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 Ni1-xFexOy (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 (similar to 302 mV) at 10 mA/cm(2) and a reduced Tafel slope (similar to 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 alpha-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.