Tailoring Local Spin Interactions in Trinuclear Iron‐Based Catalysts for Water Oxidation

Water oxidation is crucial for energy conversion and storage technologies, but its efficiency is limited by the spin‐flip characteristics of the oxygen evolution reaction (OER). Here, we present a chemical approach to spin control of trinuclear ferric clusters (Fe 3 O) as the catalytic sites for the OER, by incorporating radical molecules into a metal‒organic frameworks (MIL‐T). The spin catalyst MIL‐T provides spin‐parallel electrons for triplet O─O bonding, and exhibits a low overpotential of 238 mV@10 mA cm −2 for the OER and a long‐term stability at 500 mA cm −2 for 100 h in an electrolyzer device. The spin crossover of one Fe site from low spin (LS, s = 1/2) to high spin (HS, s = 5/2) breaks the symmetry of the spin electron distribution in triangular Fe 3 O and thus promotes the local spin interaction of two Fe(LS) sites in Fe(HS)Fe(LS) 2 O. The spin catalysis in MIL‐T is validated by the absence of a magnetic field effect on the OER, paving the way for practical spin catalytic technologies.