Ilmenite-Structured Ni-Doped FeCrO3; Fe1–xNixCrO3 (0 ≤ x ≤ 0.2): Dual-Site-Active Electrocatalyst for the Oxygen Evolution Reaction

Abstract

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 A_xMO₂ layered oxides, where redox-inactive A site cations do not take part in electrocatalytic activity. Ni substitution in FeCrO₃; Fe_xNi_1–xCrO₃ (0.05 ≤ x ≤ 0.2) was propitiously investigated as the OH–M^2+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 Fe_0.8Ni_0.2CrO₃ 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.