Cu₂O/ZnO p–n Junction Decorated with NiO_x as a Protective Layer and Cocatalyst for Enhanced Photoelectrochemical Water Splitting

Cuprous oxide (Cu 2 O) has attracted much interest as a photocathode for photoelectrochemical (PEC) water splitting because of its elemental abundance and the favorable band gap, but its poor stability in aqueous solutions hinders the practical PEC application. Compared to the mostly used TiO 2 and noble metal cocatalysts for coating the Cu 2 O photocathode, this work demonstrates a strategy to fabricate a noble metal-free photocathode. We construct a Cu 2 O/ZnO p-n junction photocathode decorated with the NiO x layer as both the protective layer and the hydrogen evolution reaction (HER) cocatalyst. The NiO x cocatalyst exhibits a small Tafel slope of 35.9 mV/ dec and a very low overpotential of 115 mV to drive a current of 10 mA/cm 2 , which are very close to the HER activity of the noble metal platinum. With decorated NiO x , the Cu 2 O/ZnO/NiO x photocathode exhibits significantly improved stability and photocurrent density with a Faradaic efficiency of H 2 gas evolution of 95 4%, distinctly outperforming the Cu 2 O, Cu 2 O/ZnO, and Cu 2 O/ZnO/TiO 2 photocathodes. Moreover, electrochemical impedance analysis evidenced that NiO x as a cocatalyst also facilitates the transfer of photogenerated electrons across the electrode/electrolyte interface for water reduction. This work demonstrates that NiO x is not only a stable protective layer against corrosion but also a highly active H 2 evolution cocatalyst. These findings provide new insights for the design of noble metalfree photocathodes toward solar fuel development.