Modelling Anion-Exchange Membrane-Electrode Assembly Systems for CO2 Reduction

Gas diffusion electrodes (GDEs) have drastically increased the achievable CO2 reduction (CO2R) current density by overcoming the significant mass transport limitations observed in aqueous planar electrodes.1-3 The thin diffusion layer in GDEs, typically in the order of 10-100 nm, also allows operation of CO2R in alkaline conditions, which have been shown to suppress the side reaction, hydrogen evolution reaction (HER), and reduce the onset potentials of CO2R products such as CO and C2H4.4-7 With this increase in total current density, conventional cell designs for planar electrodes become severely limited by the ohmic drop across the cell, making membrane-electrode assemblies (MEAs) an attractive alternative. MEAs have smaller cell resistances as they do not have aqueous electrolyte compartments and can greatly reduce the distance between the two electrodes. In this talk, we explore the mechanisms and limitations through multiphysics modeling of two MEA designs: one with gaseous feeds at both the anode and cathode (full-MEA); the other with an aqueous anode feed (KHCO­3 or KOH exchange solution) and a gaseous cathode feed (exchange-MEA). We discuss important cell design considerations such as the distribution of the applied voltage, water management, and CO2 utilization efficiencies. Finally, we examine the effects of temperature and the trade-off between CO2 solubility and water supply for a full-MEA. R. L. Cook, R. C. Macduff and A. F. Sammells, J. Electrochem. Soc., 1990, 137, 607-608. B. Kim, S. Ma, H.-R. Molly Jhong and P. J. A. Kenis, Electrochim. Acta, 2015, 166, 271-276. L. C. Weng, A. T. Bell and A. Z. Weber, Phys. Chem. Chem. Phys., 2018, 20, 16973-16984. S. Verma, X. Lu, S. Ma, R. I. Masel and P. J. Kenis, Phys. Chem. Chem. Phys., 2016, 18, 7075-7084. C. T. Dinh, T. Burdyny, M. G. Kibria, A. Seifitokaldani, C. M. Gabardo, F. P. G. de Arquer, A. Kiani, J. P. Edwards, P. De Luna, O. S. Bushuyev, C. Q. Zou, R. Quintero-Bermudez, Y. J. Pang, D. Sinton and E. H. Sargent, Science, 2018, 360, 783-787. C.-T. Dinh, F. P. García de Arquer, D. Sinton and E. H. Sargent, ACS Energy Lett., 2018, 3, 2835-2840. D. Raciti, M. Mao, J. H. Park and C. Wang, J. Electrochem. Soc., 2018, 165, F799-F804