Modeling of a Gas-Phase, Bipolar Membrane CO2 Electrolyzer

Abstract

Aqueous electrochemical carbon dioxide reduction electrodes encounter prohibitive mass-transport limitations due to the low solubility of CO2 in aqueous electrolyte which limits cells to ~10 mA/cm2 reaction rates (1, 2). While C2+ products are desirable they remain economically challenging with low Faradaic efficiencies and high cell overpotentials (3). Alkaline environments are capable of facilitating preferential C-C bond formation (4); for example, the highest faradaic efficiencies for C2 products (5) and C3 products (6) have been reported under alkaline conditions. Unfortunately, carbon dioxide fouls alkaline reactors through the formation of carbonate species. Bipolar membrane-based CO2 electrolyzers, which combine gas phase acidic reduction and alkaline oxygen evolution, offer a unique opportunity to address the engineering and technological challenges of CO2 reduction. Such a system may preclude large anodic activation overpotentials while avoiding carbonate generation as the cathodic reaction proceeds in acid environment. In the gas-phase bipolar membrane CO2 reduction reactor, fundamental questions remain regarding optimal choice of cell architecture, catalyst, and operating conditions. A model of the cell was developed and used to explore rate-limiting processes and process parameters effects on reaction kinetics which dictate Faradaic efficiencies and, therefore, economic viability of the reactor. Fundamental benefits and challenges of the reactor were assessed and contrasted with competing cell architectures in context of the ultimate parameter to be optimized, cost per kilogram of averted CO2. R. Singh, E. L. Clark, and A. T. Bell, Phys. Chem. Chem. Phys., 17 (2015)Higgins et al., ACS Energy Letters, 4 (1) (2018)S. Bushuyev et al., Joule, 2 (2018)Wang et al., ACS Catal., 8 (8) (2018)Ma et al., J. Power Sources, 301 (2016)Ren et al., J. Phys. Chem. Lett., 7 (2016)