J. Phys. Chem. B 2000, 104, 9777-9779
9777
In Situ Ru K-Edge X-Ray Absorption Fine Structure Studies of Electroprecipitated Ruthenium Dioxide Films with Relevance to Supercapacitor Applications Yibo Mo,† Mark R. Antonio,‡ and Daniel A. Scherson*,† Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106-7078, and Chemistry DiVision, Argonne National Laboratory, 9700 Cass AVenue, Argonne, Illinois 60439-4831 ReceiVed: June 30, 2000; In Final Form: August 17, 2000
Modifications in electronic and structural aspects of RuO2 films electroprecipitated onto Au electrodes induced by changes in the applied potential have been examined in situ in aqueous 0.50 M H2SO4 by Ru K-edge X-ray absorption spectroscopy (XAS). The Fourier transform of the k3-weighted extended X-ray absorption fine structure (EXAFS), k3χ(k), for the film polarized at +1.20V vs RHE is characterized by two shells attributed to Ru-O and Ru-Ru interactions with average distances of 1.94(1) and 3.12(2) Å, respectively, in agreement with results obtained ex situ for Ru4+ in hydrous RuO2 by other groups. In contrast, films in the reduced state, i.e., +0.40 V vs RHE, yielded only a single shell ascribed to a Ru-O interaction at 2.02(1) Å with no evidence for a distant Ru-Ru shell. The long Ru-O distance is in agreement with that reported earlier for the hydrous Ru3+ ion [Ru-(OH2)6]3+ in the solid state. Moreover, the difference between the average Ru-O bond lengths for the reduced and oxidized films is consistent with the difference in the ionic radii of Ru3+ and Ru4+. On this basis it has been suggested that films in the reduced state contain Ru3+ sites, consistent with the electrochemical results, in a phase with apparently less order beyond the Ru-O coordination sphere than for hydrous RuO2.
Introduction The nature of the electrochemical processes associated with the rather unique pseudocapacitive behavior of RuO2 in strongly acidic electrolytes continues to elude unambiguous identification.1 Much of the effort in our laboratory has been aimed at monitoring in situ changes in the electronic and structural properties of this material in a Nafion-based supercapacitor type environment2 as a function of the applied potential using X-ray absorption spectroscopy (XAS). Although the feasibility of such type of measurements was clearly demonstrated, the changes observed in the Ru K-edge features as the state of charge of the capacitor was varied were found to be very small.3 This contribution presents in situ Ru K-edge X-ray absorption fine structure (XAFS) of RuO2 films electroprecipitated onto a Au/Melinex electrode in 0.50 M H2SO4 acquired in the fluorescence mode as a function of the applied potential. The results obtained revealed for the first time distinct features in the extended X-ray absorption fine structure (EXAFS) for the reduced and oxidized forms of the material. Experimental Section Ruthenium dioxide films were electroprecipitated on Au vapor-deposited on Melinex (12.5 × 5.5 mm) by cycling the potential repeatedly between -0.2 and +1.0 V vs SCE at a rate of 20 mV/s in an O2-saturated aqueous solution containing 2.5 mM RuCl3‚xH2O and 0.10 M KNO3. Film depositions were carried in the same cell in which in situ fluorescence XAFS measurements were performed shown schematically in Figure 1. This film growth strategy is very similar to that described recently by Hu and Huang,4 who used instead a heated Ru chloride aqueous bath at a pH of 1.96. Once the desired thickness had been achieved, the films were rinsed thoroughly † ‡
Department of Chemistry. Chemistry Division.
Figure 1. Schematic diagram of the spectroelectrochemical cell for in situ Ru K-edge XAFS in the fluorescence mode.
with pure water to remove traces of solution phase species, and the cell then filled with 0.50 M H2SO4 for the spectroelectrochemical measurements. In situ XAFS spectra were acquired at beamline 4-3 at the Stanford Synchrotron Radiation Laboratory (SSRL) operating at ring currents in the range 50-100 mA. A set of Si(220) crystals was used to monochromatize the beam. X-ray absorption spectra over the XANES and EXAFS regions were collected in the fluorescence mode with the electrode placed at an angle of 45° with respect to a 13-element Ge detector. The Ru K-edge energy was calibrated using the first inflection point of the edge region of a metallic Ru foil recorded in transmission after each fill, (Ruedge ) 22117 eV). The X-ray energy was scanned with respect to Ruedge in the range -230 to -40 eV for the pre-edge region, -40 to +70 eV for the X-ray absorption near edge
10.1021/jp002355a CCC: $19.00 © 2000 American Chemical Society Published on Web 09/30/2000
9778 J. Phys. Chem. B, Vol. 104, No. 42, 2000
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Figure 2. Cyclic voltammogram of a RuO2 film obtained after about 100 deposition recorded in 0.50 M H2SO4 at a scan rate of 20 mV/s. Electrode cross sectional area: 0.69 cm2 (1.25 × 0.55 cm2). Film average charge: q*: 22.5 mC (see text for details).
structure (XANES) region (in increments of 2 and 0.35 eV, respectively), and in steps of 0.05 Å-1 for the EXAFS region. The EXAFS analysis was performed using WinXAS, a commercially available software routine developed by T. Ressler.5 The fitting was performed in k-space on the k3-weighted reduced EXAFS data using single-scattering theoretical phase and amplitude data calculated from FEFF (version 8.0).6 Electrochemical data were collected using a Pine RDE3 potentiostat connected to a portable PC equipped with a Keithley DAS-1000 data acquisition system. Measurements were performed in 0.50 M H2SO4 using a high-area carbon electrode supplied by ICET as a counter electrode and a reversible hydrogen electrode (RHE) in the same solution as a reference electrode.
Figure 3. Upper part: in situ Ru K-edge X-ray absorption near edge structure (XANES) of the RuO2 film in Figure 2 polarized at +0.40 (thick line) and +1.20 V (thin line) in 0.50 M H2SO4. Lower part: first differentials of the XANES data in the upper panel in this figure.
Results and Discussion The cyclic voltammogram of the RuO2 films grown using about 100 deposition cycles recorded in 0.50 M H2SO4 displayed a broad peak (full width at half-maximum larger than 200 mV) centered at about +0.75 V vs RHE (see Figure 2), as well as a second feature at potentials larger than +1.0 V. This curve is in very good agreement with those reported in the same electrolyte by Hu and Huang,4 who used a similar film growth procedure, as well as those obtained by Zheng et al.7 in 0.50 M H2SO4, for nonannealed RuO2 powders prepared by sol-gel techniques. In situ XAFS measurements were recorded with the electrode polarized at +1.20 V (oxidized state) and at +0.40 V vs RHE (reduced state). Cursory inspection of the XANES region (see Figure 3) indicates rather subtle changes in the shape of the edge; however, a more rigorous analysis of the corresponding derivative curves (inset of Figure 3) failed to show a clear shift in the energy of the inflection point for the film at these two potentials. The inability of K-edge XANES to distinguish between Ru4+ and Ru3+ has already been pointed out by McKeown et al.8 and references therein. Nevertheless, the lack of pre-edge features in the in situ XANES spectra strongly suggests that the O coordination environment consists of 6 O nearest neighbors in an octahedral (or distorted Oh) configuration for the reduced and oxidized films. Our ongoing collection and analysis of high-resolution Ru L-edge XANES is anticipated to provide more detailed insights about the electronic and geometric properties of Ru in the oxidized and reduced films than are available from K-edge XANES.
Figure 4. In situ Ru K-edge k3χ(k) vs k data for the film in Figure 2, collected at +1.20 V (see dotted line, lower curve) and +0.40 vs RHE (see dotted line, upper curve). The solid lines represent the best fits to the data.
The analysis of the in situ k3χ(k) EXAFS data collected at +1.20 V (dotted line, lower curve, Figure 4) and +0.40 vs RHE (dotted line, upper curve, Figure 4) yielded rather remarkable differences. In particular, the FT of the k3χ(k) EXAFS in the oxidized state reveals a two-shell EXAFS response characteristic of Ru4+ in RuO2 with average Ru-O and Ru-Ru interatomic distances of 1.94(1) and 3.12(2) Å, respectively (see dotted line, Figure 5). These bond distances, especially the presence of a single Ru-Ru shell, are in excellent agreement with those reported by McKeown et al.8 for commercially available RuO2‚ 2.32H2O (Alfa). It should be noted, however, that these authors found two Ru-O shells for this specific hydrous oxide as well as for RuO2‚0.29H2O (Aldrich) at 1.90(1) and 1.99(1) Å. Because the distance resolution of our EXAFS data is estimated to be 0.2 Å, two or more Ru-O distances separated by less than this amount will produce a single broad peak in the FT data, much like that evident at ca. 1.4 Å (before phase shift correction) in Figure 5 (bottom, solid line). Consequently, the Ru-O interactions for the oxidized film were treated conser-
Letters
J. Phys. Chem. B, Vol. 104, No. 42, 2000 9779 TABLE 1: Structural Parameters for RuO2 Film Electrodes Derived from the Analysis of EXAFS Data Recorded in Situ as a Function of the Applied Potentiala applied potential (V vs RHE) +1.20 +0.40
Figure 5. Fourier transform of the k3χ(k) vs k data in Figure 4 at +1.20 V (scattered points, lower curve) and at +0.40 V (scattered points, upper curve). The solid lines represent the best fits to the data.
vatively as a single coordination sphere with a larger than average Debye-Waller factor. Attempts to fit the single Ru-O peak observed in the in situ data by two shells, as the large Debye-Waller factor would suggest, met with limited success. Overall, the fits did not show significant improvement and the results were somewhat inconclusive as the increased number of refined parameters exceeded the maximum number of independent parameters. In rutile-type RuO2, the tetragonal distortion of the Ru-O6 octahedron is too small, 0.044 Å difference between two short Ru-O bonds of 1.942 Å and four long ones of 1.985 Å,9 to be determined from the present EXAFS analysis. In contrast with the results found for the oxidized form of the film, the FT of the k3χ(k) EXAFS recorded in situ at +0.40 V (see solid line, Figure 5) revealed a single-shell EXAFS response, which corresponds to an average Ru-O distance of 2.02(1) Å. There is no evidence for Ru-Ru backscattering. Unlike the situation encountered with the specimen in the oxidized state, attempts to fit two shells for the Ru-O interaction of this reduced film were unsuccessful. The Ru-O shell for the film at +0.40 V may be attributed to Ru in a lower oxidation state, as the electrochemical data would suggest. Support for the presence of Ru3+ in the reduced film comes from the bond distance similarity with that for Ru3+ as the hydrated ion, [Ru(OH2)6]3+, in an alum lattice.10 The 6 Ru-O bonds in the octahedron are 2.014 Å. Moreover, the 0.08 Å longer average Ru-O distance for the reduced film (2.02 Å) compared to the oxidized film (1.94 Å) is consistent with the larger ionic radius for Ru3+ than Ru4+. For 6-coordinate Ru, the ionic radius for Ru3+ is 0.68 Å, which is 0.06 Å larger than the corresponding value for Ru4+, 0.62 Å.11 It is clear from the EXAFS data of Figures 4 and 5 that polarization of the electrode at the more negative potential (reduction of the film) induces a medium-range (ca. 3 Å) disordering phenomenon. This effect was found to be reversible as the FT of the k3χ(k) EXAFS recorded at +1.20 V after the measurements at +0.40 V were completed did exhibit a clear Ru-Ru interaction. Evidence in support of an order/disorder transition as responsible for the disappearance of EXAFS shells may be found in the work of Brown et al.12 on rutile GeO2, who monitored the Ge K-edge as a function of temperature. As shown by these workers, the Ge-Ge interaction decreased in intensity as the temperature was raised to finally disappear as the material melted, despite the fact that Ge occupies networkforming positions in GeO2 glass and liquid.
shell Ru-O Ru-Ru Ru-O
r, Å 1.94(1) 3.12(1) 2.02(1)
Nb 6 0.9 6
σ2, Å2
∆E0 c
0.012(4) 0.001(2) 0.003(1)c
2.24 2.24 3.66
a An average of four scans at each applied potential were used for the EXAFS analysis. The energy threshold (E0) was set to 22 135 eV and the background was extracted using six spline sections of cubicweighted data from 1 to ca. 11 Å-1 and then sectioned from 2 to ca. 10.5 Å-1 for curve fitting with single scattering Ru-O and RuRu from FEFF8.01.6 The number of curve fitting parameters (7 for the two-shell fit of the EXAFS obtained at +1.20 V, and 4 for the one shell fit of the EXAFS obtained at +0.40 V) was less than the number of independent parameters (8). b The O coordination number for the film at the two potentials was found to be the same, 6. The scale factor extracted from our in situ Ru K-edge data (0.68) is significantly larger than that (0.275) determined by McKeown et al.8 c The Ru-O DebyeWaller factor for the film polarized at +1.20 V, i.e., 0.012 Å2 is 4 times larger than at +0.40 V, i.e., 0.003 Å2, which suggests that two or more Ru-O peaks may best represent the data at +1.20 V.
In summary, in situ Ru K-edge XAFS experiments for electrodeposited RuO2 films on a Au substrate in concentrated sulfuric acid have shown that the material in the oxidized state resembles that of highly hydrated RuO2 obtained commercially. As the material is reduced, the average Ru-O distance increased, and the degree of distant atomic ordering decreased, as judged by the disappearance of the shell ascribed to Ru-Ru interactions. As proposed by McKweon et al.,8 the overall pseudocapacity of materials of the form RuOx(OH)y reflects an interplay between ionic and electronic transport, which, in turn, are controlled by the degree of hydration and local order of the Ru octahedra. On this basis, information on the type provided in this work may be expected to help development of supercapacitor electrodes displaying optimized performance. Acknowledgment. The work at CWRU was supported by the Department of Energy, Office of Basic Energy Science. The work at ANL was supported by the U.S. DOE Basic Energy Sciences-Chemical Sciences under contract No. W-31-109ENG-38. References and Notes (1) For example, see: Liu, T. C.; Pell, W. G.; Conway, B. E. Electrochim. Acta 1997, 42, 3541 and references therein. (2) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum Publishers: New York, 1999. (3) Mo, Y.; Bae, I. T.; Scherson, D.; Sarangapani, S. Extended Abstracts, Proceedings of the 194th Electrochemical Society Meeting, Nov. 1998, Boston, MA; Electrochemical Society: Pennington, NJ, 1998. (4) Hu, C.-C.; Huang, Y.-H. J. Electrochem. Soc. 1999, 146, 2465. (5) Ressler, T. J. Phys. IV 1997, 7, C2-269. (6) (a) Rehr, J. J.; Mustre de Leon, J.; Zabinsky and Albers, R. C. J. Am. Chem. Soc. 1991,113, 5135. (b) Mustre de Leon, J.; Rehr, J. J.; Zabinsky, S. I.; Albers, R. C. Phys. ReV. B: Condens. Matter 1991, 44, 4146. (7) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699-2703. (8) McKeown, D. A.; Hagans, P. L.; Carett, P. L.; Russell, A. E.; Swider. K. E.; Rolison, D. R. J. Phys. Chem. B 1999, 103, 4825-4832. (9) (a) Haines. J.; Leger, J. M.; Schulte, O.; Hull, S. Acta Crystallogr. 1997, B53, 880-884. b. Bolzan, A. A.; Fong, C.; Kennedy, B. J.; Howard, C. J. Acta Crystallogr. 1997, B53, 373-380. (10) Best, S. P.; Bruce, F. J. J. Chem. Soc., Dalton Trans., 1990, 35073511. (11) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767. (12) Brown, G. E.; Farges, F.; Calas, G. ReV. Mineral. 1995, 32, 317410.
