Multiscale Modeling of Membrane Electrode Assemblies (MEAs) for Vapor-Fed Electrochemical Reduction of CO2

Membrane-electrode assemblies (MEAs) consist of a cation- or anion-conducting membrane on either side of which is deposited a porous catalyst layer. Such structures are attractive for carrying out the electrochemical reduction of CO₂ because they exhibit low ohmic loss and high energy efficiency. Moreover, vapor-fed MEAs can operate with current densities of > 100 mA/cm² without experiencing significant mass transfer limitations encountered in electrolyzers using an aqueous electrolyte. We have developed a multiphysics, multiscale model of MEAs and used it to understand how material properties and operating conditions affect the performance of MEA-based electrolyzers. Simulations demonstrate that maintaining membrane hydration is a critical factor and that inadequate hydration results in a large voltage drop across the membrane and a loss in system efficiency. We also find that the distribution of products during CO₂ reduction is strongly affected by the distribution of water in the catalyst layers. We also find that for this reaction the main charge-carrying species are carbonate anions and that these anions are oxidized to CO₂ at the anode side of the membrane, which results in a reduced utilization of CO₂ to produce carbon-containing fuels. Ways to minimize this effect will be discussed.