X-ray Absorption Spectroscopy Studies of the Local Atomic and Electronic Structure of Iron Incorporated into Electrodeposited Hydrous Nickel Oxide Films ² M. Balasubramanian* and C. A. Melendres Argonne National Laboratory, Chemical Technology DiVision, Argonne, Illinois 60439-4837 S. Mini Northern Illinois UniVersity, Physics Department, Dekalb, Illinois 60115 ReceiVed: June 28, 1999; In Final Form: January 5, 2000 We have utilized X-ray absorption fine structure (XAFS) spectroscopy to investigate the local atomic and electronic structure of iron incorporated into electrodeposited nickel hydroxide films. We found that cathodic codeposition from a solution containing Fe(II) and Ni(II) ions results in iron occupying Ni lattice sites in R-Ni(OH) 2 . The X-ray absorption near edge structure (XANES) shows that Fe is present as Fe(III) ions in the cathodically codeposited film. Analysis of the extended X-ray absorption fine structure (EXAFS) shows that Fe is coordinated to oxygen at ∼2.00 Å and to Ni at ∼3.11 Å. This Fe-O bond length is smaller than the Fe(II)-O bond distance found in Fe(OH) 2 (∼2.10 Å) but is in good agreement with the average Fe- (III)-O bond distance found in FeOOH (R, γ). The Fe-Ni bond distance is in agreement with that of the Ni(II)-Ni(II) bond distance found in R-Ni(OH) 2 . Moreover, the radial structure function (RSF) around Fe shows a distinct peak at ∼5.8 Å, which is a fingerprint of the brucite (R-Ni(OH) 2 ) structure. On anodic oxidation of the codeposited film in KOH, we found that the Fe ions occupied Ni lattice sites in γ-NiOOH. The XANES shows that the Fe edge shifts to higher energy values, indicating an increase in the oxidation state of Fe on charging. Analysis of the EXAFS data shows that Fe is coordinated to oxygen at ∼1.94 Å and to Ni at ∼2.84 Å. The latter value is in good agreement with the Ni(IV)-Ni(IV) bond length found in γ-NiOOH. The RSF around Fe in the oxidized film shows a distinct peak at ∼5.4 Å, just as in the RSF of Ni in γ-NiOOH. The Fe-O bond distance of ∼1.94 Å is in good agreement with the Fe(IV)-O bond distance found in SrFeO 3 . Our results strongly suggest that the Fe ions in the oxidized film is nominally tetravalent but with the Fe-O bond possessing a high degree of covalency. Introduction The nickel hydroxide electrode has been studied extensively due to its application in a number of nickel battery systems. Nickel hydroxide occurs in two polymorphic forms, R and . -Ni(OH) 2 crystallizes in the CdI 2 -type (brucite) structure and exhibits sharp diffraction peaks. R-Ni(OH) 2 is more disordered than -Ni(OH) 2 , as is reflected from its broader diffraction pattern. 1,2 However, X-ray absorption fine structure spectroscopy (XAFS) experiments have clearly shown that the medium-range local structure around Ni is similar in both R- and -Ni(OH) 2 . 3-8 The structure of both R- and -Ni(OH) 2 consists of NiO 2 layers, made up of edge-sharing (NiO 6 ) octahedra. It has been shown that R-Ni(OH) 2 intercalates water molecules and anions such as NO 3 - , CO 3 2- , and SO 4 2- , in between the brucite layers. 9,10 This accounts for the larger interlayer separation observed in R-Ni(OH) 2 (∼7.6 Å) as compared to -Ni(OH) 2 (∼4.7 Å). On charging, - and R-Ni(OH) 2 are transformed to -NiOOH and γ-NiOOH, respectively. The R/γ couple has a higher discharge capacity and better reversibility compared to the / couple. However, in strong alkali the anions are expelled out of the R-Ni(OH) 2 structure, leading to the R f  transformation. There has been considerable interest in stabilizing the R/γ couple. A common strategy to stabilize R-Ni(OH) 2 in strong alkali has been to enhance the strength of anion bonding to the brucite layers by partial replacement of Ni(II) ions with trivalent ions. It has been clearly demonstrated that replacement of Ni(II) with Al(III), Fe(III), and Co(III) leads to a more stable R/γ couple. 11-15 There have been a number of studies designed to understand the local atomic and electronic structure of additives incorpo- rated into R-Ni(OH) 2 . 11-15 Using Mossbauer spectroscopy, Axmann and Glemser have studied the incorporation of Fe(III) ions into R-Ni(OH) 2 prepared by controlled chemical precipita- tion. 11 They found that the incorporated Fe atoms in R-Ni(OH) 2 exist in the Fe(III) state. On electrochemical and chemical oxidation they found that some of the Fe atoms were oxidized to the Fe(IV) state. Delmas et al. studied the incorporation of Fe and Co in hydrous nickel oxide samples prepared by the “chimie douce” technique. 13-15 They found, by UV-vis spec- troscopy, that Co atoms incorporated into γ-NiOOH exist in the Co(III) state. 13 On chemical reduction with H 2 O 2 , they found that Co remained as Co(III) in the R-Ni(OH) 2 lattice. In the case of iron, Delmas et al., using Mossbauer spectroscopy, found that for low Fe content, Fe atoms predominantly exist as Fe- (IV) in γ-NiOOH. 14,15 For higher Fe content they suggest that Fe(III) ions are stabilized. Corrigan et al. have utilized “ex situ” XAFS to study the local structure of Co and Fe atoms in R-Ni- (OH) 2 prepared by electrochemical cathodic codeposition. 16 They found that on cathodic codeposition from a solution containing Co(II) and Ni(II) ions, Co predominantly occupies Ni lattice sites in Ni(OH) 2 and exists in the Co(III) state. It is interesting to note that Co gets oxidized to Co(III) upon ² Work supported by the U.S. Department of Energy, Office of Environmental Management and Office of Energy Research, under Contract W-31-109-ENG-38. * Corresponding author. Current Address: Brookhaven National Labora- tory, Bldg 480, Upton, NY 11973. E-mail: bmali@bnl.gov. Fax: 1 631 344-4701. 4300 J. Phys. Chem. B 2000, 104, 4300-4306 10.1021/jp9921710 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/14/2000