In situ X-ray absorption fine structure and optical reflectance studies of electrodeposited nickel hydrous oxide films in alkaline electrolytes

X-ray absorption fine structure (XAFS) and optical reflectance spectroscopy (RS) have been used to examine in situ electronic and structural aspects of nickel hydrous oxide, α-Ni(OH)₂(hyd), electrodes supported on gold in alkaline electrolytes as a function of their state of charge. The extended X-ray absorption fine structure (EXAFS) of α-Ni(OH)₂(hyd) electrodes in the uncharged (UC, or discharged) and overcharged (OC, or fully charged) states yielded, in each case, a single set of two distinct nearest-neighbor shells, with distances, d(Ni - O)₁ = 2.05 ± 0.02 Å and d(Ni - Ni)₁ = 3.11 ± 0.02 Å for UC, and d(Ni - O)₁ = 1.87 ± 0.02 Å and d(Ni - Ni)₁ = 2.83 ± 0.02 Å for OC. The in situ EXAFS of films allowed to self-discharge following overcharge could be fit with contributions from both sets of shells, suggesting that only two types of nickel sites are sufficient to account for the redox chemistry of this material. These data, in addition to information derived both from quantitative X-ray absorption near-edge structure (XANES) and optical RS in the visible range, indicate that the excess anodic charge, i.e., beyond the one-electron oxidation of Ni²⁺ sites, observed during the first oxidation of freshly prepared α-Ni(OH)₂(hyd) electrodes may not be related to oxidation state changes involving nickel sites in the lattice, and, therefore, do not support the existence of nickel sites with a formal oxidation state higher than three for charged or overcharged electrodes in this media.