Structural and Chemical State Analyses of Cu-In Catalysts on Gas Diffusion Electrodes for Electrochemical CO<sub>2</sub> Reduction

Cu-In catalysts with different mole ratios were prepared and conducted to the electrochemical CO2 reduction on gas diffusion electrodes. All the Cu-In catalysts showed higher activity for CO production than In and Cu single catalysts. The activity changed remarkably with Cu-In mole ratio, and the catalyst with Cu-In mole ratio of 20:1 showed good performance in electrocatalytic CO2 reduction giving the CO production rate of 32 µmol h−1 with a Faraday efficiency of 81.0% and a current density of 2.3 mA cm−2 at maximum under the cell voltage of 2.2 V. X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption fine structure analyses revealed the dynamic change in the chemical state and structure/morphology of Cu-In catalysts before and after the reaction, i.e., CuO and In(OH)3 or In2O3 were mainly formed before the reaction whereas CuO changed to CuCO3 and Cu(OH)2 after the reaction. In addition, Scanning electron microscope-energy dispersive X-ray spectroscopy observations clearly indicated the difference in morphology of high and low active Cu-In catalysts after the reaction; In species was segregated on the Cu surface in the former catalyst while it completely covered the surface of the latter catalyst. These results suggested that the interface of In(OH)3 or In2O3 and Cu species should play a key role in the electrocatalytic CO2 reduction.