- Email: [email protected]
On the Conversion Efficiency of CO2 Electroreduction on Gold
COMMENTARY
On the Conversion Efficiency of CO2 Electroreduction on Gold Benjamin A. Zhang,1 Cyrille Costentin,1,2,* and Daniel G. Nocera1,* Benjamin A. Zhang is a PhD student at Harvard University. His research focus is on the control of mass transport and concentration gradients in electrochemical reactions, in particular CO2 reduction, through nano- and mesostructuring of electrodes. Cyrille Costentin is Professor at Universite´ Paris Diderot. He received his undergraduate education at Ecole Normale Supe´rieure de Cachan and his PhD at Universite´ Paris Diderot. He is currently a Visiting Scientist in the group of D.G. Nocera at Harvard University. His interests include mechanisms and reactivity in electron transfer chemistry with particular emphasis on proton-coupled electron transfer and catalysis of electrochemical reactions. Daniel G. Nocera is the Patterson Rockwood Professor of Energy at Harvard University. His group studies mechanisms of biological and chemical energy conversion.
As anthropogenic CO2 emissions continue to rise, the imperative for renewable energy sources grows stronger. To this end, there has been a strong demand to find reliable methods to store renewable sources, such as solar and wind, in the form of fuels for the more consistent deployment of renewable energy.1 One fuel-forming reaction that is easily adapted to renewable electricity is electrochemical CO2 reduction (CO2RR) to produce CO, which may be translated into
liquid fuels by syngas processing. CO2RR to CO is a 2e– j 2H+ process and accordingly confronts the challenge of suppressing the more thermodynamically favorable and competing hydrogen evolution reaction (HER)2 (Scheme 1). Gold has emerged as a preferred electrode for CO2RR to CO owing to its high faradic efficiency (FE(CO)). Much attention has been devoted to understanding the competition between CO2RR and HER on gold catalysts by controlling crystal faceting, grains, nanostructuring, and mesostructuring as well as tuning of diffusion gradients and experimental conditions, all with the aim of increasing FE(CO) by minimizing HER and decreasing the overpotential required to produce CO2.2 Important yet underappreciated considerations, however, for the utilization of electrochemical CO2 reduction as a fuel-forming reaction are the homogeneous reactions between CO2 and water and hydroxide (OH–) in solution (Scheme 1) and their impact beyond FE(CO). Whereas reaction (3) is thermodynamically uphill and kinetically slow, reaction (4), under high-currentdensity conditions relevant to practical application, plays a critical role in carbon mass balance that has not been appropriately considered or simply neglected. The general trends for potential-selectivity and potential-partial-current-density relationships for CO2RR and HER for electrochemical CO2RR to CO on gold electrodes in a standard H-cell configuration are displayed in Figure 1. Under these conditions, dissolved CO2 is supplied to the electrode by diffusion through solution, and mass transport is dictated by agitation of the solution. These potential-selectivity and potential-partial-current-density relationships are classified into three regimes in which the intrinsic rates of CO2RR and HER, mass transport, and homogeneous reactions work together
to dictate CO2RR selectivity on gold.3,4 Whereas the specific morphological nature of gold (polycrystalline, nanostructured, etc.) can modestly affect the overpotentials for CO2RR and HER and hence peak FE(CO), structured electrodes have typically proven to favor CO2RR over HER as compared to planar electrodes, leading to higher FE(CO) and therefore more focus from the community.2 Regime I occurs at low overpotential and is characterized by low FE(CO) and low activity for both HER and CO2RR. As the current is initially dominated by HER, FE(CO) is low. FE(CO) begins to increase with the onset of CO production while HER activity remains low. Nonetheless, partial current densities for CO are low and consequently mass transport and homogeneous reactions do not affect CO production. Due to low FE(CO) and low current, this regime has been of little interest for practical applications. As overpotential is increased, regime II is reached where FE(CO) rises and maximizes. The rise in FE(CO) is due to the continued prominence of CO2RR against a shallow potential dependence on the current density of HER. Given the importance of the balance between CO2RR and HER, many investigations have explored regime II in order to develop a mechanistic understanding of CO2RR and to acquire insight into strategies to suppress HER. For example, oxide-derived gold5 and gold nanoneedles6 have been utilized to probe the effects of grain boundaries and local concentration of CO2, respectively. Such electrode preparations, among others, have been effective in increasing FE(CO) and positively shifting CO2RR overpotential. While FE(CO) can approach unity in regime II, the current at which CO is produced is ultimately still too low for practical applications.
Joule 3, 1565–1568, July 17, 2019 ª 2019 Elsevier Inc. 1565
Scheme 1. Reactions Involved in CO2RR (A) Electrochemical reactions in competition during CO 2 RR corresponding to (1) CO 2 reduction and (2) hydrogen evolution. HA is a general acid proton donor (H 2 O, H 3 O+ , HCO 3 – , and H2 CO3 ) and A – is its conjugate base. (B) Homogeneous hydration reactions present in solution corresponding to CO 2 reaction with water and hydroxide (OH– ). Numerous acidbase reactions of carbonate, bicarbonate, and carbonic acid are not listed.
Regime III occurs at high overpotentials and is characterized by a downturn in FE(CO). This decrease is due to the limitations imposed on CO2RR by mass transport and homogeneous reactions as well as a concurrent rise in HER. Mass transport limitations on CO production occur due to the low solubility of CO2 in aqueous solution. In addition, homogeneous reactions become increasingly important in this regime. As shown in Scheme 1, for each turnover of CO2RR and HER, two basic species are generated, and depending on the proton donor utilized, this basic species can be OH– produced directly (from H2O) or indirectly (from HCO3– or H2CO3) by acid-base reactions with water. Therefore, as partial current densities for CO (JCO) and H2 (JH2) increase, more OH– is produced at the surface of the electrode. This leads to a decrease in concentration of CO2 near the surface of the electrode by the rapid and irreversible consumption of CO2 with OH– to produce bicarbonate according to reaction (4). This is particularly problematic, as production of bicarbonate results in a viable substrate for HER and therefore further enhances HER. The positive relationship between homogeneous consumption of CO2 increasing with HER and CO2RR current
1566 Joule 3, 1565–1568, July 17, 2019
causes FE(CO) to continue to drop with increased applied overpotential, as CO2RR becomes self-inhibiting due to OH– production. The consumption of CO2 by OH– not only causes a decrease in FE(CO) but also results in a severe limitation on the amount of CO2 (i.e., CO2 conversion efficiency) that is productively reduced to CO per pass. These homogeneous reactions have been underappreciated in many studies of CO2RR, and they convolute CO2RR analysis and conclusions derived therefrom. Considering CO2RR on gold in the context of these three regimes casts a different light on the interpretation of CO2RR on gold. The general potential-selectivity and potential-partial current density behavior shown in Figure 1 is exhibited by most all gold catalysts. This general selectivity behavior is likely to be true for silver catalysts,8 and potentially other catalyst materials producing a single CO2RR product in competition with H2, due to the interplay between the thermodynamics and kinetics of CO2RR and HER on the catalyst surface.3,4 In this regard, there is lit-
tle differentiation among gold electrocatalysts—all benefit from high FE(CO) at low current densities with the caveat that rougher catalyst surfaces (nano- or mesostructured) tend to curtail HER and thus lead to higher FE(CO) than flat gold surfaces.2,5,6 Nonetheless, whereas many catalysts champion high FEs as a metric, for the reason mentioned here, there is no gold electrocatalyst that is differentiated from its counterpart in a significant way and thus attempts to distinguish one gold electrocatalyst from another are with diminished merit. High current densities (at least in excess of 100–200 mA/cm2) must be achieved in order for electrochemical CO2 reduction to reach consideration as a practical fuel-forming reaction.9–12 Thus, regime III is most relevant to the design of gold-based CO2RR electrocatalysts. In most cell configurations, mass transport and homogeneous reactions result in low FE(CO) without reaching such high current densities. To circumvent the mass transport limitation, research has recently turned toward the use of gas-diffusion electrodes (GDE) in CO2
Figure 1. Typical FE(CO) and Partial Current Densities for CO2RR and HER Faradic efficiency for CO and partial current densities for CO 2 RR and HER (J CO and J H2 , respectively) observed on gold in bicarbonate solution in standard electrochemical H-cell 2,5,6 and gas-diffusion electrode (GDE)-based flow electrolyzer configurations. 7 Three general potential regions describe the observed selectivity based on control by intrinsic activity, mass transport, and homogeneous reaction.
Scheme 2. Homogeneous Reactions in Highly Basic Solution In highly basic solution CO 2 is consumed by OH – to form carbonate (CO 3 2– ).
electrolyzers with some success.10,11 In this configuration, dissolved CO2 is supplied directly to the electrode at the interface of the gas-diffusion layer, providing rapid delivery of CO2 to the electrode surface, therefore mitigating the mass transport limitations and partially overcoming homogeneous CO2 consumption issues encountered in traditional H-cell configurations. The electrolyte in the electrolyzer is also typically flowed across the electrode surface to maintain reaction conditions local to the electrode. As shown in Figure 1, such a configuration has allowed higher current densities to be attained while maintaining reasonable FE(CO), therefore improving the conversion efficiency of CO2 reduction to CO compared with a standard H-cell configuration. However, this research path raises several considerations. What is the maximum overpotential the catalyst can be run at to produce a single product? What is the optimal catalyst loading density? Even in a rapid mass transport regime, will the current be high enough with GDEs? Given the desired high current density, what electrolyte flow rate is needed to prevent OH– accumulation proximate to the electrode and hence homogeneous CO2 consumption? In an attempt to address some of these considerations, GDE-based catalysts have been studied under highly basic conditions where the HCO3– electrolyte is replaced with OH– to elevate FE(CO).11,12 The basic electrolyte offers the obvious benefit that HER is suppressed by removing HCO3– as a proton source for HER, with the unexpected result that H2O is not a competent proton source for HER. Thus, the
overpotential at which the catalyst operating in base is poised can be increased without HER interfering in the CO2RR reaction. However, again we emphasize that this strategy neglects the complication presented by reaction (4) and portrayed in regime III. In highly basic solution, regardless of electrolyte flow, consumption of CO2 by OH– to produce CO32– (Scheme 2) is rapid. Thus, with regard to carbon mass balance, the reduction of CO2 to CO is accompanied by the significant consumption of CO2 to produce CO32–. In such a system that is selective for a single product, this acid-base-driven reaction of CO2 is not manifested in FE(CO) loss, as the faradic efficiency only reflects the utilization of current passed to produce a desired product; instead, it limits the conversion efficiency of CO2. Such conversion efficiency has largely been neglected and was only recently investigated regarding hydrocarbon production via fermentation of electrogenerated syngas13 and CO gas diffusion electrolysis.14 More importantly, this effect results in the severe, unproductive loss of CO2, therefore introducing a penalty attributed to low mass balance for CO2 conversion to CO and thus to the attendant additional energetic inputs that are required for the separation of CO32– from basic electrolyte and the regeneration of OH– and CO2. Furthermore, due to the rapid consumption of CO2 in strongly basic solution, the concentration of CO2 within the GDE catalyst layer diminishes precipitously away from the CO2 source, and therefore, only a thin portion of a catalyst layer may be active in CO2 electrolyzers operated in a strong base, presenting concerns of catalyst material inefficiency and, therefore, limiting the mass activity for CO production.12 Evaluation of the performance of GDE-based catalysts in CO2 electrolyzers by FE(CO) alone is thus misrepresentative, as the fundamental homogeneous reactions that affect the
mass balance of carbon are overlooked. Instead, a proper evaluation of these catalysts requires a full systems analysis that appropriately weighs factors that fall outside the scope of FE(CO), including the energetic inputs required to separate CO32– and regenerate OH– and CO2. While gold has emerged as the standard for electrochemical CO2 reduction to CO, the analysis represented by the three regimes of Figure 1 indicate that there is little differentiation among gold electrocatalysts. Most electrodes will display high FE(CO) production in regime II where current densities remain low. Progressing toward high current densities requisite for practical applications leads to regime III where CO2RR is limited by mass transport, self-inhibited by homogeneous consumption of CO2, and ultimately dominated by HER leading to low FE(CO). Studies to overcome slow mass transport have led to the emergence of GDEs as a possible solution. Certainly, high FE(CO) at high current densities is attained owing to increased mass transport; however, homogeneous consumption of CO2 via reaction (4) and its substantial implications on the mass balance of carbon in the system have largely been ignored. As research turns toward the engineering and optimization of GDE-based CO2RR systems to achieve high current and high CO2 conversion, the critical loss of CO2 to HCO3– and CO32– must addressed. Whereas the direct gas phase reduction of CO215 or reduction of CO with GDEs14 can circumvent the loss of carbon, the reduction of CO2 with GDEs will continue to need novel chemical and engineering solutions to become a practical route to renewable fuel generation.
ACKNOWLEDGMENTS This material is based on work supported under the Solar Photochemistry Program of the Chemical Sciences, Geosciences
Joule 3, 1565–1568, July 17, 2019 1567
and Biosciences Division, Office of Basic Energy Sciences of the U.S. Department of Energy DE-SC0017619.
1. Nocera, D.G. (2017). Solar fuels and solar chemicals industry. Acc. Chem. Res. 50, 616–619. 2. Zhao, S., Jin, R., and Jin, R. (2018). Opportunities and challenges in CO2 reduction by gold- and silver- based electrocatalysts: from bulk metals to nanoparticles and atomically precise nanoclusters. ACS Energy Lett. 3, 452–462. 3. Wuttig, A., Yoon, Y., Ryu, J., and Surendranath, Y. (2017). Bicarbonate is not a general acid in Au-catalyzed CO2 electroreduction. J. Am. Chem. Soc. 139, 17109–17113. 4. Wuttig, A., Yaguchi, M., Motobayashi, K., Osawa, M., and Surendranath, Y. (2016). Inhibited proton transfer enhances Aucatalyzed CO2-to-fuels selectivity. Proc. Natl. Acad. Sci. USA 113, E4585–E4593. 5. Chen, Y., Li, C.W., and Kanan, M.W. (2012). Aqueous CO2 reduction at very low overpotential on oxide-derived Au nanoparticles. J. Am. Chem. Soc. 134, 19969–19972. 6. Liu, M., Pang, Y., Zhang, B., De Luna, P., Voznyy, O., Xu, J., Zheng, X., Dinh, C.T., Fan,
1568 Joule 3, 1565–1568, July 17, 2019
F., Cao, C., et al. (2016). Enhanced electrocatalytic CO2 reduction via fieldinduced reagent concentration. Nature 537, 382–386. 7. Weng, L.C., Bell, A.T., and Weber, A.Z. (2018). Modeling gas-diffusion electrodes for CO2 reduction. Phys. Chem. Chem. Phys. 20, 16973–16984. 8. Yoon, Y., Hall, A.S., and Surendranath, Y. (2016). Tuning of silver catalyst mesostructured promotes selective carbon dioxide conversion into fuels. Angew. Chem. Int. Ed. Engl. 55, 15282–15286. 9. Chen, C., Khosrowabadi Kotyk, J.F., and Sheehan, S.W. (2018). Progress toward commercial application of electrochemical carbon dioxide reduction. Chem 4, 1–16. 10. Weekes, D.M., Salvatore, D.A., Reyes, A., Huang, A., and Berlinguette, C.P. (2018). Electrolytic CO2 reduction in a flow cell. Acc. Chem. Res. 51, 910–918. 11. Dinh, C.-T., Garcı´a de Arquer, F.P., Sinton, D., and Sargent, E.H. (2018). High rate, selective, and stable electroreduction of CO2 to CO in basic and neutral media. ACS Energy Lett. 3, 2835–2840. 12. Verma, S., Hamasaki, Y., Kim, C., Huang, W., Lu, S., Jhong, H.-R.M., Gewirth, A.A., Fujigaya, T., Nakashmia, N., and Kenis, P.J.A. (2017). Insights into the low overpotential electroreduction of CO2 to
CO on a supported gold catalyst in an alkaline flow electrolyzer. ACS Energy Lett. 3, 193–198. 13. Haas, T., Krause, R., Weber, R., Demler, M., and Schmid, G. (2018). Technical photosynthesis involving CO2 electrolysis and fermentation. Nat. Catal. 1, 32–39. 14. Ripatti, D.S., Veltman, T.R., and Kanan, M.W. (2019). Carbon monoxide gas diffusion electrolysis that produces concentrated C2 products with high single-pass conversion. Joule 3, 240–256. 15. Li, Y.C., Zhou, D., Yan, Z., Gonc¸alves, R.H., Salvatore, D.A., Berlinguette, C.P., and Mallouk, T.E. (2016). Electrolysis of CO2 to syngas in bipolar membrane-based electrochemical cells. ACS Energy Lett 1, 1149–1153. 1Department of Chemistry
and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA
2Laboratoire
d’Electrochimie Mole´culaire, Unite´ Mixte de Recherche Universite´ - CNRS N 7591, Baˆtiment Lavoisier, Universite´ Paris Diderot, Sorbonne Paris Cite´, 15 rue Jean de Baı¨f, 75205 Paris Cedex 13, France *Correspondence: [email protected] (C.C.), [email protected] (D.G.N.) https://doi.org/10.1016/j.joule.2019.05.017
On the Conversion Efficiency of CO2 Electroreduction on Gold Benjamin A. Zhang,1 Cyrille Costentin,1,2,* and Daniel G. Nocera1,* Benjamin A. Zhang is a PhD student at Harvard University. His research focus is on the control of mass transport and concentration gradients in electrochemical reactions, in particular CO2 reduction, through nano- and mesostructuring of electrodes. Cyrille Costentin is Professor at Universite´ Paris Diderot. He received his undergraduate education at Ecole Normale Supe´rieure de Cachan and his PhD at Universite´ Paris Diderot. He is currently a Visiting Scientist in the group of D.G. Nocera at Harvard University. His interests include mechanisms and reactivity in electron transfer chemistry with particular emphasis on proton-coupled electron transfer and catalysis of electrochemical reactions. Daniel G. Nocera is the Patterson Rockwood Professor of Energy at Harvard University. His group studies mechanisms of biological and chemical energy conversion.
As anthropogenic CO2 emissions continue to rise, the imperative for renewable energy sources grows stronger. To this end, there has been a strong demand to find reliable methods to store renewable sources, such as solar and wind, in the form of fuels for the more consistent deployment of renewable energy.1 One fuel-forming reaction that is easily adapted to renewable electricity is electrochemical CO2 reduction (CO2RR) to produce CO, which may be translated into
liquid fuels by syngas processing. CO2RR to CO is a 2e– j 2H+ process and accordingly confronts the challenge of suppressing the more thermodynamically favorable and competing hydrogen evolution reaction (HER)2 (Scheme 1). Gold has emerged as a preferred electrode for CO2RR to CO owing to its high faradic efficiency (FE(CO)). Much attention has been devoted to understanding the competition between CO2RR and HER on gold catalysts by controlling crystal faceting, grains, nanostructuring, and mesostructuring as well as tuning of diffusion gradients and experimental conditions, all with the aim of increasing FE(CO) by minimizing HER and decreasing the overpotential required to produce CO2.2 Important yet underappreciated considerations, however, for the utilization of electrochemical CO2 reduction as a fuel-forming reaction are the homogeneous reactions between CO2 and water and hydroxide (OH–) in solution (Scheme 1) and their impact beyond FE(CO). Whereas reaction (3) is thermodynamically uphill and kinetically slow, reaction (4), under high-currentdensity conditions relevant to practical application, plays a critical role in carbon mass balance that has not been appropriately considered or simply neglected. The general trends for potential-selectivity and potential-partial-current-density relationships for CO2RR and HER for electrochemical CO2RR to CO on gold electrodes in a standard H-cell configuration are displayed in Figure 1. Under these conditions, dissolved CO2 is supplied to the electrode by diffusion through solution, and mass transport is dictated by agitation of the solution. These potential-selectivity and potential-partial-current-density relationships are classified into three regimes in which the intrinsic rates of CO2RR and HER, mass transport, and homogeneous reactions work together
to dictate CO2RR selectivity on gold.3,4 Whereas the specific morphological nature of gold (polycrystalline, nanostructured, etc.) can modestly affect the overpotentials for CO2RR and HER and hence peak FE(CO), structured electrodes have typically proven to favor CO2RR over HER as compared to planar electrodes, leading to higher FE(CO) and therefore more focus from the community.2 Regime I occurs at low overpotential and is characterized by low FE(CO) and low activity for both HER and CO2RR. As the current is initially dominated by HER, FE(CO) is low. FE(CO) begins to increase with the onset of CO production while HER activity remains low. Nonetheless, partial current densities for CO are low and consequently mass transport and homogeneous reactions do not affect CO production. Due to low FE(CO) and low current, this regime has been of little interest for practical applications. As overpotential is increased, regime II is reached where FE(CO) rises and maximizes. The rise in FE(CO) is due to the continued prominence of CO2RR against a shallow potential dependence on the current density of HER. Given the importance of the balance between CO2RR and HER, many investigations have explored regime II in order to develop a mechanistic understanding of CO2RR and to acquire insight into strategies to suppress HER. For example, oxide-derived gold5 and gold nanoneedles6 have been utilized to probe the effects of grain boundaries and local concentration of CO2, respectively. Such electrode preparations, among others, have been effective in increasing FE(CO) and positively shifting CO2RR overpotential. While FE(CO) can approach unity in regime II, the current at which CO is produced is ultimately still too low for practical applications.
Joule 3, 1565–1568, July 17, 2019 ª 2019 Elsevier Inc. 1565
Scheme 1. Reactions Involved in CO2RR (A) Electrochemical reactions in competition during CO 2 RR corresponding to (1) CO 2 reduction and (2) hydrogen evolution. HA is a general acid proton donor (H 2 O, H 3 O+ , HCO 3 – , and H2 CO3 ) and A – is its conjugate base. (B) Homogeneous hydration reactions present in solution corresponding to CO 2 reaction with water and hydroxide (OH– ). Numerous acidbase reactions of carbonate, bicarbonate, and carbonic acid are not listed.
Regime III occurs at high overpotentials and is characterized by a downturn in FE(CO). This decrease is due to the limitations imposed on CO2RR by mass transport and homogeneous reactions as well as a concurrent rise in HER. Mass transport limitations on CO production occur due to the low solubility of CO2 in aqueous solution. In addition, homogeneous reactions become increasingly important in this regime. As shown in Scheme 1, for each turnover of CO2RR and HER, two basic species are generated, and depending on the proton donor utilized, this basic species can be OH– produced directly (from H2O) or indirectly (from HCO3– or H2CO3) by acid-base reactions with water. Therefore, as partial current densities for CO (JCO) and H2 (JH2) increase, more OH– is produced at the surface of the electrode. This leads to a decrease in concentration of CO2 near the surface of the electrode by the rapid and irreversible consumption of CO2 with OH– to produce bicarbonate according to reaction (4). This is particularly problematic, as production of bicarbonate results in a viable substrate for HER and therefore further enhances HER. The positive relationship between homogeneous consumption of CO2 increasing with HER and CO2RR current
1566 Joule 3, 1565–1568, July 17, 2019
causes FE(CO) to continue to drop with increased applied overpotential, as CO2RR becomes self-inhibiting due to OH– production. The consumption of CO2 by OH– not only causes a decrease in FE(CO) but also results in a severe limitation on the amount of CO2 (i.e., CO2 conversion efficiency) that is productively reduced to CO per pass. These homogeneous reactions have been underappreciated in many studies of CO2RR, and they convolute CO2RR analysis and conclusions derived therefrom. Considering CO2RR on gold in the context of these three regimes casts a different light on the interpretation of CO2RR on gold. The general potential-selectivity and potential-partial current density behavior shown in Figure 1 is exhibited by most all gold catalysts. This general selectivity behavior is likely to be true for silver catalysts,8 and potentially other catalyst materials producing a single CO2RR product in competition with H2, due to the interplay between the thermodynamics and kinetics of CO2RR and HER on the catalyst surface.3,4 In this regard, there is lit-
tle differentiation among gold electrocatalysts—all benefit from high FE(CO) at low current densities with the caveat that rougher catalyst surfaces (nano- or mesostructured) tend to curtail HER and thus lead to higher FE(CO) than flat gold surfaces.2,5,6 Nonetheless, whereas many catalysts champion high FEs as a metric, for the reason mentioned here, there is no gold electrocatalyst that is differentiated from its counterpart in a significant way and thus attempts to distinguish one gold electrocatalyst from another are with diminished merit. High current densities (at least in excess of 100–200 mA/cm2) must be achieved in order for electrochemical CO2 reduction to reach consideration as a practical fuel-forming reaction.9–12 Thus, regime III is most relevant to the design of gold-based CO2RR electrocatalysts. In most cell configurations, mass transport and homogeneous reactions result in low FE(CO) without reaching such high current densities. To circumvent the mass transport limitation, research has recently turned toward the use of gas-diffusion electrodes (GDE) in CO2
Figure 1. Typical FE(CO) and Partial Current Densities for CO2RR and HER Faradic efficiency for CO and partial current densities for CO 2 RR and HER (J CO and J H2 , respectively) observed on gold in bicarbonate solution in standard electrochemical H-cell 2,5,6 and gas-diffusion electrode (GDE)-based flow electrolyzer configurations. 7 Three general potential regions describe the observed selectivity based on control by intrinsic activity, mass transport, and homogeneous reaction.
Scheme 2. Homogeneous Reactions in Highly Basic Solution In highly basic solution CO 2 is consumed by OH – to form carbonate (CO 3 2– ).
electrolyzers with some success.10,11 In this configuration, dissolved CO2 is supplied directly to the electrode at the interface of the gas-diffusion layer, providing rapid delivery of CO2 to the electrode surface, therefore mitigating the mass transport limitations and partially overcoming homogeneous CO2 consumption issues encountered in traditional H-cell configurations. The electrolyte in the electrolyzer is also typically flowed across the electrode surface to maintain reaction conditions local to the electrode. As shown in Figure 1, such a configuration has allowed higher current densities to be attained while maintaining reasonable FE(CO), therefore improving the conversion efficiency of CO2 reduction to CO compared with a standard H-cell configuration. However, this research path raises several considerations. What is the maximum overpotential the catalyst can be run at to produce a single product? What is the optimal catalyst loading density? Even in a rapid mass transport regime, will the current be high enough with GDEs? Given the desired high current density, what electrolyte flow rate is needed to prevent OH– accumulation proximate to the electrode and hence homogeneous CO2 consumption? In an attempt to address some of these considerations, GDE-based catalysts have been studied under highly basic conditions where the HCO3– electrolyte is replaced with OH– to elevate FE(CO).11,12 The basic electrolyte offers the obvious benefit that HER is suppressed by removing HCO3– as a proton source for HER, with the unexpected result that H2O is not a competent proton source for HER. Thus, the
overpotential at which the catalyst operating in base is poised can be increased without HER interfering in the CO2RR reaction. However, again we emphasize that this strategy neglects the complication presented by reaction (4) and portrayed in regime III. In highly basic solution, regardless of electrolyte flow, consumption of CO2 by OH– to produce CO32– (Scheme 2) is rapid. Thus, with regard to carbon mass balance, the reduction of CO2 to CO is accompanied by the significant consumption of CO2 to produce CO32–. In such a system that is selective for a single product, this acid-base-driven reaction of CO2 is not manifested in FE(CO) loss, as the faradic efficiency only reflects the utilization of current passed to produce a desired product; instead, it limits the conversion efficiency of CO2. Such conversion efficiency has largely been neglected and was only recently investigated regarding hydrocarbon production via fermentation of electrogenerated syngas13 and CO gas diffusion electrolysis.14 More importantly, this effect results in the severe, unproductive loss of CO2, therefore introducing a penalty attributed to low mass balance for CO2 conversion to CO and thus to the attendant additional energetic inputs that are required for the separation of CO32– from basic electrolyte and the regeneration of OH– and CO2. Furthermore, due to the rapid consumption of CO2 in strongly basic solution, the concentration of CO2 within the GDE catalyst layer diminishes precipitously away from the CO2 source, and therefore, only a thin portion of a catalyst layer may be active in CO2 electrolyzers operated in a strong base, presenting concerns of catalyst material inefficiency and, therefore, limiting the mass activity for CO production.12 Evaluation of the performance of GDE-based catalysts in CO2 electrolyzers by FE(CO) alone is thus misrepresentative, as the fundamental homogeneous reactions that affect the
mass balance of carbon are overlooked. Instead, a proper evaluation of these catalysts requires a full systems analysis that appropriately weighs factors that fall outside the scope of FE(CO), including the energetic inputs required to separate CO32– and regenerate OH– and CO2. While gold has emerged as the standard for electrochemical CO2 reduction to CO, the analysis represented by the three regimes of Figure 1 indicate that there is little differentiation among gold electrocatalysts. Most electrodes will display high FE(CO) production in regime II where current densities remain low. Progressing toward high current densities requisite for practical applications leads to regime III where CO2RR is limited by mass transport, self-inhibited by homogeneous consumption of CO2, and ultimately dominated by HER leading to low FE(CO). Studies to overcome slow mass transport have led to the emergence of GDEs as a possible solution. Certainly, high FE(CO) at high current densities is attained owing to increased mass transport; however, homogeneous consumption of CO2 via reaction (4) and its substantial implications on the mass balance of carbon in the system have largely been ignored. As research turns toward the engineering and optimization of GDE-based CO2RR systems to achieve high current and high CO2 conversion, the critical loss of CO2 to HCO3– and CO32– must addressed. Whereas the direct gas phase reduction of CO215 or reduction of CO with GDEs14 can circumvent the loss of carbon, the reduction of CO2 with GDEs will continue to need novel chemical and engineering solutions to become a practical route to renewable fuel generation.
ACKNOWLEDGMENTS This material is based on work supported under the Solar Photochemistry Program of the Chemical Sciences, Geosciences
Joule 3, 1565–1568, July 17, 2019 1567
and Biosciences Division, Office of Basic Energy Sciences of the U.S. Department of Energy DE-SC0017619.
1. Nocera, D.G. (2017). Solar fuels and solar chemicals industry. Acc. Chem. Res. 50, 616–619. 2. Zhao, S., Jin, R., and Jin, R. (2018). Opportunities and challenges in CO2 reduction by gold- and silver- based electrocatalysts: from bulk metals to nanoparticles and atomically precise nanoclusters. ACS Energy Lett. 3, 452–462. 3. Wuttig, A., Yoon, Y., Ryu, J., and Surendranath, Y. (2017). Bicarbonate is not a general acid in Au-catalyzed CO2 electroreduction. J. Am. Chem. Soc. 139, 17109–17113. 4. Wuttig, A., Yaguchi, M., Motobayashi, K., Osawa, M., and Surendranath, Y. (2016). Inhibited proton transfer enhances Aucatalyzed CO2-to-fuels selectivity. Proc. Natl. Acad. Sci. USA 113, E4585–E4593. 5. Chen, Y., Li, C.W., and Kanan, M.W. (2012). Aqueous CO2 reduction at very low overpotential on oxide-derived Au nanoparticles. J. Am. Chem. Soc. 134, 19969–19972. 6. Liu, M., Pang, Y., Zhang, B., De Luna, P., Voznyy, O., Xu, J., Zheng, X., Dinh, C.T., Fan,
1568 Joule 3, 1565–1568, July 17, 2019
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and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA
2Laboratoire
d’Electrochimie Mole´culaire, Unite´ Mixte de Recherche Universite´ - CNRS N 7591, Baˆtiment Lavoisier, Universite´ Paris Diderot, Sorbonne Paris Cite´, 15 rue Jean de Baı¨f, 75205 Paris Cedex 13, France *Correspondence: [email protected] (C.C.), [email protected] (D.G.N.) https://doi.org/10.1016/j.joule.2019.05.017