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

Photocatalytic reduction of CO2 to produce fuels is a promising way to reduce CO2 emission and address the energy crisis. However, the H-2 evolution reaction competes with CO2 photoreduction, which would lower the overall selectivity for carbonaceous products. Cu2O 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 Cu2O. This paper describes a simple strategy using Cu2O as a dark cathode and TiO2 as a photoanode to achieve stable aqueous CO2 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 Cu2O. Therefore, Cu2O was used as a dark cathode to minimize the adverse effects of holes, by which an improved stability was achieved compared to the Cu2O photocathode under illumination. Additionally, direct exposure of the Cu2O surface to the electrolyte was identified as a critical factor for the high selectivity for carbonaceous products.