Operando Observation of Cobalt Catalysts for Oxygen Evolution By Electrochemical XAFS Technique Using Soft X-Ray

Abstract

Photoelectrochemical water splitting using solar energy is an attractive candidate to produce hydrogen gas from water. This system consists of two half reactions of hydrogen and oxygen evolutions. Among them, the rate of oxygen evolution is generally slower than that of hydrogen evolution. Thus, the development of efficient oxygen evolution catalyst has been required toward highly active energy conversion system. Recently, a cobalt-borate electrodeposited from a dilute Co2+ solution in a borate-buffered electrolyte (Co-Bi) was reported to function as an efficient electrocatalyst for oxygen evolution reaction [1]. The structural information was investigated by X-ray pair distribution function (PDF), which indicates that Co-Bi catalyst is formed by coherent domains consisting of 3−4 nm cobaltate clusters with up to three layers [1]. However, the correlation between structure and activity is still unclear. Therefore, in this study, the Co-Bi catalyst was investigated by in-situ O K-edge XAFS measurements under potential control conditions. Electrochemical XAFS measurements with transmission mode using soft X-rays were performed at BL3U in the UVSOR Synchrotron, according to the previous works [2]. A home-made electrochemical cell was used with Au/Cr/SiC thin film substrates as working electrodes, a Pt mesh counter electrode, and a Ag/AgCl (saturated KCl) reference electrode. Co-Bi catalyst was prepared on the Au/Cr/SiC working electrode at 1.0 V in 0.1 M potassium borate (K-Bi) electrolyte containing 0.5 mM Co(NO3)2. Figure 1 shows the linear sweep voltammograms of bare and Co-Bi-modified Au electrodes in 0.1 M K-Bi electrolyte. The current density of oxygen evolution reaction for Co-Bi-modified electrode was higher than that for bare Au electrode, which exhibits that Co-Bi can function as an efficient oxygen evolution catalyst. Figure 2 shows the O K-edge XAFS spectra for Co-Bi catalyst at 0.9 and 0.0 V in 0.1 M K-Bi. Two absorption peaks assigned to the oxygen species in the CoOOH were observed around 530 and 531 eV. Meanwhile, an absorption peak of the lower energy was detected around 529 eV for the spectrum of Co-Bi at 0.9 V. Empirically, the O K-edge absorption peak position shifts to lower energies with increasing the formal oxidation state. Thus, the lowest energy absorption peak for Co-Bi sample at 0.9 V is probably attributed to Co4+ species, and the formation of the Co4+ is likely to contribute to oxygen evolution reaction. In this presentation, we will discuss more detailed assignment for the XAFS spectra and relationship with the activity of oxygen evolution reaction. [1] D. G. Nocera et al., J. Am. Chem. Soc. 135 (2013) 6403. [2] M. Nagasaka et al., J. Electron. Spectrosc. Relat. Phenom. 177 (2010) 130., J. Phys. Chem. C 117 (2013) 16343., Rev. Sci. Instrum. 85 (2014) 104105. Figure 1