Nanostructured copper molybdates as promising bifunctional electrocatalysts for overall water splitting and CO₂ reduction

Overall water splitting and CO₂ reduction are two very important reactions from the environmental viewpoint. The former produces hydrogen as a clean fuel and the latter decreases the amount of CO₂ emissions and thus reduces greenhouse effects. Here, we prepare two types of copper molybdate, CuMoO₄ and Cu₃Mo₂O₉, and electrochemically investigate them for water splitting and CO₂ reduction. Our findings show that Cu₃Mo₂O₉ is a better electrocatalyst for full water splitting compared to CuMoO₄. It provides overpotentials, which are smaller than the overpotentials of CuMoO₄ by around 0.14 V at a current density of 1 mA cm^-2 and 0.10 V at -0.4 mA cm^-2, for water oxidation and hydrogen evolution reactions, respectively. However, CuMoO₄ adsorbs CO₂ and the reduced intermediates/products more strongly than Cu₃Mo₂O₉. Such different behaviors of these electrocatalysts can be attributed to their different unit cells.