Tailoring Octahedral Coordination Environment of Co ₃ O ₄ with Facile Cation-Exchange Synthesis for Efficient and Long-Term Alkaline Freshwater/Seawater Splitting

The collaborative strategy involving heteroatom doping and oxygen vacancy engineering of spinel oxides holds considerable promise for hydrogen production through water electrolysis. However, significant challenges still remain regarding both its synthesis and catalytic mechanism. Herein, we propose a simple and scalable one-step hydrothermal strategy to fabricate a self-supporting hierarchical cation–substituted spinel Co3O4 nanorod array rich in oxygen vacancies in situ grown on nickel foam (denoted as Fe-Co3O4|Vo/NF) for overall freshwater and seawater splitting. Only by adjusting the feeding molar of heteroatom Fe have we successfully prepared Co3O4 with controllable morphology and tunable oxygen vacancy content, which simultaneously increases the specific surface area, enhances electrical conductivity, and ultimately modifies the electronic structure. Consequently, the resultant Fe-Co3O4|Vo/NF electrocatalyst exhibits favorable electrocatalytic activity with the hydrogen evolution reaction (HER) overpotential at 100 mA cm–2 of only 223 mV, oxygen evolution reaction (OER) overpotential at 100 mA cm–2 of 238 mV, and the cell voltage of 1.53 V at 10 mA cm–2 when applied to overall freshwater splitting (OFS). Furthermore, in alkaline-simulated seawater and natural seawater, the assembled Fe-Co3O4|Vo/NF electrode requires only 1.58 and 1.59 V, respectively, to drive 10 mA cm–2 for overall seawater splitting (OSS), along with superior long-term durability exceeding 100 h. Density functional theory (DFT) calculations reveal that the synergistic effect of Fe doping and oxygen vacancy not only significantly tailor the electronic structure of Co active sites and optimize the adsorption free energy of reactants but also comprehensively lead to a poor adsorption capacity for Cl– and inhibition of Cl– corrosion, thus enhancing the intrinsic HER/OER kinetics. This study synergistically combines doping and vacancy engineering strategies, offering valuable design insights for developing efficient catalysts for electrolytic hydrogen production from both freshwater and seawater.