Pinpointing the active sites where electrocatalytic reactions occur requires atomic resolution and is fundamental to rational catalyst design. In water electrolysis, the oxygen evolution reaction (OER) is driven by highly oxidizing potentials that typically transform electrocatalysts into metastable structures with high oxidation states, often reaching +4, even to +6. However, these metastable structures relax back once the potential is removed, complicating the identification of the true active structure and undermining long-term catalyst stability 13. Using atomic-resolution cryogenic scanning transmission electron microscopy (Cryo-STEM), we directly visualized the metastable structure of a widely used OER electrocatalyst, nickel–iron (oxy)hydroxides, under operating conditions. Combining in-situ X-ray absorption spectroscopy, Cryo-STEM and electrochemical measurements, we find low(er) oxidation states (Fe3+ and Ni2+) are preserved during catalysis and are sufficient to drive the reaction efficiently, contrary to conventional expectations that high-oxidation-state sites are essential for strongly oxidizing reactions. Meanwhile, this electrocatalyst exhibits 2.2 A/cm2 at 1.8 V, in anion exchange membrane water catalysts, a state-of-the-art performance. These findings experimentally demonstrate an apparent thermodynamic anomaly: that low(er)-oxidation-state sites can efficiently drive highly oxidizing reactions, paving the way to design durable OER electrocatalysts.
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