Electrochemical behaviour of the β(II)-Ni(OH)2/β(III)-NiOOH redox couple upon potentiodynamic cycling conditions

The electrochemical behaviour of pure and Co-doped Ni(OH)2 powders is investigated under potentiodynamic cycling conditions. Evolution of some β-Ni(OH)2 physical properties as a function of the oxidation state is followed by ex situ Surface Enhanced μ-Raman Spectroscopy and in situ Ellipsometry. Effects related to the first and second reduction steps, as well as to the sweeping rate, are qualitatively studied. When compared to the results that have been recently obtained under galvanostatic conditions [S. Deabate, F. Henn, Electrochim. Acta 50 (2005) 2823], the present work confirms that the second reduction process is strongly associated to the occurrence of a reduced phase, i.e. NiO2H2−ɛ, with 0 < ɛ < ∼0.3, at the surface of the active mass grains. This reduced phase exhibits spectroscopic features very similar to the initial hydroxide which is known to be highly insulating in regards to both electrons and protons transfer.