Understanding the Role of Spin State in Cobalt Oxyhydroxides for Water Oxidation.

Although the electronic state of catalysts is strongly corrected with their oxygen evolution reaction (OER) performances, understanding the role of spin state in dynamic electronic structure evolution during OER process is still challenging. Herein, we developed a spin state regulation strategy to boost the OER performance of CoOOH through elemental doping (CoMOOH, M=V, Cr, Mn, Co and Cu). Experimental results including magnetic characterization, in situ X-ray absorption spectroscopy, in situ Raman and density functional theory calculations unveil that Mn doping could successfully increase the Co sites from low spin state to intermediate spin state, leading to the largest lattice distortion and smallest energy gap between dxy and dz 2 orbitals among the obtained CoMOOH electrocatalysts. Benefiting from the promoted electron transfer from dxy to dz 2 orbital, facilitated formation of active high-valent *O-Co(IV) species at applied potential, and reduced energy barrier of rate-determining step, the CoMnOOH exhibits the highest OER performance. Our work provides significant insight into the correction between dynamic electronic structure evolution and OER performance by understanding the role of spin state regulation in metal oxyhydroxides, paving a new avenue for rational design of high-activity electrocatalysts.