Cu ₂ONiO _x nanocomposite as an inexpensive photocathode in photoelectrochemical water splitting

A photoelectrochemical (PEC) cell for overall water splitting made of a Cu ₂O nanowire photocathode modified with a thin film of NiO _x coupled to a WO ₃ nanosheet photoanode is presented. The photocathode was prepared by thermal annealing of Cu(OH) ₂ nanowires on a Cu foil under N ₂, followed by the deposition of a 10 nm NiO _x film on the Cu ₂O nanowires (aspect ratio > 40). XPS spectra revealed that the surface species of NiO _x is a mixture of NiO and Ni(OH) ₂, which enhances charge separation in photoexcited Cu ₂O, as confirmed by electrochemical impedance spectroscopy. The optimized NiO _x modified Cu ₂O electrode shows a photocurrent density up to -4.98 mA cm ^-2 at -0.33 V and -0.56 mA cm ^-2 at 0.1 V vs. the normal hydrogen electrode (NHE) under white-light irradiation (26 mW cm ^-2) in an aqueous electrolyte solution at pH 6 and 25 °C. The formation of H ₂ gas was only observed by gas chromatography for NiO _x-modified Cu ₂O and was not detectable for unmodified Cu ₂O electrodes during prolonged irradiation. The nanocomposite structure also resulted in a three-fold increase in photostability of Cu ₂O; 72 ± 3% of the initial photocurrent density remained for the NiO _x modified Cu ₂O electrode after 20 min irradiation at 0.1 V vs. NHE. The optimized photocathode was subsequently used in a two-electrode PEC cell with an n-type WO ₃ nanosheet photoanode for overall water splitting. The different band gap of Cu ₂O (2 eV) and WO ₃ (2.6 eV) permits for efficient and complementary light absorption and sunlight-driven water splitting. The p/n heterojunction PEC cell operates with a small output of electricity even in the absence of an external bias. We demonstrate that a Cu ₂O-based electrode for H ₂ evolution can be prepared free of noble metals and we show its utilization in a PEC water splitting cell made solely from earth abundant elements.