Understanding three-dimensionally interconnected porous oxide-derived copper electrocatalyst for selective carbon dioxide reduction

In this work, we have investigated a hierarchical CuO-derived inverse opal (CuO-IO) catalyst with high CO selectivity up to 80-90% and minimal H-2 evolution at moderate potentials for CO2 electroreduction. The three-dimensionally (3D) structured, porous catalyst was composed of small CuO nanoparticles and exhibited a peak CO faradaic efficiency (FE) of 72.5% (+/- 1.8), complete suppression of H-2 formation, and good stability over 24 hours operation at -0.6 V versus the reversible hydrogen electrode (RHE). In situ Raman, X-ray absorption spectroscopy and X-ray diffraction measurements indicated reduction of the catalyst into metallic Cu-0 oxidation state with dominant Cu(111) orientation under electrocatalytic conditions. We suggest that rapid depletion of CO2 and protons at the highly roughened catalyst surface likely increased the local pH during the electrolysis. The combination of C-1 favoring Cu(111) surfaces and reduced local proton/CO2 availability facilitated selective conversion of CO2 into CO and reduced H-2 and C-2 products. Our work provides additional understanding of the structure-property relationships of 3D porous electrocatalysts for CO2 reduction applications by evaluating the crystallographic orientation, oxidation state, and crystallite size of a CO-selective CuO-IO catalyst under realistic working conditions.