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

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journal contribution
posted on 22.10.2020, 22:11 by Jingxin Jian, Raj Kumar, Jianwu Sun
Cuprous oxide (Cu2O) 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 TiO2 and noble metal cocatalysts for coating the Cu2O photocathode, this work demonstrates a strategy to fabricate a noble metal-free photocathode. We construct a Cu2O/ZnO p–n junction photocathode decorated with the NiOx layer as both the protective layer and the hydrogen evolution reaction (HER) cocatalyst. The NiOx 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/cm2, which are very close to the HER activity of the noble metal platinum. With decorated NiOx, the Cu2O/ZnO/NiOx photocathode exhibits significantly improved stability and photocurrent density with a Faradaic efficiency of H2 gas evolution of 95 ± 4%, distinctly outperforming the Cu2O, Cu2O/ZnO, and Cu2O/ZnO/TiO2 photocathodes. Moreover, electrochemical impedance analysis evidenced that NiOx as a cocatalyst also facilitates the transfer of photogenerated electrons across the electrode/electrolyte interface for water reduction. This work demonstrates that NiOx is not only a stable protective layer against corrosion but also a highly active H2 evolution cocatalyst. These findings provide new insights for the design of noble metal-free photocathodes toward solar fuel development.