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

Here, copper electrodes, prepared by reduction of oxidized metallic copper, have been reported to exhibit higher activity for the electrochemical reduction of CO<sub>2</sub> and better selectivity toward C<sub>2</sub> and C<sub>3</sub> (C<sub>2+</sub>) 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 CO<sub>2</sub> reduction, with particular attention given to the selectivity to C<sub>2+</sub> products. All catalysts were tested for CO<sub>2</sub> reduction in 0.1 M KHCO<sub>3</sub> and 0.1 M CsHCO<sub>3</sub> at applied voltages in the range from –0.7 to –1.0 V vs RHE. The best performing oxide-derived catalysts show up to ~70% selectivity to C<sub>2+</sub> products and only ~3% selectivity to C<sub>1</sub> products at –1.0 V vs RHE when CsHCO<sub>3</sub> is used as the electrolyte. In contrast, the selectivity to C<sub>2+</sub> products decreases to ~56% for the same catalysts tested in KHCO<sub>3</sub>. 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 C<sub>2+</sub> 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 CO<sub>2</sub>. On the basis of recent theoretical studies, a high pH suppresses the formation of C<sub>1</sub> relative to C<sub>2+</sub> products. At the same time, however, a high local CO<sub>2</sub> concentration is necessary for the formation of C<sub>2+</sub> products.