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A robust water oxidation catalyst based on copper oxide was prepared by facile electrodeposition of Cu²⁺ from borate buffer solution under near neutral conditions. The Cu–B_i thin film exhibits high activity and long-term stability in Cu²⁺-free pH 9 borate buffer. A steady current density of 1.2 mA/cm² was sustained for at least 10 h at 1.3 V versus NHE without iR compensation, which sets a new benchmark for copper-based OEC.