Spin-State Engineering of Ni Centers by Dual-Ligand Competitive Coordination for Superior Oxygen Evolution Reaction

Modulating electronic spin states of metal active centers is an effective strategy to address the sluggish oxygen evolution reaction (OER) kinetics. Herein, we utilize a dual-ligand competitive coordination strategy to induce lattice expansion, generate abundant oxygen vacancies and unsaturated coordination sites, and restructure the NiO₆ octahedron. This triggers a pivotal transition of Ni from intermediate-spin (Ni²⁺) to high-spin (Ni³⁺) states, which enhances adsorption of OH^- and oxygen-containing intermediates, but also tailors the interfacial microenvironment by enriching free water, thus accelerating OER kinetics. In situ x-ray adsorption spectroscopy further verifies the accelerated adsorption and transformation of oxygen intermediates enabled by this spin reconfiguration. Consequently, the optimized nickel-thiophene-2,5-dicarboxylic acid_0.6-1,4-dicarboxybenzene_0.4 dual-ligand metal-organic framework catalyst (marked as Ni-TDC_0.6BDC_0.4) delivers excellent OER performance (230 mV@10 mA cm^-2). Using Ni-TDC_0.6BDC_0.4 as the anode, the assembled anion exchange membrane water electrolyzer achieves a low cell voltage of 2.40 V at 1.5 A cm^-2 and maintains stability for 500 h at 300 mA cm^-2. Its photovoltaic-integrated overall water splitting device also attains a 13.23% solar-to-hydrogen efficiency with robust stability. This work provides an innovative synthesis pathway for designing high-performance OER electrocatalysts by tailoring electron spin states.