Voltage- and time-dependent valence state transition in cobalt oxide catalysts during the oxygen evolution reaction

• Published in: Nature communications Vol. 11; no. 1; p. 1984
• Main Authors: Zhou, Jing, Zhang, Linjuan, Huang, Yu-Cheng, Dong, Chung-Li, Lin, Hong-Ji, Chen, Chien-Te, Tjeng, L. H., Hu, Zhiwei
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 24-04-2020; Nature Publishing Group; Nature Portfolio
• Subjects: 119/118; 140/133; 639/301/299/161/886; 639/301/930/12; 639/638/563/979; 639/638/77/886; Catalysts; Chemical reactions; Cobalt; Cobalt oxides; Density functional theory; Electric potential; Electrochemistry; Electronic structure; Evolution; Humanities and Social Sciences; Ions; multidisciplinary; Oxygen; Oxygen evolution reactions; Reaction mechanisms; Science; Science (multidisciplinary); Soft X ray spectroscopy; Soft x rays; Spectrum analysis; Time dependence; Valence; Voltage; X-ray spectroscopy
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-020-15925-2

The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li
2
Co
2
O
4
under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and time-dependent valence state transition from Co
3+
to Co
4+
accompanied by spontaneous delithiation, whereas the edge-shared Co–O network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co
4+
site, rather than the Co
3+
site or the oxygen vacancy site, is mainly responsible for the high OER activity.
Determining catalyst electronic structures during electrochemical reactions is crucial to understand mechanisms. Here authors perform in operando soft X-ray spectroscopy on a cobalt oxide catalyst during O
2
evolution and observe voltage and time-dependent valence state transitions.