In situ x-ray absorption fine structure studies of foreign metal ions in nickel hydrous oxide electrodes in alkaline electrolytes

Aspects of the structural and electronic properties of hydrous oxide films of Ni and of composite (9:1) Ni/Co and (9:1) Ni/Fe, prepared by electrodeposition, have been examined in alkaline electrolytes using in situ X-ray absorption fine structure (XAFS). An analysis of the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) for the Co and Fe K-edges of these composite hydrous oxides revealed that, regardless of the oxidation state of nickel sites in the films, the guest metal ions are present as Co³⁺ and Fe³⁺ and that the cobalt-oxygen distance d(Co-O) = 1.90 ± 0.02 Å and d(Fe-O) = 1.92 ± 0.02 Å. The latter values are in excellent agreement with d(Me-O) (Me = Co or Fe) in CoOOH and β- and γ-FeOOH, respectively, determined by conventional X-ray diffraction. Two clearly defined Me-Ni first coordination shells could be observed in the Fourier transforms (FT) of the K-edge EXAFS of the guest metal recorded at a potential at which both Ni²⁺ and Ni³⁺ sites are expected to be present. The relative intensities of these FT features could be varied by changing the applied potential or, equivalently, the relative population of the two nickel sites. On the basis of these results, the Me-Ni shells are ascribed to Co³⁺ adjacent to Ni²⁺ and Ni³⁺ sites. Furthermore, d(Ni-Co) and d(Ni-Fe) for a given nickel oxidation state are found to be essentially the same as those observed for d(Ni-Ni) in pure nickel hydrous oxide films. This provides evidence that Co³⁺ and Fe³⁺ ions replace Ni sites in the hydrous oxide lattice, forming single-phase materials.