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

Abstract

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 (PO4 3–) 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 (P2O7 4–), 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.

Operando Raman spectroscopy tracks oxidation-state changes in an amorphous Co oxide material for electrocatalysis of the oxygen evolution reaction

“…64 The polyphosphates, which likely surround the Co-oxo clusters, can form a hydrogen-bond network, which likely favors proton transport inside the catalyst material. 63,65,66 On the contrary, the 990 cm −1 solution-peak (dotted black line) has no visible correspondence in the dry-CoCat spectrum, suggesting that phosphate is present inside the CoCat film mainly in the H 2 PO 4 − form. To test the assignment of these bands to phosphate, we changed the deposition solution from KPi to KBi buffer at pH 9.2.…”

“…At these concentrations, phosphate starts forming dimers or polymers forming a hydrogen-bond network, which, in the CoCat, can be functional for proton transport. 63,65,66…”

Amorphous CoV Phosphate Nanosheets as Efficient Oxygen Evolution Electrocatalyst

“…The results of Raman spectra confirm that V element is successfully introduced. The strength of M−O bonds are very weak, which indates amorphous structure of CoV−Pi catalyst [29] . The bonding nature of CoV−Pi, CoV−O, Co−Pi and Co 3 O 4 was analyzed by FT‐IR spectroscopy (Figure 2b).…”

“…The strength of MÀ O bonds are very weak, which indates amorphous structure of CoVÀ Pi catalyst. [29] The bonding nature of CoVÀ Pi, CoVÀ O, CoÀ Pi and Co 3 O 4 was analyzed by FT-IR spectroscopy (Figure 2b). Obviously, the peak at 1037 cm À 1 indicates the vibration of P=O bond, which proves the existence of phosphate groups.…”

Electrochemical In Situ/<i>operando</i> Spectroscopy and Microscopy Part 1: Fundamentals

“…66 Eom et al reported that structural information and formation mechanism of amorphous cobalt oxides in phosphate electrolyte (CoPi) could be successfully observed because stimulated Raman spectroscopy, which is a thirdorder non-linear phenomenon, could dramatically enhance Raman signals of the phosphate bands. 67 Further application of the nonlinear Raman spectroscopy to the electrochemical systems is expected in future studies.…”