In situ CoK-edge X-ray absorption fine structure of cobalt hydroxide film electrodes in alkaline solutions

Structural and electronic properties of electrodeposited Cu(OH)(2) films on Au electrodes in alkaline solutions have been characterized in situ by Co K-edge X-ray absorption spectroscopy (XAS). The results obtained have shown that freshly prepared films undergo spontaneous partial and irreversible oxidation in 1 M KOH solution even when polarized at -0.58 V vs. Hg/HgO,OH-, as evidenced by the presence of contributions (ca. 20%) from Co-O and Co-Co shells in the extended X-ray absorption fine structure (EXAFS) attributed to CoOOH. Subsequent polarization of the electrode at + 0.6 V oxidizes a significant fraction, but not all of the Co(OH)(2) into CoOOH, as judged by the occurrence of Co-O and Co-Co shells in the EXAFS ascribed to Co(OH)(2). Further reduction of the same film at -0.58 V vs. Hg/HgO,OH- increased the amount of Co(OH)(2) from 28 to 43%, indicating that the fraction of electrochemically active sites does not exceed ca. 15%. On this basis, the redox activity of Co hydroxide is probably associated with surface rather than bulk Co2+/Co3+ sites in the lattice. These results suggest that Co(OH)(2) present as a distinct pure phase in technical Ni oxide electrodes does not contribute significantly to charge storage during cycling, nor to changes in the redox properties of the electrochemically active material. (C) 2000 The Electrochemical Society. All rights reserved.