The photocatalytic reduction of CO₂ into chemicals and fuels is a promising way to address global warming and energy crisis. However, the hydrogen evolution reaction (HER) severely competes with CO₂ reduction for electrons, leading to a low selectivity for carbonaceous products. To suppress H₂ evolution, Cu₂O has been applied as the cathode owing to its active sites for the preferential conversion of CO₂ molecules. Unfortunately, Cu₂O suffers from poor stability since its redox potentials lay within its band gap. A common strategy to improve the stability of Cu₂O cathode is to deposit a protective layer (such as TiO₂) onto Cu₂O. But the protective layer will also prohibit the Cu₂O surface from suppression the HER, which sacrifices the selectivity for carbonaceous products.

To address this dilemma, Prof. Jinlong Gong and his colleagues at Tianjin University proposed a simple and effective system for photocatalytic CO₂ reduction, using Cu₂O as the dark cathode and TiO2 as a model photoanode. By shadowing the cathode from illumination, the Cu₂O surface could be directly exposed to the electrolytes to suppress H₂ evolution. With a set of carefully designed comparison study, they have identified that the oxidation of Cu₂O into CuO by photo-generated holes is the main reason for the instability of Cu2O photocathode, although it has been long suggested that the instability is caused by the reduction of Cu₂O into metallic Cu.

With Cu₂O as the dark cathode and TiO₂ as photoanode, their photocatalytic CO2 reduction system exhibits a stable performance over a 6-hour period, with an extremely high selectivity of 92.6% for carbonaceous products, whichis among the highest values ever reported.

This work has been published in Angew. Chem.as a VIP paper, and selected as the cover story.