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

Abstract

Aspects of the structural and electronic properties of hydrous oxide films of Ni and of composite (9: 1) NUCo and (9:l) 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-0) = 1.90 f 0.02 8, and d(Fe-0) = 1.92 f 0.02 A. The latter values are in excellent agreement with d(Me-0) (Me = Co or Fe) in CoOOH and p-and y-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 NiZ+ and Ni3+ sites are expected to be present. The relative intensities of these F T 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.

``In-situ'' spectro-electrochemical studies of radionuclide-contaminated surface films on metals

“…To within the sensitivity of the EXAFS technique, the local structure of Ni remains the same in both α-Ni(OH) 2 and γ-NiOOH upon incorporation of Fe. This is similar to results from the Ni EXAFS in other co-deposited systems [28,29,30].…”

“…As the peak at ~3.10 Å is relatively weak we find that this interaction can also be represented as a Cr-Cr correlation at 3.44 (5) There has been a number of studies designed to understand the local atomic and electronic structure of iron and other metal ions incorporated into α-Ni(OH) 2 [23][24][25][26][27][28][29]. For brevity, we omit discussion of previous works here.…”

In Situ Co K-Edge X-Ray Absorption Fine Structure of Cobalt Hydroxide Film Electrodes in Alkaline Solutions

“…Although somewhat speculative, the redox activity of Co oxide, which involves no more than 15% of the total amount of material, is probably associated with surface rather than bulk Co 2ϩ /Co 3ϩ sites. As was suggested based on earlier in situ XAFS studies conducted in this laboratory, 12 the most significant effects of Co as an additive in Ni oxide electrodes appear to be derived from Co sites which replace Ni sites in the nickel oxide lattice forming a solid-state solution for which the properties are unlike those found for the two pure materials. The fraction of Co 2ϩ and Co 3ϩ sites was estimated based on the relative Co-O (and Co-Co) coordination numbers, as previously employed by Manceau et al 28 c The sum of the coordination numbers for the Co-O(1) and Co-O(2) shells, as well as for Co-Co(1) and Co-Co(2) shells, for all three stages was set to six during the fitting.…”

Simple and Convenient Design of a Spectroelectrochemical Cell for In Situ XANES Measurements of Adsorbed Species in Transmission Mode

“…In analogy with Ni and Co hydrous oxides, the reaction may be written as follows in alkaline solutions. 16,18 FeOOH + H 2 O + e − ↔ Fe(OH) 2 + OH − Polarization at potentials where FeOOH and Fe(OH) 2 are formed gives rise to a series of XANES spectra with energy shift as expected (Fig. 5).…”

“…Nitrate reduction near the working electrode raises pH which in turn makes iron hydrous oxide film formed on the surface. 16 After washing the film with water, the electrode was immediately put into the in situ cell and examined by cyclic voltammetry in 1.0 M KOH solution.…”