(639h) Investigation of Vapor-Fed Carbon Dioxide Reduction at the Solid Electrolyte-Catalyst Interface Using Zero-Gap Membrane Electrode Assemblies

Recent research efforts in the development of systems for electrochemical reduction of carbon dioxide (CO₂) has opened possibilities for utilizing emitted carbon to generate useful chemicals and fuels. Using electrode structures that are relatively simple, various metal surfaces have been studied to understand their intrinsic activity and product selectivity towards CO₂ reduction. While these electrodes provide a simple two phase (solid/liquid) interface for studying the intrinsic properties of the catalysts, the degree of activity (observed current density) is often limited by the amount of CO₂ gas that can be solubilized in the liquid electrolyte. In order to improve mass transport of CO₂ and render systems that are practically viable for large-scale applications, gas diffusion electrodes (GDEs), such as those used in fuel cells and water electrolyzers must be used. Specifically, the use of GDEs allows a direct conversion of vapor-phase CO₂ at the three-phase (catalyst-electrolyte-CO₂) boundary with significantly improved mass transport, resulting in much improved current densities compared to those observed with planar metal electrodes. This work therefore utilizes GDEs to fabricate membrane electrode assemblies (MEAs) using polymer-based solid electrolyte membranes to convert vapor-phase CO₂ to generate current densities greater than 50 mA cm^-2 at moderate overpotentials (> 150 mA cm^-2 at the highest potential tested). This is more than an order of magnitude higher compared to the numbers typically obtained using planar electrodes. To further understand the behavior of GDEs during CO₂ reduction, electrode fabrication parameters such as catalyst thickness and ionomer content, and cell operation parameters such as backpressure and flow rate of CO₂ have been investigated and the results are discussed.