In-Situ X-ray Spectroscopy study of Cobalt Oxide Amorphous Catalysts

In this talk, we present spectroscopic evidence of valence state evolution during the OER in CoPi and CoBi, using in-situ x-ray spectroscopy techniques. Resonant x-ray emission spectra (RXES) show that low-spin octahedral (O_h) Co(III) species dominate in both materials at open circuit potential (OCP), with substantial conversion of Co(II) to Co(III). However, more residual Co(II) content remains detectable at OCP in CoPi than CoBi. These valence state changes correlate with changes in formal oxidation state and bond angles. A notable observation is the absence of oxygen-mediated metal-metal delocalization in the OCP spectrum of CoPi, contrasting with CoBi, which suggests that electron and proton conductivity in in-situ condition is poorer in CoPi compared to CoBi. In the OER active condition, RXES spectra of both materials reveal a new excitation feature, closely resembling that of delithiated LiCoO₂. Resonant inelastic x-ray scattering (RIXS) spectra indicate that this feature corresponds to the presence of low spin O_h Co(IV), providing direct spectroscopic evidence of Co(IV) during the OER. Thermodynamic calculations on a model structure show that O_h Co(III) is both energetically and kinetically more favorable than T_d Co(II). On the other hand, there is no significant energetic advantage for a two-site mechanism over a single-site mechanism, despite the potential activity of the former in CoBi due to the presence of a Co-O-Co bridging structure. Our findings suggest that differences in electron-proton conductivity play a central role in the OER performance of these cobalt-based amorphous catalysts.