Recently, hydrogen production system by electrochemical water splitting has received attractive attention to realize a sustainable society that does not depend on fossil fuels. For commercial applications, the improvement of overall water splitting efficiency has been required by the development of highly active oxygen evolution electrocatalyst. In this situation, Nocera and colleagues reported that a cobalt oxide electrodeposited from a dilute Co²⁺ solution in a potassium phosphate buffered electrolyte (Co-KP_i) is efficient electrocatalyst for oxygen evolution reaction (OER) [1]. The local structure of Co species in the Co-KP_i catalyst was investigated by Co K-edge X-ray absorption fine structure (XAFS) spectroscopy and X-ray pair distribution function (PDF) analysis, and it was revealed that the Co-KP_i is composed of nano-sized clusters with an edge-sharing CoO₆ octahedral structure [2]. However, the detailed information about oxygen species in this electrocatalyst under working condition is not yet obtained enough. Thus, we have investigated the reaction mechanism by measuring operando O K-edge XAFS spectra for Co-KP_iand similar catalysts under electrochemical potential control.

Electrochemical O K-edge XAFS measurements using soft X-rays were performed with transmission mode at BL3U in the UVSOR Synchrotron, according to the previous works [3]. A home-made electrochemical cell was used with Au/Cr/SiC thin film substrates as working electrodes, a Pt counter electrode, and a Ag/AgCl reference electrode. Co-KP_i electrocatalyst was electrodeposited on the Au/Cr/SiC working electrode in potassium phosphate buffered electrolyte containing Co(NO₃)₂.

The O K-edge XAFS spectra for Co-KP_i catalyst were taken under electrode potential control in potassium phosphate buffered electrolyte (Figure 1). At 0.0 V, an absorption peak attributed to oxygen species of CoOOH was detected around 531 eV. When the electrode potential was changed from 0.0 to 1.1 V, a new absorption peak assigned to oxygen species of CoO₂ was observed around 529 eV, indicating that the part of CoOOH cluster was oxidized to CoO₂ with the high-valent cobalt species (Co⁴⁺). Next, to discuss the relationship between spectroscopic result and catalytic activity, the O K-edge XAFS spectra were taken for the low active cobalt oxide catalysts with CoOOH local structures electrodeposited in a potassium acetate (Co-KOAc) or a potassium chloride (Co-KCl) aqueous solution. Then, the intensities of CoO₂ species for Co-KOAc and Co-KCl were much lower than that for Co-KP_i. Therefore, we found that Co-KP_i can function as highly active OER electrocatalysts due to the presence of CoO₂species.

[1] D. G. Nocera et al, J. Am. Chem. Soc. 131, 2615 (2009).

[2] (a) D. G. Nocera et al, J. Am. Chem. Soc. 132, 13692 (2010). (b) J. Am. Chem. Soc. 135, 6403 (2013).

[3] (a) M. Nagasaka et al, J. Phys. Chem. C 117 16343 (2013). (b) M. Yoshida et al, J. Phys. Chem. C 119, 19279 (2015).