Electroreduction of CO 2 (eCO 2 RR) is a potentially sustainable approach for carbon-based chemical production. Despite significant progress, performing eCO 2 RR economically at scale is challenging. Here we report meeting key technoeconomic benchmarks simultaneously through electrolyte engineering and process optimization. A systematic flow electrolysis studyperforming eCO 2 RR to CO on Ag nanoparticles as a function of electrolyte composition (cations, anions), electrolyte concentration, electrolyte flow rate, cathode catalyst loading, and CO 2 flow rate -resulted in partial current densities of 417 and 866 mA/cm 2 with faradaic efficiencies of 100 and 98 % at cell potentials of À 2.5 and À 3.0 V with full cell energy efficiencies of 53 and 43 %, and a conversion per pass of 17 and 36 %, respectively, 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 eCO 2 RR to CO.[a] S.

The renewable electricity-powered electrolysis of CO 2 could be a viable carbon-neutral method for producing carbon-based value-added chemicals like carbon monoxide, formic acid, ethylene, and ethanol. A typical CO 2 electrolyzer suffers, however, from the high power requirements, mainly due to the energy-intense anode reaction. In this work, we decrease the anode overpotential and thus reduce the overall cell energy consumption by using a NiFe-based bimetallic catalyst at the anode and applying a magnetic field. For a CO 2 electrolysis process producing CO in a gas diffusion electrode-based flow electrolyzer, we demonstrate that power savings in the range from 7% to 64% can be achieved at CO partial current densities exceeding −300 mA/cm 2 using a NiFe catalyst at the anode and/or by using a magnetic field at the anode. We achieve a maximum CO partial current density of −565 mA/cm 2 at a full cell energy efficiency of 45% with 2 M KOH as the electrolyte.

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The Cover Feature illustrates how mechanistic insights can used to engineer the electrolyte composition and formulate system design rules for intensified electroreduction of CO2 to CO. More information can be found in the Aricle by S. S. Bhargava et al.

Addition of about 14.7 Gigatons per year of net CO2 to the atmosphere has resulted in the atmospheric CO2 concentrations to increase from 316 ppm to 413 ppm over the past 60 years.1, 2 Excess CO2 can be remediated by capturing the excess CO2 emissions and subsequently utilizing them to make value-added products.3 CO2 electroreduction (ECR) is a potential method for converting a fraction of the excess CO2 emissions into carbon-based value-added chemicals such as carbon monoxide, hydrocarbons, and oxygenates.4 Academic and industrial research over the past decade has resulted in active and selective catalysts as well as energy-efficient cell configurations for ECR, however, significant improvements are still needed in electrochemical performance for ECR to become economically feasible at scale.5-7 The first part of this talk will focus on the role of electrolyte composition in the mechanism of ECR to CO on Ag nanoparticles. Specifically, the effects of pH, electrolyte identity, electrolyte concentration, and applied potential on the rate determining step will be discussed. A combination of measurements based on onset potentials, Tafel slopes, and electrochemical impedance spectroscopy can explain the promotional behavior of cations and the overpotential reduction for cell operation under high pH.8 The second part of this talk will cover a systematic process optimization of ECR to CO on Ag nanoparticles in a gas diffusion electrode based flow electrolyzer resulting in state-of-the-art electrochemical performance: a CO partial current density exceeding 850 mA/cm2 with a CO Faradaic efficiency of 98% at a cell potential of -3 V corresponding to a full cell energy efficiency exceeding 40% for a conversion per pass of CO2 exceeding 35%.8 Finally, this talk will also discuss how establishing quantitative/qualitative functional property relationships between system parameters (e.g.: pH, electrolyte flow rate) and system performance (e.g.: cathode overpotentials, CO partial current density) can help in rational system design for ECR to CO on Ag nanoparticles.8 References IPCC, Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2013. https://www.esrl.noaa.gov/gmd/ccgg/trends/, (accessed 17 December 2020). S. Pacala and R. Socolow, Science, 2004, 305, 968. O. S. Bushuyev, P. De Luna, C. T. Dinh, L. Tao, G. Saur, J. van de Lagemaat, S. O. Kelley and E. H. Sargent, Joule, 2018, 2, 825-832. B. Endrődi, G. Bencsik, F. Darvas, R. Jones, K. Rajeshwar and C. Janáky, Progress in Energy and Combustion Science, 2017, 62, 133-154. S. Verma, B. Kim, H.-R. M. Jhong, S. Ma and P. J. A. Kenis, Chemsuschem, 2016, 9, 1972-1979. M. Jouny, W. Luc and F. Jiao, Industrial & Engineering Chemistry Research, 2018, 57, 2165-2177. S. S. Bhargava, F. Proietto, D. Azmoodeh, E. R. Cofell, D. A. Henckel, S. Verma, C. J. Brooks, A. A. Gewirth and P. J. A. Kenis, ChemElectroChem, 2020, 7, 2001-2011.

Atmospheric CO2 concentrations measured at the ESRL (NOAA, Mauna Loa, HI) have risen to 413 ppm in the past 60 years as a result of addition of about 14.7 Gigatons of net CO2 to the atmosphere per year.1, 2 A fraction of the excess CO2 emissions can be utilized to synthesize carbon-based value-added chemicals (e.g.: CO, ethylene, ethanol) via the electroreduction of CO2 (ECR).3 Prior research efforts have resulted in active and selective catalysts for ECR, however, significant improvements in full cell energy requirement are still needed for ECR to be economically viable at industrial scales.4, 5 Some prior efforts have used magnetic fields as a process intensification tool to improve the electrochemical performance of water electrolysis.6 This talk will cover the use of magnetic fields in CO2 electrolysis to significantly reduce the full cell energy requirement in a gas diffusion electrode-based flow electrolyzer. We demonstrate that the application of a magnetic field leads to a reduction in cell potentials and/or increase in current densities, translating to an improvement in full cell energy efficiencies and reduction in full cell energy requirements. In a 2 M KOH solution, CO partial current densities of 468 mA/cm2 and 345 mA/cm2 were obtained at -3 V cell potential in the presence and absence of a magnetic field, respectively, corresponding to full cell energy savings exceeding 35% due to the application of a magnetic field. A combination of polarization plots and Tafel slope analysis reveals that the reduction in cell potentials and/or increase in current densities is caused by an improvement in the mass transfer of the electroactive species in the electrolyte due to the magnetohydrodynamic effect. References IPCC, Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2013. https://www.esrl.noaa.gov/gmd/ccgg/trends/, (accessed 17 December 2020). O. S. Bushuyev, P. De Luna, C. T. Dinh, L. Tao, G. Saur, J. van de Lagemaat, S. O. Kelley and E. H. Sargent, Joule, 2018, 2, 825-832. B. Endrődi, G. Bencsik, F. Darvas, R. Jones, K. Rajeshwar and C. Janáky, Progress in Energy and Combustion Science, 2017, 62, 133-154. M. Jouny, W. Luc and F. Jiao, Industrial & Engineering Chemistry Research, 2018, 57, 2165-2177. F. A. Garcés-Pineda, M. Blasco-Ahicart, D. Nieto-Castro, N. López and J. R. Galán-Mascarós, Nature Energy, 2019, 4, 519-525.

Every year about 14.7 Gigatons of net CO2 are added to the atmosphere.1 Atmospheric CO2 concentrations measured at the ESRL (NOAA, Mauna Loa HI) show an increase from 316 ppm to 409 ppm over the past 60 years.2 For the remediation of excess CO2, various studies suggest that one approach is to capture the excess CO2 emissions and subsequently utilize them to make value-added products.3 Electroreduction of CO2 (CO2RR) is a potential method for utilizing a fraction of the excess CO2 emissions by converting them into various carbon-based chemicals such as methanol, formic acid, ethylene, and carbon monoxide.4 Research efforts in academia and industry have developed catalysts, electrodes, electrolytes, and have also optimized reactor configurations and associated operating conditions to allow for high-rate and selective CO2RR.5 However, significant improvements in electrolyzer performance (catalyst activity and selectivity) are still needed for CO2RR to be technoeconomically feasible at scale.6, 7 This talk will cover multiple system-level approaches that can be further explored to intensify the performance of a flow electrolyzer for CO2RR. The first part will discuss how electrolyte engineering based on the role of electrolyte composition and the rate determining step (RDS) can guide the systematic process optimization of CO2RR to CO on Ag nanoparticles (NPs). The effects of pH and of cation identity as well as the identity of the RDS explain how CsOH as the electrolyte can be used to optimize CO2RR performance, resulting in CO partial current densities (jCO) exceeding 850 mA/cm2 at a cathode potential of -1 V vs. RHE with 98% Faradaic efficiency (FE) and full cell energetic efficiency for CO production (EE) exceeding 40% at a conversion per pass of CO2 to CO of 36%. These process optimization insights led to the formulation of system design rules pertaining to jCO, CO FEs, CO EEs, cathode overpotentials, catalyst mass activities, conversion per pass, and system costs for CO2RR to CO on Ag NPs. The second part briefly discusses the effects of using multiple alkali metals or multivalent cations in the electrolyte composition and the effect of electrolyte composition on performance stability over multiple hours for CO2RR to CO on Ag NPs. The third part focuses on improving the overall cell performance by intensifying the anode performance with approaches such as using a Ni and Fe based bimetallic anode catalyst instead of the typically used IrO2 catalyst or using a magnet at the anode to enhance mass transport. These approaches led to about 200 mV reduction in cell overpotential and thus, enhanced CO EEs. Lastly, the fourth part will discuss the effect of operating temperature on jCO, CO FEs, and CO EEs to understand the role of temperature as a process lever for process intensification of CO2RR. References IPCC, Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2013. https://www.esrl.noaa.gov/gmd/ccgg/trends/ (accessed 12 November 2019). S. Pacala and R. Socolow, Science, 2004, 305, 968. O. S. Bushuyev, P. De Luna, C. T. Dinh, L. Tao, G. Saur, J. van de Lagemaat, S. O. Kelley and E. H. Sargent, Joule, 2018, 2, 825-832. B. Endrődi, G. Bencsik, F. Darvas, R. Jones, K. Rajeshwar and C. Janáky, Progress in Energy and Combustion Science, 2017, 62, 133-154. S. Verma, B. Kim, H.-R. M. Jhong, S. Ma and P. J. A. Kenis, Chemsuschem, 2016, 9, 1972-1979. M. Jouny, W. Luc and F. Jiao, Industrial & Engineering Chemistry Research, 2018, 57, 2165-2177.

Every year about 14.7 Gigatons of net CO2 are added to the atmosphere.1 Atmospheric CO2 concentrations measured at the ESRL (NOAA, Mauna Loa HI) show an increase from 316 ppm to 409 ppm over the past 60 years.2 For the remediation of excess CO2, various studies suggest that one approach is to capture the excess CO2 emissions and subsequently utilize them to make value-added products.3 Electroreduction of CO2 (CO2RR) is a potential method for utilizing a fraction of the excess CO2 emissions by converting them into various carbon-based chemicals such as methanol, formic acid, ethylene, and carbon monoxide.4 Research efforts in academia and industry have developed catalysts, electrodes, electrolytes, and have also optimized reactor configurations and associated operating conditions to allow for high-rate and selective CO2RR.5 However, significant improvements in electrolyzer performance (catalyst activity and selectivity) are still needed for CO2RR to be technoeconomically feasible at scale.6, 7 This talk will cover multiple system-level approaches that can be further explored to intensify the performance of a flow electrolyzer for CO2RR. The first part will discuss how electrolyte engineering based on the role of electrolyte composition and the rate determining step (RDS) can guide the systematic process optimization of CO2RR to CO on Ag nanoparticles (NPs). The effects of pH and of cation identity as well as the identity of the RDS explain how CsOH as the electrolyte can be used to optimize CO2RR performance, resulting in CO partial current densities (jCO) exceeding 850 mA/cm2 at a cathode potential of -1 V vs. RHE with 98% Faradaic efficiency (FE) and full cell energetic efficiency for CO production (EE) exceeding 40% at a conversion per pass of CO2 to CO of 36%. These process optimization insights led to the formulation of system design rules pertaining to jCO, CO FEs, CO EEs, cathode overpotentials, catalyst mass activities, conversion per pass, and system costs for CO2RR to CO on Ag NPs. The second part briefly discusses the effects of using multiple alkali metals or multivalent cations in the electrolyte composition and the effect of electrolyte composition on performance stability over multiple hours for CO2RR to CO on Ag NPs. The third part focuses on improving the overall cell performance by intensifying the anode performance with approaches such as using a Ni and Fe based bimetallic anode catalyst instead of the typically used IrO2 catalyst or using a magnet at the anode to enhance mass transport. These approaches led to about 200 mV reduction in cell overpotential and thus, enhanced CO EEs. Lastly, the fourth part will discuss the effect of operating temperature on jCO, CO FEs, and CO EEs to understand the role of temperature as a process lever for process intensification of CO2RR. References 1. IPCC, Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2013. 2. https://www.esrl.noaa.gov/gmd/ccgg/trends/ (accessed 12 November 2019). 3. S. Pacala and R. Socolow, Science, 2004, 305, 968. 4. O. S. Bushuyev, P. De Luna, C. T. Dinh, L. Tao, G. Saur, J. van de Lagemaat, S. O. Kelley and E. H. Sargent, Joule, 2018, 2, 825-832. 5. B. Endrődi, G. Bencsik, F. Darvas, R. Jones, K. Rajeshwar and C. Janáky, Progress in Energy and Combustion Science, 2017, 62, 133-154. 6. S. Verma, B. Kim, H.-R. M. Jhong, S. Ma and P. J. A. Kenis, Chemsuschem, 2016, 9, 1972-1979. 7. M. Jouny, W. Luc and F. Jiao, Industrial & Engineering Chemistry Research, 2018, 57, 2165-2177.