Substrate-Mediated Boosting of the Co ³⁺ /Co ²⁺ Pair for Highly Active and Durable Oxygen Evolution Electrocatalysis

Boosting the trivalent/divalent metal (M³⁺/M²⁺) redox pair in 3d metal compounds (TMCs) can facilitate the surface reconstruction during the oxygen evolution reaction (OER), transforming TMCs into trivalent metal oxyhydroxide (MOOH) species, which has been demonstrated to be the essential phase of the active substance. Herein, we examined an indirect strategy by the proof-of-concept of cobalt phosphide (CoP), that is, leveraging the strong substrate effect of Ti₃C₂T_<i>x</i> MXene to boost the Co³⁺/Co²⁺ pair, validated by X-ray absorption near-edge spectroscopy (XANES). The resulting CoP@Ti₃C₂T_<i>x</i> electrocatalyst achieves an ultralow overpotential of 218 mV at 10 mA cm^-2 and a small Tafel slope of 52 mV dec^-1 on the glassy carbon electrode and can operate stably over 500 h in a water electrolyzer, outperforming most non-noble metal electrocatalysts. Operando attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), operando Raman spectra, and theoretical calculations reveal an adsorbate evolution mechanism (AEM) for CoP@Ti₃C₂T_<i>x</i>, suggesting the high activity typically associated with lattice oxygen-mediated mechanism (LOM) catalysts while inheriting the excellent stability of AEM-driven systems. This work highlights substrate engineering as an effective indirect strategy to boost the M³⁺/M²⁺ pair, providing a paradigm for designing OER electrocatalysts with both high activity and long-term stability.