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The development of a highly active manganese-based water oxidation catalyst in the design of an ideal artificial photosynthetic device operating under neutral pH conditions remains a great challenge, due to the instability of pivotal Mn³⁺ intermediates. We report here defective and “c-disordered” layered manganese oxides (MnO_x-300) formed on a fluorine-doped tin oxide electrode by constant anodic potential deposition and subsequent annealing, with a catalytic onset (0.25 mA/cm²) at an overpotential (η) of 280 mV and a benchmark catalytic current density of 1.0 mA/cm² at an overpotential (η) of 330 mV under neutral pH (1 M potassium phosphate). Steady current density above 8.2 mA/cm² was obtained during the electrolysis at 1.4 V versus the normal hydrogen electrode for 20 h. Insightful studies showed that the main contributing factors for the observed high activity of MnO_x-300 are (i) a defective and …