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Langmuir 1999, 15, 780-785
Voltammetric Characterization of Ruthenium Oxide-Based Aerogels and Other RuO2 Solids: The Nature of Capacitance in Nanostructured Materials Jeffrey W. Long,† Karen E. Swider,† Celia I. Merzbacher,‡ and Debra R. Rolison*,† Surface Chemistry and Optical Physics, Naval Research Laboratory, Washington, D.C. 20375 Received June 30, 1998. In Final Form: November 24, 1998 Ruthenium dioxide is an important electrode material for applications in electrocatalysis and power sources. High surface areas are achieved in hydrous RuO2 precipitates and in mixed ruthenium oxidetitanium oxide, (Ru-Ti)Ox, aerogels (in which nanoscale domains are networked to form a highly porous structure). The electrochemical properties of (Ru-Ti)Ox aerogels, RuO2, and hydrous RuO2 are examined by direct pressing of sub-milligram quantities of the solid onto the surface of a conductive carbon/wax composite. Voltammetric measurements in acidic electrolyte confirm a pseudocapacitive response for all the RuOx-based materials. Despite an improvement in BET surface area, as compared with other RuO2 materials, the (Ru-Ti)Ox aerogel displays a low specific capacitance, which correlates to the high degree of crystallinity of the aerogel. Nanocrystallites of rutile RuO2, formed during thermal treatment of the sol-gel Ru/Ti precursors, deleteriously affect the specific capacitance of the material; however, all RuOx domains in the aerogel are voltammetrically addressable. The influence of proton-donating species on the observed capacitance for the (Ru-Ti)Ox aerogel is evident from the dependence of the voltammetric charge in acidic electrolyte on the potential scan rate.
Introduction Ruthenium dioxide is recognized as an important material for a wide range of electrochemical applications1 because of a set of properties nearly unique to RuO2. Anhydrous RuO2 is a d-band metallic conductor, with a single-crystal conductivity of 104 S cm-1.2 Hydrous forms of RuO2sdesignated as RuOxHy or as RuO2‚xH2Osare also conductive, on the order of 1 S cm-1.3 RuO2 (or rather the hydrous surface of crystalline, rutile RuO2) is a desirable electrocatalyst, due to its low overpotentials for O2 and Cl2 evolution. Mixed RuO2-TiO2 electrodes, or dimensionally stable anodes (DSA), are currently the anode materials used in the chlor-alkali process in which brine is electrolyzed to produce commodity NaOH and Cl2.1 Ruthenium dioxide has also been investigated as a cathode material for H2 evolution, having the advantage of lower sensitivity to poisoning than the more familiar noble-metal catalysts.4 More recent studies demonstrate the importance of hydrous RuO2 in Pt-Ru direct methanol fuel cell catalysts.5,6 RuO2 electrodes are also of interest for supercapacitor applications due to the ability of RuOxHy to act as a “proton condenser”.1,7,8 RuOxHy surface sites are reversibly oxi* To whom correspondence may be addressed. E-mail: rolison@ nrl.navy.mil. † Surface Chemistry. ‡ Optical Physics. (1) Trasatti, S. Electrochim. Acta 1991, 36, 225. (2) Schafer, H.; Schneidereit, G.; Gerhardt, W. Z. Anorg. Allg. Chem. 1963, 319, 372. (3) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (4) Ko¨tz, E. R.; Stucki, S. J. Appl. Electrochem. 1987, 17, 1190. (5) Hagans, P. L.; Swider, K. E.; Rolison, D. R. In Electrode Materials and Processes for Energy Conversion and Storage IV, McBreen, J., Srinivasan, S., Eds.; The Electrochemical Society: Pennington, NJ, 1997; Vol. 97-13, pp 86-105. (6) Swider, K. E.; Hagans, P. L.; Long, J. W.; Rolison, D. R. Langmuir 1999, 15, 774. (7) Trasatti, S.; Buzzanca, G. J. Electroanal. Chem. 1971, 29, Appl. 1.
dized and reduced with the simultaneous exchange of protons with the contacting solution according to the reaction
RuOx(OH)y + δH+ + δe- S RuOx-δ(OH)y+δ
(1)
Such electrochemical reactions result in capacitive, nearly featureless voltammograms for the hydrous surface of RuO2 in aqueous electrolyte between H2 and O2 evolution potentials.9 Specific capacitances as high as 720 F/g have been reported for amorphous RuOxHy.3,10 These properties make RuO2 an ideal (although expensive) candidate for supercapacitor applications.11 For both electrocatalytic and supercapacitive applications, it will be advantageous to prepare RuO2 in stable, high surface area forms, such as aerogels. Aerogels are composed of a three-dimensional network of nanoscale particles surrounded by a large volume of mesoporosity.12-14 As a consequence of this structure, aerogels retain very high specific surface areas (up to 1000 m2/g for SiO2). Metal oxide aerogels are typically prepared from sol-gel chemistry, where metal alkoxide precursors undergo a series of hydrolysis and condensation reactions to form the networked structure of the gel. The free volume of the gel is retained by removing the liquid phase under supercritical conditions to form the aerogel. The combination of high specific surface areas and mesoporosity, which allow effective mass transport of molecules through the gel structure, makes aerogels (8) Doblhofer, K.; Metikos, M.; Ogumi, Z.; Gerisher, H. Ber. BunsenGes. Phys. Chem. 1978, 82, 1046. (9) Conway, B. E. J. Electrochem. Soc. 1991, 138, 1539. (10) Zheng, J. P.; Jow, T. R. J. Electrochem. Soc. 1995, 142, L6. (11) Trasatti, S. Platinum Metals Rev. 1994, 38, 46. (12) Gesser, H. D.; Goswami, P. C. Chem. Rev. 1989, 89, 765. (13) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, 1990. (14) Hu¨sing, N.; Schubert, U. Angew. Chem., Int. Ed. Engl. 1998, 37, 22.
10.1021/la980785a CCC: $18.00 © 1999 American Chemical Society Published on Web 01/07/1999
Ruthenium Oxide-Based Aerogels
attractive as materials for applications in heterogeneous catalysis.15-17 The high surface areas and mesoporosity of aerogels are also highly desired properties in electrode materials. These desirable characteristics have prompted research using aerogels, such as vanadium pentaoxide or carbon, either as the electrochemically active component or as an electrode support. High surface area V2O5 aerogels have shown an improvement in lithium-ion intercalation over bulk V2O5 or V2O5 xerogels.18-20 Carbon aerogels can be prepared in conductive, ultrahigh surface area forms and are obvious candidates for supercapacitors.21 The specific capacitance of carbon aerogels can be improved by the incorporation of ruthenium dioxide to the aerogel structure.22,23 Aerogels of mixed ruthenium oxide-titanium oxide, (Ru-Ti)Ox, should be useful in energy applications.24 These aerogels have low densities and high surface areas (85 m2/g).25 Bulk characterization analyses such as X-ray diffraction and small-angle neutron scattering have confirmed that the (Ru-Ti)Ox aerogels are composed of ∼10-nm particles of crystalline RuO2 and TiO2, with the TiO2 component forming the aerogel backbone.24,26 Whereas bulk RuO2 is an electronic conductor, when RuO2 is expressed in this high surface area aerogel form, the electrical conductivity becomes defined by the defective, hydrous RuOxHy surface, which is a mixed proton-electron conductor.6,24,27 In this report we describe the electrochemistry of (RuTi)Ox aerogels immobilized without binders onto the surface of a carbon/wax substrate. The voltammetric response in acidic electrolyte is compared with those of various RuO2 powders; all RuOx-based solids are characterized in terms of specific capacitance. We also examine the effects of the Ru content of the aerogel and the potential scan rate on the observed voltammetry. Experimental Procedures Chemicals. Anhydrous RuCl3 (Alfa Aesar, Ru assay ) 48.91%), titanium(IV) isopropoxide (Aldrich), RuCl3‚xH2O (Alfa Aesar, Ru assay ) 38.92%), eicosane (Aldrich, 99%), acetylene black (Alfa), anhydrous RuO2 (Alfa Aesar, Ru assay ) 75.5%), and platinum black (Alfa Aesar, fuel-cell grade) were used as received. The 0.5 M H2SO4 electrolyte was prepared from concentrated H2SO4 (Ultrex, J. T. Baker) and 18 mΩ cm water (Barnstead NANOpure). Materials. (Ru-Ti)Ox aerogels were prepared as described previously.24 Ru is introduced into a literature preparation for TiO2 aerogels28 via an anhydrous RuCl3 precursor. The supercritically dried (Ru-Ti) aerogels are annealed in a humidified argon atmosphere to 400 °C and cooled under oxygen to convert (15) Pajonk, G. M. Appl. Catal. 1991, 72, 217. (16) Ko, E. I. CHEMTECH 1993, 23, 31. (17) Schneider, M.; Baiker, A. Catal. Rec.-Sci. Eng. 1995, 37, 515. (18) Le, D. B.; Passerini, S.; Tipton, A. L.; Owens, B. B.; Smyrl, W. H. J. Electrochem. Soc. 1995, 142, L102. (19) Salloux, K.; Chaput, F.; Wong, H. P.; Dunn, B.; Breiter, M. W. J. Electrochem. Soc. 1995, 142, L191. (20) Le, D. B.; Passerini, S.; Guo, J.; Ressler, J.; Owens, B. B.; Smyrl, W. H. J. Electrochem. Soc. 1996, 143, 2099. (21) Farmer, J. C.; Dix, D. V.; Mack, G. V.; Pekala, R. W.; Poco, J. F. J. Electrochem. Soc., 1996, 143, 159. (22) Miller, J. M.; Dunn, B.; Tran, T. D.; Pekala, R. W. J. Electrochem. Soc. 1997, 144, L309. (23) Miller, J. M.; Dunn, B. Langmuir, 1999, 15, 799. (24) Swider, K. E.; Merzbacher, C. I.; Hagans, P. L.; Rolison, D. R. Chem. Mater. 1997, 9, 1248. (25) On a basis of m2/mol, 85 m2/g for (Ru0.32-Ti0.68)O2 would equal 137 m2/g of SiO2. (26) Merzbacher, C. I.; Barker, J. G., Swider, K. E.; Rolison, D. R. Adv. Colloid Interface Sci. 1998, 76-77, 57. (27) Swider, K. E.; Merzbacher, C. I.; Hagans, P. L.; Rolison, D. R. J. Non-Cryst. Solids 1998, 225, 348. (28) Dagan, G.; Tomkiewicz, M. J. Phys. Chem. 1993, 97, 12651.
Langmuir, Vol. 15, No. 3, 1999 781 to (Ru-Ti)Ox. The ruthenium content of the aerogel was estimated by thermogravimetric measurements of the decomposition of RuO2 to ruthenium metal and molecular oxygen.27,29 RuO2 solids were prepared by precipitation of RuOxHy from aqueous RuCl3‚ xH2O solutions at controlled pH according to the procedure of Jow and co-workers.3 Annealing temperatures of 150 and 400 °C were chosen to convert this amorphous solid to RuO2‚0.5H2O and RuO2‚0.03H2O, respectively. The extent of hydration of all RuOx solids was determined from thermogravimetric measurement of water loss in flowing argon. Electrochemical Measurements. For electrochemical measurements, powders of the (Ru-Ti)Ox aerogels and various RuOx solids were immobilized onto the surface of a carbon/wax composite, designated “sticky carbon”. This procedure is a modification of one reported by Yang and co-workers.30 The sticky carbon was prepared from acetylene black carbon and eicosane wax (35:65 mass ratio) and then packed into the empty well of a rotating disk electrode and polished flat on standard weighing paper. When introducing a powder to the surface of the sticky carbon, the sample was first weighed (∼0.4 mg) and the powder gathered (without piling) near the center of the weigh paper. The surface of the electrode was firmly pressed onto the powder for transfer. The paper was then reweighed to determine the quantity of powder transferred to the electrode. Electrochemical measurements were performed in 0.5 M H2SO4 in a conventional three-compartment cell with an SCE reference electrode and platinum foil auxiliary electrode. The electrolyte was thoroughly purged with argon prior to measurements. Potential control was maintained through a Bioanalytical Systems CV-27 potentiostat. For impedance measurements, powders of the (Ru-Ti)Ox aerogels were hand-pressed into 0.5 mm thick pellets using a 4-mm diameter die.24 Aerogel pellets were spring-loaded between platinum foil electrodes in a ceramic cell. Two-probe impedance measurements were performed using a Solartron 1260 Impedance/Gain Phase Analyzer. Instrumentation. Thermogravimetric measurements were performed using a Rheometrics Scientific simultaneous thermal analyzer. BET surface areas were measured using a Micromeritics ASAP 2010 accelerated surface area and porosimetry system. X-ray diffraction measurements were performed with a Siemens D5005 X-ray diffractometer (Cu anode; powders were placed in a sample cup for analysis).
Results and Discussion Voltammetry of Pt Black on Sticky Carbon. Before the voltammetric response of either the (Ru-Ti)Ox aerogels or the various RuOx solids in aqueous acid were assessed, the integrity of the sticky carbon itself was examined. As a control experiment, fuel-cell-grade platinum black was immobilized on the sticky carbon electrode and the voltammetry recorded in 0.5 M H2SO4 as shown in Figure 1. The expected voltammetric waves and resolution for oxidation/oxide stripping and hydrogen adsorption/desorption on high surface area platinum are evident. The voltammetric peaks remain comparably defined over time as compared to the initial response and show only a slight decrease in current, even after extended exposure (24 h) of the Pt black to both electrolyte and the sticky carbon substrate. This experiment indicates that no significant contamination of a high surface area powder is caused by the carbon/wax composite during the voltammetric characterization in aqueous acidic electrolyte. Voltammetry of (Ru-Ti)Ox Aerogels. Representative voltammograms for a (Ru0.32-Ti0.68)Ox aerogel immobilized on sticky carbon are displayed in Figure 2. The faradaic features are broad and capacitive, consistent with the electrochemical mechanisms expected for the RuOxHy (29) Campbell, P. F.; Ortner, M. H.; Anderson, C. J. Anal. Chem. 1961, 33, 58. (30) Yang, Y.-F.; Zhou, Y.-H.; Cha, C.-S. Electrochim. Acta 1995, 40, 2579.
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Long et al.
Figure 1. Voltammograms (at 100 mV/s) for 0.30 mg of fuelcell-grade platinum black immobilized on the sticky carbon electrode. Measurements are taken as a function of time of immersion in 0.5 M H2SO4 electrolyte.
Figure 3. (A) Voltammograms (at 100 mV/s) for (Ru0.32-Ti0.68)Ox aerogel on sticky carbon electrode and unmodified sticky carbon electrode. (B) Voltammograms (at 100 mV/s) for (Ru0.14Ti0.86)Ox aerogel on sticky carbon electrode and unmodified sticky carbon electrode. Measurements recorded in 0.5 M H2SO4.
Figure 2. Voltammograms (at 20 mV/s) for 0.29 mg of (Ru0.32Ti0.68)Ox aerogel immobilized on the sticky carbon electrode. Measurements are recorded as a function of time immersed in 0.5 M H2SO4 electrolyte.
surface according to eq 1.7 The voltammetric response was also examined as a function of the time in which the aerogel-modified sticky carbon was immersed in 0.5 M H2SO4. With longer exposure times the currents decrease slightly and the shape of the voltammogram becomes less resistive. The observed voltammetric response in the aerogel can be correlated to its Ru content as seen in Figure 3 for (Ru0.32-Ti0.68)Ox (Figure 3a) and (Ru0.14-Ti0.86)Ox (Figure 3b). At 32 atom % Ru, the electrochemical response is as expected for hydrous ruthenium oxide, and the background current from the sticky carbon substrate is negligible. For the aerogel with 14 atom % Ru, no response from the RuOx domains can be discerned above the voltammetric background of the sticky carbon electrode. The voltammetric results are consistent with impedance measurements on pressed pellets of these (Ru-Ti)Ox aerogels. The conductivities of these specific (Ru0.32-Ti0.68)Ox and (Ru0.14Ti0.86)Ox aerogels under ambient conditions are 7 × 10-4 and 3 × 10-7 S cm-1, respectively. The difference in conductivities can be ascribed to the presence of a conductive percolation network between RuO2 domains in the aerogel with the higher Ru content.
The electrical conductivity of the (Ru0.14-Ti0.86)Ox aerogel resembles that observed for pure TiO2 aerogels, suggesting that the RuOx domains in the (Ru0.14-Ti0.86)Ox aerogel are electrically isolated. Without an electrical percolation path through the particle, only the small fraction of RuOx domains in intimate contact with the sticky carbon substrate are electrochemically active. This comparison also demonstrates the limits of using the sticky carbon substrate to characterize high surface area solids in that transconductive samples are required to achieve an electrochemical signal above that of the substrate. Comparison with RuOx Solids. The structure and voltammetric behavior of the (Ru0.32-Ti0.68)Ox aerogel were compared to those of a set of RuOx solids: RuOxHy precipitate, annealed in air at 150 and 400 °C to prepare RuO2‚0.5H2O and RuO2‚0.03H2O, respectively, and a commercial anhydrous RuO2 (Alfa Aesar). X-ray diffraction of anhydrous RuO2, a (Ru0.32-Ti0.68)Ox aerogel, and the precipitated and annealed RuOxHy solids are shown in Figure 4. The diffraction pattern of anhydrous RuO2 has the well-defined, sharp peaks expected for a material having polycrystals >0.1 µm, and it matches well to the literature pattern for RuO2.31 The (Ru0.32-Ti0.68)Ox aerogel has broad peaks which have previously been identified as ∼10-nm particles of rutile RuO2 and anatase TiO2.24 The 400 °C annealed RuOxHy precipitates have some sharp peaks characteristic of anhydrous RuO2 as well as the broad peaks expected for (31) JCPDS database #43-1027; International Center for Diffraction Data.
Ruthenium Oxide-Based Aerogels
Figure 4. X-ray diffraction patterns for commercial anhydrous RuO2, (Ru0.32-Ti0.68)Ox aerogel (annealed at 400 °C), RuO2‚ 0.03H2O (annealed at 400 °C), and RuO2‚0.5H2O (annealed at 150 °C).
the highly disordered hydrous oxides.32 This mingling of broad and sharp diffraction features suggests that the material annealed at 400 °C is a mixture of anhydrous RuO2 crystallites which are >0.1 µm in diameter and other domains of hydrous RuO2. In contrast, the 150 °C annealed RuOxHy precipitates have only two broad peaks centered around 2θ values of ∼28° and ∼58°. These positions correspond to the (110) and (220) reflections of RuO2 (at 28.124° and 58.123°,31 respectively), which could indicate that the hydrous material is ordered along (110) planes, but these peaks may also arise from X-ray scattering. The structure of RuOxHy materials as determined by extended X-ray absorption fine structure (EXAFS) is addressed elsewhere.33 The resulting voltammograms (at 2 mV/s) for the RuOx solids are compared with that of the (Ru0.32-Ti0.68)Ox aerogel in Figure 5. Each material exhibits a pseudocapacitive response. The hydrous materials have nearly featureless voltammograms, unlike the anhydrous materials (RuO2 and RuO2‚0.03H2O). The nature of the faradaic features in RuO2 has previously been discussed.9,34 The capacitive currents decrease in the order RuO2‚0.5H2O . RuO2‚0.03H2O > (Ru0.32-Ti0.68)Ox aerogel > anhydrous RuO2. Specific capacitances (as derived from the voltammetric response35 at 2 mV/s) and multipoint BET surface areas for each of these materials are recorded in Table 1. The capacitance for the amorphous RuO2‚0.5H2O (900 F/g) is more than 1000 times greater than that for the highly crystalline, anhydrous RuO2 (0.75 F/g), despite only a 2-fold difference in their surface areas. The specific capacitances of RuOx solids are known to depend on their degree of hydration and degree of crystallinity (i.e., (32) Columban, P.; Novak, A. In Proton Conductors: Solids, Membranes and GelssMaterials and Devices, Columban, P., Ed.; Cambridge University Press: Cambridge, 1992; p 282. (33) McKeown, D. A.; Hagans, P. L.; Charette, L. P. L.; Russell, A. E.; Swider, K. E.; Rolison, D. R. Submitted for publication in J. Phys. Chem. B. (34) Liu, T.; Pell, W.G.; Conway, B.E. Electrochim. Acta 1997, 2324, 3541. (35) Capacitance measurements are derived from the respective voltammograms (2 mV/s), according to the equation, capacitance ) current/scan rate. Capacitances are averaged by measuring the current at every 100 mV from 0.2 to 0.7 V in the voltammetric sweep.
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formation of rutile structure).3,10 The very low specific capacitance for the (Ru0.32-Ti0.68)Ox aerogel, despite an enhancement in the BET surface area, could suggest that a significant fraction of RuOx domains in the aerogel are voltammetrically inaccessible. We know from prior impedance studies (in which proton and electron conductivity were demonstrated for this material) that an amorphous hydrous oxide is also present,24,27 in keeping with its molar composition of (RuO2-TiO2)‚0.25H2O.24 The X-ray diffraction pattern of the aerogel, however, is more similar to that of the commercial anhydrous RuO2 than to that of the hydrous RuOx-derived RuO236 heated to 400 °C (as was the aerogel). On the basis of this structural similarity, the specific capacitance of the aerogel can be normalized to the anhydrous solid to estimate the fraction of connectivity in the aerogel structure. With this normalization (see Table 1), it appears that all of the available RuOx in the (Ru0.32-Ti0.68)Ox aerogel is voltammetrically addressable.37 Because rutile RuO2 centers cannot undergo proton insertion, the aerogel cannot have a high specific capacitance, despite its partial hydrous component. This combination of hydrous ruthenium oxide and crystalline RuO2 in the aerogel also explains why its voltammetry (Figure 5d) is relatively featurelesssthe hydrous, capacitive surface screens the faradaic features seen for anhydrous RuO29 (see also Figure 5c) and starting to grow in for RuO2‚0.03H2O (Figure 5b). Also listed in Table 1 are the specific capacitance values reported by Jow and co-workers3 for RuO2‚0.5H2O (720 F/g) and RuO2‚0.03H2O (19 F/g). The capacitance measurements obtained in our laboratory for these same materials are ∼25% larger, presumably due to the different electrode structure we used to measure the specific capacitance. In the previous report, RuOxHy powders were assembled as electrodes by mixing with 5 wt % Teflon binder and rolling into a thin film. The authors asserted that the Teflon binder blocked potential RuO2 sites.3 Because the sticky carbon electrode is modified by handpressing a high surface area solid onto its surface, less blocking or occlusion of the active material due to organic or polymeric binders should occur. It is also appealing that the active material need not be dispersed into solvents to prepare it for electroanalysis. In that the specific capacitance (900 F/g) obtained in this laboratory for RuO2‚ 0.5H2O using the sticky carbon method equals the theoretical value (∼900 F/g),3 this method may provide a cleaner approach to examine electrode materials of this type. Electrode preparation with this method is also rapid and simple and requires only minimal quantities of material (
