Ilmenite-Structured Ni-Doped FeCrO₃; Fe_1–<i>x</i>Ni_<i>x</i>CrO₃ (0 ≤ <i>x</i> ≤ 0.2): Dual-Site-Active Electrocatalyst for the Oxygen Evolution Reaction

As an alternative to costly precious-noble-metal-based RuO2 and IrO2, the development of a superior cost-efficient electrocatalyst for oxygen evolution reaction (OER) is key to producing green hydrogen through electrolysis of water using electricity generated through renewable sources or storing in large-scale meta-air batteries for effective utilization of renewable energy sources to reduce the dependency on fossil fuels. We envisaged Ilmenite-type M1M2O3 as the efficient family of electrocatalysts for OER, as both M1O6 and M2O6 octahedra can act as active centers for the OER, unlike ABO3 perovskite and AxMO2 layered oxides, where redox-inactive A site cations do not take part in electrocatalytic activity. Ni substitution in FeCrO3; FexNi1–xCrO3 (0.05 ≤ x ≤ 0.2) was propitiously investigated as the OH–M2+δ bond strength is the lowest for Ni and the incorporation of Ni3+ at Fe3+ is known to significantly improve electrical conductivity and density of active sites as the higher electronegativity of Ni than that of Fe (Ni3+ (∼1.695) > Fe3+ (∼1.651)) drives the transition metal 3d states closer to the O(2p) energy level and lowers the Fermi level, promoting metal–oxygen hybridization to result in superior electrocatalytic activity toward OER in alkaline media. Exclusively with the help of UV-DRS and X-ray photoelectron spectroscopy studies, we found that the Ni3+ doping contents can provide more occupied metal–adsorbate states and a stronger interaction of active sites due to the mixing of Ni(3d)/(Fe 3d)/Cr(3d) states with the O(2p) orbitals, resulting in a continuum band for enhanced adsorption of OH– and facilitation of greater electrons for the spinel OER reaction. The developed Fe0.8Ni0.2CrO3 electrocatalyst demonstrated superior electrocatalytic activity toward OER in the alkaline electrolyte, together with a low overpotential of 258 mV at a 10 mA cm–2 current density with a Tafel slope of 47 mV dec–1; it possesses superior activity than RuO2 (overpotential: 330 mV, Tafel slope: 70 mV dec–1), a well-known electrocatalyst for OER.