High-spin active center enables magnetic-field-enhanced photocatalytic overall water splitting

Abstract Photocatalytic reaction efficiency is governed by the spin states of active center and reaction intermediates, as it allows selective formation of desired products through spin-dependent reaction pathways. As a representative process, photocatalytic overall water splitting remains limited by water oxidation reaction, where spin-dependent formation of triplet-state O2 is thermodynamically and kinetically unfavorable. This work describes an atom-specific spin modulation strategy by selectively substituting tetrahedral Co2+ with Ni2+ in spinel Co3O4 to form NiCo2O4, which reconstructs the local coordination environment and charge distribution of octahedral Co3+, constructing a high-spin configuration with increased eg occupancy. Under the constraint of external magnetic field, high-spin Co3+ atom generates aligned parallel ·OH radicals, favoring the formation of triplet-state O2 while suppressing singlet byproducts. Meanwhile, the enhanced eg occupancy weakens the adsorption of oxygenated intermediates, lowering the Gibbs free energy barrier of the rate-determining step and accelerating water oxidation. Hence, the NiCo2O4/BiVO4 photocatalyst achieves a water oxidation rate of 148.5 ± 3.7 μmol h−1, which is 5.6 times higher than the pristine BiVO4. Furthermore, it enables near-stoichiometric overall water splitting with a solar-to-hydrogen efficiency of up to 0.309 ± 0.003%. This work underscores the importance of atom-specific spin state regulation to promote spin-polarization-dependent photocatalytic water splitting.