Mesoporous nickel–iron binary oxide nanorods for efficient electrocatalytic water oxidation

The design and fabrication of low-cost, high-efficiency, and stable oxygen-evolving catalysts are essential for promoting the overall efficiency of water electrolysis. In this study, mesoporous Ni_1–x Fe _x O _y (0 ≤ x ≤ 1, 1 ≤ y ≤ 1.5) nanorods were synthesized by the facile thermal decomposition of Ni–Fe-based coordination polymers. These polymers passed their nanorod-like morphology to oxides, which served as active catalysts for oxygen evolution reaction (OER). Increasing the Fe-doping amount to 33 at.% decreased the particle size and charge-transfer resistance and increased the surface area, resulting in a reduced overpotential (~302 mV) at 10 mA/cm² and a reduced Tafel slope (~42 mV/dec), which were accompanied by a far improved OER activity compared with those of commercial RuO₂ and IrO₂ electrocatalysts. At Fe-doping concentrations higher than 33 at.%, the trend of the electrocatalytic parameters started to reverse. The shift in the dopant concentration of Fe was further reflected in the structural transformation from a NiO (< 33 at.% Fe) rock-salt structure to a biphasic NiO/NiFe₂O₄ (33 at.% Fe) heterostructure, a NiFe₂O₄ (66 at.% Fe) spinel structure, and eventually to α-Fe₂O₃ (100 at.% Fe). The efficient water-oxidation activity is ascribed to the highly mesoporous one-dimensional nanostructure, large surface area, and optimal amounts of the dopant Fe. The merits of abundance in the Earth, scalable synthesis, and highly efficient electrocatalytic activity make mesoporous Ni–Fe binary oxides promising oxygen-evolving catalysts for water splitting.