Precise modulation of electron spin states in metal-organic framework towards exceptional oxygen evolution reaction

Spin configuration and coordination environment changes have emerged as promising strategies to boost the oxygen evolution reaction (OER) activity. However, achieving the precise and gradual regulation of both spin states and coordination environment to elucidate the structure-activity relationship remains a key priority and is rarely reported. In this work, we successfully induce the gradual transition of spin states of Fe sites from low spin state to a medium spin state and ultimately to high spin state by meticulously adjusting coordination environment within NiFe-MOF, while the Ni sites still keep a low spin state. Experimental and theoretical calculations confirm the precise regulation of spin polarization and electrons migration from the Fe-t _2g to the Fe-e _g orbitals with a reduced coordination saturation, which optimizes the spin orbital exchange interactions between Fe and Ni ions and facilitates adsorption of reaction intermediates. The NiFe-MOF-D ₃ with unique NiO ₆ -FeO ₄ geometric structure exhibits low overpotential of 328 mV@1 A cm ^-2 and 365 mV@1.5 A cm ^-2 in alkaline medium. Furthermore, the assembled alkaline electrolyzer also presents remarkable activity (1.77 V@500 mA cm ^-2 ) and lower cost ($ 0.94) than the target of U.S. Department of Energy ($ 2.00).