Electrochemical CO₂ Reduction on Polycrystalline Copper by Modulating Proton Transfer with Fluoropolymer Composites

Electrochemical reduction of CO2 to alcohols and hydrocarbon fuels offers a sustainable pathway toward mitigating atmospheric CO2 while concomitantly generating value-added products. Our group recently demonstrated that Nafion-modified electrodes give an extraordinarily high yield of CH4 [a Faradaic efficiency of 88% at −0.4 V vs reversible hydrogen electrode (RHE) at room temperature] through the stabilization of a metal-bound CO intermediate. In this work, we fabricate Cu electrodes with a polymer blend of Nafion and polyvinylidene fluoride (PVDF) to modulate proton transfer to the metal-CO intermediate. Electrodes modified with hydrophobic PVDF blended into Nafion generate high yields of formate (a Faradaic efficiency of 58% at −0.15 V vs RHE). In addition, the total proton concentration in the electrolyte is decreased by adding an aprotic solvent to slow down proton-transfer rates at the electrode–polymer interface. This control over proton-transfer rate results in higher yields of C2+ products including ethylene, ethanol, and 1-propanol. We demonstrate that a Cu electrode with a 15 μm Nafion overlayer in an acetonitrile/bicarbonate electrolyte results in a higher yield of total carbon-containing products than an analogous unmodified Cu electrode. The total yield of carbon-containing products on these electrodes is as high as nearly 100%, indicating that hydrogen evolution does not occur under properly controlled conditions. Taken together, these results demonstrate how the selectivity of Cu-based CO2 reduction electrocatalysts can be tuned by controlling proton-transfer dynamics.