Achieving feasible photoelectrochemical water splitting devices requires the combination of three key properties: material abundance, efficiency, and stability. Unfortunately, known Earth-abundant materials such as metal oxides often behave quite poorly in terms of the latter two requirements. Here we show a progression of strategies using nanostructure morphologies and tailored interfaces to improve the stability and efficiency of copper oxide (Cu₂O) photocathodes to benchmark levels. Nanowire arrays, prepared by a simple anodization and annealing, lead to significant improvement of achievable photocurrent densities due to improved light absorption and charge separation. Forming a buried p-n heterojunction between Cu₂O and Ga₂O₃ minimized the conduction band offset, resulting in a doubling of the device photovoltage. Combining these two approaches, and adding Ni-Mo catalyst to the surface, resulted in an all Earth-abundant photocathode with benchmark efficiency.

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