Optimizing C-C Coupling on Oxide-Derived Copper Catalysts for Electrochemical CO2 Reduction

Copper electrodes, prepared by reduction of oxidized metallic copper, have been reported to exhibit higher activity for the electrochemical reduction of CO2 and better selectivity toward C-2 and C-3 (C2+) products than metallic copper that has not been preoxidized. We report here an investigation of the effects of four different preparations of oxide-derived electrocatalysts on their activity and selectivity for CO2 reduction, with particular attention given to the selectivity to products. All catalysts were tested for CO2 reduction in 0.1 M KHCO3 and 0.1 M CsHCO3 at applied voltages in the range from -0.7 to -1.0 V vs RHE. The best performing oxide-derived catalysts show up to similar to 70% selectivity to C2+ products and only similar to 3% selectivity to C-1 products at -1.0 V vs RHE when CsHCO3 is used as the electrolyte. In contrast, the selectivity to C2+ products decreases to similar to 56% for the same catalysts tested in KHCO3. By studying all catalysts under identical conditions, the key factors affecting product selectivity could be discerned. These efforts reveal that the surface area of the oxide-derived layer is a critical parameter affecting selectivity. A high selectivity to C2+ products is attained at an overpotential of -1 V vs RHE by operating at a current density sufficiently high to achieve a moderately high pH near the catalyst surface but not so high as to cause a significant reduction in the local concentration of CO2. On the basis of recent theoretical studies, a high pH suppresses the formation of C-1 relative to C2+ products. At the same time, however, a high local CO2 concentration is necessary for the formation of C2+ products.