Copper plating was performed using 0.01 mol/L CuSO4. The CuSO4 electrolyte was prepared freshly from CuSO4.5H2O powder (Sigma Aldrich, USA). Liquid electrolyte solutions of 0.01 or 0.1 mol/L KHCO3 or K2SO4 were used for the CO2 ER experiments. The electrolytes were prepared from dry powders immediately before each experiment (Sigma Aldrich, USA). The electrolyte was saturated with CO2 or N2, prior their use for CO2 ER experiments, through continuous vigorous bubbling at ambient conditions for >1 h .