Molecular\nO<sub>2</sub> Dimers and Lattice Instability\nin a Perovskite Electrocatalyst

Structural\ndegradation of oxide electrodes during the electrocatalytic\noxygen evolution reaction (OER) is a major challenge in water electrolysis.\nAlthough the OER is known to induce changes in the surface layer,\nlittle is known about its effect on the bulk of the electrocatalyst\nand its overall phase stability. Here, we show that under OER conditions,\na highly active SrCoO<sub>3–<i>x</i></sub> electrocatalyst\ndevelops bulk lattice instability, which results in the formation\nof molecular O<sub>2</sub> dimers inside the bulk and nanoscale amorphization\ninduced via chemo-mechanical coupling. Using high-resolution resonant\ninelastic X-ray scattering and first-principles calculations, we unveil\nthe potential-dependent evolution of lattice oxygen inside the perovskite\nand demonstrate that O<sub>2</sub> dimers are stable in a densely\npacked crystal lattice, thus challenging the assumption that O<sub>2</sub> dimers require sufficient interatomic spacing. We also show\nthat the energy cost of local atomic rearrangements in SrCoO<sub>3–<i>x</i></sub> becomes very low under the OER conditions, leading\nto an unusual amorphization under intercalation-induced stress. As\na result, we propose that the amorphization energy can be calculated\nfrom the first principles and can be used to assess the stability\nof electrocatalysts. Our study demonstrates that extreme oxidation\nof electrocatalysts under OER can intrinsically destabilize the lattice\nand result in bulk anion redox and disorder, suggesting why some oxide\nmaterials are unstable and develop a thick amorphous layer under water\nelectrolysis conditions.