In situ x-ray absorption fine structure studies of foreign metal ions in

Jul 20, 1994 - Ni(OH)2 to NiOOH in the negative direction4 and improves the ...... funding was provided by Eveready Battery Co., Westlake, OH. The res...
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J. Phys. Chem. 1994,98, 10269-10276

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In Situ X-ray Absorption Fine Structure Studies of Foreign Metal Ions in Nickel Hydrous Oxide Electrodes in Alkaline Electrolytes Sunghyun Kim2 Donald A. Tryk,? Mark R. Antonio,” Roger Cam,* and Daniel Scherson**t Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, Argonne National Laboratory, Chemistry Division, Argonne, Illinois 60439, Stanford Synchrotron Radiation Laboratory, Stanford, California 94309 Received: July 20, 1994@

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) = 1.92f 0.02A. 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 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.

Introduction The presence of certain metal cations in hydrated nickel oxides and oxyhydroxides, referred to generically as nickel hydrous oxides, can profoundly affect the electrochemical characteristics of this interesting electrode material.’ Because of their technological importance, nickel hydrous oxides containing either iron or cobalt have received special attention. In particular, composite Ni/Fe hydrous oxides have been found to promote oxygen evolution and, as such, may be of practical value in alkaline water electrolysis.2*3 In contrast, the incorporation of cobalt, instead of iron, into the nickel hydrous oxide leads to a shift of the potential associated with the oxidation of Ni(OH)2 to NiOOH in the negative direction4 and improves the charge acceptance of practical nickel electrodes in alkaline media.’ Insight into the structural and electronic properties of both the host and foreign metals in such composite metal hydrous oxides can be gained by the use of in situ X-ray absorption fine structure (XAFS).5 The virtues of this technique for studying this class of systems are numerous. Specifically: (1) The K absorption edges of transition metals of the first (and second) row can be examined in detail using high intensity, tunable, synchrotron radiation. (2) X-rays with energies in the range of interest (ca. 7-25 keV) can penetrate thin electrolyte layers as well as low-Z window materials, such as organic polymers. This enables measurements to be performed in situ, that is, with the electrode under potential control in an appropriately designed electro-

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Experimental Section

Case Western Reserve University.

The XAFS measurements were conducted at lines 4-1 and 6-2 at the Stanford Synchrotron Radiation Laboratory (SSRL), operating with beam currents in the range 50- 100 mA. A set

Stanford Synchrotron Radiation Laboratory. Abstract published in Advance ACS Absrracfs, September 1, 1994.

of two Si(ll1) crystals was used to monochromatize the radiation in the energy region of interest. Harmonic rejection

* Argonne National Laboratory. 5

chemical cell. Such an approach removes the ambiguities associated with information derived from experiments performed ex situ. This is a very critical issue for studies involving systems of the type examined in this work, as the removal of electrodes from the cell and their further exposure to air, vacuum, and/or electron and photon beams (as in AES and XPS) can lead to a loss of potential control and structural modifications due primarily to changes in the oxidation state and/or dehydration. (3) XAFS provides structural information without relying on long range order. This factor is of crucial importance, as high charge storage capacities can only be achieved by employing materials in high-area form, which are in many cases amorphous or consist of particles that are too small to achieve sufficient coherence for standard X-ray diffraction techniques to be very useful. (4)An analysis of the absorption edge features affords insight into various electronic aspects of metal ions in the lattice, including site symmetry and oxidation state. This work will present in situ Ni, Co, and Fe K-edge fluorescence XAFS results of pure Ni and composite 9:l Ni/ Co and 9:1 Ni/Fe hydrous oxide films prepared by cathodic deposition on a solid graphite support (edge plane) as a function of the oxidation state of nickel. Of particular relevance to the present study is the seminal work of McBreen, O’Grady, Hoffman, and co-workers,6-10 who examined a variety of nickelbased battery materials using XAFS both ex situ and in situ.

0022-3654/94/2098-10269$04.50/00 1994 American Chemical Society

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was achieved by detuning the monochromator to reduce the incident X-ray intensity by 50%. Nitrogen-filled ion-chamber detectors were used for measuring the intensity of the incident beam and for recording transmission spectra of well-defined Ni and Fe compounds which served as references for the EXAFS analysis. All in situ XAFS spectra were recorded in the fluorescence mode using an ion chamber detector" (EXAFS Co.) filled with Ar and with Mn, Fe, or Co filters (EXAFS Materials) for measurements involving Fe, Co, and Ni K-edges, respectively, three absorption lengths thick, interposed between the sample and the detector. The X-ray energy was scanned with step sizes of 0.25-0.3 eV in the X-ray absorption near edge structure (XANES) region and 0.05 A-1 in the EXAFS region. The data collection time was increased in the high-energy region in order to improve the signal-to-noise ratio. Background removal and normalization was effected by subtracting the extrapolated pre-edge data through the EXAFS region and then dividing the resulting data set by the magnitude of the edge step (see supplementary material). The EXAFS, ~ ( k )was , obtained by fitting the postedge background of these data with either three or five cubic spline functions, which were then subtracted from the spectra. Glitches were removed and replaced by values interpolated from neighboring data points. Before the Fourier transforms (FT') were evaluated, the ~ ( kdata ) were multipled by k3 to enhance contributions arising from weak, high-energy extended fine structure. The FT'spectra of all reference compounds, namely Ni," NiO,lLFe304,lZand co304,l3were of high quality and in excellent agreement with data reported in the literature. The backscattering amplitudes (Fj(k)) and phase shifts (+,(k)) extracted from the FT data of the corresponding first Ni-0 shell, the first Ni-Ni shell of NiO, and the fourth shell of metallic nickel were used to fit respectively the first Ni-0, second Ni-Ni, and third Ni-Ni shells of electrodeposited pure nickel and iron and cobalt composite films, measured in situ, using a numerical method based on single-electron, singlescattering theory.I4 Best fits were achieved by nonlinear least squares refinements using the routines available in the XFPAKG package. In order to obtain the most accurate interatomic distances, no restrictions were applied to coordination numbers (CN) and Debye-Waller factors (a)throughout the fitting. This approach is valid because there is little interparameter correlation between the EXAFS phase (Le., d, A&) and amplitude (a, N) parameters. A similar procedure was utilized for the analysis of the Fe and Co EXAFS. The first shell in this case was fitted using F,(k) and +,(k) values extracted from the first shell of Fe304 and Co304, which are due to Fe-0 and Co-0, respectively. Since the second and third shells for the composite hydrous oxides involve Fe-Ni or Co-Ni interactions, for which no standards were available, theoretical ~$j(k)and Fj(k) values were obtained from FEFF 3.25.15 The electrochemical cell for in situ fluorescence experiments was identical to that employed for studies involving iron porphyrins adsorbed on high-area carbon.I6 The composite 9:1 Ni/Co and 9:1 Ni/Fe hydrous oxides were prepared by cathodic electrodeposition3J7 directly on the solid graphite electrode (cross sectional area = 0.74 cm2) from metal nitrate solutions containing 0.05 M Ni(N03)2 and either 0.0055 M Co(N03)~or 0.0055 M Fe(N03)3 by passing a current of 1 mA for 5 min for Ni/Co and 10 min for Ni/Fe films. Pure nickel hydrous oxide films were prepared using the same procedure from 0.05 M Ni(N03)~solutions at 1 mA for 15 min. This methodology has been found to yield the a form of Ni(OH)2.l8 After deposition,

Kim et al. ,

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Potential (V vs SCE) Figure 1. Cyclic voltammetry curves for pure Ni (curve A) and composite Ni/Fe (curve B) and NdCo (curve C) hydrous oxides in deaerated 1.0 M KOH: scan rate, 5 mVh; cross sectional electrode area, 0.74 cm2.

all films were rinsed with water and immediately examined by cyclic voltammetry in the XAFS cell in deaerated 1.0 M KOH at a scan rate of 5 mV/s (vide infra) using a saturated calomel reference electrode (SCE). In situ XAFS measurements were conducted in that same deaerated electrolyte with freshly deposited, voltammetrically characterized films in the discharged (Le., reduced) and partially and nominally fully charged (i.e., oxidized) states. For the latter experiments, the potential was first scanned to a value more positive than the onset of oxygen evolution and then reversed to a value sufficiently negative for the current to drop essentially to zero but still positive with respect to the onset of NiOOH reduction (0.3 V for Ni and N W o and 0.35 V vs SCE for Ni/ Fe films). This strategy eliminated problems associated with oxygen bubble formation during spectral acquisition, which was found to displace the thin electrolyte layer trapped between the window and the electrode, leading to sudden shifts in X-ray absorptivity. The in situ XAFS spectra for the reduced form of these films was obtained always at -0.3 V vs SCE. For measurements involving partially oxidized films, the potential was scanned up to a value on the rising part of the Ni(0H)z oxidation peak and then reversed to the no-current voltages, 0.27 V for Ni/Co and 0.32 V for Ni/Fe films.

Results and Discussion

I. Electrochemistry. The cyclic voltammetric curves for pure Ni composite Ni/Fe and Ni/Co hydrous oxides in 1.0 M KOH (see curves A-C in Figure 1) displayed characteristic oxidation and reduction peaks associated with the Ni(OH)2/ NiOOH redox process. In agreement with the observations of Corrigan? albeit at variance with those of Cordoba et al.,3 the overpotential for oxygen evolution was found to decrease in the sequence Ni > 9:l NVCo > 9:l Ni/Fe. The rather high electrocatalytic activity of the 9:l Ni/Fe film for oxygen evolution (see curve B in Figure 1)in the potential range positive of the peak maxima made it impossible to achieve a full conversion of Ni(0H)Z to NiOOH (vide infra). Also in harmony with earlier data3 is the shift of the overall redox features in the negative direction (ca. 50 mV) associated with the Ni(OH)z/ NiOOH couple induced by the presence of Co in the Ni hydrous

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Energy (eV) Figure 2. Ni K-edge XANES for a nickel hydrous oxide film in the reduced (or discharged, curve a) and oxidized (or nominally charged, curve b) forms recorded at -0.3 and f 0 . 3 V vs SCE, respectively. Curve c in this figure shows the Ni K-edge XANES obtained for a 9: 1 NilFe hydrous oxide film in a partially oxidized state (0.32 V vs SCE). oxide film. As pointed out by Corrigan and B e n d e ~ t ,no ~ voltammetry features associated with the Co(OH)z/CoOOH couple can be observed for this composite Ni/Co film, despite the fact that the redox peaks for a pure Co(0H)z film in this electrolyte occur at 0.09 V vs SCE (see below). 11. XANES. A. Nickel K-Edge. The Ni K-edge XANES for an a nickel hydrous oxide film in the reduced (fully discharged) and oxidized (nominally charged) states obtained at -0.3 and f 0 . 3 V vs SCE in 1.O M KOH are shown in curves a and b in Figure 2, respectively. These curves are nearly identical to those reported by Pandya et aL8 for @-Ni(OH)zin 1.0 M KOH. Particularly noticeable is the shift in the overall absorption edge region toward higher X-ray energies for the oxidized (Ni3+) film compared to the reduced (Ni2+) film, including the peak at about 8363 eV. Also shown in this figure (see curve c in Figure 2) is the Ni K-edge XANES obtained for a partially oxidized Ni/Fe hydrous oxide film, for which the absorption edge occurs, as expected, at a value intermediate between those for the fully reduced and nominally fully oxidized states. As discussed by Pandya et al.,s the pre-edge feature at about 8332 eV is ascribed to the 1s 3d electronic transition, for which the intensity is found to be larger for the oxidized than for the reduced state. However, the small magnitude of these features for both oxidation states is indicative of an octahedral environment with only slight distortion. B . Cobalt K-Edge. Unlike the behavior observed for Ni, the Co K-edge XANES for the film in the oxidized and reduced states were found to be very similar (see curves a and b in Figure 3), displaying a very small pre-edge peak. A comparison of these Ni/Co data with that of a pure cobaltous hydrous oxide film prepared by the same electrodeposition method obtained in situ in the same electrolyte (see curve c in Figure 3) indicates that the cobalt is present in the Ni/Co films in the 3+ formal oxidation state. This is somewhat surprising, since the Co(OH)z/ CoOOH redox peaks observed in such pure Co hydrous oxide films in the same alkaline electrolyte occur at a potential only slightly more negative than that of the Ni(OH)2/NiOOH ~ o u p l e , ~ in pure nickel films. Although it is well-known that the nature of the ligands can profoundly alter the redox potential of = 0.108 V, transition metal ions, e.g. E'[CO(NH~)~]~+'~+ = 1.83 V,I9 it seems rather remarkwhereas E"[CO(H~O)#+/~+ able that such a large shift in the redox potential can be achieved by seemingly subtle lattice changes. This is especially so since the Ni-0 distance (d(Ni-0)) and d(Co-0) in NiOOH and CoOOH are practically the same. The presence of cobaltic sites

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Energy (eV) Figure 3. Co K-edge XANES for a 9: 1 NUCo composite hydrous oxide in the oxidized (+0.3 V) and reduced (-0.3 V) states (curves a and b, respectively). Also shown in this figure is the correspondingspectrum for a pure Co hydrous film prepared by the same electrodeposition procedure described in the Experimental Section in the reduced state (curve c).

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Energy (ev) Figure 4. Fe K-edge XANES for a 9: 1 NiFe composite hydrous oxide film in the oxidized i.e. +0.35 V (dotted curve) and reduced i.e. -0.3 V (solid curve) states. in composite Ni/Co hydrous oxides has indeed been claimed in the literature, although without much substantiation.20 C. Iron K-Edge. In analogy with the case of Co in the Ni/ Co films, the Fe K-edge XANES for the Ni/Fe film in the oxidized and reduced states were essentially identical (see Figure 4), indicating that the state of charge of the electrode does not perturb the oxidation state of the iron sites in this material. On the basis of a comparison with the XANES of genuine FeOOH obtained in situ (not shown in this work) and other evidence as well (see below), all iron ions in this hydrous nickel lattice are in the ferric state. The pre-edge features at about 7114.8 eV are attributed to a 1s 3d electronic transition. As discussed elsewhere,21the intensity of this absorption has been found to increase as the symmetry of the X-ray-absorbing atom site is reduced. The small value of the integrated peak area, i.e. 0.1 1 eV, compared, for example, with ca. 0.24 eV for y-oxobis[iron meso-tetraphenylp~rphyrin],~~ a square pyramidal compound, (although larger than the corresponding pre-edge peak for Ni and Co) strongly suggests that the environment of iron, as that of nickel, is highly symmetric (vide infra). 111. EXAFS. A. Nickel K-Edge. The background-subtracted k32(k) Ni K-edge EXAFS and the corresponding FTs (without phase shift correction) of these data for the pure Ni, NWe, and Ni/Co hydrous oxide films in the reduced (-0.30 V

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TABLE 1: Curve-Fitting Results for the Fourier-Filtered k3%(k)Ni K-Edge in Situ EXAFS of Pure Ni and Composite 9:l NUCo and 9:l Fe/Ni Hydrous Oxides in Alkaline Electrolytes Obtained on the Basis of Model Compounds" sample shell Ak,b A-1 Ar,' A CNd r,e A a!A 2 AEog Fh 4-15 pure Ni oxidized Ni-O(1) 1.0-1.9 9.2 1.91 0.0033 2.4 0.23 Ni-Ni( 1) 2.1-3.1 7.9 2.82 4-15 0.0025 3.0 0.16 Ni-Ni(2) 0.9 3.14 4-15 0.0000 -3.4 0.0000 -6.9 0.49 4.9-5.7 Ni-Ni(3) 7.4 5.70 4-15 pure Ni reduced 4-15 0.0025 -4.0 0.17 1.4-2.0 Ni-O(1) 3.8 2.08 2.4-3.2 Ni-Ni( 1) 2.9 3.11 4-15 0.0025 -0.1 0.38 5.3-5.9 Ni-Ni(3) 2.8 6.15 4-15 0.0025 4.8 0.06 4- 14 0.0000 NUCo oxidized 3.1 0.54 1.1-1.9 Ni-O(1) 6.9 1.91 0.0016 5.5 2.83 2.0-3.2 Ni-Ni( 1) 4-14 -1.5 0.20 6.3 5.65 Ni-Ni(3) 4.7-5.7 4- 14 0.0000 -3.8 0.25 4- 14 NUCo reduced 0.0021 0.2 0.10 7.1 2.09 Ni-O(1) 1.4-2.0 0.0036 8.5 3.10 2.4-3.2 Ni-Ni( 1) 4-14 -0.3 0.17 Ni-O(1) Ni/Co partially oxidized 4-14 5.8 1.85 24.8 0.21 0.0036 1.0-1.7 2.0 2.83 2.2-3.2 Ni-Ni( 1) 4- 14 o.oO0o -3.9 0.04 Ni-Ni(2) 4-14 0.0016 -7.8 2.1 3.13 4.8-5.6 4.7 5.61 Ni-Ni(3) 4-14 0.0000 -5.4 0.15 4-14 Ni/Fe oxidized 11.1 1.84 0.0049 1.1-1.8 Ni-O(1) 16.8 0.17 7.9 2.83 2.2-3.2 Ni-Ni( 1) 4-14 0.0040 2.1 0.47 Ni-Ni(2) 4-14 0.0000 -12.0 1.9 3.18 4.8-5.6 5.6 5.66 2.5 0.24 Ni-Ni(3) 4- 14 0.0000 1.3-2.0 Ni/Fe reduced Ni-O(1) 4-15 0.0041 11.3 2.06 5.1 0.12 0.0050 2.3-3.2 11.2 3.10 Ni-Ni( 1) 4-15 1.1 0.16 5.3-5.9 Ni-Ni(3) 4-15 0.0000 5.5 6.13 1.4 0.07 4-15 Ni/Fe reduced' 0.0058 16 2.05 1.2-2.1 Ni-O( 1) 5.5 0.21 2.3-3.4 14.4 3.11 Ni-Ni( 1) 4-15 0.0053 0.1 0.39 Ni-Ni(3) 4-15 0.0020 -2.1 0.31 11.1 6.18 5.2-6.3 Ni/Fe partially oxidized 1.1-1.7 6.3 1.89 Ni-O(1) 4-15 0.0044 14.8 0.23 3.0 2.82 2.2-3.3 Ni-Ni( 1) 4-15 0.0008 0.05 1.1 Ni-Ni(2) 4-15 0.0016 -3.4 2.9 3.13 4.8-5.5 4.5 5.66 Ni-Ni(3) 4-15 0.0003 -2.4 0.11 a The backscattering amplitudes (F,(k))and phase shifts (4/(k))extracted from the FT data of the corresponding first Ni-0 shell, the first Ni-Ni shell of NiO, and the fourth shell of metallic nickel were used to fit, respectively, the first Ni-0, second Ni-Ni, and third Ni-Ni shells of electrodeposited pure nickel and iron and cobalt composite films, measured in situ, using a numerical method based on single-electron, singlescattering theory.I4 Best fits were achieved by nonlinear least square refinements using the routines available in the XFPAKG package, assuming a constant scale factor of 0.5. Ak, range of k values over which the curve fitting was performed. Ar, window for inverse Fourier transform. CN, coordination number. e r, distance in A. f a , Debye-Waller factor in Az. g AE,, energy threshold difference in eV. F, goodness of fit. These data were obtained in an independent run using a different beam line. I

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k (A-') Figure 5. Background-subtracted k3x(k) Ni K-edge EXAFS of the k3x(k)Ni K-edge EXAFS for pure Ni (panel A) and 9: 1 Ni/Fe (panel B) and 9:1 Ni/Co (panel C) hydrous oxide films in the reduced (-0.25 V vs SCE, curve a) and oxidized (+0.35 V, curve b) states. Curves c in panels B and C are the corresponding FTs for the partially oxidized composite hydrous oxide films obtained at +0.32 V. vs SCEj and oxidized (+0.30 V) states (see curves a and b in panels A-C in Figures 5 and 6, respectively) are very similar to those reported by Pandya et a1.6-8 for the oxidized and reduced forms of battery-type p nickel hydrous oxide electrodes.

In accordance with their observations, d(Ni-0) and d(Ni-Nij, within the sheet-like NiO2 layers (first and second major shells, respectively), were found to be smaller for the nominally oxidized film, d(Ni-0) = 1.89-1.91 A, d(Ni-Ni)l = 2.822.84 A, shown in curves b in Figure 6, compared to its reduced counterpart, d(Ni-Oj = 2.05-2.09 A, d(Ni-Ni)l = 3.11-3.13 A (see Table 1). The close similarity between the results obtained for the pure and composite nickel hydrous oxide films indicates that, within the sensitivity of this technique, the presence of iron or cobalt at nominally 10 metal atom % does not appear to affect the structure of the nickel hydrous oxide. It is interesting to note that Pandya et al.738have reported the presence of a shoulder on the right side of the first Ni-Ni shell for nominally fully charged P-Ni(OH)Z electrodes. According to these authors, two explanations could account for this feature: (i) a decreased coordination symmetry within the lattice and (ii) the incomplete oxidation of Ni2+ to Ni3+. Support for the second of these possibilities is provided by the FT of the Ni K-edge EXAFS of the composite hydrous oxides films in a mixed oxidation state (see curves c in Figure 6), which display two fully developed peaks of about the same height in that distance region. A two-shell fit to the overlapping peaks yielded for both Ni/Fe and Ni/Co films values of d(Ni-Nij of 2.822.83 and 3.13 A. These distances are in excellent agreement with those observed for the second shells of pure nickel hydrous oxide in the oxidized and reduced states, respectively (see Table 1). The individual Ni-Ni interactions are clearly deconvolved into two peaks by FT techniques as shown in curves a and b in panel D in Figure 6. Unlike the behavior observed for the Ni/

Metal Ions in Nickel Hydrous Oxide Electrodes

J. Phys. Chem., Vol. 98, No. 40, 1994 10273

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Figure 6. Phase-uncorrected FT for the k3x(k)Ni K-edge EXAFS in Figure 5 for pure Ni (panel A) and 9: 1 Ni/Fe (panel B) and 9: 1 NUCo (panel C) hydrous oxide films in the reduced (-0.25 V vs SCE, curve a) and oxidized (+0.35V, curve b) states. Curves c in panels B and C are the corresponding FTs for the partially oxidized composite hydrous oxide films obtained at +0.32 V. The components of the two-shell fit for the and b (Ni/Co) in panel D. Ni-Ni shell for the latter films are shown in curves a ("e)

Fe, film, for which contributions due to Ni2+ sites could be found for nominally fully oxidized films, both pure nickel and 9: 1 composite Ni/Co films under similar conditions displayed only one sharply defined shell. This indicates that the nickel in these a-Ni( Om2 films can be completely oxidized, leaving no residual Ni2+ ions in the lattice (vide infra). B . Cobalt K-Edge. The background-subtracted k3x(k) Co K-edge EXAFS for the reduced, partially reduced, and oxidized 9:l Ni/Co composite film are shown in Figure 7, and the corresponding phase shift uncorrected FTs of these data are given in Figure 8. The composite film in the fully reduced and fully oxidized states (see curves a and c in Figure 8) displays two main peaks in the radial distribution function attributed to the Co-0 and Co-Ni shells. The actual value for d(Co-0) derived from the statistical fit (see Table 2) was found to be 1.90 f 0.02 A for both states (see Table 2) and thus, within experimental error, identical to d(Co-0) in crystalline CoOOH, i.e. 1.90 A.23 Furthermore, d(Co-Ni) in the reduced (3.06 A) and oxidized (2.80 A) states compares very well with d(NiNi) in pure Ni hydrous oxide (3.11 and 2.83 A, respectively). Measurements in which the Ni(OH)2 film was charged only

partially gave rise to two Ni-Co peaks in the FT data (see curve b in Figure 8). The fit to these spectra yielded components at 2.83 and 3.06 A, which compare very well with 2.80 and 3.06 8, obtained for the film in the oxidized and reduced forms, respectively (see Table 2). Therefore these features can be attributed to Co3+ sites adjacent to Ni3+ and Ni2+ sites in the lattice, respectively. These observations provide unambiguous evidence that the co-electrodeposition procedure generates a single-phase, mixed metal hydrous oxide, in which cobalt cations occupy nickel sites in the Ni02 sheet, and not two intermixed phases, each consisting of a single metal hydrous oxide. The formation of a NUCo hydrous oxide solution has also been proposed by Cornilsen et a1.22on the basis of ex situ Raman spectroscopy of similar films grown on Ni foils. C . Iron K-Edge. On the basis of the analysis of the background-subtracted k3x(k) Fe K-edge EXAFS (see Figure 9) and their corresponding phase-shift-uncorrected FTs (see Figure lo), the overall behavior of the composite Ni/Fe films was found to be somewhat similar to that found for the NiKo counterparts. In particular, Ni/Fe films in the reduced state (see curve a in Figure 10) exhibit two main peaks in the radial

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k (A-’) Figure 7. Background-subtractedk3x(k)Co K-edge EXAFS for a 9:l NilCo composite hydrous oxide film in the reduced (curve a), nearly fully oxidized (curve b), and partially oxidized (curve c) states. Other conditions are given in the caption to Figure 6 .

the scan in the positive direction was reversed had to be carefully controlled so as to avoid excessive oxygen evolution (and thereby comprise the integrity of the film) and therefore may have not been high enough to carry out the oxidation to completion. (ii) The active sites for the electrocatalytic evolution of oxygen may involve Fe3+-modified Ni3+ sites; hence, upon reversing the scan, a fraction of these sites reacts with the solvent to generate dioxygen, consuming a fraction of the Ni3+ produced at the positive potential limit. The fact that the relative heights of the corresponding FT components in curves b and c in Figure 10 are comparable holds true, despite the fact that the potential at which the scan was reversed was sufficient to oxidize about three quarters of the total nickel sites in the film. In situ 57Fe Mossbauer effect spectroscopy (MES) data obtained in this laboratory for essentially identical Ni/Fe composite films26in the same electrolyte yielded for the reduced state a quadrupole split doublet with an isomer shift characteristic of a ferric species, which upon nominal oxidation converted into what was regarded as a broad singlet. These changes appear to be consistent with the in situ XAFS results reported in this work. Specifically, d(Fe-0) = 1.92 f 0.028, (regardless of the oxidation state of the nickel in the film) is very similar to d(Ni-0) = 1.90f 0.02 for the film in the oxidized state; hence, Fe3+ can replace Ni3+in the lattice without major lattice distortions. Such a close to octahedral environment would lead to a very small quadrupole splitting (as in, for example, Fe~[Mo04]3),~’ as the MES data shows. As the film is reduced, the oxygen atoms surrounding the Fe3+ site must readjust to the expanded Ni hydrous oxide lattice without, however, changing the Fe3+-0 distance. One plausible structural rearrangement consistent with this effect would involve a squashing of the Fe-06 octahedron, so that there is a movement of the upper and lower oxygens toward the metal ion plane, leading to a decrease in the corresponding 0-Fe-0 angle. This atomic displacement will distort the octahedral symmetry around the iron site, generating a non-zero gradient of the electric field at the nucleus, and thus account for the quadrupole split doublet observed in the MES spectra. It may be argued that the lowering in the symmetry of the Fe3+ following reduction of the film should have brought about an increase in the intensity of the Fe3+ 1s 3d electronic transition. The subtle structural changes observed with MES, however, may be too small to be clearly detected in the corresponding XANES. The lack of a clear Fe-Ni shell at about 5.6 8, would suggest that FeOOH may be adsorbed on the nickel hydrous oxide sheet. On the basis of energetic considerations, however, it is not possible to adsorb a metal cation (such as ferric species) on the layer either in tetrahedral or octahedral sites due to the close proximity of the nickel cations in the layer.28 Such an adsorption could occur if the metal cation in the layer directly below the adsorbed cation is removed,28but if that is the case, there would be no scattering species at the distance observed. Furthermore, the fact that d(Fe-Ni) changes with the oxidation state of the nickel sites in the layer implies that the iron sites are in close proximity to the nickel sites and not forming a separate phase. The fact that the third nearest neighbor could not be clearly resolved could be derived from the overall quality of the Fe K-edge data, for which the signal-to-noisewas inferior to that obtained in the case of the Co K-edge.

a

0

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r’ (4 Figure 8. Phase-uncorrectedFT for the k3x(k) Co K-edge EXAFS of a 9:l CoFe composite hydrous oxide film in the reduced (curve a), fully oxidized (curve b), and partially oxidized (curve c) states. Other conditions are given in the caption to Figure 6. distribution function, at about 1.5 and 2.88,, attributed to Fe-0 and Fe-Ni shells. The actual values derived from the statistical fit yielded d(Fe-0) = 1.89-1.92 A and d(Fe-Ni) = 3.093.10A (see Table 3). The magnitude of d(Fe-0) is essentially identical to that of Fe-0 in p- and y-FeOOH obtained from X-ray diffraction (XRD) measurements reported earlier in the literat~re.~~.~~ The FT data for the Fe EXAFS in the composite Ni/Fe hydrous oxide in the nominally fully oxidized state, shown in curve b in Figure 10,consist of two closely spaced (as opposed to a single) peaks in the range 2.1 < r’ < 3.1 A. These can be attributed to Fe-Ni shells, for which the actual Fe-Ni distances (see Table 3)match reasonably well those for the Ni-Ni shells in the oxidized and reduced states in the corresponding Ni K-edge spectra (see Table 1). A similar behavior was observed for films that had been charged only partially (see curve c in Figure 10, and Table 3), except that the relative heights of the peaks were slightly different. In analogy with the model put forward in sections 1II.A and 1II.B above, the two Ni-Fe shells are associated with ferric sites neighboring Ni3+ and Ni2+ sites. Two factors may account for the incomplete oxidation of the film: (i) The potential at which

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Summary The in situ Ni, Co, and Fe K-edge XAFS results presented in this work for nickel and 9:l composite Ni/Fe and Ni/Co

J. Phys. Chem., Vol. 98, No. 40, 1994 10275

Metal Ions in Nickel Hydrous Oxide Electrodes

TABLE 2: Curve-Fitting Results for the Fourier-Filtered k3&) Co K-Edge in Situ EXAFS of Composite 9:l NVCo Hydrous Oxides in Alkaline Electrolytes Obtained on the Basis of Model Compounds" and Theoretical Phase Shifts and Scattering Amplitude9 sample shell Ak,c A-1 Ar: 8, CN' rf A u2,gA 2 AEOh F 0.9-2.0 6.8 1.90 0.0046 -0.9 0.62 4- 12.5 Co/Ni oxidized co-0 2.0-2.9 6.5 2.80 0.0025 4.5 0.37 4- 12.5 Co-Ni(1) 0.9-2.0 7.9 1.90 0.0058 1.8 0.42 4-12.5 Co/Ni reduced co-0 2.5-3.1 1.6 3.06 0.0009 5 0.32 4- 12.5 Co-Ni( 1) 1.0-2.0 7.1 1.89 0.0036 4- 12.5 3.7 0.60 Co/Ni partially oxidized co-0 2.83 0.0036 4- 12.5 4.5 0.28 2.1-3.2 4.8 Co-Ni( 1) 3.06 0.0016 5 4- 12.5 Co-Ni(2) 1.9 5.80 0.0049 4-12.5 4.7-5.8 6.5 10 0.54 Co-Ni(3) e The first shell was fitted using Fj(k) and +,(k) values extracted from the first shell of Co304, which is due to Co-0. The second and third shells involving Ni-Co interactions were fitted using 4,{k) and F,(k) values obtained from FEFF 3.25.15 Best fits were achieved by nonlinear least square refinements using the routines available in the XFPAKG package, assuming a constant scale factor of 0.5. Ak, range of k values over which the curve fitting was performed. Ar, window for inverse Fourier transform. CN coordination number. f r, distance in A. 8 a, DebyeWaller factor in A2. AE.,,energy threshold difference in eV. F, goodness of fit.

TABLE 3: Curve-Fitting Results for the Fourier-Filtered k3x(k)Fe K-Edge in Situ EXAFS of Composite 9:l NilCo Hydrous Oxides in Alkaline Electrolytes Obtained on the Basis of Model Compound* and Theoretical Phase Shifts and Scattering Amplitude9 sample shell Ak: A r t 8, CN' rfA u2,gA 2 AEOh F' -8.3 0.19 7.4 1.92 0.0074 1.0-2.0 Fe/Ni reduced Fe-0 4-13.5 2.4 3.09 0.0081 0 0.16 4-13.5 2.3-3.4 Fe-Ni 0.0046 -3.1 0.32 1.0-2.0 8.3 1.89 Fe/Ni reduce& Fe-0 4-13.5 2.5-3.3 1.9 3.10 0.0049 0 0.20 Fe-Ni 4-13.5 5.5 1.94 0.0056 -13.3 0.15 4-13.5 1.1-1.9 Fe/Ni oxidized Fe-0 2.2-3.3 0.8 2.87 0.0004 0 0.06 Fe-Ni( 1) 4-13.5 Fe-Ni( 2) 3.3 3.06 0.0081 0 Fe-Ni partially oxidized Fe-0 4-13.5 0.9-2.0 6.4 1.92 0.0059 -10.2 0.17 Fe-Ni( 1) 4-13.5 2.2-3.3 2.4 2.89 0.0016 0 Fe-Ni(2) 4-13.5 1.8 3.08 0.0025 0 0.06 a The first shell was fitted using Fj(k) and +,(k) values extracted from the first shell of Fe304 due to Fe-0. The second and third shells involving Ni-Fe interactions were fitted using +j(k) and Fj(k) values obtained from FEW 3.25.15 Best fits were achieved by nonlinear least square refinements using the routines available in the XFPAKG package, assuming a constant scale factor of 0.5. Ak, range of k values over which the curve fitting was performed. Ar, window for inverse Fourier transform. ' CN, coordination number. f r , distance in A. 8 u,Debye-Waller factor in A2. A&, energy threshold difference in eV. F, goodness of fit. j These data were obtained in an independent run using a different beam line.

I 0

.

4

2

.

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.

I

6

.

I

8

S

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~

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k (A-') Figure 9. Background-subtracted k 3 ~ ( kFe ) K-edge EXAFS for a 9:l Ni/Fe composite hydrous oxide film in the reduced (curve a), nearly fully oxidized (curve b), and partially oxidized (curve c) states. Other conditions are given in the caption to Figure 6. hydrous oxide films electrodeposited onto solid pyrolytic graphite electrodes (edge plane) have demonstrated the following: (i) The Ni K-edge XAFS spectra of composite films display characteristics similar to those of pure nickel films. Unlike the behavior observed for pure Ni and Ni/Co in the nominally fully oxidized state, for which only a single Ni-Ni shell corresponding to Ni3+ sites could be observed, the corresponding Ni/Fe films exhibit a substantial contribution due to Ni2+ sites. This effect is most probably associated with the electrocatalytic activity of Ni/Fe films for oxygen evolution, a reaction that

0

2

4

6

r' (A) Figure 10. Phase-uncorrected FT for the k 3 ~ ( kFe ) K-edge EXAFS of a 9:l NiFe composite hydrous oxide film in the reduced (curve a), nearly fully oxidized (curve b), and partially oxidized (curve c) states. Other conditions are given in the caption to Figure 6. consumes a fraction of the Ni3+ sites at the potential at which the measurements were performed. (ii) The guest metal ions in the composite hydrous oxides are present as Co3+ and Fe3+, regardless of the oxidation state of nickel sites in the films. (iii) The Co-0 (d(Co-0)) and d(Fe-0) are 1.90 f.0.02 b; and 1.92 f 0.02 A, respectively, and are therefore in excellent

10276 J. Phys. Chem., Vol. 98, No. 40, 1994

agreement with the corresponding distances determined for crystalline CoOOH and ,if?- and y-FeOOH by X-ray diffraction. (iv) The two Me-Ni shells clearly observed in the Fourier transforms of the K-edge EXAFSs of the guest metals recorded at a potential at which both Ni2+ and Ni3+ sites are expected to be present have been ascribed to either Co3+ or Fe3+ adjacent to Ni2+ and Ni3+ sites. (v) For a given nickel oxidation state, d(Ni-Co) and d(NiFe) are found to be essentially identical to 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.

Acknowledgment. This work was supported in part by the Department of Energy, Basic Energy Sciences and by a subcontract from the Lawrence Berkeley Laboratory. Additional funding was provided by Eveready Battery Co., Westlake, OH. The research was carried out at the Stanford Sychrotron Radiation Laboratory, which is operated by the US Department of Energy, Office of Basic Energy Sciences. Supplementary Material Available: Figures showing the normalized K-edge XAFS for pure nickel and 9:l Ni/Fe and 9: 1 NUCo composite hydrous oxide films (6 pages). Ordering information is given on any current masthead page. References and Notes (1) McBreen, J. In Modern Aspects of Electrochemistry; Conway, B., White, R., Bockris, J. O’M., Eds.; Plenum Press: New York, 1991; Vol. 23. (2) Corrigan, D. A. J . Electrochem. Soc. 1987, 134, 377. (3) Cordoba, S. I.; Carbonio, R. E.; Lopez Teijelo, M.; Macagno, V. A. Electrochim. Acta 1986, 31, 1321. (4) Comgan, D. A.; Bendert, R. M. J . Electrochem. Soc. 1989, 136, 723. (5) Abruna, H. D. In Electrochemical Znterfaces; Abruna, H. D., Ed.; VCH: New York, 1991; Chapter 1. (6) McBreen, J.; O’Grady, W. E.; Pandya, K. I.; Hoffman, R. W.; Sayers, D. E. Lnngmuir 1987, 3, 428.

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