(47c) Modeling CO2 reduction and Crossover in Membrane Electrode Assemblies

Electrochemical CO₂ reduction is a promising means of using renewable electricity to upgrade waste CO₂ gas into valuable chemicals. To design a CO₂ electrolyzer capable of achieving industrially-relevant product formation rates, one must employ catalyst and membrane materials that enable efficient transport of the reactants (CO₂ and water) and products (CO and hydroxide) to and from the catalyst surface. However, the coupled transport and kinetic processes (e.g., electroosmosis and (bi)carbonate formation) within CO₂ electrolyzers makes selection of these materials challenging because of the tradeoff between product formation rate and utilization of the CO₂ reactant.

This presentation focuses on the development of a 1-dimensional continuum model that is used to resolve CO₂ reduction, (bi)carbonate crossover, and water management in a membrane electrode assembly (MEA) used for converting CO₂ into CO at high current densities. The model is validated using experimentally-measured CO and H₂ formation rates and CO₂ crossover fluxes for an electrolyzer composed of a silver cathode catalyst, anion exchange membrane, iridium anode catalyst, and serpentine flow plates. The model shows how the dominant charge-carrying species in the membrane changes as a function of current density and how the coupling of ion and water transport impacts hydration of the membrane and of the ionomer within the catalyst layers. Simulations reveal the effect of the membrane ion exchange capacity, catalyst layer specific surface area, and electrolyte pH on the CO formation rate and CO₂ utilization in the cell. This talk provides insight into material design strategies for CO₂ electrolyzers and demonstrates the utility of chemical engineering principles for resolving energy losses in electrochemical systems.