Towards membrane-electrode assembly systems for CO2 reduction: a modeling study

Abstract

Membrane-electrode assemblies (MEAs) are an attractive cell design for the electrochemical reduction of CO₂ because they exhibit low ohmic loss and high energy efficiency. We describe here the development and application of a multiphysics model to investigate the fundamental limitations of two MEA designs: one with gaseous feeds at both the anode and cathode (full-MEA), and the other with an aqueous anode feed (KHCO₃ or KOH exchange solution) and a gaseous cathode feed (exchange-MEA). The total current density for the three cases follows the order: KOH-MEA > KHCO₃-MEA > full-MEA. This trend is established by examining the distribution of the applied voltage. We show that the main charge-carrying species are carbonate anions for an MEA that uses an anion-exchange membrane (AEM). The amount of CO₂ consumed but not converted to CO decreases with increasing current densities above 100 mA cm⁻² for a full-MEA, but converges to 50% for exchange-MEAs. The full-MEA becomes limited by ohmic resistance as the membrane dehydrates with increasing cell temperature, and eventually becomes limited due to water mass transport. The exchange-MEAs can maintain membrane hydration and the local ion concentration at the anode, but are limited by salt precipitation at the cathode, as well as a higher tendency to flood. Finally, we explore the effects of temperature and discuss the possibility of increasing water supply to the full-MEA to improve its performance at elevated temperatures. The MEA model and the understanding of MEA performance for the electrochemical reduction of CO₂ presented in this study should help guide the design of next-generation CO₂ reduction cells.