Carbon Aerogel

The structure-property relationships of nanostructured Ru/carbon aerogel ... the specific capacitance of the carbon aerogels in 1.0 M H2SO4 from ≈10...
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Langmuir 1999, 15, 799-806

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Morphology and Electrochemistry of Ruthenium/Carbon Aerogel Nanostructures J. M. Miller*,† and B. Dunn Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California 90095 Received July 1, 1998. In Final Form: November 30, 1998 The structure-property relationships of nanostructured Ru/carbon aerogel composite materials have been evaluated. These new materials were prepared via a novel two-step metal vapor impregnation method which enables one to control the Ru loading through repetition of the process. The resulting microstructure is characterized by highly dispersed Ru particles (≈20-30 Å in diameter) attached to the carbon aerogel surface and distributed homogeneously throughout the material; the open pore structure of the carbon aerogels remains largely unaffected. Electrochemical characterization indicates that it is possible to increase the specific capacitance of the carbon aerogels in 1.0 M H2SO4 from ≈100 F/g to greater than 250 F/g for samples with >50 wt % Ru due to the pseudocapacitive properties of hydrous RuO2. A volumetric capacitance greater than 140 F/cm3 was observed for some samples. In addition, these materials show good cycling characteristics. Electrochemical impedance spectroscopy was unable to distinguish between double-layer capacitance and pseudocapacitance in these materials.

1. Introduction Hydrous ruthenium dioxide has been recognized as one of the most promising candidates for electrodes in aqueous electrochemical capacitors. The superior performance of this material in sulfuric acid has been attributed to surfacedriven reversible redox processes involving the intercalation of protons into the structure.1 One unique feature of this material is that the degree of faradaic conversion of a particular electrode varies linearly with the potential of that electrode, as indicated in eq 1.

RuO2 + δH+ + δe- h RuO2-δ(OH)δ 0eδe2

(1)

Furthermore, the material generates a relatively constant current under potentiodynamic conditions over the entire 1.2 V aqueous stability window. This type of energy storage mechanism has been termed pseudocapacitance since the response of this type of electrode is similar to that generated by an electrostatic capacitor. Pseudocapacitances as high as 250 µF/cm2 have been reported for hydrous ruthenium dioxide prepared from the controlled precipitation of RuO2-δ(OH)δ from a solution of RuCl3‚ H2O and NaOH.2,3 One signifcant disadvantage of the ruthenium-based materials is the cost associated with the ruthenium metal. Competing electrochemical capacitors utilize high surface area carbon materials such as activated carbon, activated carbon fiber cloth, activated carbon/carbon composites, and carbon aerogels/foams as the electrodes.4-7 These devices, termed electric double-layer capacitors * To whom correspondence should be addressed. † Present address: T/J Technologies, Inc., Ann Arbor, MI 48106. (1) Conway, B. E. J. Electrochem. Soc. 1991, 138, 1539. (2) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (3) Zheng, J. P.; Jow, T. R. J. Electrochem. Soc. 1995, 142, L6. (4) Tanahashi, I.; Yoshida, A.; Nishino, A. J. Electrochem. Soc. 1990, 137, 3052. (5) Tabuchi, J.; Saito, T.; Kibi, Y.; Ochi, A. IEEE Trans. Compon., Hybrids, Manuf. Technol. 1993, 16 (no. 4), 431. (6) Kanbara, T.; Nishimura, K.; Yamamoto, T. J. Power Sources 1990, 32, 165-74.

(EDLCs), store energy via separation of charge across a polarized liquid electrolyte/solid electrode interface, i.e., in the electric double-layer, although it is generally agreed that protonation of the chemical functionalities on the surface of the carbon electrodes contributes a small faradaic component to the overall capacitance of the material. With the best carbons exhibiting only 25-30 µF/cm2,8 there is a significant discrepancy between the performance of the carbon and the performance of the hydrous ruthenium dioxide materials. Moreover, when one compares the gravimetric capacitance (F/g) of the two electrode materials, the hydrous ruthenium dioxide generates a capacitance nearly four times greater than the highest capacitance carbons. Recently, we reported the preparation and characterization of a new type of nanostructured electrode material for use in electrochemical capacitors based on the combination of high capacitance hydrous ruthenium dioxide and high surface area carbon aerogels.9 Carbon aerogels are a unique form of carbon which can be characterized as having an open microstructure consisting of covalently linked interconnected chains of porous carbon microspheres forming a continuous three-dimensional ultraporous network. The physical properties of carbon aerogels include densities ranging from 0.04 to 1.0 g/cm3, specific surface areas typically in the range 400-1100 m2/g, and electrical resistivity less than 40 mΩ cm.10-12 In the nanostructured electrode, nanoparticles of ruthenium were deposited onto the surface of carbon aerogels by a novel two-step metal impregnation process to produce a material which exhibited capacitances as high as 250 F/g. This value is more than a factor of 2 greater than the capacitance of the original carbon aerogels. (7) Mayer, S. T.; Pekala, R. W.; Kaschmitter, J. L. J. Electrochem. Soc. 1993, 140, 446. (8) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; John Wiley & Sons: New York, 1988. (9) Miller, J. M.; Dunn, B.; Tran, T. D.; Pekala, R. W. J. Electrochem. Soc. 1997, 144, L309. (10) Hulsey, S. S.; Alviso, C. T.; Kong, F. M.; Pekala, R. W. Mater. Res. Soc. Symp. Proc. 1992, 270, 53. (11) Pekala, R. W.; Schaefer, D. W. Macromolecules 1993, 26, 5487. (12) Pekala, R. W.; Alviso, C. T. Mater. Res. Soc. Symp. Proc. 1992, 270, 3.

10.1021/la980799g CCC: $18.00 © 1999 American Chemical Society Published on Web 01/13/1999

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In the present paper, we focus on understanding two issues related to the electrochemical characteristics of these new materials: (1) the synthesis and morphology of these nanostructured materials and (2) the influence of the hydrous metal oxide on the mechanisms and kinetics of charge storage in carbon aerogels. The structural evolution of the materials was monitored primarily using transmission electron microscopy and nitrogen gas adsorption surface area/pore size measurements. The electrochemical properties of the materials were evaluated using potentiodynamic and galvanostatic techniques in combination with electrochemical impedance spectroscopy (EIS). 2. Experimental Section 2.A. Materials Preparation. The preparation of carbon aerogels was described previously.10,12,13 To prepare the hydrous RuO2/carbon aerogel nanocomposite samples, monolithic rods of carbon aerogels were first sliced into 1 mm thick disks with a diamond saw. Several different carbon aerogels were used as substrates for impregnation, but the majority of the samples used were carburized resorcinol/formaldehyde aerogels. Each carbon aerogel sample was ≈0.065 cm3. To impregnate the samples with ruthenium metal, an excess (≈100 mg) of ruthenium(III) 2,4-pentanedionate, Ru(acac)3 (25.4% Ru, Johnson Matthey Alfa/Aesar Co.), was placed in a 100 mL roundbottom flask with up to four aerogel disks. Once loaded, the flask was attached to a glass vacuum manifold and evacuated to p ) 5 × 10-4 Torr before sealing the container. The evacuated and sealed flask was then completely immersed (excluding the neck) in a silicone oil bath which was subsequently heated to 190 °C for 2 h. During this period of time, the ruthenium precursor sublimes14 and impregnates the aerogel substrate. It is important to note that the aerogels must reach a sufficiently high temperature to achieve a uniform distribution of the precursor within the sample. After the 2 h soak at 190 °C, the flask was cooled and subsequently vented. The Ru(acac)3impregnated carbon aerogels were then transferred to a tube furnace for thermal decomposition of the organometallic precursor. The samples were heated to 320 °C under flowing argon (5 cm3/min) to ensure complete decomposition of the organometallic ruthenium precursor. Thermogravimetric analysis (TGA) of the impregnated aerogels confirmed that the decomposition of the precursor occurred over the range 250-300 °C. Each sample gained approximately 10 wt % during the two-step impregnation/ decomposition procedure, independent of the sample size and/or bulk density of the carbon aerogel. The total amount of metal loading in the carbon aerogels could be increased incrementally by repeating this chemical vapor impregnation (CVI) procedure. 2.B. Microstructural Characterization. Microstructural characterization of the nanocomposite materials was conducted both before and after electrochemical characterization in order to identify any changes related to the cycling of the material. Transmission electron microscopy (TEM) was performed on a Phillips model 420T electron microscope equipped with a Kevex Quantifier detector for energy-dispersive spectroscopy (EDS). The nanocomposite samples were prepared for TEM by crushing lightly with a mortar and pestle, suspending the resulting powder (13) Pekala, R. W. In Ultrastructure Processing of Advanced Materials; Uhlmann, D. R., Ulrich, D. R., Eds.; John Wiley & Sons: New York, 1992. (14) Green, M. L.; Gross, M. E.; Papa, L. E.; Schnoes, K. J.; Brasen, D. J. Electrochem. Soc. 1985, 132, 2677.

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in a small amount of methanol, and depositing a drop of the suspension via a transfer pipet onto a holey-carboncoated Cu TEM grid (SPI supplies). Samples subjected to electrochemical characterization were dried in an oven at 55 °C overnight to prepare the TEM specimens. Images were obtained from the edges of the crushed composite aerogel fragments. Nitrogen gas adsorption experiments (Micromeritics ASAP 2010) were used to determine changes in surface area and microporosity as a function of metal loading in the nanostructured materials. Each nanocomposite sample was degassed at 300 °C for approximately 6 h prior to analysis to remove any adsorbed moisture or other gases. Pore size distribution was analyzed using the density functional theory (DFT) developed by Micromeritics Instrument Corp. A slit-shaped pore analysis for the DFT provided the most reproducible results. 2.C. Electrochemical Characterization. All electrochemical measurements were conducted in a threeelectrode cell consisting of a four-neck 500 mL roundbottom flask equipped with a standard calomel reference electrode (SCE) with luggin capillary, two graphite rods for the counter electrode, a Teflon sample holder with platinum current collector to house the nanocomposite sample working electrode, and a purging tube with a glass frit. The electrolyte was 1.0 M H2SO4 for all experiments. Measurements were carried out using a computercontrolled potentiostat/galvanostat (EG&G/Princeton Applied Research model 273A with Corrware software) and an impedance analyzer (EG&G/PAR model 6310 with EG&G software). Prior to electrochemical characterization, the Ru/carbon aerogel nanocomposite samples were immersed in the sulfuric acid electrolyte and placed under vacuum (5 × 10-1 Torr) for 3 h to ensure that all gas trapped within the pores of the sample was removed. Each sample was then individually transferred to the cell for characterization. Since the impregnated Ru was not preoxidized, it was necessary to subject the samples to an electrochemical oxidation procedure to convert the metal deposit to its pseudocapacitive hydrous oxide form. The samples were electrochemically oxidized under anodic polarization (0.75 V vs SCE) until the anodic current had dropped below 0.1 mA. Several electrochemical measurements were performed on each sample after electrochemical oxidation. Cyclic voltammetry was conducted between 0 and 0.8 V vs SCE for several sweep rates ranging from 0.2 to 20.0 mV/s. The details of the relevant experiments will be given in the context of the results. The anodic and cathodic capacitance of each sample was evaluated from cyclic voltammetry using the relationship C ) i/ν, where i (mA) is the average current in the linear region of either the cathodic or anodic response and ν is the applied sweep rate in mV/s. It should be noted that this relationship is only valid if the current generated during the poteniodynamic scans remains relatively constant over the capacitive voltage window. Electrochemical impedance spectroscopy (EIS) was performed on the nanocomposite samples immediately prior to electrochemical oxidation as well as afterward. These measurements employed an ac signal with a 5 mV amplitude over the frequency range 1.5 mHz to 100 kHz. The majority of the EIS experiments were performed at a dc bias of 20 mV vs open circuit potential (OCP), which ranged from 0.4 to 0.6 V vs SCE. The impedance behavior of pure carbon aerogels was also measured to provide a comparison for the nanocomposite samples. The experimental impedance data were numerically modeled using

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Table 1. Ruthenium Impregnation in Carbon Aerogels of Different Densitiesa

sample identity

carbon aerogel density (g/cm3)

no. of CVI cycles

∆ mass (mg)

14 15 16 10 17 27 29 39 47 42 41

0.178 0.178 0.178 0.178 0.178 0.178 0.178 0.142 0.278 0.417 0.557

1 2 3 4 8 9 14 5 5 5 5

1.43 2.42 4.99 6.21 11.61 12.03 21.87 2.51 8.06 12.71 20.83

wt % Ru

BET surface area (m2/g)

specific capacity (F/g)

12.09 18.50 28.64 32.98 49.51 50.15 62.81 32.77 34.34 34.10 35.32

546 487 452 434 373 348 280 430 369 334 328

96 153 192.3 234.5 248.4 268.9 272.3 212 224 217 221

a The influence of Ru loading on surface area and specific capacitance is also indicated.

the Equivalent Circuit software program,15 which uses a nonlinear least-squared (NLLS) fitting routine to optimize the impedance characteristics of a proposed equivalent circuit to fit the experimental data. 3. Results and Discussion 3.A. Morphological and Structural Characteristics. The two-step chemical vapor impregnation (CVI) of ruthenium into carbon aerogels described in this work is a novel method which enhances the energy storage capabilities of high surface area electrode materials. This process enables one to achieve high metal loading with good dispersion in the carbon aerogel host. Nanocomposite samples with greater than 60 total wt % ruthenium were achieved by multiple impregnations. The series of samples listed in Table 1 indicates the number of CVI cycles required to achieve the various ruthenium loading levels. High-resolution transmission electron microscopy shows that the ruthenium is deposited onto the surface of the carbon aerogels in the form of nanosized particles. This structure is shown in Figure 1 for two samples with 33 and 50 wt % Ru prior to electrochemical characterization. The metal clusters appear as ≈20-30 Å dark spots embedded in the aerogel matrix. We term the resulting material a Ru/carbon aerogel “nanocomposite”. For comparison, a micrograph of a pure carbon aerogel sample, i.e., with no metal particles, is shown in Figure 1C. The nanoparticles observed in parts A and B of Figure 1 appear to be distributed homogeneously throughout the material and adherent to the carbon surface. EDS confirmed the chemical identity of the particles as ruthenium-rich. The specific composition of the particles was not determined due to poor spatial resolution of the detector at this magnification (560000×). It is assumed, however, that there is a finite amount of oxygen associated with the metal particles due to the propensity of ruthenium metal to oxidize. Electron diffraction patterns of the nanocomposite samples indicate that the particles are amorphous. TEM images taken of samples after electrochemical oxidation show no distinct differences compared to the preoxidized samples, which is not surprising due to the small size of the particles; volume changes associated with the formation of the pseudocapacitive oxide were too small to be detected within the resolution capability of the TEM. The TEM images of the Ru/carbon aerogel materials shown in parts A and B of Figure 1 are representative of the type of nanostructure observed for all of the samples (15) Boukamp, B. A. Equivalent Circuit Users Manual; 2nd ed.; University of Twente: The Netherlands, 1989.

Figure 1. TEM micrographs of Ru/carbon aerogel nanocomposite materials: (A) 33 wt % Ru; (B) 50 wt % Ru. The Ru metal particles appear as ≈20-30 Å dark spots embedded in the aerogel structure. A pure carbon aerogel sample is shown in (C) for comparison.

examined. This nanostructure is observed throughout the bulk of the material as indicated by images taken on several sections of each sample. The primary distinction

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Figure 2. Variation in gas adsorption surface area as a function of wt % Ru in the Ru/carbon aerogel nanocomposite samples. The surface area has been normalized both per unit mass (BET surface area in m2/g; filled triangles on left axis) and per unit volume (volumetric surface area in m2/cm3, open circles on right axis).

between samples with different metal loading is the packing density of the particles observed in each TEM image. This difference is illustrated nicely in parts A and B of Figure 1 where it is evident that the number of particles per unit area in Figure 1B is significantly greater than the number of particles in Figure 1A. It is important to note that even at the high metal loading (Figure 1B), the ruthenium nanoparticles remain isolated and neither ripen nor aggregate despite repeated heating to temperatures above 300 °C. This observation suggests that there is a mechanism which anchors the particles to the surface and prevents their growth. There is a limit to this anchoring effect; the ruthenium metal particles migrated to the exterior surface of the carbon aerogel and formed a thick coating which spalled off the surface when samples were heated to temperatures greater than 500 °C. Nitrogen gas adsorption measurements of the different ruthenium/carbon aerogel nanocomposite samples provide another means of characterizing the morphological changes occurring from the metal deposition process. The specific surface area of the nanocomposite samples decreases with increasing metal loading as shown in Figure 2 for samples utilizing host carbon aerogels with a density of ≈0.18 g/cm3. The untreated carbon aerogel sample has a BET surface area of 700 m2/g while the sample with 64 wt % ruthenium has a surface area of 280 m2/g. This decreasing trend is not surprising since this property is normalized per unit weight (m2/g) and ruthenium has a large atomic mass relative to carbon. A more insightful comparison is to normalize the surface area per unit volume (m2/cm3) for the same samples, although this is not a conventional representation. The volume of each sample was determined by multiplying the original mass of the host carbon aerogel by the reciprocal of the bulk density of that aerogel. We term this normalized surface area as the “volumetric surface area” and plot it as a function of ruthenium metal loading in Figure 2. In this representation, the volumetric surface area initially decreases as one impregnates ruthenium metal and then gradually increases as more nanoparticles are deposited onto the surface of the aerogel. The change in the volumetric surface area with metal loading can be explained from the pore size distributions determined using the DFT analysis. The pore size distributions for a 34 wt % Ru and a reference carbon aerogel sample (F ≈ 0.18 g/cm3) are compared in Figure 3. The cumulative surface area (m2/g) as a function of pore width is plotted in Figure 3A while the incremental surface area as a function of pore width is plotted in Figure 3B. The primary difference between the pore size distri-

Figure 3. Comparison of surface area as a function of pore width for a Ru/carbon aerogel nanocomposite sample (34 wt % Ru) and a pure carbon aerogel sample: (A) cumulative surface area; (B) incremental surface area. Both distributions were calculated from DFT analysis using a slit-shaped pore geometry. The Ru nanocomposite was prepared using the same carbon aerogel material as that of the reference sample.

butions of the two samples is that the carbon aerogel reference sample possesses pores which are less than 20 Å in width. The surface area of these pores contributes significantly to the cumulative surface area of the sample. In contrast, no pores are observed below 25 Å for the Ru/ carbon aerogel nanocomposite sample. One plausible explanation for the disappearance of the micropores is that the metal particles “block” (i.e., prevent the access of nitrogen) all the pores in the carbon aerogel smaller than 25 Å, hence reducing the overall surface area per unit volume of the Ru/carbon aerogel nanocomposite. As more metal particles accumulate on the pore walls of the carbon aerogel, the metal particles themselves begin to contribute to the total surface area of the material. Thus, at high metal loadings, the volumetric surface area of the nanocomposite samples exceed the volumetric surface area of the untreated carbon aerogel. The decomposition of the precursor inside the carbon aerogel leads to a relatively homogeneous distribution of ruthenium nanoparticles. The tendency of the ruthenium metal to deposit onto the surface of the carbon aerogels in small metal clusters suggests that there are preferential nucleation sites for the nanoparticles on the carbon aerogel surface which must be homogeneously distributed throughout the material. One possible source of nucleation sites for the ruthenium nanoparticles is the micropores of the carbon aerogels. The pore size distribution of the modified carbon aerogel (Figure 3) is consistent with this claim since the microporosity present in the untreated aerogel sample is conspicuously absent in the modified sample. If the micropores serve as nucleation sites for the metal nanoparticles, they may also serve to anchor the particles to the surface and establish good particle-substrate contact.

Ruthenium/Carbon Aerogel Nanostructures

Figure 4. Capacitance of Ru/carbon aerogel nanocomposite electrode materials as a function of Ru loading (total wt %).

3.B. Electrochemical Characteristics. 3.B.1. Cyclic Voltammetry and Capacitance. Cyclic voltammetry (CV) measurements were used to examine the influence of both microstructure-related variables (metal loading, aerogel density) and operational parameters (sweep rate, cycling) on the capacitance of the Ru/carbon aerogel nanocomposites. Prior to carrying out the CV measurements, a chronoamperometric technique was used to oxidize the Ru nanoparticles. Once oxidized, the CV closely resemble the shape reported for hydrous RuO2.2,3 Our initial publication showed that it was possible to double the effective gravimetric capacitance by incorporating Ru nanoparticles into the carbon aerogel structure.9 As shown in Figure 4, this level has now been increased. Nanocomposite samples with 50 wt % Ru loading exhibit a gravimetric capacitance of ≈250 F/g and a volumetric capacitance of ≈80 F/cm3 as compared to 95 F/g and 20 F/cm3 for the untreated carbon aerogel. Figure 4 compares the capacitance for a series of nanocomposite samples prepared from the same carbon aerogel matrix (F ≈ 0.18 g/cm3). There is a relatively linear relationship between the capacitance increase and the wt % Ru in these nanocomposite materials which suggests that the contribution of the hydrous RuO2 to the overall capacitance of the electrode material can be determined from the slope. When the capacitance of the pure carbon aerogel is subtracted from the total capacitance of the electrodes, the contribution of the ruthenium nanoparticles is calculated to be 335 ( 42 F/g. This value is comparable to values reported for RuO2; it is, however, considerably less than the highest values (>750 F/g) obtained with hydrous RuO2.2,3 One possible explanation for this dispcrepancy is that the nanoparticles are actually small crystallites. It has been reported that when the processing temperature of the hydrous RuO2 materials exceeded 175 °C, the material crystallizes and the capacitance decreases rapidly.2 Voltammetry measurements were also used to characterize the cycling behavior and kinetics of the Ru/carbon aerogel nanocomposites. Several samples were examined for up to 75 charge/discharge cycles. The cycling response for a sample with 32 wt % Ru is shown in Figure 5. The capacitance decrease of 15% is interesting in that it falls between the 5% value reported for hydrous RuO22 and the much higher value reported for pure carbon aerogels (≈25%).7 In the latter case, the irreversibility was attributed to oxidation/reduction of loosely bound surface groups on the carbon aerogel. The influence of sweep rate on the performance of the nanocomposite electrodes is shown in Figure 6. It is evident that the potentiodynamic response becomes increasingly influenced by the capacitive (RC) time constant of the sample at the higher sweep rates. At 20 mV/s, the re-

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Figure 5. Variation in capacitance as a function of cycle number for Ru/carbon aerogel nanocomposite electrode (30 wt % Ru).

Figure 6. Cyclic voltammetry of Ru/carbon aerogel nanocomposite sample (33 wt % Ru) in 1.0 M H2SO4 showing the effect of sweep rate on the charging behavior.

sponse is dominated by the RC time constant such that a capacitive charging region is not observed. One can compare an effective capacitance at each sweep rate by integrating the total charge passed by the sample and dividing by the voltage window, i.e.

C)

∫i dt V

(2)

With this method, the anodic capacitance for the sample shown in Figure 6 (with a 0.8 V window) decreases from 9.03 F at 0.2 mV/s to 5.0 F at 2.0 mV/s to 1.75 F at 20.0 mV/s. The cathodic capacitance follows a similar trend since Qa/Qc ≈ 1. Thus, the energy storage capabilities of the Ru/carbon aerogel nanocomposite samples are reduced at higher sweep rates due to the increased influence of the RC time constant; i.e., the power density is limited. In addition to sweep rate dependence, the potentiodynamic response is also influenced by the carbon aerogel density. The fact that the loading of the Ru in the nanocomposite samples was unaffected by the bulk density of the aerogel enabled us to prepare electrodes with different carbon aerogel densities but with similar Ru loadings (see Table 1). The CV characteristics for four different nanocomposite samples with similar Ru loading levels (30-35 wt %) are compared in Figure 7. The current is given as mA/cm3 in this figure to accentuate the density related variation in the sample response. The principal distinction in the CVs is the increase in RC time constant observed for those nanocomposites using higher density carbon aerogels. The variation in the capacitance of these samples is plotted as a function of aerogel density for both the specific capacitance (F/g) and the volumetric capaci-

804 Langmuir, Vol. 15, No. 3, 1999

Figure 7. Cyclic voltammograms of Ru/carbon aerogel nanocomposite samples with different host aerogel densities and similar Ru loading (30-35 wt % Ru). The ordinate axis has been normalized per unit volume to emphasize the variation in capacitance density for a given volume sample. All CVs were recorded at a sweep rate of 2.0 mV/s in 1.0 M H2SO4.

Miller and Dunn

The results shown here establish that the inclusion of the Ru nanoparticles in the carbon aerogel adds a pseudocapacitance which is responsible for the significant increase in electrode capacitance. For example, a sample which incorporates 33 wt % Ru into the carbon aerogel structure exhibits a volumetric capacitance of ≈60 F/cm3 as compared to the pure carbon aerogel (F ≈ 0.18 g/cm3) which exhibits a capacitance of ≈17 F/cm3 (Figure 4). As shown previously, the gas adsorption surface area of the nanocomposite electrodes does not vary dramatically from that of the pure carbon aerogels. Specifically, the volumetric surface area of a particular sample with 33 wt % ruthenium is approximately 110 m2/cm3 while the reference sample of the same carbon aerogel type has a volumetric surface area of ≈130 m2/cm3 (Figure 2). On the basis of this information, the capacitance per unit BET surface area, Ceff (µF/cm2), of the nanocomposite sample is 54 µF/cm2 whereas the double-layer capacitance of the carbon aerogel is only 13 µF/cm2. This analysis indicates that the Ru nanoparticles introduce a pseudocapacitive, or faradaic, process which is responsible for the large increase in the total capacitance of the nanocomposite material. 3.B.2. Electrochemical Impedance Spectroscopy (EIS). The principal objective of the EIS experiments is to gain insight concerning the dominant time-dependent electrochemical processes occurring in the nanostructured electrode materials. We are especially interested in the prospect of using EIS to resolve the double layer and pseudocapacitance contributions to the electrochemical capacitance. The impedance spectra show few obvious differences between the Ru/carbon aerogel nanocomposite and the untreated carbon aerogel. The similarity in frequency response is shown in Figure 9 for (A) a 34 wt % Ru nanocomposite and (B) an untreated carbon aerogel. The equivalent circuit used to model this behavior incorporates both ideal and distributed circuit elements as shown in Figure 10 with each component representing some timedependent process associated with the electrochemical response (vide supra). The two subcircuits are comprised of series-parallel combinations of resistance, capacitance, and distributed impedance (denoted by Z) where Zw stands for a Warburg mass transport limited impedance and Zcpe represents a constant phase element (CPE).16 The latter manifests itself by altering the expected shape of ideal capacitor/resistor combinations and is expressed as

ZCPE ) [B(jω)n]-1 Figure 8. Capacitance of Ru/carbon aerogel nanocomposites (30-35 wt % Ru) as a function of host carbon aerogel density (denoted by filled triangles): (A) specific capacitance (F/g); (B) volumetric capacitance (F/cm3). The capacitances of the pure carbon aerogels (denoted by ×) have been included for comparison.

tance (Figure 8); the capacitance of untreated carbon aerogels has been included for comparison. The specific capacitance for all of the Ru/carbon aerogel samples (3035 wt % Ru) presented in Figure 8A is approximately 200 F/g regardless of aerogel density. This value is twice the capacitance of the host aerogel. In Figure 8B, the volumetric capacitance of each of the nanocomposite samples is approximately four times greater than the capacitance of the respective untreated aerogel. Volumetric capacitances exceeding 150 F/cm3 were achieved at this Ru loading when carbon aerogels with a density of 0.56 g/cm3 were used.

(3)

where B and n (0 < n e 1) are frequency independent proportionality constants.16 When n ) 1, the CPE can be expressed as a capacitance. The nonlinear least-squared (NLLS) fits of this equivalent circuit model to the impedance characteristics of the nanocomposite aerogel and the pure carbon aerogel samples are shown in Figure 9. The value for each circuit element for several nanocomposite samples and the untreated carbon aerogel is listed in Table 2. The equivalent circuit shown in Figure 10 is a modification of the well-known Randles17 circuit, which approximates the behavior of a solid electrode participating in single-electron faradaic charge-transfer processes. The high-frequency response (>10 kHz) is dominated by the bulk resistance associated with the electrochemical mea(16) Brett, C. M. A.; Brett, A. M. O. Electrochemistry: Principles, Methods, and Applications; Oxford University Press: New York, 1993. (17) Randles, J. E. B. Discuss. Faraday Soc. 1947, 1, 11.

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Table 2. Nonlinear Least-Squared (NLLS) Fit of the Equivalent Circuit Model Shown in Figure 10a wt % Ru carbon aerogel 34 19 40 50 a

Rel (Ω) 1.49 1.57 1.47 1.40 1.43

Rct (Ω) 1.01 2.21 2.12 2.47 4.85

Zw (Ω-1) 2.94 2.13 1.49 2.17 2.25

Zcpe (Ω-1) 10-4

2.9 × 1.38 × 10-3 1.49 × 10-4 5.32 × 10-4 4.29 × 10-4

n

Rleak (Ω)

Cechem (F)

0.86 0.74 0.87 0.78 0.88

1062 2568 1978 1216 1890

0.94 3.19 1.37 1.66 4.27

The carbon aerogel density was the same for all the nanocomposite samples (F ≈ 0.18 g/cm3).

Figure 9. Nyquist representation of the frequency dipersion (1.5 mHz to 100 kHz) of (A) a Ru/carbon aerogel nanocomposite sample and (B) a pure carbon aerogel sample. The inset shown in (A) is an enlargement of the high-frequency dispersion of that sample. The NLLS fit of the equivalent circuit shown in Figure 10 for each data set has been overlaid on the respective plots.

Figure 10. Equivalent circuit model of the electrochemical impedance behavior of carbon-aerogel-based electrodes in aqueous electrolytes.

surement (Rel). This includes the resistance of the electrolyte and the bulk resistance of the nanocomposite aerogel sample, which has a finite thickness of approximately 1 mm.

The subcircuit operating over the 1 Hz to 10 kHz regime consists of a series combination of a resistance and a Warburg impedance in parallel with a constant phase element. The resistance in this subcircuit represents the charge-transfer resistance associated with the oxidation and reduction of electroactive surface species. The constant phase element is representative of the double-layer capacitance of the geometric surface area of the carbon aerogel sample. It is evident that the CPE value (Table 2) is not proportional to the electrochemical capacitance observed in the cyclic voltammetry as the magnitude is much too low. In addition, a porous electrode generates a much lower exponent value (n ≈ 0.5) than the value for the carbon aerogel sample (n ≈ 0.8-0.9) suggesting that the sample is not behaving as a porous electrode but rather as a rough surface18 at these frequencies. The tortuous morphology of the porous carbon aerogel network is consistent with this interpretation. The Warburg element manifests itself in the frequency regime 1-10 Hz as shown in Figure 9. The presence of the Warburg impedance in this intermediate frequency range is important for our discussion as it signifies that the capacitive charging process is mass transport limited. At frequencies below 1 Hz, a parallel resistancecapacitance combination dominates the sample impedance. In this subcircuit, the capacitance represents the electrochemical capacitance of the electrode. The capacitance values at 1.5 mHz determined from the NLLS simulation are given in Table 2. The specific capacitance (F/g) of a sample at this frequency is obtained by dividing by the capacitance value by the sample mass. On the basis of this analysis, the specific capacitance for the 34 wt % Ru loaded sample is 165 F/g, which is approximately 20% lower than the value of 206 F/g obtained from the cyclic voltammetry measurement. A 20 to 25% deviation between the specific capacitance values measured by the two methods is typically observed. There are several reasons for this with the most evident one being that the ac measurement represents a significantly smaller perturbation from equilibrium (10 mV) as compared to the CV measurement.18 Contributions from processes other than capacitive charging are likely to occur upon subjecting the system to greater potentiodynamic variations such as in CV measurements. The resistance in parallel with the low-frequency capacitance represents a small leakage current at the electrode/electrolyte interface. Ideally, the electrode would behave as a blocking electrode at low frequencies and terminate with a limiting capacitance. In these measurements, the presence of a finite resistance (>2500 Ω) in parallel with the capacitance suggests that the interface undergoes relaxation processes at very low frequencies. The physical origin of this resistance is not well understood, but other researchers have attributed similar behavior in porous solids to transmission line characteristics.1,17,19-21 The deeper that ions penetrate into the pores, (18) deLevie, R. Electrochim. Acta 1963, 8, 751. (19) Thomas, M. G.; Bruce, P. G.; Goodenough, J. B. J. Electrochem. Soc. 1985, 132, 1521.

806 Langmuir, Vol. 15, No. 3, 1999

the larger the contribution of the resistive component of the transmission line network. Rather than derive an expression for this network, we have decided to express the resistive contributions associated with the transmission line model as a single quantity to better determine the influence of this resistance on the energy storage characteristics of the material. The small current associated with the parallel resistance reduces the efficiency of the nanocomposite aerogels as electrode materials for electrochemical capacitors and requires that an equal and opposite current be applied to the sample to hold the electrode in a particular charged state. The circuit parameter values obtained from the NLLS fit of the EIS measurements (Table 2) offer an opportunity to compare the untreated carbon aerogels with the Ru/ carbon aerogel nanocomposites. While it is important to not overinterpret these results, certain trends are evident. The most significant change associated with the impregnation of the ruthenium nanoparticles into the carbon aerogel structure is the value of the capacitance in the low-frequency subcircuit. The capacitance value increases from 0.94 F for the untreated carbon aerogel to 3.2 F for the sample with 34 wt % Ru to 4.2 F for the 50 wt % Ru sample at 1.5 mHz. All of these samples have approximately the same volume. This trend is in general agreement with the electrochemical behavior of the materials as determined by cyclic voltammetry (Figure 4). We tried to use the time-dependent information supplied by EIS to distinguish between double layer and pseudocapacitance contributions to the nanocomposite electrodes. The data indicate, however, that the addition of Ru nanoparticles does not lead to any difference in the time dependence of the charging process. This behavior suggests that the energy storage processes in both the nanocomposite samples and the untreated carbon aerogel materials are kinetically limited by diffusion of electrolyte ions in the pores and not by the capacitive charging mechanisms. Thus, it is reasonable to interpret the low-frequency capacitance of the ruthenium/carbon aerogel nanocomposite electrodes as two capacitances in parallel with one another: the double-layer capacitance of the surface of the electrode and the pseudocapacitance of the hydrous RuO2. These parallel capacitances have a single RC time constant and can be lumped into a single effective capacitance. (20) Anderson, D. P.; Warren, L. F. J. Electrochem. Soc. 1984, 131, 347. (21) Dini, J. W. Electrodeposition; Noyes Publications: Park Ridge, NJ, 1993; p 366.

Miller and Dunn

4. Conclusions The impregnation of Ru into carbon aerogels has proven to be an effective method of significantly improving the energy storage capabilities of this high surface area material. The chemical vapor impregnation process enables one to obtain well dispersed, nanometer dimensional ruthenium particles which adhere extremely well to the carbon aerogel matrix. At high Ru loadings, the 20-30 Å metal nanoparticles remain anchored to the surface of the carbon aerogel support as shown in TEM photos even after repeated heating to 300 °C. TEM images obtained after electrochemical testing show a similar nanostructure, and there was no evidence of Ru dissolution during measurement. The long-term electrical contact of the two materials is an important test of good particle/substrate adhesion in this type composite electrode material. We confirmed good cycling behavior for the materials for up to 75 charge/discharge cycles. The incorporation of Ru metal particles significantly increases the energy density of the nanocomposite electrode materials, both from a gravimetric and a volumetric perspective. Capacitances greater than 250 F/g and 140 F/cm3 were observed for samples with >50 wt % Ru. In addition, one can tailor the capacitance of these new materials by varying the Ru loading and/or by varying the density of the host carbon aerogel. The nanostructure of the resulting composite materials allows for good electrochemical kinetic reponse; there are minimal changes to the overall pore structure of the material and the highly dispersed metal (and hydrous metal oxide) nanoparticles promote rapid charge-transfer kinetics. Electrochemical impedance spectroscopy was unable to distinguish between double layer and pseudocapacitance charging modes in the Ru/carbon aerogel nanocomposite materials. This suggests that the energy storage process of these new materials, as well as the pure carbon aerogels, is limited by mass transport of electrolyte ions in the pores of the carbon aerogel and not by capacitive mechanisms. Acknowledgment. The authors thank Dr. Debra Rolison and Dr. Alan Berry from the Naval Research Laboratory and Dr. Tri D. Tran and Dr. Richard W. Pekala from Lawrence Livermore National Laboratory for their insightful discussions on this topic. This work was supported by Lawrence Livermore National Laboratory and by the Office of Naval Research. LA980799G