Although Ru/Ir-based catalysts achieve high efficiencies in polymer electrolyte membrane electrolysers, Co-based electrocatalysts are more economically viable, with high activity in a broad pH range from neutral to alkaline electrolytes 6, 7, 8, 9, 10, 11, 12, 13. In water electrolysis, the anodic oxygen evolution reaction (OER) remains the bottleneck, which so far limits the overall efficiency and hinders broader industrial application. Only recently, the decreasing cost of electricity from renewable power sources and the possibility to couple electrolysers directly with clean energy sources has become more economically appealing 3, 4, 5. The fossil fuel-free production of hydrogen (green H 2) through water electrolysis has been the topic of intense research efforts for several decades 1, 2. This contrasts the long-standing view of high-valent metal ions driving the OER, and thus, our advanced operando spectroscopy study provides much needed fundamental understanding of the oxygen-evolving near-surface chemistry. We find that accumulation of oxidative charge within the surface Co 3+O 6 units triggers an electron redistribution and an oxyl radical as predominant surface-terminating motif.
We uncover a superior intrinsic OER activity of sub-5 nm nanoparticles and a size-dependent oxidation leading to a near-surface Co–O bond contraction during OER. Here using operando X-ray absorption spectroscopy and density functional theory calculations, we provide quantitative near-surface structural insights into oxygen-evolving CoO x(OH) y nanoparticles by tracking their size-dependent catalytic activity down to 1 nm and their structural adaptation to OER conditions. Nonetheless, the near-surface structure of electrocatalysts during the anodic oxygen evolution reaction (OER) is still largely unknown, which hampers knowledge-driven optimization. Water electrolysis is a key technology to establish CO 2-neutral hydrogen production.
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