Effects of Surface Roughness on the Electrochemical Reduction of CO₂ over Cu

We have investigated the role of surface roughening on the CO<sub>2</sub> reduction reaction (CO<sub>2</sub>RR) over Cu. The activity and product selectivity of Cu surfaces roughened by plasma pretreatment in Ar, O<sub>2</sub>, or N<sub>2</sub> were compared with that of electrochemically polished Cu samples. Differences in total and product current densities, the ratio of current densities for HER (the hydrogen evolution reaction) to CO<sub>2</sub>RR, and the ratio of current densities for C<sub>2+</sub> to C<sub>1</sub> products depend on the electrochemically active surface and are nearly independent of plasma composition. Theoretical analysis of an electropolished and roughened Cu surface reveals a higher fraction of undercoordinated Cu sites on the roughened surface, sites that bind CO preferentially. Roughened surfaces also contain square sites similar to those on a Cu(100) surface but with neighboring step sites, which adsorb OC-COH, a precursor to C<sub>2+</sub> products. Here, these findings explain the increases in the formation of oxygenates and hydrocarbons relative to CO and the ratio of oxygenates to hydrocarbons observed with increasing surface roughness.