Modulating the potential-determining step in oxygen evolution reaction by regulating the cobalt valence in NiCo2O4 via Ru substitution

Highlights: • NiCo₂O₄ nano-flakes with different dopant concentrations of Ruthenium (NiCo_2-xRu_xO₄) are synthesized. • Ru dopant changes the valance state of Co, and adjust the ratio of Co²⁺/Co³⁺. • The free energy of the potential-determining step is reduced. • The optimized Ru-doped NiCo₂O₄ exhibits excellent OER activities and smaller charge transfer resistance. Spinel oxide has a unique open structure, the existence of numerous empty interstitial sites is conducive to cation migration, so the valence of transition metal in spinel oxide is modifiable. Optimizing the valence state on the spinel surface has always been the focus of research because it is key for realizing efficient oxygen evolution reaction. In this paper, we introduced metal Ru into spinel oxide NiCo₂O₄ to adjust the valence state of the cations on the spinel surface, achieving a suitable ion ratio of Co²⁺/Co³⁺. The catalytic performance is the best when the doping concentration of Ru is 5.7% (NiCo_1.7Ru_0.3O₄). In 1.0 M KOH, NiCo_1.7Ru_0.3O₄ required only 280 mV overpotential to drive the current of 10 mA·cm⁻². The incorporation of Ru induces more Co²⁺ on the octahedral site and changes the valence state of Co, optimizing the adsorption of the oxygen intermediate. In addition, the coordinated charge transfer between Ru, Co, and Ni will also accelerate the reaction. These results confirm that the Ru doping process can not only change the electrochemical performance of spinel oxides but also provide new insights into the design of OER (Oxygen evolution reaction) catalysts.