Membrane-electrode assembly design parameters for optimal CO2 reduction

Commercial-scale generation of carbon-containing chemicals and fuels by means of electrochemical CO₂ reduction (CO₂R) requires electrolyzers operating at high current densities and product selectivities. Membrane-electrode assemblies (MEAs) have been shown to be suitable for this purpose. In such devices, the cathode catalyst layer controls both the rate of CO₂R and the distribution of products. In this study, we investigate how the ionomer-to-catalyst ratio (I:Cat), catalyst loading, and catalyst-layer thickness influence the performance of a cathode catalyst layer containing Ag nanoparticles supported on carbon. In this paper, we explore how these parameters affect the cell performance and establish the role of the exchange solution (water vs. CsHCO₃) behind the anode catalyst layer in cell performance. We show that a high total current density is best achieved using an I:Cat ratio of 3 at a Ag loading of 0.01–0.1 mg_Ag/cm² and with a 1.0 M solution of CsHCO₃ circulated behind the anode catalyst layer. For these conditions, the optimal CO partial current density depends on the voltage applied to the MEA. The work also reveals that the performance of the cathode catalyst layer is limited by a combination of the electrochemically active surface area and the degree to which mass transfer of CO₂ to the surface of the Ag nanoparticles and the transport of OH⁻ anions away from it limit the overall catalyst activity. Hydration of the ionomer in the cathode catalyst layer is found not to be an issue when using an exchange solution. The insights gained allowed for a Ag CO₂R MEA that operates between 200 mA/cm² and 1 A/cm² with CO faradaic efficiencies of 78–91%, and the findings and understanding gained herein should be applicable to a broad range of CO₂R MEA-based devices.