Permeability Matters When Reducing CO₂ in an Electrochemical Flow Cell

The highest performing cathodes for CO2 electrolyzers are porous and exhibit high specific surface areas that serve to increase the density of CO2 reduction catalyst sites. While porous electrodes increase CO2 reduction activity, their high surface area can negatively impact the mass transport of products and reactants at high current densities (i.e., >100 mA cm–2). We demonstrate here the trade-off between the density of CO2 reduction catalyst sites (specific surface area) and mass transport (permeability) for porous electrodes used for mediating the conversion of KHCO3(aq) into CO(g) in an electrolyzer. This demonstration relied on porous electrodes containing a silver electrocatalyst deposited on carbon fibers of varying fiber diameters (0.5, 1, and 10 μm). Porous electrodes with the intermediate specific surface areas (fiber diameter = 1 μm) yielded the highest CO formation rates. This result shows that permeability should be considered when designing porous electrodes for CO2 electrolysis.