Stable Aqueous Photoelectrochemical CO2 Reduction by a Cu2O Dark Cathode with Improved Selectivity for Carbonaceous Products

Abstract Photocatalytic reduction of CO 2 to produce fuels is a promising way to reduce CO 2 emission and address the energy crisis. However, the H 2 evolution reaction competes with CO 2 photoreduction, which would lower the overall selectivity for carbonaceous products. Cu 2 O has emerged as a promising material for suppressing the H 2 evolution. However, it suffers from poor stability, which is commonly regarded as the result of the electron‐induced reduction of Cu 2 O. This paper describes a simple strategy using Cu 2 O as a dark cathode and TiO 2 as a photoanode to achieve stable aqueous CO 2 reduction with a high Faradaic efficiency of 87.4 % and a selectivity of 92.6 % for carbonaceous products. We have shown that the photogenerated holes, instead of the electrons, primarily account for the instability of Cu 2 O. Therefore, Cu 2 O was used as a dark cathode to minimize the adverse effects of holes, by which an improved stability was achieved compared to the Cu 2 O photocathode under illumination. Additionally, direct exposure of the Cu 2 O surface to the electrolyte was identified as a critical factor for the high selectivity for carbonaceous products.