Rational Lattice Engineering of Spinel Co _x Rh_3-x O₄ Solid Solution Expediting Oxygen Evolution Reaction

Electrochemical water splitting holds great promise as a viable method to produce a sustainable hydrogen fuel. Spinel crystal structure (AB(2)O(4)) is regarded as a promising electrocatalyst for the anodic oxygen evolution reaction (OER) of water electrolysis. Fine-tuning of metal cations' composition at the tetrahedral (A) and octahedral (B) sites within the well-defined spinel structure plays a critical role in determining the electroactivities for electrochemical reactions, including the OER. Herein, we report the rational incorporation of rhodium ions into the B sites of the spinel lattice of Co3O4 to form the CoxRh3-xO4 solid solution via an ecofriendly acid-base reaction between metal (Co, Rh) chlorides and NaOH in an aqueous solution, followed by the thermal annealing process. Among the CoxRh3-xO4 series, Co1.47Rh1.53O4 nanoparticles represented superior OER catalytic performances in alkaline conditions, verified by the lowest onset potential, small Tafel slope, and excellent long-term stability. The combination of experimental data with theoretical simulations suggests that the moderate d-band center (epsilon(d)) energy levels are responsible for the enhanced activity by tuning the adsorption and desorption strengths of oxygen-containing intermediates, such as *OH, *O, and *OOH species. Our findings introduce a straightforward and environmentally friendly synthetic methodology for a single phase of spinel Co1.47Rh1.53O4 nanoparticles, resulting in a rational lattice structure that can be applied as an effective OER catalyst electrode.