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.
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