Importance of Substrate Pore Size and Wetting Behavior in Gas Diffusion Electrodes for CO₂ Reduction

We study the effect of substrate pore size and wetting behavior on the performance of polymer-based Ag gas diffusion electrodes (GDEs) for electrochemical CO2 reduction. We find a strong correlation between the pore size of the substrate and the GDE product selectivity and performance stability during CO2 electrolysis. We attribute this correlation to the GDE’s wetting behavior, in particular, to its resistance to aqueous electrolyte penetration. We quantify this resistance by the water entry pressure, that is, the minimum pressure required to push water through a hydrophobic porous substrate, which depends on the pore size of the substrate and on the hydrophobicity of the substrate polymer. While substrates with low water entry pressure yield GDEs that exhibit poor CO selectivity and stability, high water entry pressure substrates lead to greatly improved Faradaic efficiency toward CO (up to 95% at 100 mA/cm2) and remarkably longer performance stability (97% of initial CO selectivity retained after >40 h). We also assess the sensitivity to surface Cu contaminants and find that substrates with high water entry pressures lead to GDEs that are more resilient to impurities, with an almost unvaried selectivity even when contaminated with >1 at % Cu vs Ag. On this basis, we propose the water entry pressure as a metric to assess GDE quality. These results highlight how acting on the substrate is a powerful and scalable handle for improving the performance of GDEs for the electrochemical CO2 reduction.