In Situ Stimulated Raman Spectroscopy Reveals the Phosphate Network in the Amorphous Cobalt Oxide Catalyst and Its Role in the Catalyst Formation

Amorphous oxides are one of the most active catalysts for the oxygen evolution reaction (OER). However, very little is known about the structure of the amorphous oxide catalyst during OER, especially the structural detail of the low-atomic number groups (e.g., O, P-containing species). Herein, we report in situ stimulated Raman spectroscopy (SRS) of an amorphous cobalt oxide deposited in phosphate electrolyte (CoPi), one of the most active OER catalysts in neutral pH. In situ SRS reveals the presence of orthophosphates (PO43–) in CoPi, despite the species being unstable at the studied pH. 18O labeling of water during the CoPi electrodeposition substantially shifts the vibrational spectra of the phosphate bands, even though the phosphate groups were not labeled. The new vibrational positions match best to the phosphate network, for example, pyrophosphates (P2O74–), implying that the phosphates polymerize like a phosphate glass. We propose that the CoPi formation starts by electro-generating high-valence Co that subsequently react with water and phosphate to form CoPi. In 18O water, the kinetic isotope effect slows down the Co reactivity toward water. As a result, the high-valence Co reacts preferably with phosphates, polymerizing them into a phosphate network. Our finding provides a mechanistic view of how the buffer ions affect the structure of an amorphous oxide, which may explain why the activity is sensitive to the deposition procedure.