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Nickel, cobalt, and manganese oxides are easily obtainable non-noble metal catalysts for water splitting. However, the relationship between composition and catalysts' performance still needs systematic studies. Herein, guided by theoretical calculations, a low overpotential, easily prepared Mn-doped Co3O4 was deposited on graphite plates for water splitting. The 30% Mn-doped Co3O4 (Co2.1Mn0.9O4) required the lowest overpotential for oxygen evolution reaction (OER), in which the Co2.1Mn0.9O4 reached 20, 30, and 50 mA cm−2 in the overpotentials of 425, 451, and 487 mV, respectively, with 90% IR compensation. Under overall water-splitting conditions, the current density reached 30 mA cm−2 at an overpotential of 0.78 V without IR compensation. Charge density difference analysis illustrates that doped Mn provides electrons for O atoms, and that Mn doping also promotes the electron fluctuation of Co atoms. XPS analysis reveals that Mn-doping increases the chemical valence of the Co atom, and that the doped Mn atom also exhibits higher chemical valence than the Mn of Mn3O4, which is advantageous to boost the form of based-OOH* radical, then decrease the overpotential. Considering the particular simplicity of growing the Co2.1Mn0.9O4 on graphite plates, this work is expected to provide a feasible way to develop the high-performance Co-Mn bimetallic oxide for water splitting. [ABSTRACT FROM AUTHOR]