In Situ X-ray Spectroscopic Identification and Kinetic Control of Highly Hybridized Co(IV) States in Cobalt Oxide Amorphous Catalysts

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Abstract
Valence dynamics of transition metals are central to oxygen evolution reaction (OER) mechanisms, yet in situ spectroscopic studies capturing these dynamics within amorphous catalysts remain limited. Here, we present in situ X-ray spectroscopy of amorphous catalysts, where CoBi exhibits higher OER performance compared to CoPi. While our data shows successive valence changes toward high-valent states in both catalysts, distinct initial electro-structural states dictate the intermediate evolution. Co(III) dominates in both catalysts at open-circuit potential, yet the Co(III)–O–Co(III) bridge structure observed in CoBi is absent in CoPi. Under active OER, Co(IV) species are identified as distinguishable spectral intensities; these are assigned to highly hybridized low-spin (LS) Co(IV) with reduced electron–electron repulsion, as validated by resonant inelastic X-ray spectroscopy (RIXS) analysis of Li0.75CoO2. Density functional theory supports the conversion from tetrahedral (Td) Co(II) to octahedral (Oh) Co(III) and suggests that factors other than thermodynamic pathways are likely responsible for the performance gap. The lower formal oxidation, larger Co–O length, smaller Co–O–Co angle, prominent pre-edge dipole character and absent oxo-bridging in CoPi suggest a kinetically inefficient Co(IV) state. These findings demonstrate that the initial electro-structure strongly influences the formation of productive high-valent active species and the resulting OER activity.
Funding Information
  • National Energy Research Scientific Computing Center (BES-ERCAP0024218)

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