Engineering High-Spin State Cobalt Cations in Sulfide Spinel for Enhancing Water Oxidation

The spin states of active sites have a significant impact on the adsorption/desorption ability of the reaction intermediates during the oxygen evolution reaction (OER). Sulfide spinel is not generally considered a highly efficient OER catalyst owing to the low spin state of Co³⁺ and the lack of unpaired electrons available for adsorption of reaction intermediates. Herein, it is proposed a novel Nd-evoked valence electronic adjustment strategy to engineer the spin state of Co ions. The unique f-p-d orbital electronic coupling effect stimulates the rearrangement of Co d orbital electrons and increases the e_g electron filling to achieve high-spin state Co ions, which promotes charge transport by propagating a spin channel and generates a high number of active sites for intermediate adsorption. The optimized CuCo_1.75 Nd_0.25 S₄ catalyst exhibits outstanding electrocatalytic properties with a low overpotential of 320 mV at 500 mA cm^-2 and a 48 h stability at 300 mA cm^-2 . In situ synchrotron radiation infrared spectra confirm the quick accumulation of key *OOH and *O intermediates. This work deepens the comprehensive understanding of the relationship between OER activity and spin configurations of Co ions and offers a new design strategy for spinel compound catalysts.