrolyte. Under these conditions the individual product-current efficiencies were CH4 16.3%, CH2:CH2 21.2%, EtOH 10.9%, PrOH 1.5%, and HCHO 0.1%.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL2sXlsFertbk%253D&md5=5dff48ac27936d9d164f9287818f5bdb
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García de Arquer, F. P. ; Dinh, C.-T. ; Ozden, A. ; Wicks, J. ; McCallum, C. ; Kirmani, A. R. ; Nam, D.-H. ; Gabardo, C. ; Seifitokaldani, A. ; Wang, X. ; Li, Y. C. ; Li, F. ; Edwards, J. ; Richter, L. J. ; Thorpe, S. J. ; Sinton, D. ; Sargent, E. H. CO₂ electrolysis to multicarbon products at activities greater than 1 A cm^–2 . Science 2020, 367 , 661– 666, DOI: 10.1126/science.aay4217

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CO2 electrolysis to multicarbon products at activities greater than 1 A cm-2

Garcia de Arquer, F. Pelayo; Dinh, Cao-Thang; Ozden, Adnan; Wicks, Joshua; McCallum, Christopher; Kirmani, Ahmad R.; Nam, Dae-Hyun; Gabardo, Christine; Seifitokaldani, Ali; Wang, Xue; Li, Yuguang C.; Li, Fengwang; Edwards, Jonathan; Richter, Lee J.; Thorpe, Steven J.; Sinton, David; Sargent, Edward H.

Science (Washington, DC, United States) (2020), 367 (6478), 661-666CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)

Electrolysis offers an attractive route to upgrade greenhouse gases such as carbon dioxide (CO2) to valuable fuels and feedstocks; however, productivity is often limited by gas diffusion through a liq. electrolyte to the surface of the catalyst. Here, we present a catalyst:ionomer bulk heterojunction (CIBH) architecture that decouples gas, ion, and electron transport. The CIBH comprises a metal and a superfine ionomer layer with hydrophobic and hydrophilic functionalities that extend gas and ion transport from tens of nanometers to the micrometer scale. By applying this design strategy, we achieved CO2 electroredn. on copper in 7 M potassium hydroxide electrolyte (pH about 15) with an ethylene partial c.d. of 1.3 A per square centimeter at 45% cathodic energy efficiency.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXisFKksr8%253D&md5=e82b9abd305407660b80f9164d88b68f
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Dinh, C.-T. ; Burdyny, T. ; Kibria, M. G. ; Seifitokaldani, A. ; Gabardo, C. M. ; García de Arquer, F. P. ; Kiani, A. ; Edwards, J. P. ; De Luna, P. ; Bushuyev, O. S. ; Zou, C. ; Quintero-Bermudez, R. ; Pang, Y. ; Sinton, D. ; Sargent, E. H. CO₂ electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface. Science 2018, 360 , 783– 787, DOI: 10.1126/science.aas9100

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Carbon dioxide electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface

Dinh, Cao-Thang; Burdyny, Thomas; Kibria, Md Golam; Seifitokaldani, Ali; Gabardo, Christine M.; Garcia de Arquer, F. Pelayo; Kiani, Amirreza; Edwards, Jonathan P.; De Luna, Phil; Bushuyev, Oleksandr S.; Zou, Chengqin; Quintero-Bermudez, Rafael; Pang, Yuanjie; Sinton, David; Sargent, Edward H.

Science (Washington, DC, United States) (2018), 360 (6390), 783-787CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)

Carbon dioxide (CO2) electroredn. could provide a useful source of ethylene, but low conversion efficiency, low prodn. rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alk. electrolyte reduces CO2 to ethylene with 70% faradaic efficiency at a potential of -0.55 V vs. a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO2 redn. and carbon monoxide (CO)-CO coupling activation energy barriers; as a result, onset of ethylene evolution at -0.165 V vs. an RHE in 10 M potassium hydroxide occurs almost simultaneously with CO prodn. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between sep. hydrophobic and conductive supports, providing const. ethylene selectivity for an initial 150 operating hours.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXpsVCgsL0%253D&md5=0beec1cdcc8939b3eb057cb6b26742f6
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De Gregorio, G. L. ; Burdyny, T. ; Loiudice, A. ; Iyengar, P. ; Smith, W. A. ; Buonsanti, R. Facet-Dependent Selectivity of Cu Catalysts in Electrochemical CO₂ Reduction at Commercially Viable Current Densities. ACS Catal. 2020, 10 , 4854– 4862, DOI: 10.1021/acscatal.0c00297

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Facet-Dependent Selectivity of Cu Catalysts in Electrochemical CO2 Reduction at Commercially Viable Current Densities

De Gregorio, Gian Luca; Burdyny, Thomas; Loiudice, Anna; Iyengar, Pranit; Smith, Wilson A.; Buonsanti, Raffaella

ACS Catalysis (2020), 10 (9), 4854-4862CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

Despite substantial progress in the electrochem. conversion of CO2 into value-added chems., the translation of fundamental studies into com. relevant conditions requires addnl. efforts. Here, the authors study the catalytic properties of tailored Cu nanocatalysts under com. relevant current densities in a gas-fed flow cell. Their facet-dependent selectivity is retained in this device configuration with the advantage of further suppressing H prodn. and increasing the faradaic efficiencies toward the CO2 redn. products compared to a conventional H-cell. The combined catalyst and system effects result in state-of-the art product selectivity at high current densities (in the range 100-300 mA/cm2) and at relatively low applied potential (≥-0.65 V vs. RHE). Cu cubes reach an ethylene selectivity of up to 57% with a corresponding mass activity of 700 mA/mg, and Cu octahedra reach a methane selectivity of up to 51% with a corresponding mass activity of 1.45 A/mg in 1 M KOH.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXlvVGnsrs%253D&md5=21b5d57cb69ca1ef68e892115cddd01e
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Hoang, T. T. H. ; Verma, S. ; Ma, S. ; Fister, T. T. ; Timoshenko, J. ; Frenkel, A. I. ; Kenis, P. J. A. ; Gewirth, A. A. Nanoporous Copper–Silver Alloys by Additive-Controlled Electrodeposition for the Selective Electroreduction of CO₂ to Ethylene and Ethanol. J. Am. Chem. Soc. 2018, 140 , 5791– 5797, DOI: 10.1021/jacs.8b01868

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Nanoporous Copper-Silver Alloys by Additive-Controlled Electrodeposition for the Selective Electroreduction of CO2 to Ethylene and Ethanol

Hoang, Thao T. H.; Verma, Sumit; Ma, Sichao; Fister, Tim T.; Timoshenko, Janis; Frenkel, Anatoly I.; Kenis, Paul J. A.; Gewirth, Andrew A.

Journal of the American Chemical Society (2018), 140 (17), 5791-5797CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

Electrodeposition of CuAg alloy films from plating baths contg. 3,5-diamino-1,2,4-triazole (DAT) as an inhibitor yields high surface area catalysts for the active and selective electroredn. of CO2 to multicarbon hydrocarbons and oxygenates. EXAFS shows the co-deposited alloy film to be homogeneously mixed. The alloy film contg. 6% Ag exhibits the best CO2 electroredn. performance, with the faradaic efficiency for C2H4 and EtOH prodn. reaching nearly 60 and 25%, resp., at a cathode potential of just -0.7 V vs. RHE and a total c.d. of approx. - 300 mA/cm2. Such high levels of selectivity at high activity and low applied potential are the highest reported to date. In situ Raman and electroanal. studies suggest the origin of the high selectivity toward C2 products to be a combined effect of the enhanced stabilization of the Cu2O overlayer and the optimal availability of the CO intermediate due to the Ag incorporated in the alloy.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXntFWit7g%253D&md5=031c8a7848bd522daf01d9e00fe63864
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Vennekötter, J.-B. ; Scheuermann, T. ; Sengpiel, R. ; Wessling, M. The electrolyte matters: Stable systems for high rate electrochemical CO₂ reduction. J. CO2 Util. 2019, 32 , 202– 213, DOI: 10.1016/j.jcou.2019.04.007
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Wang, X. ; Wang, Z. ; García de Arquer, F. P. ; Dinh, C.-T. ; Ozden, A. ; Li, Y. C. ; Nam, D.-H. ; Li, J. ; Liu, Y.-S. ; Wicks, J. ; Chen, Z. ; Chi, M. ; Chen, B. ; Wang, Y. ; Tam, J. ; Howe, J. Y. ; Proppe, A. ; Todorović, P. ; Li, F. ; Zhuang, T.-T. ; Gabardo, C. M. ; Kirmani, A. R. ; McCallum, C. ; Hung, S.-F. ; Lum, Y. ; Luo, M. ; Min, Y. ; Xu, A. ; O'Brien, C. P. ; Stephen, B. ; Sun, B. ; Ip, A. H. ; Richter, L. J. ; Kelley, S. O. ; Sinton, D. ; Sargent, E. H. Efficient electrically powered CO₂-to-ethanol via suppression of deoxygenation. Nat. Energy 2020, 5 , 478– 486, DOI: 10.1038/s41560-020-0607-8

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Efficient electrically powered CO2-to-ethanol via suppression of deoxygenation

Wang, Xue; Wang, Ziyun; Garcia de Arquer, F. Pelayo; Dinh, Cao-Thang; Ozden, Adnan; Li, Yuguang C.; Nam, Dae-Hyun; Li, Jun; Liu, Yi-Sheng; Wicks, Joshua; Chen, Zitao; Chi, Miaofang; Chen, Bin; Wang, Ying; Tam, Jason; Howe, Jane Y.; Proppe, Andrew; Todorovic, Petar; Li, Fengwang; Zhuang, Tao-Tao; Gabardo, Christine M.; Kirmani, Ahmad R.; McCallum, Christopher; Hung, Sung-Fu; Lum, Yanwei; Luo, Mingchuan; Min, Yimeng; Xu, Aoni; O'Brien, Colin P.; Stephen, Bello; Sun, Bin; Ip, Alexander H.; Richter, Lee J.; Kelley, Shana O.; Sinton, David; Sargent, Edward H.

Nature Energy (2020), 5 (6), 478-486CODEN: NEANFD; ISSN:2058-7546. (Nature Research)

The carbon dioxide electroredn. reaction (CO2RR) provides ways to produce ethanol but its Faradaic efficiency could be further improved, esp. in CO2RR studies reported at a total c.d. exceeding 10 mA cm-2. Here we report a class of catalysts that achieve an ethanol Faradaic efficiency of (52 ± 1)% and an ethanol cathodic energy efficiency of 31%. We exploit the fact that suppression of the deoxygenation of the intermediate HOCCH* to ethylene promotes ethanol prodn., and hence that confinement using capping layers having strong electron-donating ability on active catalysts promotes C-C coupling and increases the reaction energy of HOCCH* deoxygenation. Thus, we have developed an electrocatalyst with confined reaction vol. by coating Cu catalysts with nitrogen-doped carbon. Spectroscopy suggests that the strong electron-donating ability and confinement of the nitrogen-doped carbon layers leads to the obsd. pronounced selectivity towards ethanol.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXpt1yrsL4%253D&md5=c6a13deacdd9f00711528a1a48151e54
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Zhong, M. ; Tran, K. ; Min, Y. ; Wang, C. ; Wang, Z. ; Dinh, C.-T. ; De Luna, P. ; Yu, Z. ; Rasouli, A. S. ; Brodersen, P. ; Sun, S. ; Voznyy, O. ; Tan, C.-S. ; Askerka, M. ; Che, F. ; Liu, M. ; Seifitokaldani, A. ; Pang, Y. ; Lo, S.-C. ; Ip, A. ; Ulissi, Z. ; Sargent, E. H. Accelerated discovery of CO₂ electrocatalysts using active machine learning. Nature 2020, 581 , 178– 183, DOI: 10.1038/s41586-020-2242-8

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Accelerated discovery of CO2 electrocatalysts using active machine learning

Zhong, Miao; Tran, Kevin; Min, Yimeng; Wang, Chuanhao; Wang, Ziyun; Dinh, Cao-Thang; De Luna, Phil; Yu, Zongqian; Rasouli, Armin Sedighian; Brodersen, Peter; Sun, Song; Voznyy, Oleksandr; Tan, Chih-Shan; Askerka, Mikhail; Che, Fanglin; Liu, Min; Seifitokaldani, Ali; Pang, Yuanjie; Lo, Shen-Chuan; Ip, Alexander; Ulissi, Zachary; Sargent, Edward H.

Nature (London, United Kingdom) (2020), 581 (7807), 178-183CODEN: NATUAS; ISSN:0028-0836. (Nature Research)

The rapid increase in global energy demand and the need to replace carbon dioxide (CO2)-emitting fossil fuels with renewable sources have driven interest in chem. storage of intermittent solar and wind energy1,2. Particularly attractive is the electrochem. redn. of CO2 to chem. feedstocks, which uses both CO2 and renewable energy3-8. Copper has been the predominant electrocatalyst for this reaction when aiming for more valuable multi-carbon products9-16, and process improvements have been particularly notable when targeting ethylene. However, the energy efficiency and productivity (c.d.) achieved so far still fall below the values required to produce ethylene at cost-competitive prices. Here we describe Cu-Al electrocatalysts, identified using d. functional theory calcns. in combination with active machine learning, that efficiently reduce CO2 to ethylene with the highest Faradaic efficiency reported so far. This Faradaic efficiency of over 80 per cent (compared to about 66 per cent for pure Cu) is achieved at a c.d. of 400 mA per square centimetre (at 1.5 V vs. a reversible hydrogen electrode) and a cathodic-side (half-cell) ethylene power conversion efficiency of 55 ± 2 per cent at 150 mA per square centimetre. We perform computational studies that suggest that the Cu-Al alloys provide multiple sites and surface orientations with near-optimal CO binding for both efficient and selective CO2 redn.17. Furthermore, in situ X-ray absorption measurements reveal that Cu and Al enable a favorable Cu coordination environment that enhances C-C dimerization. These findings illustrate the value of computation and machine learning in guiding the exptl. exploration of multi-metallic systems that go beyond the limitations of conventional single-metal electrocatalysts.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXpt1Kksrs%253D&md5=408b83b4424ac9ef8742d9d4627c3863
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Chen, X. ; Chen, J. ; Alghoraibi, N. M. ; Henckel, D. A. ; Zhang, R. ; Nwabara, U. O. ; Madsen, K. E. ; Kenis, P. J. A. ; Zimmerman, S. C. ; Gewirth, A. A. Electrochemical CO₂-to-ethylene conversion on polyamine-incorporated Cu electrodes. Nat. Catal. 2021, 4 , 20– 27, DOI: 10.1038/s41929-020-00547-0

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Electrochemical CO2-to-ethylene conversion on polyamine-incorporated Cu electrodes

Chen, Xinyi; Chen, Junfeng; Alghoraibi, Nawal M.; Henckel, Danielle A.; Zhang, Ruixian; Nwabara, Uzoma O.; Madsen, Kenneth E.; Kenis, Paul J. A.; Zimmerman, Steven C.; Gewirth, Andrew A.

Nature Catalysis (2021), 4 (1), 20-27CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

Electrochem. conversion of CO2 into value-added chems. holds promise to enable the transition to carbon neutrality. Enhancing selectivity for a specific hydrocarbon product is challenging, however, due to numerous possible reaction pathways of CO2 electroredn. Here we present a Cu-polyamine hybrid catalyst, developed through co-electroplating, that significantly increases the selectivity for ethylene prodn. The Faradaic efficiency for ethylene prodn. is 87% ± 3% at -0.47 V vs. reversible hydrogen electrode, with full-cell energetic efficiency reaching 50% ± 2%. Raman measurements indicate that the polyamine entrained on the Cu electrode results in higher surface pH, higher CO content and higher stabilization of intermediates compared with entrainment of additives contg. little or no amine functionality. More broadly, this work shows that polymer incorporation can alter surface reactivity and lead to enhanced product selectivity at high current densities.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhtVerur%252FK&md5=d50600ff8aaf884a1ead360c89dc99c8
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Li, F. ; Thevenon, A. ; Rosas-Hernández, A. ; Wang, Z. ; Li, Y. ; Gabardo, C. M. ; Ozden, A. ; Dinh, C. T. ; Li, J. ; Wang, Y. ; Edwards, J. P. ; Xu, Y. ; McCallum, C. ; Tao, L. ; Liang, Z.-Q. ; Luo, M. ; Wang, X. ; Li, H. ; O'Brien, C. P. ; Tan, C.-S. ; Nam, D.-H. ; Quintero-Bermudez, R. ; Zhuang, T.-T. ; Li, Y. C. ; Han, Z. ; Britt, R. D. ; Sinton, D. ; Agapie, T. ; Peters, J. C. ; Sargent, E. H. Molecular tuning of CO₂-to-ethylene conversion. Nature 2020, 577 , 509– 513, DOI: 10.1038/s41586-019-1782-2

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Molecular tuning of CO2-to-ethylene conversion

Li, Fengwang; Thevenon, Arnaud; Rosas-Hernandez, Alonso; Wang, Ziyun; Li, Yilin; Gabardo, Christine M.; Ozden, Adnan; Dinh, Cao Thang; Li, Jun; Wang, Yuhang; Edwards, Jonathan P.; Xu, Yi; McCallum, Christopher; Tao, Lizhi; Liang, Zhi-Qin; Luo, Mingchuan; Wang, Xue; Li, Huihui; O'Brien, Colin P.; Tan, Chih-Shan; Nam, Dae-Hyun; Quintero-Bermudez, Rafael; Zhuang, Tao-Tao; Li, Yuguang C.; Han, Zhiji; Britt, R. David; Sinton, David; Agapie, Theodor; Peters, Jonas C.; Sargent, Edward H.

Nature (London, United Kingdom) (2020), 577 (7791), 509-513CODEN: NATUAS; ISSN:0028-0836. (Nature Research)

The electrocatalytic redn. of carbon dioxide, powered by renewable electricity, to produce valuable fuels and feedstocks provides a sustainable and carbon-neutral approach to the storage of energy produced by intermittent renewable sources. However, the highly selective generation of economically desirable products such as ethylene from the carbon dioxide redn. reaction (CO2RR) remains a challenge. Tuning the stabilities of intermediates to favor a desired reaction pathway can improve selectivity, and this has recently been explored for the reaction on copper by controlling morphol., grain boundaries, facets, oxidn. state and dopants. Unfortunately, the Faradaic efficiency for ethylene is still low in neutral media (60% at a partial c.d. of 7 mA per square centimetre in the best catalyst reported so far), resulting in a low energy efficiency. Here a mol. tuning strategy - the functionalization is presented of the surface of electrocatalysts with org. mols.-that stabilizes intermediates for more selective CO2RR to ethylene. Using electrochem., operando/in situ spectroscopic and computational studies, the effect is investigated of a library of mols., derived by electrodimerization of arylpyridiniums, adsorbed on copper. It was found that the adhered mols. improve the stabilization of a 'atop-bound' CO intermediate (i.e., an intermediate bound to a single copper atom), thereby favoring further redn. to ethylene. As a result of this strategy, the CO2RR to ethylene is reported with a Faradaic efficiency of 72% at a partial c.d. of 230 mA per square centimetre in a liq.-electrolyte flow cell in a neutral medium. stable ethylene electrosynthesis is reported for 190 h in a system based on a membrane-electrode assembly that provides a full-cell energy efficiency of 20%. It is anticipated that this may be generalized to enable mol. strategies to complement heterogeneous catalysts by stabilizing intermediates through local mol. tuning.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXis1Sit7Y%253D&md5=c89fc8be9ee70d8a3584e21f01a811ec
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Ma, W. ; Xie, S. ; Liu, T. ; Fan, Q. ; Ye, J. ; Sun, F. ; Jiang, Z. ; Zhang, Q. ; Cheng, J. ; Wang, Y. Electrocatalytic reduction of CO₂ to ethylene and ethanol through hydrogen-assisted C–C coupling over fluorine-modified copper. Nat. Catal. 2020, 3 , 478– 487, DOI: 10.1038/s41929-020-0450-0

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Electrocatalytic reduction of CO2 to ethylene and ethanol through hydrogen-assisted C-C coupling over fluorine-modified copper

Ma, Wenchao; Xie, Shunji; Liu, Tongtong; Fan, Qiyuan; Ye, Jinyu; Sun, Fanfei; Jiang, Zheng; Zhang, Qinghong; Cheng, Jun; Wang, Ye

Nature Catalysis (2020), 3 (6), 478-487CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

Electrocatalytic redn. of CO2 into multicarbon (C2+) products is a highly attractive route for CO2 utilization; however, the yield of C2+ products remains low because of the limited C2+ selectivity at high CO2 conversion rates. Here we report a fluorine-modified copper catalyst that exhibits an ultrahigh c.d. of 1.6 A cm-2 with a C2+ (mainly ethylene and ethanol) Faradaic efficiency of 80% for electrocatalytic CO2 redn. in a flow cell. The C2-4 selectivity reaches 85.8% at a single-pass yield of 16.5%. We show a hydrogen-assisted C-C coupling mechanism between adsorbed CHO intermediates for C2+ formation. Fluorine enhances water activation, CO adsorption and hydrogenation of adsorbed CO to CHO intermediate that can readily undergo coupling. Our findings offer an opportunity to design highly active and selective CO2 electroredn. catalysts with potential for practical application.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXnsFartbo%253D&md5=7d2cb4bc97471c2ba325c3463293c4c1
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Ozden, A. ; Li, F. ; Garcia de Arquer, F. P. ; Rosas-Hernández, A. ; Thevenon, A. ; Wang, Y. ; Hung, S.-F. ; Wang, X. ; Chen, B. ; Li, J. ; Wicks, J. ; Luo, M. ; Wang, Z. ; Agapie, T. ; Peters, J. C. ; Sargent, E. H. ; Sinton, D. High-Rate and Efficient Ethylene Electrosynthesis Using a Catalyst/Promoter/Transport Layer. ACS Energy Lett. 2020, 5 , 2811– 2818, DOI: 10.1021/acsenergylett.0c01266

[ACS Full Text ], [CAS], Google Scholar

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High-Rate and Efficient Ethylene Electrosynthesis Using a Catalyst/Promoter/Transport Layer

Ozden, Adnan; Li, Fengwang; Garcia de Arquer, F. Pelayo; Rosas-Hernandez, Alonso; Thevenon, Arnaud; Wang, Yuhang; Hung, Sung-Fu; Wang, Xue; Chen, Bin; Li, Jun; Wicks, Joshua; Luo, Mingchuan; Wang, Ziyun; Agapie, Theodor; Peters, Jonas C.; Sargent, Edward H.; Sinton, David

ACS Energy Letters (2020), 5 (9), 2811-2818CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

Carbon dioxide (CO2) electroredn. to valuable chems. such as ethylene is an avenue to store renewable electricity and close the carbon cycle. Membrane electrode assembly (MEA) electrolyzers have attracted recent interest in light of their high stability and despite low productivity (a modest partial c.d. in CO2-to-ethylene conversion of approx. 100 mA cm-2). Here we present an adlayer functionalization catalyst design: a catalyst/tetrahydro-phenanthrolinium/ionomer (CTPI) interface in which the catalytically active copper is functionalized using a phenanthrolinium-derived film and a perfluorocarbon-based polymeric ionomer. We find, using electroanal. tools and operando spectroscopies, that this hierarchical adlayer augments both the local CO2 availability and the adsorption of the key reaction intermediate CO on the catalyst surface. Using this CTPI catalyst, we achieve an ethylene Faradaic efficiency of 66% at a partial c.d. of 208 mA cm-2-a 2-fold increase over the best prior MEA electrolyzer report-and an improved full-cell energy efficiency of 21%.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhsFyrs73P&md5=f572caaceed69b2c22dab2f9e3af3f93
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She, X. ; Zhang, T. ; Li, Z. ; Li, H. ; Xu, H. ; Wu, J. Tandem Electrodes for Carbon Dioxide Reduction into C2+ Products at Simultaneously High Production Efficiency and Rate. Cell Reports Phys. Sci. 2020, 1 , 100051, DOI: 10.1016/j.xcrp.2020.100051
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Tan, Y. C. ; Lee, K. B. ; Song, H. ; Oh, J. Modulating Local CO₂ Concentration as a General Strategy for Enhancing C-C Coupling in CO₂ Electroreduction. Joule 2020, 4 , 1104– 1120, DOI: 10.1016/j.joule.2020.03.013

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43

Modulating Local CO2 Concentration as a General Strategy for Enhancing C-C Coupling in CO2 Electroreduction

Tan, Ying Chuan; Lee, Kelvin Berm; Song, Hakhyeon; Oh, Jihun

Joule (2020), 4 (5), 1104-1120CODEN: JOULBR; ISSN:2542-4351. (Cell Press)

Flow electrolyzers based on gas-diffusion electrodes (GDEs) have been increasingly employed to advance toward industry-relevant electrochem. CO2 redn. reaction (CO2RR) performance, though fundamental understanding of the GDE system is still lacking. Here, we propose that regulating local CO2 concn. on copper (Cu) surfaces is an effective and general strategy to promote C-C coupling in CO2RR. Local CO2 concn. could influence the surface coverage of *CO2, *H, and *CO, which affects the reaction pathways toward multi-carbon (C2+) products. Guided by mass-transport modeling, we have identified three approaches to modulate the local CO2 concn. in GDE-based electrolyzers: (1) catalyst layer structure, (2) feed CO2 concn., and (3) feed flow rate. Utilizing Cu2O nanoparticles as the model catalysts, modulation of local CO2 concn. enabled an optimized faradaic efficiency toward C2+ products of up to 75.5% at 300 mA cm-2 and C2+ partial c.d. of up to 342 mA cm-2 in 1.0 M KHCO3.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhtVCmsrvM&md5=d623f07ecf5d3b4f088c4238e4178a4c
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Wang, Y. ; Wang, Z. ; Dinh, C.-T. ; Li, J. ; Ozden, A. ; Golam Kibria, M. ; Seifitokaldani, A. ; Tan, C.-S. ; Gabardo, C. M. ; Luo, M. ; Zhou, H. ; Li, F. ; Lum, Y. ; McCallum, C. ; Xu, Y. ; Liu, M. ; Proppe, A. ; Johnston, A. ; Todorovic, P. ; Zhuang, T.-T. ; Sinton, D. ; Kelley, S. O. ; Sargent, E. H. Catalyst synthesis under CO₂ electroreduction favours faceting and promotes renewable fuels electrosynthesis. Nat. Catal. 2020, 3 , 98– 106, DOI: 10.1038/s41929-019-0397-1

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44

Catalyst synthesis under CO2 electroreduction favors faceting and promotes renewable fuels electrosynthesis

Wang, Yuhang; Wang, Ziyun; Dinh, Cao-Thang; Li, Jun; Ozden, Adnan; Golam Kibria, Md; Seifitokaldani, Ali; Tan, Chih-Shan; Gabardo, Christine M.; Luo, Mingchuan; Zhou, Hua; Li, Fengwang; Lum, Yanwei; McCallum, Christopher; Xu, Yi; Liu, Mengxia; Proppe, Andrew; Johnston, Andrew; Todorovic, Petar; Zhuang, Tao-Tao; Sinton, David; Kelley, Shana O.; Sargent, Edward H.

Nature Catalysis (2020), 3 (2), 98-106CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

The electrosynthesis of C2+ hydrocarbons from CO2 has attracted recent attention in light of the relatively high market price per unit energy input. Today's low selectivities and stabilities towards C2+ products at high current densities curtail system energy efficiency, which limits their prospects for economic competitiveness. Here we present a materials processing strategy based on in situ electrodeposition of copper under CO2 redn. conditions that preferentially expose and maintain Cu(100) facets, which favor the formation of C2+ products. We observe capping of facets during catalyst synthesis and achieve control over faceting to obtain a 70% increase in the ratio of Cu(100) facets to total facet area. We report a 90% Faradaic efficiency for C2+ products at a partial c.d. of 520 mA cm-2 and a full-cell C2+ power conversion efficiency of 37%. We achieve nearly const. C2H4 selectivity over 65 h operation at 350 mA cm-2 in a membrane electrode assembly electrolyzer.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXisVajsLvO&md5=a20a62418fb8f177170b5cd8faf00539
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Romero Cuellar, N. ; Wiesner-Fleischer, K. ; Fleischer, M. ; Rucki, A. ; Hinrichsen, O. Advantages of CO over CO₂ as reactant for electrochemical reduction to ethylene, ethanol and n-propanol on gas diffusion electrodes at high current densities. Electrochim. Acta 2019, 307 , 164– 175, DOI: 10.1016/j.electacta.2019.03.142

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Advantages of CO over CO2 as reactant for electrochemical reduction to ethylene, ethanol and n-propanol on gas diffusion electrodes at high current densities

Romero Cuellar, N. S.; Wiesner-Fleischer, K.; Fleischer, M.; Rucki, A.; Hinrichsen, O.

Electrochimica Acta (2019), 307 (), 164-175CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)

The electrochem. conversion of CO2 to value-added chems. is a technol. gaining broader interest as society moves towards a carbon-neutral circular economy. Nonetheless, there are still several challenges to overcome before this technol. can be applied as an industrial process. In the reaction path of the electrochem. redn. of CO2 with Cu as an electrocatalyst, it is known that carbon monoxide is the key intermediate to chems. such as ethylene, ethanol, and n-propanol. However, a better understanding of the electrochem. redn. of CO is still necessary to improve selectivity and efficiency at high current densities. In this work, the electrochem. redn. of CO2 and CO towards C2 and C3 products is investigated using gas diffusion electrodes in a flow cell. Thereby the electrochem. reaction is not limited by the soly. of the feed gas in the electrolyte, and current densities of industrial relevance can be achieved. The electrodes are prepd. using com. Cu-powders consisting either of nano- or microparticles that are deposited on gas diffusion layers. Potentiostatic expts. show that with CO as the reactant, higher current densities for C2 and C3 products can be achieved at lower working electrode potentials compared to CO2 as the reactant. Galvanostatic CO electrochem. redn. at -300 mA cm-2 with Cu-nanoparticles (40-60 nm) results in a cumulative Faradaic efficiency of 89% for C2 and C3 products. This represents a two-fold increase in selectivity to ethylene and a three-fold increase towards ethanol and n-propanol compared to the selectivity obtained with CO2 as the reactant. This enhancement of selectivity for C2 and C3 products at current densities of industrial relevance with CO as reactant provides a new perspective regarding a two-step electrochem. redn. of CO2.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXmslOnu70%253D&md5=93b32c2a5974c6d255d14802330985f4
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Romero Cuellar, N. ; Scherer, C. ; Kaçkar, B. ; Eisenreich, W. ; Huber, C. ; Wiesner-Fleischer, K. ; Fleischer, M. ; Hinrichsen, O. Two-step electrochemical reduction of CO₂ towards multi-carbon products at high current densities. J. CO2 Util. 2020, 36 , 263– 275, DOI: 10.1016/j.jcou.2019.10.016

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46

Two-step electrochemical reduction of CO2 towards multi-carbon products at high current densities

Romero Cuellar, N. S.; Scherer, C.; Kackar, B.; Eisenreich, W.; Huber, C.; Wiesner-Fleischer, K.; Fleischer, M.; Hinrichsen, O.

Journal of CO2 Utilization (2020), 36 (), 263-275CODEN: JCUOAJ; ISSN:2212-9839. (Elsevier Ltd.)

Two-step electrochem. redn. of CO2 is investigated as an alternative to increase selectivity towards C2 and C3 products. In this type of proposed cascade electrocatalytic operation, CO is produced in a first step and subsequently reduced to multi-carbon products in a second step with significantly higher Faradaic efficiencies compared to a one-step process. Taking into account that the study of this technol. at high current densities is crucial for industrial applicability, gas diffusion electrodes and flow-cells were used for operation at current densities above -200 mA cm-2 . Firstly, each step was characterized sep., the first using a silver gas diffusion electrode to generate a mixt. of humidified CO, H2, and unreacted CO2; the second step using copper nanoparticles on a carbon-based gas diffusion structure to obtain C2 and C3 products. Furthermore, expts. with isotope labeled 13CO2 and 13CO were performed in order to obtain some insights on the (electrochem.) reaction path of gas mixts. contg. CO2 and CO. With this set-up a total Faradaic efficiency towards C2 and C3 products of 62% at a total c.d. of -300 mA cm-2 was achieved. The results confirm the need for a gas sepn. technique between the two steps for a feasible two-step electrochem. redn. of CO2.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXit1KqsrzK&md5=2f2516ea23cdbfce197d9f9cc94f0233
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Jouny, M. ; Luc, W. ; Jiao, F. High-rate electroreduction of carbon monoxide to multi-carbon products. Nat. Catal. 2018, 1 , 748– 755, DOI: 10.1038/s41929-018-0133-2

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High-rate electroreduction of carbon monoxide to multi-carbon products

Jouny, Matthew; Luc, Wesley; Jiao, Feng

Nature Catalysis (2018), 1 (10), 748-755CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

Carbon monoxide electrolysis has previously been reported to yield enhanced multi-carbon (C2+) Faradaic efficiencies of up to ∼55%, but only at low reaction rates. This is due to the low soly. of CO in aq. electrolytes and operation in batch-type reactors. Here, we present a high-performance CO flow electrolyzer with a well controlled electrode-electrolyte interface that can reach total current densities of up to 1 A cm-2, together with improved C2+ selectivities. Computational transport modeling and isotopic C18O redn. expts. suggest that the enhanced activity is due to a higher surface pH under CO redn. conditions, which facilitates the prodn. of acetate. At optimal operating conditions, we achieve a C2+ Faradaic efficiency of ∼91% with a C2+ partial c.d. over 630 mA cm-2. Further investigations show that maintaining an efficient triple-phase boundary at the electrode-electrolyte interface is the most crit. challenge in achieving a stable CO/CO2 electrolysis process at high rates.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtFGisL%252FK&md5=d0e97e6e1ae97bbde8a7c4b473693abc
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Jouny, M. ; Lv, J.-J. ; Cheng, T. ; Ko, B. H. ; Zhu, J.-J. ; Goddard, W. A. ; Jiao, F. Formation of carbon–nitrogen bonds in carbon monoxide electrolysis. Nat. Chem. 2019, 11 , 846– 851, DOI: 10.1038/s41557-019-0312-z

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48

Formation of carbon-nitrogen bonds in carbon monoxide electrolysis

Jouny, Matthew; Lv, Jing-Jing; Cheng, Tao; Ko, Byung Hee; Zhu, Jun-Jie; Goddard, William A., III; Jiao, Feng

Nature Chemistry (2019), 11 (9), 846-851CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)

The electroredn. of CO2 is a promising technol. for carbon use. Although electrolysis of CO2 or CO2-derived CO can generate important industrial multicarbon feedstocks such as ethylene, ethanol, n-propanol and acetate, most efforts were devoted to promoting C-C bond formation. Here, C-N bonds can be formed through coelectrolysis of CO and NH3 with acetamide selectivity of nearly 40% at industrially relevant reaction rates. Full-solvent quantum mech. calcns. show that acetamide forms through nucleophilic addn. of NH3 to a surface-bound ketene intermediate, a step that is in competition with OH- addn., which leads to acetate. The C-N formation mechanism was successfully extended to amide products through amine nucleophilic attack on the ketene intermediate. This strategy enables the authors to form carbon-heteroatom bonds through the electroredn. of CO, expanding the scope of products available from CO2 redn.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhs1KktrvO&md5=f5ff62fd23152386ec694f95703d24b1
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Li, J. ; Wang, Z. ; McCallum, C. ; Xu, Y. ; Li, F. ; Wang, Y. ; Gabardo, C. M. ; Dinh, C.-t. ; Zhuang, T.-t. ; Wang, L. ; Howe, J. Y. ; Ren, Y. ; Sargent, E. H. ; Sinton, D. Constraining CO coverage on copper promotes high-efficiency ethylene electroproduction. Nat. Catal. 2019, 2 , 1124– 1131, DOI: 10.1038/s41929-019-0380-x

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49

Constraining CO coverage on copper promotes high-efficiency ethylene electroproduction

Li, Jun; Wang, Ziyun; McCallum, Christopher; Xu, Yi; Li, Fengwang; Wang, Yuhang; Gabardo, Christine M.; Dinh, Cao-Thang; Zhuang, Tao-Tao; Wang, Liang; Howe, Jane Y.; Ren, Yang; Sargent, Edward H.; Sinton, David

Nature Catalysis (2019), 2 (12), 1124-1131CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

The availability of inexpensive industrial CO gas streams motivates efficient electrocatalytic upgrading of CO to higher-value feedstocks such as ethylene. However, the electrosynthesis of ethylene by the CO redn. reaction (CORR) has suffered from low selectivity and energy efficiency. Here we find that the recent strategy of increasing performance through use of highly alk. electrolyte-which is very effective in CO2RR-fails in CORR and drives the reaction to acetate. We then observe that ethylene selectivity increases when we constrain (decrease) CO availability. Using d. functional theory, we show how CO coverage on copper influences the reaction pathways of ethylene vs. oxygenate: lower CO coverage stabilizes the ethylene-relevant intermediates whereas higher CO coverage favors oxygenate formation. We then control local CO availability exptl. by tuning the CO concn. and reaction rate; we achieve ethylene Faradaic efficiencies of 72% and a partial c.d. of >800 mA cm-2. The overall system provides a half-cell energy efficiency of 44% for ethylene prodn.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXisVWqtb7K&md5=a10f917de77f689868bca274bdff20fa
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Luc, W. ; Fu, X. ; Shi, J. ; Lv, J.-J. ; Jouny, M. ; Ko, B. H. ; Xu, Y. ; Tu, Q. ; Hu, X. ; Wu, J. ; Yue, Q. ; Liu, Y. ; Jiao, F. ; Kang, Y. Two-dimensional copper nanosheets for electrochemical reduction of carbon monoxide to acetate. Nat. Catal. 2019, 2 , 423– 430, DOI: 10.1038/s41929-019-0269-8

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50

Two-dimensional copper nanosheets for electrochemical reduction of carbon monoxide to acetate

Luc, Wesley; Fu, Xianbiao; Shi, Jianjian; Lv, Jing-Jing; Jouny, Matthew; Ko, Byung Hee; Xu, Yaobin; Tu, Qing; Hu, Xiaobing; Wu, Jinsong; Yue, Qin; Liu, Yuanyue; Jiao, Feng; Kang, Yijin

Nature Catalysis (2019), 2 (5), 423-430CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

Upgrading carbon dioxide to high-value multicarbon (C2+) products is one promising avenue for fuel and chem. prodn. Among all the monometallic catalysts, copper has attracted much attention because of its unique ability to convert CO2 or CO into C2+ products with an appreciable selectivity. Although numerous attempts have been made to synthesize Cu materials that expose the desired facets, it still remains a challenge to obtain high-quality nanostructured Cu catalysts for the electroredn. of CO2/CO. Here we report a facile synthesis of freestanding triangular-shaped two-dimensional Cu nanosheets that selectively expose the (111) surface. In a 2 M KOH electrolyte, the Cu nanosheets exhibit an acetate Faradaic efficiency of 48% with an acetate partial c.d. up to 131 mA cm-2 in electrochem. CO redn. Further anal. suggest that the high acetate selectivity is attributed to the suppression of ethylene and ethanol formation, probably due to the redn. of exposed (100) and (110) surfaces.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXpvVyqsro%253D&md5=7cc8a7868adddc50aed9d76d51189bfe
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Ozden, A. ; Wang, Y. ; Li, F. ; Luo, M. ; Sisler, J. ; Thevenon, A. ; Rosas-Hernández, A. ; Burdyny, T. ; Lum, Y. ; Yadegari, H. ; Agapie, T. ; Peters, J. C. ; Sargent, E. H. ; Sinton, D. Cascade CO₂ electroreduction enables efficient carbonate-free production of ethylene. Joule 2021, 5 , 706– 719, DOI: 10.1016/j.joule.2021.01.007

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51

Cascade CO2 electroreduction enables efficient carbonate-free production of ethylene

Ozden, Adnan; Wang, Yuhang; Li, Fengwang; Luo, Mingchuan; Sisler, Jared; Thevenon, Arnaud; Rosas-Hernandez, Alonso; Burdyny, Thomas; Lum, Yanwei; Yadegari, Hossein; Agapie, Theodor; Peters, Jonas C.; Sargent, Edward H.; Sinton, David

Joule (2021), 5 (3), 706-719CODEN: JOULBR; ISSN:2542-4351. (Cell Press)

CO2 electroredn. provides a route to convert waste emissions into chems. such as ethylene (C2H4). However, the direct transformation of CO2-to-C2H4 suffers from CO2 loss to carbonate, consuming up to 72% of energy input. A cascade approach-coupling a solid-oxide CO2-to-CO electrochem. cell (SOEC) with a CO-to-C2H4 membrane electrode assembly (MEA)-would eliminate CO2 loss to carbonate. However, this approach requires a CO-to-C2H4 MEA with energy efficiency well beyond demonstrations to date. Focusing on the MEA, we find that an N-tolyl substituted tetrahydro-bipyridine film improves the stabilization of key reaction intermediates, while an SSC ionomer enhances CO transport to the Cu surface, enabling a C2H4 faradaic efficiency of 65% at 150 mA cm-2 for 110 h. Demonstrating a cascade SOEC-MEA approach, we achieve CO2-to-C2H4 with a ∼48% redn. in energy intensity compared with the direct route. We further reduce the energy intensity by coupling CO electroredn. (CORR) with glucose electrooxidn.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXks1GisLk%253D&md5=04d35df4e8bacb017d5c4d991bcb558b
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Ren, S.; Fink, A.; Lees, E.; Zhang, Z.; Wu, W. Y.; Dvorak, D. J.; Berlinguette, C. Molecular electrocatalysts transform CO into C2+ products effectively in a flow cell [preprint]. Researchsquare.com, October 16, 2020. DOI: 10.21203/rs.3.rs-83176/v1 .
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Ripatti, D. S. ; Veltman, T. R. ; Kanan, M. W. Carbon Monoxide Gas Diffusion Electrolysis that Produces Concentrated C₂ Products with High Single-Pass Conversion. Joule 2019, 3 , 240– 256, DOI: 10.1016/j.joule.2018.10.007

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Carbon Monoxide Gas Diffusion Electrolysis that Produces Concentrated C2 Products with High Single-Pass Conversion

Ripatti, Donald S.; Veltman, Thomas R.; Kanan, Matthew W.

Joule (2019), 3 (1), 240-256CODEN: JOULBR; ISSN:2542-4351. (Cell Press)

Electrochem. CO conversion is crit. for the development of alternative fuel and chem. syntheses. To be efficient, electrosynthesis must make concd. product streams at high rates with modest potentials, but the combination of these features has not been established for CO or the related CO2 electrolysis. Here we investigate CO electrolysis with gas diffusion electrodes (GDEs) supplied by interdigitated flow fields in electrochem. cells with different ion transport properties. By optimizing gas and ion transport, we show that it is possible to simultaneously achieve high c.d., high selectivity, and high single-pass conversion at moderate cell potentials. Using a cell with the GDE directly contacting a Nafion membrane, we demonstrate >100 mA cm-2 CO redn. to C2 products and direct prodn. of 1.1 M acetate at a cell potential of 2.4 V over 24 h. Our results reveal crit. design features for maximizing the efficiency of C2 electrosynthesis.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsValt7c%253D&md5=d301c4721ed676d3fe66d8524ee6e8d1
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Zhu, P. ; Xia, C. ; Liu, C.-Y. ; Jiang, K. ; Gao, G. ; Zhang, X. ; Xia, Y. ; Lei, Y. ; Alshareef, H. N. ; Senftle, T. P. ; Wang, H. Direct and continuous generation of pure acetic acid solutions via electrocatalytic carbon monoxide reduction. Proc. Natl. Acad. Sci. U. S. A. 2021, 118 , e2010868118 DOI: 10.1073/pnas.2010868118

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Direct and continuous generation of pure acetic acid solutions via electrocatalytic carbon monoxide reduction

Zhu, Peng; Xia, Chuan; Liu, Chun-Yen; Jiang, Kun; Gao, Guanhui; Zhang, Xiao; Xia, Yang; Lei, Yongjiu; Alshareef, Husam N.; Senftle, Thomas P.; Wang, Haotian

Proceedings of the National Academy of Sciences of the United States of America (2021), 118 (2), e2010868118CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)

Electrochem. CO2 or CO redn. to high-value C2+ liq. fuels is desirable, but its practical application is challenged by impurities from cogenerated liq. products and solutes in liq. electrolytes, which necessitates cost- and energy-intensive downstream sepn. processes. By coupling rational designs in a Cu catalyst and porous solid electrolyte (PSE) reactor, here we demonstrate a direct and continuous generation of pure acetic acid solns. via electrochem. CO redn. With optimized edge-to-surface ratio, the Cu nanocube catalyst presents an unprecedented acetate performance in neutral pH with other liq. products greatly suppressed, delivering a maximal acetate Faradaic efficiency of 43%, partial current of 200 mA·cm-2, ultrahigh relative purity of up to 98 wt%, and excellent stability of over 150 h continuous operation. D. functional theory simulations reveal the role of stepped sites along the cube edge in promoting the acetate pathway. Addnl., a PSE layer, other than a conventional liq. electrolyte, was designed to sep. cathode and anode for efficient ion conductions, while not introducing any impurity ions into generated liq. fuels. Pure acetic acid solns., with concns. up to 2 wt% (0.33 M), can be continuously produced by employing the acetate-selective Cu catalyst in our PSE reactor.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhsVCitrk%253D&md5=d848a6b7f794e860370144548261c3dc
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Xia, R. ; Lv, J.-J. ; Ma, X. ; Jiao, F. Enhanced multi-carbon selectivity via CO electroreduction approach. J. Catal. 2021, 398 , 185– 191, DOI: 10.1016/j.jcat.2021.03.034

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55

Enhanced multi-carbon selectivity via CO electroreduction approach

Xia, Rong; Lv, Jing-Jing; Ma, Xinbin; Jiao, Feng

Journal of Catalysis (2021), 398 (), 185-191CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)

Electrochem. CO2 redn. reaction (eCO2RR) attracted much attention as potential pathways for carbon utilization and sustainable chem. prodn. Many efforts have been devoted into improving eCO2RR selectivity to multi-carbon (C2+) products as well as energetic efficiency, which often involve the use of a highly alk. electrolyte. The employment of alk. electrolyte in eCO2RR inevitably causes the formation of carbonates and loss of CO2 feedstock. Electrochem. CO redn. reaction (eCORR) has been proposed as a potential strategy to mitigate the carbonate formation issues. In this study, we conducted a detailed comparison of the electrocatalytic behaviors of Cu catalysts in both eCO2RR and eCORR using a microfluidic flow cell under alk. electrolyte conditions. Single-pass conversion of both reactions was studied under feedstock-deficient conditions through varying the feeding rates of CO2 or CO and their partial pressures. In eCO2RR, the Cu catalysts showed a relatively low carbon efficiency (i.e., the amt. of carbon ended in the desired products divided by the total amt. of CO2 consumed) of less than 23% due to the formation of carbonate, whereas the catalysts exhibited a significantly higher carbon efficiency (up to 84%) in eCORR. Among all three Cu catalysts, the oxide-derived Cu plates showed the highest C2+ Faradaic efficiency of 83% at -0.59 V vs. RHE in eCORR, corresponding to a C2+ partial current densities of 166 mA cm-2.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhtVeltbbK&md5=028d1bce55c6523eb5a926c6e95f3416
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Gao, D. ; Arán-Ais, R. M. ; Jeon, H. S. ; Roldan Cuenya, B. Rational catalyst and electrolyte design for CO2 electroreduction towards multicarbon products. Nat. Catal. 2019, 2 , 198– 210, DOI: 10.1038/s41929-019-0235-5

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Rational catalyst and electrolyte design for CO2 electroreduction towards multicarbon products

Gao, Dunfeng; Aran-Ais, Rosa M.; Jeon, Hyo Sang; Roldan Cuenya, Beatriz

Nature Catalysis (2019), 2 (3), 198-210CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

A review. The CO2 electroredn. reaction (CO2RR) to fuels and feedstocks is an attractive route to close the anthropogenic carbon cycle and store renewable energy. The generation of more reduced chems., esp. multicarbon oxygenate and hydrocarbon products (C2+) with higher energy densities, is highly desirable for industrial applications. However, selective conversion of CO2 to C2+ suffers from a high overpotential, a low reaction rate and low selectivity, and the process is extremely sensitive to the catalyst structure and electrolyte. Here we discuss strategies to achieve high C2+ selectivity through rational design of the catalyst and electrolyte. Current state-of-the-art catalysts, including Cu and Cu-bimetallic catalysts, as well as some alternative materials, are considered. The importance of taking into consideration the dynamic evolution of the catalyst structure and compn. are highlighted, focusing on findings extd. from in situ and operando characterizations. Addnl. theor. insight into the reaction mechanisms underlying the improved C2+ selectivity of specific catalyst geometries and compns. in synergy with a well-chosen electrolyte are also provided.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtFGisb7F&md5=7e4b0fa961893a752cbe169ae8cffa4f
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Fan, L. ; Xia, C. ; Yang, F. ; Wang, J. ; Wang, H. ; Lu, Y. Strategies in catalysts and electrolyzer design for electrochemical CO₂ reduction toward C2+ products. Sci. Adv. 2020, 6 , eaay3111 DOI: 10.1126/sciadv.aay3111
58

Merino-Garcia, I. ; Albo, J. ; Irabien, A. Tailoring gas-phase CO₂ electroreduction selectivity to hydrocarbons at Cu nanoparticles. Nanotechnology 2018, 29 , 014001, DOI: 10.1088/1361-6528/aa994e

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58

Tailoring gas-phase CO2 electroreduction selectivity to hydrocarbons at Cu nanoparticles

Merino-Garcia, I.; Albo, J.; Irabien, A.

Nanotechnology (2018), 29 (1), 014001/1-014001/9CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)

Copper-based surfaces appear as the most active catalysts for CO2 electroredn. to hydrocarbons, even though formation rates and efficiencies still need to be improved. The aim of the present work is to evaluate the continuous gas-phase CO2 electroredn. to hydrocarbons (i.e. ethylene and methane) at copper nanoparticulated-based surfaces, paying attention to particle size influence (ranging from 25-80 nm) on reaction productivity, selectivity, and Faraday efficiency (FE) for CO2 conversion. The effect of the c.d. and the presence of a microporous layer within the working electrode are then evaluated. Copper-based gas diffusion electrodes are prepd. by airbrushing the catalytic ink onto carbon supports, which are then coupled to a cation exchange membrane (Nafion) in a membrane electrode assembly. The results show that the use of smaller copper nanoparticles (25 nm) leads to a higher ethylene prodn. (1148μmol m-2 s-1) with a remarkable high FE (92.8%), at the same time, diminishing the competitive hydrogen evolution reaction in terms of FE. This work demonstrates the importance of nanoparticle size on reaction selectivity, which may be of help to design enhancedelectrocatalytic materials for CO2 valorization to hydrocarbons.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXitFWms7jO&md5=f91b8f78dcc7baa1de5c38cf7bede2aa
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Merino-Garcia, I. ; Albo, J. ; Solla-Gullón, J. ; Montiel, V. ; Irabien, A. Cu oxide/ZnO-based surfaces for a selective ethylene production from gas-phase CO₂ electroconversion. J. CO2 Util. 2019, 31 , 135– 142, DOI: 10.1016/j.jcou.2019.03.002

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59

Cu oxide/ZnO-based surfaces for a selective ethylene production from gas-phase CO2 electroconversion

Merino-Garcia, Ivan; Albo, Jonathan; Solla-Gullon, Jose; Montiel, Vicente; Irabien, Angel

Journal of CO2 Utilization (2019), 31 (), 135-142CODEN: JCUOAJ; ISSN:2212-9839. (Elsevier Ltd.)

In this work, the application of Cu oxides/ZnO-based electrocatalytic surfaces for the continuous and selective gas-phase electroredn. of CO2 to ethylene in a filter-press type electrochem. cell is studied. The prepd. catalytic materials are characterized by transmission electron microscopy, X-ray diffraction and XPS. Then, the Cu oxides/ZnO-based gas diffusion electrodes are electrochem. characterized by cyclic voltammetry and Tafel plot analyses. The ethylene formation rate and Faradaic efficiency are as high as 487.9μmol m-2s-1 and 91.1% when a c.d. of 7.5 mAcm-2 (-2.5 V vs. Ag/AgCl) is applied to the system, with an ethylene/methane prodn. ratio of 139, showing a better performance than previous electrocatalytic systems for the prodn. of ethylene from CO2 conversion. Consequently, the use of Cu oxides/ZnO-based electrocatalysts for gas-phase CO2 redn. is a step forward in the prodn. of C2 products, such as ethylene.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXltFCltbg%253D&md5=ff461350bbe003926153220e4e0e4315
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Wang, X. ; de Araújo, J. F. ; Ju, W. ; Bagger, A. ; Schmies, H. ; Kühl, S. ; Rossmeisl, J. ; Strasser, P. Mechanistic reaction pathways of enhanced ethylene yields during electroreduction of CO₂–CO co-feeds on Cu and Cu-tandem electrocatalysts. Nat. Nanotechnol. 2019, 14 , 1063– 1070, DOI: 10.1038/s41565-019-0551-6

[Crossref], [PubMed], [CAS], Google Scholar

60

Mechanistic reaction pathways of enhanced ethylene yields during electroreduction of CO2-CO co-feeds on Cu and Cu-tandem electrocatalysts

Wang, Xingli; de Araujo, Jorge Ferreira; Ju, Wen; Bagger, Alexander; Schmies, Henrike; Kuehl, Stefanie; Rossmeisl, Jan; Strasser, Peter

Nature Nanotechnology (2019), 14 (11), 1063-1070CODEN: NNAABX; ISSN:1748-3387. (Nature Research)

Unlike energy efficiency and selectivity challenges, the kinetic effects of impure or intentionally mixed CO2 feeds on the catalytic reactivity of the direct electrochem. CO2 redn. reaction (CO2RR) were poorly studied. Given that industrial CO2 feeds are often contaminated with CO, a closer study of the CO2RR under CO2/CO co-feed conditions is warranted. Here, the authors report mechanistic insights into the CO2RR reactivity of CO2/CO co-feeds on Cu-based nanocatalysts. Kinetic isotope-labeling expts.-performed in an operando differential electrochem. mass spectrometry capillary flow cell with millisecond time resoln.-showed an unexpected enhanced prodn. of C2H4, with a yield increase of almost 50%, from a cross-coupled 12CO2-13CO reactive pathway. The results suggest the absence of site competition between CO2 and CO mols. on the reactive surface at the reactant-specific sites. The practical significance of sustained local interfacial CO partial pressures under CO2 depletion is demonstrated by metallic/nonmetallic Cu/Ni-N-doped C tandem catalysts. The authors' findings show the mechanistic origin of improved C2 product formation under co-feeding, but also highlight technol. opportunities of impure CO2/CO process feeds for H2O/CO2 coelectrolyzers.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhvFWqur3E&md5=0a8f47428c5592320c802a0a85dc0e89
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Wang, X. ; Xu, A. ; Li, F. ; Hung, S.-F. ; Nam, D.-H. ; Gabardo, C. M. ; Wang, Z. ; Xu, Y. ; Ozden, A. ; Rasouli, A. S. ; Ip, A. H. ; Sinton, D. ; Sargent, E. H. Efficient Methane Electrosynthesis Enabled by Tuning Local CO₂ Availability. J. Am. Chem. Soc. 2020, 142 , 3525– 3531, DOI: 10.1021/jacs.9b12445

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Efficient Methane Electrosynthesis Enabled by Tuning Local CO2 Availability

Wang, Xue; Xu, Aoni; Li, Fengwang; Hung, Sung-Fu; Nam, Dae-Hyun; Gabardo, Christine M.; Wang, Ziyun; Xu, Yi; Ozden, Adnan; Rasouli, Armin Sedighian; Ip, Alexander H.; Sinton, David; Sargent, Edward H.

Journal of the American Chemical Society (2020), 142 (7), 3525-3531CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

The electroredn. of carbon dioxide (CO2RR) to valuable chems. is a promising avenue for the storage of intermittent renewable electricity. Renewable methane, obtained via CO2RR using renewable electricity as energy input, has the potential to serve as a carbon-neutral fuel or chem. feedstock, and it is of particular interest in view of the well-established infrastructure for its storage, distribution, and utilization. However, CO2RR to methane still suffers from low selectivity at com. relevant current densities (>100 mA cm-2). D. functional theory calcns. herein reveal that lowering *CO2 coverage on the Cu surface decreases the coverage of the *CO intermediate, and then this favors the protonation of *CO to *CHO, a key intermediate for methane generation, compared to the competing step, C-C coupling. We therefore pursue an exptl. strategy wherein we control local CO2 availability on a Cu catalyst by tuning the concn. of CO2 in the gas stream and regulate the reaction rate through the c.d. We achieve as a result a methane Faradaic efficiency (FE) of (48 ± 2)% with a partial c.d. of (108 ± 5) mA cm-2 and a methane cathodic energy efficiency of 20% using a dil. CO2 gas stream. We report stable methane electrosynthesis for 22 h. These findings offer routes to produce methane with high FE and high conversion rate in CO2RR and also make direct use of dil. CO2 feedstocks.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhsl2rtrY%253D&md5=541e46a5d117c2785fff86b3d1a53880
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Jeanty, P. ; Scherer, C. ; Magori, E. ; Wiesner-Fleischer, K. ; Hinrichsen, O. ; Fleischer, M. Upscaling and continuous operation of electrochemical CO₂ to CO conversion in aqueous solutions on silver gas diffusion electrodes. J. CO2 Util. 2018, 24 , 454– 462, DOI: 10.1016/j.jcou.2018.01.011

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62

Upscaling and continuous operation of electrochemical CO2 to CO conversion in aqueous solutions on silver gas diffusion electrodes

Jeanty, Philippe; Scherer, Christian; Magori, Erhard; Wiesner-Fleischer, Kerstin; Hinrichsen, Olaf; Fleischer, Maximilian

Journal of CO2 Utilization (2018), 24 (), 454-462CODEN: JCUOAJ; ISSN:2212-9839. (Elsevier Ltd.)

For the upscaling of the electrochem. conversion of CO2 to CO using Ag catalyst in water, gas diffusion electrodes (GDE) are needed for a sufficient supply of CO2 to obtain currents in the 100mAcm-2 range. The effects of an upscale of one order of magnitude starting with 10cm2 GDE size are presented. The penetration of electrolyte through the GDE needs to be regulated to balance the pos. effects (avoid salt deposition) and the detrimental impacts (access blocking of CO2 to the GDE). Using a control of the partial pressure at the GDE to monitor the electrolyte penetration and enhancing CO2 feed by recirculation and turbulence promoters at the larger GDE. The first step of scale-up could be achieved without loss in performance. With three-compartment cells and 0.4M K2SO4 electrolytes, the process is run at a c.d. of 150mAcm-2 over a couple of hundred hours with a Faradaic efficiency for CO (FECO) of approx. 60% on 100cm2 electrode area. It is discussed how to improve the performance by a management of the perspiration rate through the GDE to lay the scientific foundations for an industrial use of this technol.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXjt12ntLY%253D&md5=3ea050fbb36ec4c317f1a4b7941a3805
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Duarte, M. ; De Mot, B. ; Hereijgers, J. ; Breugelmans, T. Electrochemical Reduction of CO₂: Effect of Convective CO₂ Supply in Gas Diffusion Electrodes. ChemElectroChem 2019, 6 , 5596– 5602, DOI: 10.1002/celc.201901454

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63

Electrochemical reduction of CO2: Effect of convective CO2 supply in gas diffusion electrodes

Duarte, Miguel; De Mot, Bert; Hereijgers, Jonas; Breugelmans, Tom

ChemElectroChem (2019), 6 (22), 5596-5602CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)

The electrochem. redn. of carbon dioxide (CO2) is a promising technol. in light of energy transition and industrial electrification. In this study, two different electrolyzer configurations, flow-through and flow-by modes, were analyzed for the prodn. of carbon monoxide to resolve the CO2 mass-transfer limitation problem at high current densities in gas diffusion electrodes. These two configurations resp. state convective and diffusive flow inside the gas diffusion layer, and their effect was studied on the cathodic performance of the electrolyzer by varying the operating conditions: cathodic potential, electrocatalyst loading, and Nafion content. In flow-through configuration, a c.d. of 220 mA/cm2 could be achieved at a faradaic efficiency of 90%; whereas, in the flow-by configuration, the c.d. was at the same faradaic efficiency limited to 140 mA/cm2. However, the flow-through configuration has a few limitations, such as lower energy efficiency, owing to the higher ohmic drop and the faster deactivation caused by crystn. of electrolyte salts inside the gas diffusion electrode. Therefore, flow-by mode is currently the most adequate configuration for the long-term operation of electrolyzers for the redn. of CO2 to CO. This study represents an essential step toward the application of electrolyzers for the electroredn. of CO2.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXit12jsb3E&md5=c935f9876fe14e4d0cc450d89e8d8f7c
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Endrődi, B. ; Kecsenovity, E. ; Samu, A. ; Darvas, F. ; Jones, R. V. ; Török, V. ; Danyi, A. ; Janáky, C. Multilayer Electrolyzer Stack Converts Carbon Dioxide to Gas Products at High Pressure with High Efficiency. ACS Energy Lett. 2019, 4 , 1770– 1777, DOI: 10.1021/acsenergylett.9b01142

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64

Multilayer Electrolyzer Stack Converts Carbon Dioxide to Gas Products at High Pressure with High Efficiency

Endrodi, B.; Kecsenovity, E.; Samu, A.; Darvas, F.; Jones, R. V.; Torok, V.; Danyi, A.; Janaky, C.

ACS Energy Letters (2019), 4 (7), 1770-1777CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

Electrochem. redn. of CO2 is a value-added approach to both decrease the atm. emission of CO2 and form valuable chems. The authors present a zero gap electrolyzer cell, which continuously converts gas phase CO2 to products without using any liq. catholyte. This is the 1st report of a multilayer CO2 electrolyzer stack for scaling up the electrolysis process. CO formation with partial current densities >250 mA cm-2 were achieved routinely, which was further increased to 300 mA cm-2 (with ∼95% faradic efficiency) by pressurizing the CO2 inlet (up to 10 bar). Evenly distributing the CO2 gas among the layers, the electrolyzer operates identically to the sum of multiple single-layer electrolyzer cells. When passing the CO2 gas through the layers consecutively, the CO2 conversion efficiency increased. The electrolyzer simultaneously provides high partial c.d., low cell voltage (-3.0 V), high conversion efficiency (up to 40%), and high selectivity for CO prodn.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXht1KhtL3K&md5=369417146f8d07d57b9c3b3d6eae0dd3
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Dinh, C.-T. ; García de Arquer, F. P. ; Sinton, D. ; Sargent, E. H. High Rate, Selective, and Stable Electroreduction of CO₂ to CO in Basic and Neutral Media. ACS Energy Lett. 2018, 3 , 2835– 2840, DOI: 10.1021/acsenergylett.8b01734

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65

High Rate, Selective, and Stable Electroreduction of CO2 to CO in Basic and Neutral Media

Dinh, Cao-Thang; Garcia de Arquer, F. Pelayo; Sinton, David; Sargent, Edward H.

ACS Energy Letters (2018), 3 (11), 2835-2840CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

The electroredn. of carbon dioxide (CO2) to chems. such as carbon monoxide (CO) shows great potential for renewable fuel and chem. prodn. Significant progress in individual performance metrics such as reaction rate, selectivity, and stability has been achieved, yet the simultaneous achievement of each of these key metrics within a single system, and in a wide range of operating conditions, has yet to be demonstrated. Here a composite multilayered porous electrode is reported consisting of a polytetrafluoroethylene gas distribution layer, a conformal Ag catalyst, and a carbon current distributor. Sepg. the gas and current distribution functions provides endurance, and further reconstructing the catalyst to carbonate-derived Ag provides flexibility in terms of electrolyte. The resulting electrodes reduce CO2 to CO with a Faradaic efficiency over 90% at current densities above 150 mA/cm2, in both neutral and alk. media for over 100 h of operation. This represents an important step toward the deployment of CO2 electroduction systems using electrolyzer technologies.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvFChsb%252FO&md5=937b8744cb07868d8d67772b0cbff746
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Dufek, E. J. ; Lister, T. E. ; Stone, S. G. ; McIlwain, M. E. Operation of a Pressurized System for Continuous Reduction of CO₂ . J. Electrochem. Soc. 2012, 159 , F514– F517, DOI: 10.1149/2.011209jes

[Crossref], [CAS], Google Scholar

66

Operation of a pressurized system for continuous reduction of CO2

Dufek, Eric J.; Lister, Tedd E.; Stone, Simon G.; McIlwain, Michael E.

Journal of the Electrochemical Society (2012), 159 (9), F514-F517CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)

A pressurized electrochem. system equipped for continuous redn. of CO2 is presented. At elevated pressures, using a Ag-based cathode, the quantity of CO which can be generated is 5 times that obsd. at ambient pressure with faradaic efficiencies ≤92% obsd. at 350 mA cm-2. For operation at 225 mA cm-2 and 60° the cell voltage at 18.5 atm was 0.4 V below that obsd. at ambient pressure. Increasing the temp. further to 90° led to a cell voltage <3 V (18.5 atm and 90 °C), which equates to an elec. efficiency of 50%.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsVylu7jM&md5=36f6bdfdfb2647859661879ef8898e92
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Haas, T. ; Krause, R. ; Weber, R. ; Demler, M. ; Schmid, G. Technical photosynthesis involving CO₂ electrolysis and fermentation. Nat. Catal. 2018, 1 , 32– 39, DOI: 10.1038/s41929-017-0005-1

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67

Technical photosynthesis involving CO2 electrolysis and fermentation

Haas, Thomas; Krause, Ralf; Weber, Rainer; Demler, Martin; Schmid, Guenter

Nature Catalysis (2018), 1 (1), 32-39CODEN: NCAACP; ISSN:2520-1158. (Nature Research)

Solar-powered electrochem. redn. of CO2 and H2O to syngas, followed by fermn., could lead to sustainable prodn. of useful chems. However, due to insufficient elec. current densities and instabilities of current CO2-to-CO electrolyzers, a practical, scalable artificial photosynthesis remains a major challenge. Here, we address these problems using a com. available silver-based gas diffusion electrode (used in industrial-scale chlorine-alk. electrolysis) as the cathode in the CO2 electrolyzer. Elec. current densities up to 300 mA cm-2 were demonstrated for more than 1,200 h with continuous operation. This CO2 electrolyzer was coupled to a fermn. module, where the out-coming syngas from the CO2 electrolyzer was converted to butanol and hexanol with high carbon selectivity. Conversion of photovoltaic electricity, CO2 and H2O to the desired alcs. achieved close to 100% Faradaic efficiency. Industrial prodn. of useful and high-value chems. via artificial photosynthesis is closer than expected with the proposed scalable hybrid system.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtFGisLzJ&md5=0deda489a443a913959f2698b5e8ede9
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Ma, S. ; Luo, R. ; Gold, J. I. ; Yu, A. Z. ; Kim, B. ; Kenis, P. J. A. Carbon nanotube containing Ag catalyst layers for efficient and selective reduction of carbon dioxide. J. Mater. Chem. A 2016, 4 , 8573– 8578, DOI: 10.1039/C6TA00427J

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68

Carbon nanotube containing Ag catalyst layers for efficient and selective reduction of carbon dioxide

Ma, Sichao; Luo, Raymond; Gold, Jake I.; Yu, Aaron Z.; Kim, Byoungsu; Kenis, Paul J. A.

Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (22), 8573-8578CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)

Over the last few decades significant progress was made in the development of catalysts for efficient and selective electroredn. of CO2. These improvements in catalyst performance were of the extent that identifying electrodes of optimum structure and compn. has become key to further improve throughput levels in the electrolysis of CO2 to CO. Here the authors report on a simple 1-step method to incorporate multi-walled C nanotubes (MWCNT) in the catalyst layer to form gas diffusion electrodes with different structures: (i) a mixed catalyst layer in which the Ag nanoparticle catalyst and MWCNTs are homogeneously distributed; and (ii) a layered catalyst layer comprised of a layer of MWCNTs covered with a layer of Ag catalyst. Both approaches improve performance in the electroredn. of CO2 compared to electrodes that lack MWCNTs. The mixed layer performed best: an electrolyzer operated at a cell potential of -3 V using 1 M KOH as the electrolyte yielded unprecedented high levels of CO prodn. of up to 350 mA cm-2 at high faradaic efficiency (>95% selective for CO) and an energy efficiency of 45% under the same condition. Electrochem. impedance spectroscopy measurements indicate that the obsd. differences in electrode performance can be attributed to a lower charge transfer resistance in the mixed catalyst layer. A simple optimization of electrode structure and compn., i.e. incorporation of MWCNTs in the catalyst layer of a GDE, has a profound beneficial effect on their performance in electrocatalytic conversion of CO2, while allowing for a lower precious metal catalyst loading with improved performance.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XnvVGmtb4%253D&md5=b34411c51d7b4f87920e6a5ca7e135bc
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Wang, R. ; Haspel, H. ; Pustovarenko, A. ; Dikhtiarenko, A. ; Russkikh, A. ; Shterk, G. ; Osadchii, D. ; Ould-Chikh, S. ; Ma, M. ; Smith, W. A. ; Takanabe, K. ; Kapteijn, F. ; Gascon, J. Maximizing Ag Utilization in High-Rate CO₂ Electrochemical Reduction with a Coordination Polymer-Mediated Gas Diffusion Electrode. ACS Energy Lett. 2019, 4 , 2024– 2031, DOI: 10.1021/acsenergylett.9b01509

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69

Maximizing Ag Utilization in High-Rate CO2 Electrochemical Reduction with a Coordination Polymer-Mediated Gas Diffusion Electrode

Wang, Riming; Haspel, Henrik; Pustovarenko, Alexey; Dikhtiarenko, Alla; Russkikh, Artem; Shterk, Genrikh; Osadchii, Dmitrii; Ould-Chikh, Samy; Ma, Ming; Smith, Wilson A.; Takanabe, Kazuhiro; Kapteijn, Freek; Gascon, Jorge

ACS Energy Letters (2019), 4 (8), 2024-2031CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

The authors report the prepn. and electrocatalytic performance of Ag-contg. gas diffusion electrodes (GDEs) derived from a Ag coordination polymer (Ag-CP). Layer-by-layer growth of the Ag-CP onto porous supports was applied to control Ag loading. Subsequent electro-decompn. of the Ag-CP resulted in highly selective and efficient CO2-to-CO GDEs in aq. CO2 electroredn. Afterward, the metal-org. framework (MOF)-mediated approach was transferred to a gas-fed flow electrolyzer for high c.d. tests. The in situ formed GDE, with a low Ag loading of 0.2 mg cm-2, showed a peak performance of jCO ≈ 385 mA cm-2 at ∼-1.0 V vs. RHE and stable operation with high FECO (>96%) at jTotal = 300 mA cm-2 over a 4 h run. The MOF-mediated approach offers a facile route for manufg. uniformly dispersed Ag catalysts for CO2 electrochem. redn. by eliminating ill-defined deposition steps (drop-casting, etc.) while allowing control of the catalyst structure through self-assembly.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsVGntbbI&md5=f0bcf5f81ad7cc224c49d1ddf8f84776
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Salvatore, D. A. ; Weekes, D. M. ; He, J. ; Dettelbach, K. E. ; Li, Y. C. ; Mallouk, T. E. ; Berlinguette, C. P. Electrolysis of Gaseous CO₂ to CO in a Flow Cell with a Bipolar Membrane. ACS Energy Lett. 2018, 3 , 149– 154, DOI: 10.1021/acsenergylett.7b01017

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70

Electrolysis of Gaseous CO2 to CO in a Flow Cell with a Bipolar Membrane

Salvatore, Danielle A.; Weekes, David M.; He, Jingfu; Dettelbach, Kevan E.; Li, Yuguang C.; Mallouk, Thomas E.; Berlinguette, Curtis P.

ACS Energy Letters (2018), 3 (1), 149-154CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

The conversion of CO2 to CO is demonstrated in an electrolyzer flow cell contg. a bipolar membrane at current densities of 200 mA/cm2 with a faradaic efficiency of 50%. Electrolysis was carried out by delivering gaseous CO2 at the cathode with a Ag catalyst integrated in a C-based gas diffusion layer. Nonprecious Ni foam in a strongly alk. electrolyte (1 M NaOH) was used to mediate the anode reaction. While a configuration where the anode and cathode were sepd. by only a bipolar membrane is unfavorable for robust CO2 redn., a modified configuration with a solid-supported aq. layer inserted between the Ag-based catalyst layer and the bipolar membrane enhanced the cathode selectivity for CO2 redn. to CO. The authors report higher current densities (200 mA/cm2) than previously reported for gas-phase CO2 to CO electrolysis and demonstrate the dependence of long-term stability on adequate hydration of the CO2 inlet stream.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhvFyrtLfE&md5=c8659dc2acfdcabb94fc13652155bc0d
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Edwards, J. P. ; Xu, Y. ; Gabardo, C. M. ; Dinh, C.-T. ; Li, J. ; Qi, Z. ; Ozden, A. ; Sargent, E. H. ; Sinton, D. Efficient electrocatalytic conversion of carbon dioxide in a low-resistance pressurized alkaline electrolyzer. Appl. Energy 2020, 261 , 114305, DOI: 10.1016/j.apenergy.2019.114305

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71

Efficient electrocatalytic conversion of carbon dioxide in a low-resistance pressurized alkaline electrolyzer

Edwards, Jonathan P.; Xu, Yi; Gabardo, Christine M.; Dinh, Cao-Thang; Li, Jun; Qi, ZhenBang; Ozden, Adnan; Sargent, Edward H.; Sinton, David

Applied Energy (2020), 261 (), 114305CODEN: APENDX; ISSN:0306-2619. (Elsevier Ltd.)

Electrochem. carbon dioxide conversion offers a means to utilize carbon dioxide and simultaneously store excess renewable energy. To be economical, industrial carbon dioxide electroredn. systems require high energy efficiencies to minimize elec. input. To this end, these systems need high product selectivity at low cell voltages and industrially viable current densities. Here, a liq. phase flow cell electrolyzer using a silver catalyst for carbon dioxide conversion to carbon monoxide is reported. Significant improvements in cell efficiency are demonstrated through the synergistic combination of three factors: minimal electrode spacing (0.25 mm flow field), pressurization (50 bar), and alky. (5 M KOH). Diminished electrode spacings reduce ohmic losses, pressurization increases carbon monoxide selectivities, and alk. conditions improve reaction kinetics. The combination of these three factors enables an uncorrected full cell energy efficiency of 67% at 202 mA/cm2, the highest reported above 150 mA/cm2. This system maintains a competitive energy efficiency of 47% at a high c.d. of 941 mA/cm2.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXisVGgtLbI&md5=e3a076a60e75f34a6e565f12f4026e7a
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Reinisch, D. ; Schmid, B. ; Martić, N. ; Krause, R. ; Landes, H. ; Hanebuth, M. ; Mayrhofer, K. J. ; Schmid, G. Various CO₂-to-CO Electrolyzer Cell and Operation Mode Designs to avoid CO₂-Crossover from Cathode to Anode. Z. Phys. Chem. 2020, 234 , 1115– 1131, DOI: 10.1515/zpch-2019-1480

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72

Various CO2-to-CO Electrolyzer Cell and Operation Mode Designs to avoid CO2-Crossover from Cathode to Anode

Reinisch, David; Schmid, Bernhard; Martic, Nemanja; Krause, Ralf; Landes, Harald; Hanebuth, Marc; Mayrhofer, Karl J. J.; Schmid, Guenter

Zeitschrift fuer Physikalische Chemie (Muenchen, Germany) (2020), 234 (6), 1115-1131CODEN: ZPCFAX; ISSN:0942-9352. (Oldenbourg Wissenschaftsverlag GmbH)

The electrochem. CO2 redn. reaction (CO2RR) towards CO allows to turn CO2 and renewable energy into feedstock for the chem. industry. Previously shown electrolyzers are capable of continuous operation for more than 1000 h at high faradaic efficiencies and industrially relevant current densities. However, the crossover of educt CO2 into the anode gas has not been investigated in current cell designs, Carbonates (HCO3- and CO32-) are formed at the cathode during CO2RR and are subsequently neutralized at the anode. Thus, CO2 mixes into the anodically evolved O2, which is undesired from com. perspectives. In this work this chem. transport was suppressed by using a carbonate-free electrolyte. However, a second transport mechanism via phys. dissolved gases became apparent. A transport model based on chem. and phys. absorption of CO2 and O2 will be proposed and two solns. were exptl. investigated: the use of an anode GDL (A-GDL) and degassing the anolyte with a membrane contactor (MC). Both solns. further reduce the CO2 crossover to the anode below 0.1 CO2 for each cathodically formed CO while still operating at industrially relevant current densities of 200 mA/cm2.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXht1Wnu7rJ&md5=8cdb20df2d6a308edf0146e0708261bf
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Bhargava, S. S. ; Proietto, F. ; Azmoodeh, D. ; Cofell, E. R. ; Henckel, D. A. ; Verma, S. ; Brooks, C. J. ; Gewirth, A. A. ; Kenis, P. J. A. System Design Rules for Intensifying the Electrochemical Reduction of CO₂ to CO on Ag Nanoparticles. ChemElectroChem 2020, 7 , 2001– 2011, DOI: 10.1002/celc.202000089

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73

System design rules for intensifying the electrochemical reduction of CO2 to CO on Ag nanoparticles

Bhargava, Saket S.; Proietto, Federica; Azmoodeh, Daniel; Cofell, Emiliana R.; Henckel, Danielle A.; Verma, Sumit; Brooks, Christopher J.; Gewirth, Andrew A.; Kenis, Paul J. A.

ChemElectroChem (2020), 7 (9), 2001-2011CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)

Electroredn. of CO2 (eCO2RR) is a potentially sustainable approach for carbon-based chem. prodn. Despite significant progress, performing eCO2RR economically at scale is challenging. Here we report meeting key technoeconomic benchmarks simultaneously through electrolyte engineering and process optimization. A systematic flow electrolysis study - performing eCO2RR to CO on Ag nanoparticles as a function of electrolyte compn. (cations, anions), electrolyte concn., electrolyte flow rate, cathode catalyst loading, and CO2 flow rate - resulted in partial current densities of 417 and 866 mA/cm2 with faradaic efficiencies of 100 and 98% at cell potentials of -2.5 and -3.0 V with full cell energy efficiencies of 53 and 43%, and a conversion per pass of 17 and 36%, resp., when using a CsOH-based electrolyte. The cumulative insights of this study led to the formulation of system design rules for high rate, highly selective, and highly energy efficient eCO2RR to CO.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXltlertbg%253D&md5=c33c4faebebd46cb4a49bde181a710a3
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Lee, W. H. ; Ko, Y.-J. ; Choi, Y. ; Lee, S. Y. ; Choi, C. H. ; Hwang, Y. J. ; Min, B. K. ; Strasser, P. ; Oh, H.-S. Highly selective and scalable CO₂ to CO - Electrolysis using coral-nanostructured Ag catalysts in zero-gap configuration. Nano Energy 2020, 76 , 105030, DOI: 10.1016/j.nanoen.2020.105030

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74

Highly selective and scalable CO2 to CO - Electrolysis using coral-nanostructured Ag catalysts in zero-gap configuration

Lee, Woong Hee; Ko, Young-Jin; Choi, Yongjun; Lee, Si Young; Choi, Chang Hyuck; Hwang, Yun Jeong; Min, Byoung Koun; Strasser, Peter; Oh, Hyung-Suk

Nano Energy (2020), 76 (), 105030CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)

The direct electroredn. of CO2 to pure CO streams has attracted much attention for both academic research and industrial polymer synthesis development. Here, we explore catalytically very active, coral-structured Ag catalyst for the generation of pure CO from CO2-feeds in lab-bench scale zero-gap CO2 electrolyzer. Coral-shaped Ag electrodes achieved CO partial current densities of up to 312 mA cm-2, EECO of 38%, and FECO clearly above 90%. In-situ/operando X-ray Absorption Spectroscopy revealed the sustained presence of Ag+ subsurface species, whose local electronic field effects constitute likely mol. origins of the favorable exptl. kinetics and selectivity. In addn., we show how electrode flooding in zero-gap CO2 electrolyzer compromises efficient CO2 mass transfer. Our studies highlight the need for a concomitant consideration of factors related to intrinsic catalytic activity of the active phase, its porous structure and its hydrophilicity/phobicity to achieve a sustained high product yield in AEM zero-gap electrolyzer.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhtlSrtbbF&md5=1fd32365314febf0158662bd7a2eea2d
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Lee, J. ; Lee, W. ; Ryu, K. H. ; Park, J. ; Lee, H. ; Lee, J. H. ; Park, K. T. Catholyte-free electroreduction of CO₂ for sustainable production of CO: concept, process development, techno-economic analysis, and CO₂ reduction assessment. Green Chem. 2021, 23 , 2397– 2410, DOI: 10.1039/D0GC02969F

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75

Catholyte-free electroreduction of CO2 for sustainable production of CO: concept, process development, techno-economic analysis, and CO2 reduction assessment

Lee, Jaeseo; Lee, Wonhee; Ryu, Kyung Hwan; Park, Joungho; Lee, Hyojin; Lee, Jay H.; Park, Ki Tae

Green Chemistry (2021), 23 (6), 2397-2410CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)

Electrochem. CO2 redn. (ECO2R) is considered as one of the economically viable means to convert CO2 into useful products, for achieving carbon neutrality in the future. Many studies have been conducted on developing and evaluating electrochem. cells that provide high energy efficiency and conversion of CO2 for CO prodn. via ECO2R. However, com. feasible technologies are yet to be developed and demonstrated due to various tech. limitations as well as the lack of a systematic evaluation framework. In this study, CO prodn. performance is examd. by catholyte-free electrochem. CO2 redn. (CF-ECO2R), which eliminates a large ohmic drop by removing the catholyte compartment, thus lowering the energy requirement. The CF-ECO2R shows 0.6 V lower cell voltage and 17.4% higher elec. energy conversion efficiency than a typical GDE-catholyte method using 1 M KOH as the catholyte at a c.d. of 240 mA cm-2. The obtained results are used to demonstrate its tech. feasibility and to provide basic data for the evaluation of a scale-up process. A math. model is developed based on the obtained exptl. data and the model is simulated along with three different options of downstream sepn. to provide basic mass and energy balance data. Then, techno-economic and CO2 life cycle analyses are conducted to suggest the best configuration of the overall CO prodn. system in terms of the economics and net CO2 redn. It is estd. that the best configuration provides a redn. in CO2 emission by 48% compared to the conventional CO prodn. via steam methane reforming. This configuration is also assessed to be economically feasible, giving a prodn. cost (584 USD per ton CO) that is competitive with the current market price of CO. It is also demonstrated that the operating voltage of the CF-ECO2R and regional influences such as CO2 emission intensity and cost of the mixed electricity strongly affect the cost and CO2 emission of the system.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXktVOmsbk%253D&md5=c44aeb299a688e5c4dd2c135821ba3d8
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Liu, Z. ; Yang, H. ; Kutz, R. ; Masel, R. I. CO₂ Electrolysis to CO and O₂ at High Selectivity, Stability and Efficiency Using Sustainion Membranes. J. Electrochem. Soc. 2018, 165 , J3371– J3377, DOI: 10.1149/2.0501815jes

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76

CO2 electrolysis to CO and O2 at high selectivity, stability and efficiency using Sustainion membranes

Liu, Zengcai; Yang, Hongzhou; Kutz, Robert; Masel, Richard I.

Journal of the Electrochemical Society (2018), 165 (15), J3371-J3377CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)

CO2 electrolysis provides a pathway to close the anthropogenic C cycle and store renewable energy, but the stability, selectivity, efficiency and rate of such process needs to be improved. The authors explore the use of Sustainion imidazolium-functionalized membranes and ionomers to improve the performance of that process. Potentiometric runs at a fixed current of 200 mA/cm2 using Sustainion membranes and ionomers showed that one can maintain 98% selectivity at ∼3V applied potential for five months, with a voltage increase of only 3 μV/h. Other runs showed stable performance at 400 and 600 mA/cm2. These results pave the way for commercialization of CO2 electrolysis, providing a viable pathway to recycle CO2 back to fuels.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXms12gsLY%253D&md5=4a5309a0d5ab03bfd00b1ac680c58e6a
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Verma, S. ; Hamasaki, Y. ; Kim, C. ; Huang, W. ; Lu, S. ; Jhong, H.-R. M. ; Gewirth, A. A. ; Fujigaya, T. ; Nakashima, N. ; Kenis, P. J. A. Insights into the Low Overpotential Electroreduction of CO₂ to CO on a Supported Gold Catalyst in an Alkaline Flow Electrolyzer. ACS Energy Lett. 2018, 3 , 193– 198, DOI: 10.1021/acsenergylett.7b01096

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77

Insights into the Low Overpotential Electroreduction of CO2 to CO on a Supported Gold Catalyst in an Alkaline Flow Electrolyzer

Verma, Sumit; Hamasaki, Yuki; Kim, Chaerin; Huang, Wenxin; Lu, Shawn; Jhong, Huei-Ru Molly; Gewirth, Andrew A.; Fujigaya, Tsuyohiko; Nakashima, Naotoshi; Kenis, Paul J. A.

ACS Energy Letters (2018), 3 (1), 193-198CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

Cost competitive electroredn. of CO2 to CO requires electrochem. systems that exhibit partial c.d. (jCO) exceeding 150 mAcm-2 at cell overpotentials (|ηcell|) <1 V. However, achieving such benchmarks remains difficult. Here, the authors report the electroredn. of CO2 on a supported Au catalyst in an alk. flow electrolyzer with performance levels close to the economic viability criteria. Onset of CO prodn. occurred at cell and cathode overpotentials of just -0.25 and -0.02 V, resp. High jCO (∼99, 158 mAcm-2) was obtained at low |ηcell| (∼0.70, 0.94 V) and high CO energetic efficiency (∼63.8, 49.4%). The performance was stable for at least 8 h. Addnl., the onset cathode potentials, kinetic isotope effect, and Tafel slopes indicate the low overpotential prodn. of CO in alk. media to be the result of a pH-independent rate-detg. step (i.e., electron transfer) in contrast to a pH-dependent overall process.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhvF2mtLrM&md5=f3e7e1e280ae800d6977a4b08fa6188c
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Endrődi, B. ; Kecsenovity, E. ; Samu, A. ; Halmágyi, T. ; Rojas-Carbonell, S. ; Wang, L. ; Yan, Y. ; Janáky, C. High carbonate ion conductance of a robust PiperION membrane allows industrial current density and conversion in a zero-gap carbon dioxide electrolyzer cell. Energy Environ. Sci. 2020, 13 , 4098– 4105, DOI: 10.1039/D0EE02589E

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78

High carbonate ion conductance of a robust PiperION membrane allows industrial current density and conversion in a zero-gap carbon dioxide electrolyzer cell

Endrodi, B.; Kecsenovity, E.; Samu, A.; Halmagyi, T.; Rojas-Carbonell, S.; Wang, L.; Yan, Y.; Janaky, C.

Energy & Environmental Science (2020), 13 (11), 4098-4105CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)

A poly(aryl piperidinium)-based anion exchange membrane (PiperION) with high carbonate conductance is employed for CO2 electrolysis to CO in conjunction with a tailored electrolyzer cell structure. This combination results in unprecedentedly high partial current densities in zero-gap cells (jCO > 1.0 A cm-2), while maintaining high conversion (20-45%), selectivity (up to 90%) and low cell voltage (2.6-3.4 V).

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhvVymtbrE&md5=faf2a9033fe24e57713e58c707965912
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Endrődi, B. ; Samu, A. ; Kecsenovity, E. ; Halmágyi, T. ; Sebők, D. ; Janáky, C. Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolysers. Nat. Energy 2021, 6 , 439– 448, DOI: 10.1038/s41560-021-00813-w

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79

Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolysers

Endrodi, B.; Samu, A.; Kecsenovity, E.; Halmagyi, T.; Sebok, D.; Janaky, C.

Nature Energy (2021), 6 (4), 439-448CODEN: NEANFD; ISSN:2058-7546. (Nature Portfolio)

Continuous-flow electrolyzers allow CO2 redn. at industrially relevant rates, but long-term operation is still challenging. One reason for this is the formation of ppts. in the porous cathode from the alk. electrolyte and the CO2 feed. Here we show that while ppt. formation is detrimental for the long-term stability, the presence of alkali metal cations at the cathode improves performance. To overcome this contradiction, we develop an operando activation and regeneration process, where the cathode of a zero-gap electrolyzer cell is periodically infused with alkali cation-contg. solns. This enables deionized water-fed electrolyzers to operate at a CO2 redn. rate matching those using alk. electrolytes (CO partial c.d. of 420 ± 50 mA cm-2 for over 200 h). We deconvolute the complex effects of activation and validate the concept with five different electrolytes and three different com. membranes. Finally, we demonstrate the scalability of this approach on a multicell electrolyzer stack, with an active area of 100 cm2 per cell.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhvVCrsLzP&md5=884b3d933c47da1ae90e57e4b201ad94
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Monteiro, M. C. O. ; Philips, M. F. ; Schouten, K. J. P. ; Koper, M. T. M. Efficiency and selectivity of CO₂ reduction to CO on gold gas diffusion electrodes in acidic media. Nat. Commun. 2021, 12 , 4943, DOI: 10.1038/s41467-021-24936-6

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80

Efficiency and selectivity of CO2 reduction to CO on gold gas diffusion electrodes in acidic media

Monteiro, Mariana C. O.; Philips, Matthew F.; Schouten, Klaas Jan P.; Koper, Marc T. M.

Nature Communications (2021), 12 (1), 4943CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)

Abstr.: The electrochem. redn. of CO2 to CO is a promising technol. for replacing prodn. processes employing fossil fuels. Still, low energy efficiencies hinder the prodn. of CO at com. scale. CO2 electrolysis has mainly been performed in neutral or alk. media, but recent fundamental work shows that high selectivities for CO can also be achieved in acidic media. Therefore, we investigate the feasibility of CO2 electrolysis at pH 2-4 at indrustrially relevant conditions, using 10 cm2 gold gas diffusion electrodes. Operating at current densities up to 200 mA cm-2, we obtain CO faradaic efficiencies between 80-90% in sulfate electrolyte, with a 30% improvement of the overall process energy efficiency, in comparison with neutral media. Addnl., we find that weakly hydrated cations are crucial for accomplishing high reaction rates and enabling CO2 electrolysis in acidic media. This study represents a step towards the application of acidic electrolyzers for CO2 electroredn.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhvVSkt7vF&md5=700e89f27f929a8a4a404691b8bbaade
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Dinh, C.-T. ; García de Arquer, F. P. ; Sinton, D. ; Sargent, E. H. High Rate, Selective, and Stable Electroreduction of CO₂ to CO in Basic and Neutral Media. ACS Energy Lett. 2018, 3 , 2835– 2840, DOI: 10.1021/acsenergylett.8b01734

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81

High Rate, Selective, and Stable Electroreduction of CO2 to CO in Basic and Neutral Media

Dinh, Cao-Thang; Garcia de Arquer, F. Pelayo; Sinton, David; Sargent, Edward H.

ACS Energy Letters (2018), 3 (11), 2835-2840CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

The electroredn. of carbon dioxide (CO2) to chems. such as carbon monoxide (CO) shows great potential for renewable fuel and chem. prodn. Significant progress in individual performance metrics such as reaction rate, selectivity, and stability has been achieved, yet the simultaneous achievement of each of these key metrics within a single system, and in a wide range of operating conditions, has yet to be demonstrated. Here a composite multilayered porous electrode is reported consisting of a polytetrafluoroethylene gas distribution layer, a conformal Ag catalyst, and a carbon current distributor. Sepg. the gas and current distribution functions provides endurance, and further reconstructing the catalyst to carbonate-derived Ag provides flexibility in terms of electrolyte. The resulting electrodes reduce CO2 to CO with a Faradaic efficiency over 90% at current densities above 150 mA/cm2, in both neutral and alk. media for over 100 h of operation. This represents an important step toward the deployment of CO2 electroduction systems using electrolyzer technologies.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvFChsb%252FO&md5=937b8744cb07868d8d67772b0cbff746
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Keith, D. W. ; Holmes, G. ; St. Angelo, D. ; Heidel, K. A Process for Capturing CO₂ from the Atmosphere. Joule 2018, 2 , 1573– 1594, DOI: 10.1016/j.joule.2018.05.006

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83

A Process for Capturing CO2 from the Atmosphere

Keith, David W.; Holmes, Geoffrey; St. Angelo, David; Heidel, Kenton

Joule (2018), 2 (8), 1573-1594CODEN: JOULBR; ISSN:2542-4351. (Cell Press)

A review. We describe a process for capturing CO2 from the atm. in an industrial plant. The design captures ≈1 Mt-CO2/yr in a continuous process using an aq. KOH sorbent coupled to a calcium caustic recovery loop. We describe the design rationale, summarize performance of the major unit operations, and provide a capital cost breakdown developed with an independent consulting engineering firm. We report results from a pilot plant that provides data on performance of the major unit operations. We summarize the energy and material balance computed using an Aspen process simulation. When CO2 is delivered at 15 MPa, the design requires either 8.81 GJ of natural gas, or 5.25 GJ of gas and 366 kWhr of electricity, per ton of CO2 captured. Depending on financial assumptions, energy costs, and the specific choice of inputs and outputs, the levelized cost per ton CO2 captured from the atm. ranges from 94 to 232 $/t-CO2.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsFGjs7fL&md5=4f26e56f51bd5770f1e169f5da8776ea
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Kibria Nabil, S. ; McCoy, S. ; Kibria, M. G. Comparative life cycle assessment of electrochemical upgrading of CO₂ to fuels and feedstocks. Green Chem. 2021, 23 , 867– 880, DOI: 10.1039/D0GC02831B

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Comparative life cycle assessment of electrochemical upgrading of CO2 to fuels and feedstocks

Kibria Nabil, Shariful; McCoy, Sean; Kibria, Md Golam

Green Chemistry (2021), 23 (2), 867-880CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)

Development of electrochem. pathways to convert CO2 into fuels and feedstock is rapidly progressing over the past decade. Here we present a comparative cradle-to-gate life cycle assessment (LCA) of one and two-step electrochem. conversion of CO2 to eight major value-added products; wherein we consider CO2 capture, conversion and product sepn. in our process model. We measure the carbon intensity (i.e., global warming impact) of one and two-step electrochem. routes with its counterparts - thermochem. CO2 utilization and fossil-fuel based incumbent synthesis routes for those eight products. Here we show that due to inevitable carbonate formation or CO2 crossover in one-step CO2 electrolysis using neutral pH membrane electrode assembly, the two-step electrosynthesis pathways (i.e., CO2 to CO in solid oxide electrolysis cell followed by CO electroredn. in alk. flow cell) would be distinctively compelling through the lens of climate benefits. This anal. further reveals that the carbon intensity of electrosynthesis products is due to significant energy requirement for conversion (77-83% of total energy consumption for gas products) and product sepn. (30-85% of total energy consumption for liq. products) phases. Global warming impact of electrochem. route is highly sensitive to the electricity emission intensity and is compelling over incumbent routes only when coupled with low emission intensity (<0.25 kg CO2e per kWh). As the technol. advances, we identify near-term compelling products that would provide climate benefits over incumbent routes, including syngas, ethylene and n-propanol. We further identify technol. goals required for electrochem. route to be competitive, notably achieving liq. product concn. >20 wt%. It is our hope that this anal. will guide the CO2 electrosynthesis community to target achieving these technol. goals, such that when coupled with low-carbon electricity, electrochem. route would bring climate benefits in near future.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXisFWrtr%252FO&md5=be5a8c6266f8cafa8e49e4793341fc57
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Nagahama, K. ; Konishi, H. ; Hoshino, D. ; Hirata, M. Binary vapor-liquid equilibria of carbon dioxide-light hydrocarbons at low temperature. J. Chem. Eng. Jpn. 1974, 7 , 323– 328, DOI: 10.1252/jcej.7.323

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Binary vapor-liquid equilibriums of carbon dioxide-light hydrocarbons at low temperature

Nagahama, Kunio; Konishi, Hitoshi; Hoshino, Daisuke; Hirata, Mitsuho

Journal of Chemical Engineering of Japan (1974), 7 (5), 323-8CODEN: JCEJAQ; ISSN:0021-9592.

Vapor-liq. equilibrium data for binary systems contg. CO2: CO2-C2H6, CO2-C3H8, CO2-n-C4H10, CO2-iso-C4H10, CO2-C2H4, CO2-C3H6 and CO2-1-C4H8 were detd. by the vapor recirculation method in the low-temp. range (from -41.6 to 0°). The CO2-C2H4 and CO2-C2H6 systems formed min.-boiling azeotropes. The modified Redlich-Kwong equation of state was used to calc. vapor-liq. equilibria of these systems and provided fairly good agreement with extp.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE2MXhtFKrtLg%253D&md5=8f69bd3cfcc93d2a388b72b68269d710
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Burr, B.; Lyddon, L. A comparison of physical solvents for acid gas removal. Proceedings of the 87th Annual Convention of the Gas Processors Association; Gas Processors Association: 2008; pp 100– 113.
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Sander, R. Compilation of Henry's law constants (version 4.0) for water as solvent. Atmos. Chem. Phys. 2015, 15 , 4399– 4981, DOI: 10.5194/acp-15-4399-2015

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Compilation of Henry's law constants (version 4.0) for water as solvent

Sander, R.

Atmospheric Chemistry and Physics (2015), 15 (8), 4399-4981CODEN: ACPTCE; ISSN:1680-7324. (Copernicus Publications)

Many atm. chems. occur in the gas phase as well as in liq. cloud droplets and aerosol particles. Therefore, it is necessary to understand the distribution between the phases. According to Henry's law, the equil. ratio between the abundances in the gas phase and in the aq. phase is const. for a dil. soln. Henry's law consts. of trace gases of potential importance in environmental chem. have been collected and converted into a uniform format. The compilation contains 17350 values of Henry's law consts. for 4632 species, collected from 689 refs.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXotFyktrc%253D&md5=c06fd66b33b6dae9e50f0e9c1e3e60cb
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Benson, J. ; Celin, A. Recovering Hydrogen – and Profits – from Hydrogen-Rich Offgas. Chem. Eng. Prog. 2018, 55– 59

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Recovering hydrogen - and profits - from hydrogen-rich offgas

Benson, Joanna; Celin, Andrew

Chemical Engineering Progress (2018), 114 (1), 55/1-55/5CODEN: CEPRA8; ISSN:0360-7275. (American Institute of Chemical Engineers)

As demand for industrial-grade hydrogen increases, more facilities are recovering H2 from offgas and selling it. Several recovery technologies are available, each with its own strengths and limitations.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXntVynsbo%253D&md5=d173dd3395d47f22fd3ce94ddf8401d3
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Miller, G. Q.; Stoecker, J. Selection of a hydrogen separation process. National Petroleum Refiners Association Annual Meeting; 1989; technical paper AM-89-55.
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Al-Rabiah, A. A.; Timmerhaus, K. D.; Noble, R. D. Membrane Technology for Hydrogen Separation in Ethylene Plants. The 6th World Congress of Chemical Engineering, Melbourne; 2001; pp 1– 7.
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Pettersen, T. ; Lien, K. A new robust design model for gas separating membrane modules, based on analogy with counter-current heat exchangers. Comput. Chem. Eng. 1994, 18 , 427– 439, DOI: 10.1016/0098-1354(94)88021-2

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A new robust design model for gas separating membrane modules, based on analogy with counter-current heat exchangers

Pettersen, T.; Lien, K. M.

Computers & Chemical Engineering (1994), 18 (5), 427-39CODEN: CCENDW; ISSN:0098-1354.

A simple and robust algebraic design model, for hollow-fiber membrane modules used in gas sepn., was verified against numerical and exptl. results from the literature. Product purity was typically predicted to ±2%, compared with a numerical ref. soln. (R. Rautenbach and W. Dahm, 1986), although prediction of model capacity was less accurate for membranes operating at high selectivity and high trans-membrane pressure difference. Nevertheless, module capacity prediction compared favorably with other models from the literature. The model can also be extended to take into account pressure drop on the bore side of the hollow fibers. To the authors' knowledge, this is the first reliable algebraic model for gas membrane sepn. systems able to handle multicomponent mixts.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2cXislekur0%253D&md5=1b8452e0215289e7c31cedf952c40ca3
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Gentry, J. C. ; Solazzo, A. J. Recovery of Carboxylic Acids From Aqueous Streams. Environ. Prog. 1995, 14 , 61– 64, DOI: 10.1002/ep.670140123

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Gentry, Joseph C.; Solazzo, Anthony J., Jr.

Environmental Progress (1995), 14 (1), 61-4CODEN: ENVPDI; ISSN:0278-4491. (American Institute of Chemical Engineers)

A low-cost liq.-liq. extn. process and a unit designed for it are described for recovery of carboxylic acids such as acetic (I) and formic (II) acids, using a blend of phosphine oxides as a high-boiling solvent. An aq. soln. contg. 2.5% I and 1.5% II is fed to the top of a counter-current extn. column, where it comes in contact with lean solvent entering the bottom of the column. The solvent exts. the acids and carries them along with a small vol. of water into the overhead stream. The co-extd. water and any light impurities are stripped overhead in a dehydration column. The bottoms from the dehydration column are routed to an acid stripping column, where the acids are stripped overhead from the solvent. The product contains 99.85% I or 90-98.5% II.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXkvVOmt7o%253D&md5=8f8c6ae0cf2747e379629dbd3a3b9c03
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Shah, V. H. ; Pham, V. ; Larsen, P. ; Biswas, S. ; Frank, T. Liquid–Liquid Extraction for Recovering Low Margin Chemicals: Thinking beyond the Partition Ratio. Ind. Eng. Chem. Res. 2016, 55 , 1731– 1739, DOI: 10.1021/acs.iecr.5b03914

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93

Liquid-Liquid Extraction for Recovering Low Margin Chemicals: Thinking beyond the Partition Ratio

Shah, Vishesh H.; Pham, Viet; Larsen, Paul; Biswas, Sanjib; Frank, Timothy

Industrial & Engineering Chemistry Research (2016), 55 (6), 1731-1739CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)

Recovery of carboxylic acids from fermn. broths is an active area of research due to ongoing interest in utilizing renewable feedstock for chem. prodn. Several recent studies have focused on recovery via liq.-liq. extn. using reactive extn. solvents such as high mol. wt. amines because they yield significantly higher partition ratios. However, these solvents tend to be more expensive than conventional phys. extn. solvents. The liq.-liq. phase equil. behavior is measured for extg. propionic acid from aq. solns. at 26-91° using 1-butanol (phys. extn.) and a blend of trioctylamine and 1-octanol (reactive extn.). As expected, the amine-based solvent system is more effective at extg. propionic acid. Addnl. anal. shows, however, that the 1-butanol process is still preferred in spite of its lower partitioning for propionic acid due to the high cost of the amine solvent relative to the product (propionic acid). The study therefore shows that solvents must be evaluated based not only on the partition ratio but also on solvent cost, product cost, mutual solubilities, thermal stability, and ease of recovery.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtlWksbg%253D&md5=5b9b2cf8956d771430ef43b423b2fd21
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Küngas, R. Review─Electrochemical CO₂ Reduction for CO Production: Comparison of Low- and High-Temperature Electrolysis Technologies. J. Electrochem. Soc. 2020, 167 , 044508, DOI: 10.1149/1945-7111/ab7099
95

O'Brien, C. P. ; Miao, R. K. ; Liu, S. ; Xu, Y. ; Lee, G. ; Robb, A. ; Huang, J. E. ; Xie, K. ; Bertens, K. ; Gabardo, C. M. ; Edwards, J. P. ; Dinh, C.-T. ; Sargent, E. H. ; Sinton, D. Single Pass CO₂ Conversion Exceeding 85% in the Electrosynthesis of Multicarbon Products via Local CO₂ Regeneration. ACS Energy Lett. 2021, 6 , 2952– 2959, DOI: 10.1021/acsenergylett.1c01122

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95

Single Pass CO2 Conversion Exceeding 85% in the Electrosynthesis of Multicarbon Products via Local CO2 Regeneration

O'Brien, Colin P.; Miao, Rui Kai; Liu, Shijie; Xu, Yi; Lee, Geonhui; Robb, Anthony; Huang, Jianan Erick; Xie, Ke; Bertens, Koen; Gabardo, Christine M.; Edwards, Jonathan P.; Dinh, Cao-Thang; Sargent, Edward H.; Sinton, David

ACS Energy Letters (2021), 6 (8), 2952-2959CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

The carbon dioxide redn. reaction (CO2RR) presents the opportunity to consume CO2 and produce desirable products. However, the alk. conditions required for productive CO2RR result in the bulk of input CO2 being lost to bicarbonate and carbonate. This loss imposes a 25% limit on the conversion of CO2 to multicarbon (C2+) products for systems that use anions as the charge carrier-and overcoming this limit is a challenge of singular importance to the field. Here, we find that cation exchange membranes (CEMs) do not provide the required locally alk. conditions, and bipolar membranes (BPMs) are unstable, delaminating at the membrane-membrane interface. We develop a permeable CO2 regeneration layer (PCRL) that provides an alk. environment at the CO2RR catalyst surface and enables local CO2 regeneration. With the PCRL strategy, CO2 crossover is limited to 15% of the amt. of CO2 converted into products, in all cases. Low crossover and low flow rate combine to enable a single pass CO2 conversion of 85% (at 100 mA/cm2), with a C2+ faradaic efficiency and full cell voltage comparable to the anion-conducting membrane electrode assembly.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhs1KgtrjM&md5=9843e71b79c2c9e0a2c7f79c6d9be0e6
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Dutta, N. ; Patil, G. Developments in CO separation. Gas Sep. Purif. 1995, 9 , 277– 283, DOI: 10.1016/0950-4214(95)00011-Y

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Developments in CO separation

Dutta, N. N.; Patil, G. S.

Gas Separation & Purification (1995), 9 (4), 277-83CODEN: GSEPEP; ISSN:0950-4214. (Elsevier)

A review with 34 refs. describing the status of technol. developments for the sepn. and purifn. of CO from industrial gas streams. The scope and prospects for adsorptive and membrane sepn. processes are highlighted. An assessment has been made of the most promising technol., which is dependent on feed gas type and process conditions.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXpsleisbo%253D&md5=6131c754dee6959291487fcebb4f6413
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Bachman, J. E. ; Reed, D. A. ; Kapelewski, M. T. ; Chachra, G. ; Jonnavittula, D. ; Radaelli, G. ; Long, J. R. Enabling alternative ethylene production through its selective adsorption in the metal–organic framework Mn₂(m-dobdc). Energy Environ. Sci. 2018, 11 , 2423– 2431, DOI: 10.1039/C8EE01332B

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Enabling alternative ethylene production through its selective adsorption in the metal-organic framework Mn2(m-dobdc)

Bachman, Jonathan E.; Reed, Douglas A.; Kapelewski, Matthew T.; Chachra, Gaurav; Jonnavittula, Divya; Radaelli, Guido; Long, Jeffrey R.

Energy & Environmental Science (2018), 11 (9), 2423-2431CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)

The unique adsorptive properties of metal-org. frameworks open the door to new processes for energy and raw materials prodn. One such process is the oxidative coupling of methane for the generation of ethylene, which has limited viability due to the high cost of cryogenic distn. Rather than employing such a traditional sepn. route, we propose the use of a porous material that is highly selective for ethylene over a wide range of gases in an energy- and cost-effective adsorbent-based sepn. process. Here, we analyze the metal-org. frameworks M2(m-dobdc) (M = Mg, Mn, Fe, Co, Ni; m-dobdc4- = 4,6-dioxido-1,3-benzenedicarboxylate), featuring a high d. of coordinatively-unsatd. M2+ sites, along with the com. adsorbent zeolite CaX, for their ability to purify ethylene from the effluent of an oxidative coupling of methane process. Our results show that unique metal-adsorbate interactions facilitated by Mn2(m-dobdc) render this material an outstanding adsorbent for the capture of ethylene from the product mixt., enabling this potentially disruptive alternative process for ethylene prodn.

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Bains, P. ; Psarras, P. ; Wilcox, J. CO₂ capture from the industry sector. Prog. Energy Combust. Sci. 2017, 63 , 146– 172, DOI: 10.1016/j.pecs.2017.07.001
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Mantripragada, H. C. ; Zhai, H. ; Rubin, E. S. Boundary Dam or Petra Nova – Which is a better model for CCS energy supply?. Int. J. Greenhouse Gas Control 2019, 82 , 59– 68, DOI: 10.1016/j.ijggc.2019.01.004
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Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions. Energy Prices and Costs in Europe; COM/2020/951 final; European Commission: 2020; pp 1– 18.
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Kvande, H. ; Haupin, W. Cell voltage in aluminum electrolysis: A practical approach. JOM 2000, 52 , 31– 37, DOI: 10.1007/s11837-000-0044-x

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Cell voltage in aluminum electrolysis: a practical approach

Kvande, Halvor; Haupin, Warren

JOM (2000), 52 (2), 31-37CODEN: JOMMER; ISSN:1047-4838. (Minerals, Metals & Materials Society)

A review with 24 refs. During aluminum electrolysis there must be an optimum cell voltage for a given cell design at const. amperage. This optimum-voltage target depends on the goal, which may change with economic conditions. Although cell voltages were reduced steadily during the last 50 yr, they now seem to have reached a const. level at 4.1 V to 4.2 V, at least for cells larger than 200 kA. Efforts to reduce cell voltage will continue to show incremental improvements; however, it will be difficult to achieve major voltage redns. without a drained cathode combined with inert sidewalls to permit increased thermal insulation.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3cXis1yqsbc%253D&md5=f8ea625db6b16ec1ee11120340fdf556
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Kvande, H. ; Haupin, W. Inert anodes for AI smelters: Energy balances and environmental impact. JOM 2001, 53 , 29– 33, DOI: 10.1007/s11837-001-0205-6

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Inert anodes for Al smelters: Energy balances and environmental impact

Kvande, Halvor; Haupin, Warren

JOM (2001), 53 (5), 29-33CODEN: JOMMER; ISSN:1047-4838. (Minerals, Metals & Materials Society)

The energy and environmental consequences of retrofitting Al electrolysis cells with inert anodes are assessed. The energy consumption and the CO2 emissions are calcd. based on assumptions of what the cell voltage may be. It is crucial not to ignore the cell voltage increase that may be necessary to maintain the heat balance of the cell, and also the sources of elec. energy that would provide the incremental power. Thus, a global environmental anal. of the impact of retrofitting cells with inert anodes is needed.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXktVOktrs%253D&md5=30d5e6c3ab5380188804f7210380510e
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Frank, W. B.; Haupin, W. E.; Vogt, H.; Bruno, M.; Thonstad, J.; Dawless, R. K.; Kvande, H.; Taiwo, O. A. Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2009; Vol. 8, pp 255– 271.
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Woods, D. R. Rules of Thumb in Engineering Practice; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2007; pp 376– 436.
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Barecka, M. H. ; Ager, J. W. ; Lapkin, A. A. Economically viable CO₂ electroreduction embedded within ethylene oxide manufacturing. Energy Environ. Sci. 2021, 14 , 1530– 1543, DOI: 10.1039/D0EE03310C

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Economically viable CO2 electroreduction embedded within ethylene oxide manufacturing

Barecka, Magda H.; Ager, Joel W.; Lapkin, Alexei A.

Energy & Environmental Science (2021), 14 (3), 1530-1543CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)

Electrochem. conversion of CO2 (CO2R) into fuels and chems. can both reduce CO2 emissions and allow for clean manufg. in the scenario of significant expansion of renewable power generation. However, large-scale process deployment is currently limited by unfavorable process economics resulting from significant up- and down-stream costs for obtaining pure CO2, sepn. of reaction products and increased logistical effort. We have discovered a method for economically viable recycling of waste CO2 that addresses these challenges. Our approach is based on integration of a CO2R unit into an existing manufg. process: ethylene oxide (EO) prodn., which emits CO2 as a byproduct. The std. EO process separates waste CO2 from the gas stream, hence the substrate for electroredn. is available at an EO plant at no addnl. cost. CO2 can be converted into an ethylene-rich stream and recycled on-site back to the EO reactor, which uses ethylene as a raw material, and also the anode product (oxygen) can be simultaneously valorized for the EO prodn. reaction. If powered by a renewable electricity source, the process will significantly (ca. 80%) reduce the CO2 emissions of an EO manufg. plant. A sensitivity anal. shows that the recycling approach can be economically viable in the short term and that its payback time could be as low as 1-2 years in the regions with higher carbon taxes and/or with access to low-cost electricity sources.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXjtV2ks7c%253D&md5=1242b95ee61f3a65f910b7afaadf3531
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Luyben, W. L. Capital cost of compressors for conceptual design. Chem. Eng. Process. 2018, 126 , 206– 209, DOI: 10.1016/j.cep.2018.01.020

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Capital cost of compressors for conceptual design

Luyben, William L.

Chemical Engineering and Processing (2018), 126 (), 206-209CODEN: CENPEU; ISSN:0255-2701. (Elsevier B.V.)

Many process intensification techniques involve the use of gas compressors. Gas recycles is typically used to improve yield and selectivity in reactor/sepn. processes. Vapor recompression in distn. systems are becoming more widely applied, particularly with complex sepns. such as divided-wall, reactive, extractive and azeotropic distn. systems. At the conceptual design stage it is vital to have reasonable ests. of the capital cost of compressors involved in these processes. The literature correlations use only the compressor power to est. capital cost, and there are significant differences in the published methods. However, in addn. to power, it appears that suction pressure also must be considered in cost estn. The proprietary Aspen Economics program gives results that show a strong dependency of cost on the pressure of the gas being compressed. The purpose of this paper is to provide a simple method to est. compressor capital cost that incorporates both compressor power and compressor suction pressure. A case study illustrates that the conventional heuristic of using equal compression ratios in multistage compression trains does not give the optimum economic design.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXks12ht7Y%253D&md5=88f3643036b008046ec93cfabe489a19
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Lin, H. ; He, Z. ; Sun, Z. ; Kniep, J. ; Ng, A. ; Baker, R. W. ; Merkel, T. C. CO₂-selective membranes for hydrogen production and CO2 capture – Part II: Techno-economic analysis. J. Membr. Sci. 2015, 493 , 794– 806, DOI: 10.1016/j.memsci.2015.02.042

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CO2-selective membranes for hydrogen production and CO2 capture - Part II: Techno-economic analysis

Lin, Haiqing; He, Zhenjie; Sun, Zhen; Kniep, Jay; Ng, Alvin; Baker, Richard W.; Merkel, Timothy C.

Journal of Membrane Science (2015), 493 (), 794-806CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)

Syngas processing often requires sepn. of CO2 and H for H prodn., and the resulting CO2-enriched stream presents an opportunity for simultaneous CO2 capture. For example, syngas-based integrated gasification combined cycle (IGCC) power plants are envisioned as an efficient means of producing power from coal where CO2 capture technologies can be applied relatively easily. Membrane technol. is an attractive approach for CO2 capture because of inherent process advantages such as simplicity, reliability, compactness and modularity. This paper (Part II of a 2-part study) performs techno-economic anal. for CO2-selective membranes in 3 H prodn. or use applications: methane reformer/pressure swing adsorption processes, O-blown coal-fired IGCC power plants using GE gasification technol., and air-blown coal-fired IGCC power plants using Transport Integrated Gasification (TRIG) technol. These processes require membranes with CO2/H2 selectivities of 10-20, which can be provided using PolarisTM membranes. Hybrid approaches using a combination of membrane and cryogenic processes are evaluated for the prodn. of high-pressure liq. CO2 ready for use or sequestration. The optimal membrane CO2/H2 selectivity and influence of CO2 capture rate on the cost of CO2 capture are detd., providing general guidelines for future membrane development. The cost of CO2 capture with these membrane processes depends not only on the feed CO2 partial pressure, but on the feed CO2 concn. as well.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXltFWhtrs%253D&md5=a52bda36a0af2b26fc2f62538b0135ee
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Verma, S. ; Lu, S. ; Kenis, P. J. A. Co-electrolysis of CO₂ and glycerol as a pathway to carbon chemicals with improved technoeconomics due to low electricity consumption. Nat. Energy 2019, 4 , 466– 474, DOI: 10.1038/s41560-019-0374-6

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Co-electrolysis of CO2 and glycerol as a pathway to carbon chemicals with improved technoeconomics due to low electricity consumption

Verma, Sumit; Lu, Shawn; Kenis, Paul J. A.

Nature Energy (2019), 4 (6), 466-474CODEN: NEANFD; ISSN:2058-7546. (Nature Research)

The renewable electricity-driven electroredn. of carbon dioxide (CO2) offers an alternative pathway to producing carbon chems. that are traditionally manufd. using fossil fuels. Typical CO2 electroredn. approaches couple cathodic CO2 redn. with the anodic oxygen evolution reaction (OER), resulting in approx. 90% of the electricity input being consumed by the OER. Here, we explore alternatives to the OER and show that the anodic electro-oxidn. of glycerol (a byproduct of industrial biodiesel and soap prodn.) can lower electricity consumption by up to 53%. This reduces the process's operating costs and carbon footprint, thus opening avenues for a carbon-neutral cradle-to-gate process even when driven by grid electricity (∼13% renewables today), as well as economical prodn. of the 12-electron products ethylene and ethanol. This study may thus serve as a framework for the design of CO2 electroredn. processes with low electricity requirements, enhancing their CO2 utilization potential and economic viability.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXovFCqtLs%253D&md5=202fa39d3d079f1b1b42c74cd6d98bc6
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Khan, M. A. ; Nabil, S. K. ; Al-Attas, T. A. ; Hu, J. ; Kibria, M. G. Electrochemical Reduction of CO₂ to Ethylene with Coproduction of Glycolic Acid Via Glycerol Oxidation. ECS Meet. Abstr. 2021, MA2021-01 , 1277, DOI: 10.1149/MA2021-01391277mtgabs
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van Bavel, S. ; Verma, S. ; Negro, E. ; Bracht, M. Integrating CO₂ Electrolysis into the Gas-to-Liquids–Power-to-Liquids Process. ACS Energy Lett. 2020, 5 , 2597– 2601, DOI: 10.1021/acsenergylett.0c01418

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Integrating CO2 Electrolysis into the Gas-to-Liquids-Power-to-Liquids Process

van Bavel, Svetlana; Verma, Sumit; Negro, Emanuela; Bracht, Maarten

ACS Energy Letters (2020), 5 (8), 2597-2601CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)

A review. This article focuses on both gas-to-liq. (GTL) technol. derived from natural gas, and power-to-liq. (PTL) technol. derived from renewable hydrogen and carbon dioxide. In this Viewpoint, the authors focus on the integration of the CO2 electrolysis block (an emerging technol. option) into the PTL and hybrid GTL-PTL lineups, thus making the technol. a credible option to enable the energy transition. Also discussed is the need to incorporate design rules to minimize process upsets, maximize carbon as well as energy efficiency, minimize capital costs, handle intermittency, recycle off-gases, and prevent inerts build up.

https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXhsVequ73M&md5=d33019a2be3b02126854f0a935d5df53
116

Bienewald, F.; Leibold, E.; Tužina, P.; Roscher, G. Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: 2019; pp 1– 16.
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Samel, U.-R.; Kohler, W.; Gamer, A. O.; Keuser, U.; Yang, S.-T.; Jin, Y.; Lin, M.; Wang, Z.; Teles, J. H. Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2018; pp 1– 20.
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Bui, M. ; Adjiman, C. S. ; Bardow, A. ; Anthony, E. J. ; Boston, A. ; Brown, S. ; Fennell, P. S. ; Fuss, S. ; Galindo, A. ; Hackett, L. A. ; Hallett, J. P. ; Herzog, H. J. ; Jackson, G. ; Kemper, J. ; Krevor, S. ; Maitland, G. C. ; Matuszewski, M. ; Metcalfe, I. S. ; Petit, C. ; Puxty, G. ; Reimer, J. ; Reiner, D. M. ; Rubin, E. S. ; Scott, S. A. ; Shah, N. ; Smit, B. ; Trusler, J. P. ; Webley, P. ; Wilcox, J. ; Mac Dowell, N. Carbon capture and storage (CCS): The way forward. Energy Environ. Sci. 2018, 11 , 1062– 1176, DOI: 10.1039/C7EE02342A

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Carbon capture and storage (CCS): the way forward

Bui, Mai; Adjiman, Claire S.; Bardow, Andre; Anthony, Edward J.; Boston, Andy; Brown, Solomon; Fennell, Paul S.; Fuss, Sabine; Galindo, Amparo; Hackett, Leigh A.; Hallett, Jason P.; Herzog, Howard J.; Jackson, George; Kemper, Jasmin; Krevor, Samuel; Maitland, Geoffrey C.; Matuszewski, Michael; Metcalfe, Ian S.; Petit, Camille; Puxty, Graeme; Reimer, Jeffrey; Reiner, David M.; Rubin, Edward S.; Scott, Stuart A.; Shah, Nilay; Smit, Berend; Trusler, J. P. Martin; Webley, Paul; Wilcox, Jennifer; MacDowell, Niall

Energy & Environmental Science (2018), 11 (5), 1062-1176CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)

Carbon capture and storage (CCS) is broadly recognized as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, more recently, its ability to facilitate the net removal of CO2 from the atm. However, despite this broad consensus and its tech. maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO2 capture, transport, utilization and storage from a multi-scale perspective, moving from the global to mol. scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key neg. emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-tech. barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation program and consider the com. and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.

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Davies, L. L. ; Uchitel, K. ; Ruple, J. Understanding barriers to commercial-scale carbon capture and sequestration in the United States: An empirical assessment. Energy Policy 2013, 59 , 745– 761, DOI: 10.1016/j.enpol.2013.04.033
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Viebahn, P. ; Chappin, E. Scrutinising the Gap between the Expected and Actual Deployment of Carbon Capture and Storage─A Bibliometric Analysis. Energies 2018, 11 , 2319, DOI: 10.3390/en11092319
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Akerboom, S. ; Waldmann, S. ; Mukherjee, A. ; Agaton, C. ; Sanders, M. ; Kramer, G. J. Different This Time? The Prospects of CCS in the Netherlands in the 2020s. Front. Energy Res. 2021, 9 , 644796, DOI: 10.3389/fenrg.2021.644796
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Budinis, S. ; Krevor, S. ; Dowell, N. M. ; Brandon, N. ; Hawkes, A. An assessment of CCS costs, barriers and potential. Energy Strateg. Rev. 2018, 22 , 61– 81, DOI: 10.1016/j.esr.2018.08.003
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Jones, C. R. ; Olfe-Kräutlein, B. ; Naims, H. ; Armstrong, K. The Social Acceptance of Carbon Dioxide Utilisation: A Review and Research Agenda. Front. Energy Res. 2017, 5 , 11, DOI: 10.3389/fenrg.2017.00011
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Kant, M. Overcoming Barriers to Successfully Commercializing Carbon Dioxide Utilization. Front. Energy Res. 2017, 5 , 22, DOI: 10.3389/fenrg.2017.00022
125

Eide, L. I. ; Batum, M. ; Dixon, T. ; Elamin, Z. ; Graue, A. ; Hagen, S. ; Hovorka, S. ; Nazarian, B. ; Nøkleby, P. H. ; Olsen, G. I. ; Ringrose, P. ; Vieira, R. A. M. Enabling Large-Scale Carbon Capture, Utilisation, and Storage (CCUS) Using Offshore Carbon Dioxide (CO₂) Infrastructure Developments–A Review. Energies 2019, 12 , 1945, DOI: 10.3390/en12101945

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Enabling large-scale carbon capture, utilisation, and storage (CCUS) using offshore carbon dioxide (CO2) infrastructure developments-a review

Eide, Lars Ingolf; Batum, Melissa; Dixon, Tim; Elamin, Zabia; Graue, Arne; Hagen, Sveinung; Hovorka, Susan; Nazarian, Bamshad; Noekleby, Paal Helge; Olsen, Geir Inge; Ringrose, Philip; Vieira, Raphael Augusto Mello

Energies (Basel, Switzerland) (2019), 12 (10), 1945CODEN: ENERGA; ISSN:1996-1073. (MDPI AG)

A review. Presently, the only offshore project for enhanced oil recovery using carbon dioxide, known as CO2-EOR, is in Brazil. Several desk studies have been undertaken, without any projects being implemented. The objective of this review is to investigate barriers to the implementation of large-scale offshore CO2-EOR projects, to identify recent technol. developments, and to suggest non-technol. incentives that may enable implementation. We examine differences between onshore and offshore CO2-EOR, emerging technologies that could enable projects, as well as approaches and regulatory requirements that may help overcome barriers. Our review shows that there are few, if any, tech. barriers to offshore CO2-EOR. However, there are many other barriers to the implementation of offshore CO2-EOR, including: High investment and operation costs, uncertainties about reservoir performance, limited access of CO2 supply, lack of business models, and uncertainties about regulations. This review describes recent technol. developments that may remove such barriers and concludes with recommendations for overcoming non-tech. barriers. The review is based on a report by the Carbon Sequestration Leadership Forum (CSLF).

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