Two-Component Films of Fullerene and Palladium as Materials for ...
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
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0013-4651/2007/154共4兲/K1/10/$20.00 © The Electrochemical Society
Two-Component Films of Fullerene and Palladium as Materials for Electrochemical Capacitors Krzysztof Winkler,a Emilia Grodzka,a Francis D’Souza,b,* and Alan L. Balchc,*,z a
Institute of Chemistry, University of Bialystok, 15-399 Bialystok, Poland Department of Chemistry, Wichita State University, Wichita, Kansas 67260, USA Department of Chemistry, University of California, Davis, California 95616, USA
b c
The redox-active films, C60 /Pd, formed by electrochemical reduction of solutions containing palladium共II兲 acetate and C60 fullerene have been studied as active components for electrochemical capacitors. The capacitance properties of these materials have been investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The structure and electrochemical properties of films deposited on an electrode surface depend on the composition of the solution from which they are grown. Films formed in a solution with a low concentration of palladium共II兲 acetate exhibit conductivity in the potential range of the film reduction. The faradaic process of C60 reduction gives rise to pseudocapacitance. The capacitance of this polymer depends on the solvent and the size of the cations in the supporting electrolyte. For an acetonitrile solution containing only tetra共methyl兲ammonium perchlorate, the film displays a high specific capacitance, close to 300 F/g. Films formed in a solution with a high concentration of palladium共II兲 acetate also contain metallic palladium nanoparticles. Such systems exhibit conductivity at potentials less negative than the potentials for film reduction, and these films can be considered as double-layer capacitors. The specific capacitance of these films is much smaller 共about 20 F/g兲 but a large potential window 共from +800 to −2000 mV兲 is available for the performance of these capacitors. © 2007 The Electrochemical Society. 关DOI: 10.1149/1.2434683兴 All rights reserved. Manuscript submitted August 16, 2006; revised manuscript received October 31, 2006. Available electronically February 16, 2007.
In recent years, a new generation of electrochemical capacitors, the so-called supercapacitors, has been intensively investigated.1-3 These devices exhibit high-capacitance and high-power characteristics. The electrode materials employed in the construction of such capacitors include activated porous carbon,4-7 carbon nanotubes,8-15 hydrous transition-metal oxides,16-22 and conducting polymers.23-28 Composites formed from these materials are also often used as electrochemical capacitors.29-37 Two types of electrochemical capacitors are under development: double-layer and redox capacitors. In the case of double-layer capacitors, the separation of electronic and ionic charges at the interface between the electrode material and the electrolyte solution is responsible for high-energy-storage properties.1 In redox capacitors, a faradaic process within electroactive material itself gives rise to pseudocapacitance.1 Conducting polymers generally belong to the latter type of supercapacitors. The electrochemical activity of these materials and their high specific surface area make them very promising as electrode materials for secondary batteries and capacitors.38-40 The electronic charge is injected into the polymeric chain, and the ionic charge is transferred into the polymeric phase in order to maintain the charge neutrality. Therefore, p-doped or n-doped polymers can be used for constructing supercapacitor devices. The devices constructed from conducting polymers have the advantage of low cost of production, good stability, and high operating potentials range. They also exhibit relatively high capacitance. Specific capacitance of polymeric electrode materials typically ranges between 300 and 500 F/g.3 Polythiophene,23 polianiline,28 and polypyrrole29 have been the most frequently used as electroactive materials for redox capacitors. These polymers are electrochemically active at positive potentials and exhibit p-doping properties. Materials capable of cathodic n-doping are less numerous and have received less attention. Recently, conducting polymers containing fullerene moieties have been electrochemically synthesized.41-60 Because of the presence of electron-accepting fullerene moieties, these polymers are electrochemically active in the negative potential range and exhibit n-doping properties. The redox-active films formed during reduction of solutions containing C60 and selected transitionmetal complexes are particularly interesting.50 These films are insoluble in common organic solvents and adhere strongly to the
* Electrochemical Society Active Member. z
E-mail: [email protected]
electrode surface. In these materials, there is evidence that suggests that metal atoms or metal complexes are coordinated directly to the fullerenes in 2 fashion as shown below
Additional metal atoms or complexes can form cross-links between these chains. The films, C60 /Pd, formed by electroreduction from solutions containing C60 and palladium acetate have been investigated extensively.51,53-56 In this system, a polymeric network appears to be formed through covalent bonding between fullerene and palladium atoms. The films exhibit redox activity in the negative potential range due to the reduction of the C60 moieties. The electroreduction process is accompanied by cation transfer between solution and the solid film on the electrode surface.53 The composition and structure of the C60 /Pd film depend upon the concentration of the film precursors in the growth solution.54,56 Both a metallic phase and the C60 /Pd polymer are deposited simultaneously from solutions containing a relatively high concentration of the Pd共II兲 complex. Metallic Pd nanoparticles can effectively participate in the electrontransfer process. The C60 /Pd film is also relatively stable. This sort of film retains its redox activity in the course of prolonged potential cycling over a relatively wide potential range from +0.5 to −2.0 V. The composition of the growth solution also affects the film stability through its effect on the film structure.54 In this paper we report on the capacitive performance of the electrochemically formed C60 /Pd films. The effects of the composition of the growth solution on the capacitive properties of the films were investigated. The influences of the solvent and supporting electrolyte on the capacitance of electrodes covered with the C60 /Pd polymer were also studied. Experimental Palladium acetate, Pd共ac兲2 共Aldrich兲, and C60 共Southern Chemical Group兲 were used as received. The supporting electrolytes, tetra共methyl兲ammonium perchlorate, tetra共ethyl兲ammonium perchlorate, tetra共n-butyl兲ammonium perchlorate, and tetra共nhexyl兲ammonium perchlorate 共Sigma Chemical Co.兲, were dried un-
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Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
der vacuum for 24 h prior to use. Lithium perchlorate and potassium hexafluorophosphate 共Aldrich Chemical Co.兲 were dried under reduced pressure at 90°C for 24 h. Cesium hexafluoroarsenate was prepared by precipitation from an aqueous solution containing an equimolar mixture of nearly saturated CsBr 共Aldrich兲 and LiAsF6 共Aldrich兲. The CsAsF6 was recrystallized from methanol and then dried under reduced pressure at 90°C. Acetonitrile 共99.9%兲, N,N-dimethylformamide 共99.9%兲, dimethylsulfoxide 共anhydrous 99.9%兲, acetone 共99.5%兲, ethanol共HPLC/spectrophotometric grade兲, methanol 共99.8%兲, and n-propanol 共anhydrous 99.7%兲 were used as received from Aldrich Chemical Co. Toluene 共Aldrich Chemical Co.兲 was purified by distillation over sodium under a nitrogen atmosphere. Voltammetric and electrochemical impedance spectroscopy experiments were performed on a potentiostat/galvanostat model 283 and Frequency Response Detector model 1025 共EG&G Instruments兲 with a three-electrode cell. A gold disk with a diameter of 1.5 mm 共Bioanalytical Systems, Inc.兲 was used as the working electrode. Prior to the experiment, the electrode was polished with fine carborundum paper and then with a 0.5 m alumina slurry. Subsequently, the electrode was sonicated in water to remove traces of alumina from the gold surface, washed with water, and dried. A silver wire immersed in 0.010 M silver perchlorate and 0.09 M tetra共n-butyl兲ammonium perchlorate in acetonitrile that was separated from the working electrode by a ceramic tip 共Bioanalytical Systems Inc.兲 served as the reference electrode. The counter electrode was a platinum tab with an area of about 0.5 cm2. Simultaneous voltammetric and piezoelectric microgravimetry experiments were carried out with a home-built potentiostat and an electrochemical quartz crystal microbalance, EQCM 5510 共Institute of Physical Chemistry, Warsaw, Poland兲. Plano-convex quartz crystals were used. The 14 mm diameter AT-cut, plano-convex quartz crystals with a 5 MHz resonant frequency were obtained from Omig 共Warsaw, Poland兲. A 100 nm gold film, which was vacuum deposited on the quartz crystal, served as the working electrode. The projected region of this Au electrode was 5 mm in diameter. The area of the circuit center spot and two contacting radial strips was 0.24 cm2. Unpolished quartz crystals were used for better adherence of the film. The sensitivity of the mass measurement calculated from the Saurbrey equation was 17.7 ng Hz−1 cm−2. The C60 /Pd film was prepared by electroreduction of an acetonitrile/toluene 共1:4, v:v兲 solution that contained the fullerene and palladium acetate as well as the supporting electrolyte, 0.10 M tetra共n-butyl兲ammonium perchlorate. Films were grown under cyclic voltammetry conditions at a potential sweep rate of 50 mV/s. The electrochemical properties of the film were studied in a solution of the chosen solvent containing only the supporting electrolyte. In this case, the electrode covered with the film was removed from the growth solution, rinsed several times with an acetonitrile/toluene 共1:4, v:v兲, and then placed in an acetonitrile solution containing 0.10 M of supporting electrolyte. The modified electrode was allowed to equilibrate for 10 min while degassing with argon in a fresh solution before electrochemical measurements were performed.
Results and Discussion The properties of the C60 /Pd films depend on the composition of growth solution. Figure 1 shows the effect on the concentration of palladium共II兲 acetate on the redox properties of films deposited on the electrode surface. An increase in the palladium acetate concentration in the growth solution results in a decrease of the charges associated with reduction and reoxidation of the film at potentials more negative than −700 mV 共peaks R1 and O1兲. In solution with a palladium acetate concentration higher than about 8.9 mM, the peaks R1 and O1 almost completely disappear. The composition of the growth solution also influences the capacitance at potentials less negative than −700 mV. For the films formed in a solution contain-
Figure 1. Cyclic voltammograms of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 100 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 共a兲 3.35 mM Pd共ac兲2, 共b兲 6.68 mM Pd共ac兲2, and 共c兲 9.81 mM Pd共ac兲2.
ing a large excess of palladium acetate, a large increase in the capacitance at potentials less negative than −700 mV is observed. The C60 /Pd film is formed in the potential range where both fullerene and the palladium ions are reduced.51 Consequently, a parallel deposition of palladium nanoparticles and polymer on the electrode surface can take place.56 The increase of the amount of palladium nanoparticles in the polymeric phase with the increase of the palladium acetate concentration in the growth solution results in changes in the conductivity of the film. C60 /Pd films formed in solutions containing a large excess of palladium共II兲 acetate 共higher than 7 mM兲 exhibit high electronic conductivity at potentials less negative than the potential of film reduction.56 The conductivity of the C60 /Pd film formed in solution with lower concentration of Pd共ac兲2 depends on its oxidation state. For this film, the conductivity increases in the potential range of the film reduction. Such a behavior resembles conductivity properties of typical conducting polymers such as polyaniline, polypyrrole, or polythiophene.61,62 Therefore, potential applications of the following two types of capacitors are conceivable: 共i兲 Films formed from solutions with a 关Pd兴/关C60兴 ratio lower than 20:1 which exhibit conductivity in the potential range of film reduction. As shown below, the electrochemical properties of these films indicate that the faradaic process of reduction of the C60 moieties gives rise to pseudocapacitance. 共ii兲 Films formed from solutions with a 关Pd兴/关C60兴 ratio higher than 25:1 which exhibits conductivity also at potentials less negative that the potentials for film reduction. These films can be considered as double-layer capacitors, as the following data demonstrates. Capacity performance of the C60 /Pd pseudo-capacitors.— Figure 2 shows cyclic voltammograms of the C60 /Pd film in acetonitrile containing tera共n-butyl兲ammonium perchlorate as a supporting electrolyte recorded at different sweep rates. The film was cycled between −800 and −1500 mV without noticeable change in the shape of voltammograms. Voltammograms show almost pseudorectangular
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
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Figure 3. 共a兲 Multicyclic voltammograms and 共b兲 curves of the frequency change vs potential 共b兲 simultaneously recorded at the same Au/quartz electrode in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 3.6 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s.
Figure 2. 共a兲 Cyclic voltammograms of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 共1兲 20, 共2兲 50, 共3兲100, and 共4兲 200 mV/s. 共b兲 Dependence of the pseudocapacitive current on the sweep rate. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2.
cathodic and anodic profiles that are the mirror image of one another, characteristic behavior for an ideal capacitor. The departure from the ideal rectangular shape is related to the faradaic process of film reduction and reoxidation. The current at negative potentials includes capacitive and faradaic contributions. Both components depend linearly on the sweep rate. Therefore, the linear dependence of the total current on the sweep rate is expected. Such relation is shown in panel b of Fig. 2b. The specific capacitance Cs of the C60 /Pd film can be calculated according to the following equation Cs =
i 共dv /dt兲m
Fig. 4 for the first ten cycles. The amount of C60 /Pd film deposited on the electrode in each cycle decreases with the increase in the scan number. This effect can be attributed to the partial inhibition of the polymer formation on the electrode covered with the C60 /Pd film. In Fig. 5, the effect of the C60 /Pd film thickness on its voltammetric response in acetonitrile containing tera共n-butyl兲ammonium perchlorate is shown. From the slope of the i-m linear relationship, predicted by Eq. 1, the specific capacitance of the film is 170 F g−1. The capacity performance of the C60 /Pd film depends on the supporting electrolyte and solvent. The pseudocapacitance is
关1兴
where i is the pseudocapacitive current, dv /dt is the sweep rate, and m is the mass of electroactive material deposited on the electrode surface. The amount of C60 /Pd deposited on the electrode surface can be calculated on the base of results of the electrochemical quartz crystal microbalance 共EQCM兲 study. The frequency changes of the quartz crystal during the C60 /Pd film deposition are shown in Fig. 3. The frequency decreases in the cathodic cycle in the potential range of palladium cation reduction. A small increase of frequency in the positive scan direction is related to the film oxidation followed by egress of the supporting electrolyte cations from the polymer to the solution. The changes in mass of the electrode as a function of the scan number is shown in
Figure 4. Dependence of the mass of Au/quartz electrode on the cycle number during C60 /Pd film deposition in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 3.6 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s.
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
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Figure 6. Cyclic voltammograms of C60 /Pd films in acetonitrile containing 共1兲 0.10 M tetra共n-hexyl兲ammonium perchlorate, 共2兲 0.10 M tetra共n-butyl兲ammonium perchlorate, 共3兲 0.10 M tetra共ethyl兲ammonium perchlorate, and 共4兲 0.07 M tetra共methyl兲ammonium perchlorate. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2. Figure 5. 共a兲 Cyclic voltammograms of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2. The amount of C60 /Pd was 共1兲 11.5, 共2兲 21, 共3兲 28, 共4兲 41, and 共5兲 62 g/cm2. 共b兲 Dependence of the pseudocapacitive current at −1200 mV on the mass of C60 /Pd film deposited on the electrode.
strongly affected by the cation of the supporting electrolyte. The anion does not influence the capacitive currents in the negative potential range. The effect of the cation of supporting electrolyte on the pseudocapacitance of C60 /Pd film in acetonitrile is shown in Fig. 6. The decrease of the size of tetra共alkyl兲ammonium ion results in the increase of the pseudocapacitance. The specific capacitance can be calculated by the integration of cyclic voltammograms according to the equation
Ci =
冕
The effect of solvent on the capacitive performance of the C60 /Pd films was also investigated. The C60 /Pd films are electrochemically active both in protic and aprotic solvents. The voltammeric characteristics of good capacitor were observed in acetonitrile, N,N-dimethylformamide, acetone, and dimethylsulfoxide as shown in Fig. 8. For these solvents, voltammograms show pseudorectangular cathodic and anodic profiles that are of mirror-image shape. The specific capacitance depends only slightly on the solvents and changes in the range from 165 F/g for acetonitrile to 105 F/g for acetone. For dichloromethane, 1,2-dichloroethane, propylene carbonate, and benzonitrile, cyclic voltammograms exhibit less reversibility in the potential range of polymer reduction. Similar behavior is observed for most of the protic solvents 共Fig. 9兲. However, reversible behavior with a pseudorectangular shape of the voltammograms was observed for methanol and ethanol.
Table I. Specific capacitance of the C60 /Pd films in acetonitrile solutions of different tetra(n-alkyl) ammonium perchlorates.
idt
⌬Em
关2兴
where ⌬E is the potential range of integration. The values of specific capacitance obtained for different tetra共alkyl兲ammonium cations are collected in Table I. Similar studies have been done in acetonitrile solutions containing different alkali metal cation salts. Voltammograms recorded for two different potential ranges are shown in Fig. 7. The shapes of these voltammograms depend on the alkali metal cation. The specific capacitance of the C60 /Pd film is also affected by the alkali metal cations 共Table I兲. Values of the specific capacitance for alkali metal cations are comparable with those obtained for tetra共alkyl兲ammonium cations. The highest value 共375 F/g兲 was obtained for the large Cs+ cations. These cations are weakly solvated.63 They are probably transported to the polymeric phase without their solvation shell during the film reduction process. Smaller lithium and potassium cations are more strongly solvated by acetonitrile. The solvation results in the increase of effective radius of the alkali metal cation and lower values of polymer capacitance.
a
Supporting electrolyte
Specific capacitance 共F g−1兲
共n-Hx兲4NClO4 共n-Bu兲4NClO4 共Et兲4NClO4 共Me兲4NClO4 LiClO4 KPF6 CsAsF6
135a 200a 255a 295a 300a 289a 375a
21b 25b 23b
Polymer film electrochemically synthesized in 0.10 M tetra共nbutyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2, in an acetonitrile-toluene 共1:4, v:v兲 mixture. Specific capacitance was determined by the integration of voltammograms in the negative potential range. b Polymer film, rich in Pd nanoparticles, electrochemically synthesized in 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 9.15 mM Pd共ac兲2, in an acetonitrile-toluene 共1:4, v:v兲 mixture. Specific capacitance was determined by integration of voltammograms in the potential range from −500 to −700 mV.
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
Figure 7. Cyclic voltammograms of C60 /Pd films in acetonitrile containing 共a兲 0.10 M LiClO4, 共b兲 0.10 M KPF6, and 共c兲 0.10 M CsAsF6. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲, containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2.
Electrochemical impedance spectroscopy has been extensively used to study the redox processes of conducting polymers and their capacitive properties.64-68 The polymer/electrolyte interface can be represented by the following equivalent circuit69
where R1 is the resistance of the ionic conductivity of the electrolyte, Cdl is the double-layer capacitance of the external polymer/ electrolyte interface, Rct is a charge-transfer resistance related to the process of polymer oxidation or reduction, ZW is the Warburg impedance which is related to the transport of counter ions during the process of polymer oxidation or reduction, and CL the capacitance of the internal polymer chain/solution interface in the polymer microphores. The Warburg impedance is given by the equation ZW =
AW
冑 j
关3兴
where = 2f and AW is expressed as follows AW =
冑
3RL CL
关4兴
In this equation RL represents the resistance related to the iontransport process.
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Figure 8. Cyclic voltammograms of C60 /Pd films in 共a兲 acetonitrile, 共b兲 N,N-dimethylformamide, 共c兲 acetone, and 共d兲 dimethylsulfoxide containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2.
Figure 10 presents Nyquist plots for an electrode covered with C60 /Pd film in acetonitrile containing 0.1 M tetra共n-butyl兲ammonium perchlorate as a function of potential. At potentials less negative than about −700 mV, the polymer is an electronic insulator 共oxidized form兲. The contribution of diffusion process ZW and capacitance CL can be neglected in this case. In this potential region, the charge-transfer resistance, Rct, is very large, and the impedance response is dominated by the charge-transfer process of the C60 /Pd film. The electrode exhibits the double-layer characteristics of the polymer solution interface 共Fig. 10b兲. At potentials more negative than −700 mV, the impedance response of an electrode coated with a C60 /Pd film changes. Panel c of Fig. 10 shows the Nyquist plots obtained in a high frequency range. Before the semicircle becomes closed, the imaginary part of impedance increases. In this frequency range, the phase angle is close to 40 deg. This short, linear part of the Nyquist plot represents the process of counter ion diffusion inside the polymeric film. At low frequencies, the impedance depends on the capacitance of the polymer. The imaginary part of impedance 共Z⬙兲 increases rapidly with a decrease in frequency 共Fig. 10b兲. The departure of the Z⬘–Z⬙ response from the ideal capacitive behavior may be related to the porous structure of the film and its nonuniform thickness. The shapes of the experimental impedance plots at different polarization potentials were consistent with the curves simulated for the equivalent circuit. In panels a and b of Fig. 10, the experimental and calculated Z⬘–Z⬙ plots are compared. The values used to generate the simulated plots are collected in Table II. The Rct values decrease with the shift of potential toward more negative values. Such behavior indicates that the reduced C60 /Pd polymer is a good electronic conductor. The capacitance CL increases in the potential range of film reduction. At negative potentials CL reaches its limiting value corresponding to the specific capacitance of 140 F/g. This
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Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲 Table II. Faradaic impedance data for Au electrodes „1.5 mm diam… covered with C60 /Pd film formed under cyclic voltammetry conditions in acetonitrile-toluene (1:4, v:v) containing 0.10 M tetra (n-butyl) ammonium perchlorate, 4.65 mM Pd„ac…2, and 0.27 mM C60. E 共mV兲 −550 −750 −800 −850 −900 −950 −1000 −1050 −1100
R 共⍀兲
Cdl 共F兲
Rct 共⍀兲
CL 共F兲
RL 共⍀兲
320 323 332 330 315 320 310 310 314
1.20 0.37 0.60 1.10 2.40 3.60 5.59 6.55 6.20
— 275 225 160 93 72 65 60 52
0.55 7.71 19.4 40.6 61.7 62.2 63.1 61.2 61.3
— 29.8 12.0 8.8 8.2 7.5 6.0 5.1 5.5
value is lower than the value of specific capacitance obtained from cyclic voltammetry 共Table I兲. However, the film used in the faradaic impedance study was electrodeposited from a solution containing a higher concentration of palladium共II兲 acetate. Moreover, the voltammetric current in the negative potential range includes both capacitive 共iCL兲 and noncapacitive 共iF兲 contributions due to the faradaic process of film reduction.
Figure 9. Cyclic voltammograms of C60 /Pd films in 共a兲 H2O containing 0.10 M LiClO4, 共b兲 methanol containing 0.10 M tetra共n-butyl兲ammonium perchlorate, and 共c兲 n-propanol containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2.
Capacitive performance of the C60 /Pd film containing metallic palladium nanoparticles.— Figure 11a shows the voltammetric behavior of a C60 /Pd polymer in acetonitrile containing different tetra共alkyl兲ammonium perchlorates. The solid phase was formed under cyclic voltammetry conditions in a solution containing a high concentration of palladium共II兲 acetate. In this case, the film contains metallic palladium nanoparticles that participate in the chargetransfer process.56 The film is conductive in the potential range less negative than potential required for film reduction. In the negative potential range, current related to the reduction of fullerene moieties is observed. The charge of this process strongly depends on
Figure 10. Nyquist plots of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate at 共a兲 −500 and 共b兲 −800 mV. Solid lines represent simulated data. 共c兲 Z⬘Z⬙ dependences in the frequency range of semicircle formation at −750, −800, −850, −900, and −1100 mV. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 4.65 mM Pd共ac兲2.
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
Figure 11. Cyclic voltammograms of C60 /Pd films in acetonitrile containing 共1兲 0.10 M tetra共n-hexyl兲ammonium perchlorate, 共2兲 0.10 M tetra共n-butyl兲ammonium perchlorate, and 共3兲 0.10 M tetra共ethyl兲ammonium perchlorate in potential range 共a兲 from −100 to −1600 mV and 共b兲 from −100 to −775 mV. The sweep rate was 100 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 8.46 mM Pd共ac兲2, and 0.10 M tetra共nbutyl兲ammonium perchlorate.
the size of tetra共alkyl兲ammonium cations. For large tetra共n-hexyl兲ammonium cations, almost no reduction of the polymer was observed. At potentials less negative than the potential of film reduction, the capacitive current of the film is much higher than the capacitive current of the C60 /Pd polymer formed in solution with a low concentration of palladium共II兲 acetate. The capacitive current in this potential range is almost independent of the size of the cation in the supporting electrolyte 共Fig. 11b兲. Similar to the behavior observed for the film formed in a solution containing a low concentration of Pd共ac兲2 共Fig. 2兲, the capacitive current recorded at potentials less negative than −700 mV linearly depends on the sweep rate. In Fig. 12, the effect of the sweep rate on the voltammetric response of the C60 /Pd film formed in a solution of high ratio of palladium acetate to C60 兵关Pd共ac兲2兴/关C60兴 = 39:1其 is shown. The capacitive current depends linearly on the sweep rate. From the slope of this current–sweep rate relation the specific capacitance was determined. Similar studies of the effects of sweep rates on the voltammeric responses of C60 /Pd films formed from solutions with a high concentration of palladium acetate were done for acetonitrile containing other tetra共alkyl兲ammonium perchlorates. The values of the specific capacitances obtained in acetonitrile with different supporting electrolytes are collected in Table I. The mass of the film deposited on the electrode surface was
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Figure 12. 共a兲 Cyclic voltammograms of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 共1兲 20, 共2兲 50, 共3兲 100, and 共4兲 200 mV/s. 共b兲 Dependence of the capacitive current on the sweep rate. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 8.46 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate.
determined from the frequency changes of an electrochemical quartz crystal microbalance. The dependence of the mass of the film deposited as a function of scan number is shown in Fig. 13. The higher mass changes of the electrode during potential cycling as compared to that shown in Fig. 4 for the lower ratio of concentration of palladium共II兲 acetate to the concentration of fullerene can be attributed to the high amount of metallic palladium phase deposition due to the higher concentration of Pd共ac兲2 employed. The capacitance of the film was calculated by the integration of the current density–voltage 共i–E兲 curves. Values of the specific capacitance obtained for this film at potentials less negative than −700 mV are summarized in Table I. The values of Cs are almost independent of the nature of the supporting electrolyte. These values are also much smaller than the values of specific pseudocapacitance obtained for the films that do not contain palladium particles. However, it has to be taken into account that the density of material deposited on the electrode surface from a solution containing a large concentration of palladium acetate is very high. The amount of C60 /Pd polymer which gives rise to the capacitance is small in comparison to the metallic palladium phase. Figure 14 presents the results of an electrochemical impedance spectroscopy study of C60 /Pd films formed in solutions containing a relatively large concentration of Pd共ac兲2. At high frequencies, the film behaves like a resistance R. At low frequencies, the imaginary part of impedance sharply increases and the plot tends to be a vertical line. However, departure from the ideal capacitive behavior is observed. In the case of the Nyquist plot presented in Fig. 14c, a small semicircle at high frequencies is recorded. Cyclic voltammograms recorded for this layer show traces of faradaic current at
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Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲 The equivalent circuit of an electrode covered by a C60 /Pd film containing palladium nanoparticles can be represented by resistance and capacitance that are functions of the pulsation 68
The impedance of such a system is defined by the following equation Z共兲 =
1 j ⫻ C共兲
关5兴
The capacitance C共兲 can be written under its complex form C共兲 = C⬘共兲 − jC⬙共兲
关6兴
The real C⬘共兲 and imaginary C⬙共兲 part of capacitance are expressed by the following equations70 Figure 13. Dependence of the mass of Au/quartz electrode on the cycle number during C60 /Pd film deposition in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 8.46 mM Pd共ac兲2, and 0.10 M tetra共nbutyl兲ammonium perchlorate. The sweep rate was 50 mV/s.
potentials more negative than about −800 mV 共Fig. 14a兲. This faradaic process is responsible for the semicircle in the Z⬘–Z⬙ plot.
C⬘共兲 =
− Z⬙共兲 兩Z共兲兩2
关7兴
C⬙共兲 =
Z⬘共兲 兩Z共兲兩2
关8兴
The changes in the real and imaginary parts of the capacitance are shown in Fig. 15 as functions of frequency. These plots were constructed on the basis of impedance data obtained for the film formed in solutions containing 0.27 mM C60 and 9.13 mM Pd共ac兲2 共Fig. 14兲. The specific capacitance of the film calculated from the limiting value of C⬘共兲 obtained for low frequencies at potential −550 mV is 17 F/g. This value is slightly lower than the specific capacitance obtained from cyclic voltammetry 共Fig. 15a兲, which is 25 F/g. A higher limiting value of the real part of the capacitance was obtained for impedance data collected at −1100 mV. In this case, the higher value of C⬘共兲 in the low-frequency range is related to the small contribution of the pseudocapacitance due to the faradaic process of the film reduction. Figure 15b shows the evolution of the C⬙共兲 component on frequency. The imaginary part of capacitance goes through a maximum. From the frequency of the C⬙共兲 maximum, a time constant o was calculated. This time constant is associated with the dielectric relaxation time. Small values of o indicate that the system passes the frontier between resistive and capacitive behavior relatively quickly. Conclusions
Figure 14. 共a兲 Cyclic voltammogram and Nyquist diagrams obtained at 共b兲 −550 mV and 共c兲 −1100 mV for C60 /Pd film in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The film was formed under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 9.13 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate.
The redox-active C60 /Pd films are shown to be very promising electrode materials for production of supercapacitors. Depending on the conditions of film formation, two kinds of capacitors are formed. Films formed from the solutions with a low ratio of 关Pd共ac兲2兴/关C60兴 exhibit large capacitance due to the contribution from the faradaic pseudocapacitance. These capacitors function in the negative potential range where reduction of the fullerene occurs. They do not display capacitive behavior in the positive potential region. The microporous structure of the film allows for swelling of the polymer with solvent and transport of ions between solution and polymeric phase. The capacitance of this polymer is strongly affected by the solvent and supporting electrolyte. The C60 /Pd film is electrochemically active both in protic and aprotic media. The highest values of specific capacitance were found for acetonitrile. In this solvent, voltammograms show pseudorectangular cathodic and anodic profiles that are mirror images of one another, characteristic behavior for an ideal capacitor. The current linearly depends on the sweep rate. The sizes of the cations in the supporting electrolyte influence the value of the specific capacitance of these C60 /Pd films. For
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
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relatively high specific capacitance of the C60 /Pd films, there are additional advantages making them desirable for building charge storage devices. The C60 /Pd polymers can be easily formed either by electrochemical51,53-55 or chemical methods in solution.71-73 The electrochemical properties of the film can be tuned by changing the composition of the grown solution. They exhibit high conductivity in the reduced state. They can also be used in nonaqueous solvents and the range of cell voltage can be extended. In many organic solvents, the C60 /Pd films exhibit a high potential range of stability. Finally, in contrast to other electroactive polymeric materials used in literature,4-37 the C60 /Pd films are electrochemically active in the negative potential range, thus fulfilling the n-doping anode electroactive material needs. Films formed from solutions with a high ratio of concentration of Pd共ac兲2 to the concentration of C60 exhibit lower permeability to the ions of the supporting electrolyte. In this case, the charge related to the process of polymer reduction and therefore the capacitance of the polymer in negative potential range decreases. However the polymer becomes conductive in the whole potential range with increased stability. Acknowledgment Support from the Polish State Committee for Scientific Research 共grant 3T09A04626 to K.W.兲, National Science Foundation 共grants CHE0413857 to A.L.B. and 0453464 to F.D.兲, and the Petroleum Research Fund administered by the American Chemical Society is gratefully acknowledged. University of California, Davis assisted in meeting the publication costs of this article.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Figure 15. Dependence of the 共a兲 real part of capacitance, 共b兲 imaginary part of capacitance, and 共c兲 phase angle on frequency for C60 /Pd film in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate at −550 mV. The film was formed under cyclic voltammetry conditions in acetonitriletoluene 共1:4, v:v兲 containing 0.27 mM C60, 9.13 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate.
12. 13. 14. 15. 16. 17.
tetra共methyl兲ammonium ions a specific capacitance of ca. 295 F g−1 was found. This value is in the range of the values of specific capacitance obtained for other typical polymers. High values of specific capacitance of C60 /Pd films were also obtained for acetonitrile solutions containing alkali metal cations. For example, the pseudocapacitance of the C60 /Pd film is equal to 375 F g−1 in a solution containing Cs+ ions. A simulation of the measured impedance behavior of these systems was possible using an equivalent circuit model, which takes into account the double-layer capacitance, the process of electrochemical reduction of the C60 moieties, the Warburg impedance of the counter ions’ migration into the polymeric phase during its reduction, and the capacitance of the polymer micropores. The impedance measurements revealed that both double-layer capacitance and pseudocapacitance contribute to the large specific capacitance of the C60 /Pd materials. Apart from the
18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
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Y. U. Jeong and A. Manthiram, J. Electrochem. Soc., 148, A189 共2001兲. S.-L. Kuo and N.-L. Wu, Electrochem. Solid-State Lett., 6, A85 共2003兲. J. H. Park, J. M. Ko, and O. O. Park, J. Electrochem. Soc., 150, A864 共2003兲. K. Honda, M. Yoshimura, K. Kawakita, A. Fujishima, Y. Sakamoto, K. Yasui, N. Nishio, and H. Masuda, J. Electrochem. Soc., 151, A532 共2004兲. H. Kim and B. N. Popov, J. Electrochem. Soc., 150, A1153 共2003兲. A. Laforgue, P. Simon, J. F. Fauvarque, J. F. Sarrau, and P. Lailler, J. Electrochem. Soc., 148, A1130 共2001兲. H. Liang, F. Chen, R. Li, L. Wang, and Z. Deng, Electrochim. Acta, 49, 3463 共2004兲. M. Mastraagostino, R. Paraventi, and A. Zanelli, J. Electrochem. Soc., 147, 3167 共2000兲. E. Franckowiak, V. Khomenko, K. Jurewicz, K. Lota, and F. Beguin, J. Power Sources, 153, 413 共2006兲. O. Genz, M. M. Lohrengel, and J. W. Schultze, Electrochim. Acta, 39, 179 共1994兲. A. Rudge, J. Davey, I. Raistrick, and S. Gottesfeld, J. Power Sources, 47, 89 共1994兲. J. C. Carlberg and O. Inganas, J. Electrochem. Soc., 144, L61 共1997兲. T. Benincori, E. Brenna, F. Sonnicolo, L. Trimarco, G. Zoti, and P. Sozzani, Angew. Chem., Int. Ed. Engl., 33, 194 共1995兲. H. L. Anderson, C. Boudou, F. Diederich, J. P. Gisselbrecht, M. Gross, and P. Seiler, Angew. Chem., Int. Ed. Engl., 33, 1628 共1994兲. A. Cravino, G. Zerza, H. Neugebauer, M. Maggini, S. Bucella, E. Menna, M. Svensson, M. R. Andersson, C. J. Brabec, and N. S. Sarciftici, J. Phys. Chem. B, 106, 70 共2002兲. A. Carvino and N. S. Sarciftici, J. Mater. Chem., 12, 1931 共2002兲. A. Cravino, G. Zerza, M. Maggini, S. Bucella, M. Svensson, M. R. Andersson, H. Neugebauer, and N. S. Sarciftici, Chem. Commun. (Cambridge), 2000, 2487. M. Fedurco, D. A. Costa, A. L. Balch, and W. R. Fawcett, Angew. Chem., Int. Ed. Engl., 34, 194 共1995兲. K. Winkler, D. A. Costa, A. L. Balch, and W. R. Fawcett, J. Phys. Chem., 99, 17431 共1995兲. K. Winkler, D. A. Costa, W. R. Fawcett, and A. L. Balch, Adv. Mater. (Weinheim, Ger.), 9, 153 共1997兲. E. P. Krinichnaya, A. P. Moravsky, O. Efimov, J. W. Sobczak, K. Winkler, W. Kutner, and A. L. Balch, J. Mater. Chem., 15, 1468 共2005兲. K. Winkler and A. L. Balch, C. R. Chim., 9, 928 共2006兲. A. L. Balch, D. A. Costa, and K. Winkler, J. Am. Chem. Soc., 120, 9614 共1998兲.
52. A. Hayashi, A. de Bettencourt-Dias, K. Winkler, and A. L. Balch, J. Mater. Chem., 12, 2116 共2002兲. 53. K. Winkler, A. de Bettencourt-Dias, and A. L. Balch, Chem. Mater., 11, 2265 共1999兲. 54. K. Winkler, A. de Bettencourt-Dias, and A. L. Balch, Chem. Mater., 12, 1386 共2000兲. 55. K. Winkler, K. Noworyta, W. Kutner, and A. L. Balch, J. Electrochem. Soc., 147, 2597 共2000兲. 56. K. Winkler, K. Noworyta, J. W. Sobczak, C.-T. Wu, L.-C. Chen, W. Kutner, and A. L. Balch, J. Mater. Chem., 13, 518 共2003兲. 57. M. E. Plonska, A. de Bettencourt-Dias, A. L. Balch, and K. Winkler, Chem. Mater., 15, 4122 共2003兲. 58. M. E. Plonska, A. Makar, K. Winkler, and A. L. Balch, Pol. J. Chem., 78, 1431 共2004兲. 59. K. Winkler, M. E. Plonska-Brzezinska, S. Gadde, F. D’Souza, and A. L. Balch, Electroanalysis, 18, 841 共2006兲. 60. P. Strasser and M. Ata, J. Phys. Chem. B, 102, 4131 共1998兲. 61. D. Ofer, R. M. Crooks, and M. S. Wrighton, J. Am. Chem. Soc., 112, 7869 共1990兲. 62. G. Zotti, S. Zecchin, G. Schiavon, B. Vercelli, A. Berlin, E. Dalcanale, and L. Groenendaal, Chem. Mater., 15, 4642 共2003兲. 63. J. E. Gordon, The Organic Chemistry of Electrolyte Solutions, Wiley, New York 共1975兲. 64. F. Fusalba, N. El-Mehdi, L. Breau, and D. Belanger, Chem. Mater., 11, 2743 共1999兲. 65. F. Fusalba and D. Belanger, J. Phys. Chem. B, 103, 9044 共1999兲. 66. M. M. Musiani, Electrochim. Acta, 35, 1665 共1990兲. 67. X. Ren and P. G. Pickup, J. Chem. Soc., Faraday Trans., 89, 321 共1993兲. 68. X. Ren and P. G. Pickup, J. Electrochem. Soc., 139, 2097 共1992兲. 69. Z. Gao, C. Kvarknstrom, and A. Ivaska, Electrochim. Acta, 39, 1419 共1994兲. 70. P. L. Taberna, P. Simon, and J. F. Fauvarque, J. Electrochem. Soc., 150, A292 共2003兲. 71. K. Nagashima, A. Nakaoka, Y. Saito, M. Kato, T. Kawanishi, and K. Itoh, J. Chem. Soc., Chem. Commun., 1992, 377. 72. K. Nagashima, H. Yamaguchi, Y. Kato, Y. Saito, M. A. Haga, and K. Itoh, Chem. Lett., 1993, 2153. 73. K. Nagashima, Y. Kato, H. Yamaguchi, E. Kiura, T. Kawanishi, M. Kato, Y. Saito, M. Haga, and K. Itoh, Chem. Lett., 1994, 1207.
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Two-Component Films of Fullerene and Palladium as Materials for Electrochemical Capacitors Krzysztof Winkler,a Emilia Grodzka,a Francis D’Souza,b,* and Alan L. Balchc,*,z a
Institute of Chemistry, University of Bialystok, 15-399 Bialystok, Poland Department of Chemistry, Wichita State University, Wichita, Kansas 67260, USA Department of Chemistry, University of California, Davis, California 95616, USA
b c
The redox-active films, C60 /Pd, formed by electrochemical reduction of solutions containing palladium共II兲 acetate and C60 fullerene have been studied as active components for electrochemical capacitors. The capacitance properties of these materials have been investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The structure and electrochemical properties of films deposited on an electrode surface depend on the composition of the solution from which they are grown. Films formed in a solution with a low concentration of palladium共II兲 acetate exhibit conductivity in the potential range of the film reduction. The faradaic process of C60 reduction gives rise to pseudocapacitance. The capacitance of this polymer depends on the solvent and the size of the cations in the supporting electrolyte. For an acetonitrile solution containing only tetra共methyl兲ammonium perchlorate, the film displays a high specific capacitance, close to 300 F/g. Films formed in a solution with a high concentration of palladium共II兲 acetate also contain metallic palladium nanoparticles. Such systems exhibit conductivity at potentials less negative than the potentials for film reduction, and these films can be considered as double-layer capacitors. The specific capacitance of these films is much smaller 共about 20 F/g兲 but a large potential window 共from +800 to −2000 mV兲 is available for the performance of these capacitors. © 2007 The Electrochemical Society. 关DOI: 10.1149/1.2434683兴 All rights reserved. Manuscript submitted August 16, 2006; revised manuscript received October 31, 2006. Available electronically February 16, 2007.
In recent years, a new generation of electrochemical capacitors, the so-called supercapacitors, has been intensively investigated.1-3 These devices exhibit high-capacitance and high-power characteristics. The electrode materials employed in the construction of such capacitors include activated porous carbon,4-7 carbon nanotubes,8-15 hydrous transition-metal oxides,16-22 and conducting polymers.23-28 Composites formed from these materials are also often used as electrochemical capacitors.29-37 Two types of electrochemical capacitors are under development: double-layer and redox capacitors. In the case of double-layer capacitors, the separation of electronic and ionic charges at the interface between the electrode material and the electrolyte solution is responsible for high-energy-storage properties.1 In redox capacitors, a faradaic process within electroactive material itself gives rise to pseudocapacitance.1 Conducting polymers generally belong to the latter type of supercapacitors. The electrochemical activity of these materials and their high specific surface area make them very promising as electrode materials for secondary batteries and capacitors.38-40 The electronic charge is injected into the polymeric chain, and the ionic charge is transferred into the polymeric phase in order to maintain the charge neutrality. Therefore, p-doped or n-doped polymers can be used for constructing supercapacitor devices. The devices constructed from conducting polymers have the advantage of low cost of production, good stability, and high operating potentials range. They also exhibit relatively high capacitance. Specific capacitance of polymeric electrode materials typically ranges between 300 and 500 F/g.3 Polythiophene,23 polianiline,28 and polypyrrole29 have been the most frequently used as electroactive materials for redox capacitors. These polymers are electrochemically active at positive potentials and exhibit p-doping properties. Materials capable of cathodic n-doping are less numerous and have received less attention. Recently, conducting polymers containing fullerene moieties have been electrochemically synthesized.41-60 Because of the presence of electron-accepting fullerene moieties, these polymers are electrochemically active in the negative potential range and exhibit n-doping properties. The redox-active films formed during reduction of solutions containing C60 and selected transitionmetal complexes are particularly interesting.50 These films are insoluble in common organic solvents and adhere strongly to the
* Electrochemical Society Active Member. z
E-mail: [email protected]
electrode surface. In these materials, there is evidence that suggests that metal atoms or metal complexes are coordinated directly to the fullerenes in 2 fashion as shown below
Additional metal atoms or complexes can form cross-links between these chains. The films, C60 /Pd, formed by electroreduction from solutions containing C60 and palladium acetate have been investigated extensively.51,53-56 In this system, a polymeric network appears to be formed through covalent bonding between fullerene and palladium atoms. The films exhibit redox activity in the negative potential range due to the reduction of the C60 moieties. The electroreduction process is accompanied by cation transfer between solution and the solid film on the electrode surface.53 The composition and structure of the C60 /Pd film depend upon the concentration of the film precursors in the growth solution.54,56 Both a metallic phase and the C60 /Pd polymer are deposited simultaneously from solutions containing a relatively high concentration of the Pd共II兲 complex. Metallic Pd nanoparticles can effectively participate in the electrontransfer process. The C60 /Pd film is also relatively stable. This sort of film retains its redox activity in the course of prolonged potential cycling over a relatively wide potential range from +0.5 to −2.0 V. The composition of the growth solution also affects the film stability through its effect on the film structure.54 In this paper we report on the capacitive performance of the electrochemically formed C60 /Pd films. The effects of the composition of the growth solution on the capacitive properties of the films were investigated. The influences of the solvent and supporting electrolyte on the capacitance of electrodes covered with the C60 /Pd polymer were also studied. Experimental Palladium acetate, Pd共ac兲2 共Aldrich兲, and C60 共Southern Chemical Group兲 were used as received. The supporting electrolytes, tetra共methyl兲ammonium perchlorate, tetra共ethyl兲ammonium perchlorate, tetra共n-butyl兲ammonium perchlorate, and tetra共nhexyl兲ammonium perchlorate 共Sigma Chemical Co.兲, were dried un-
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der vacuum for 24 h prior to use. Lithium perchlorate and potassium hexafluorophosphate 共Aldrich Chemical Co.兲 were dried under reduced pressure at 90°C for 24 h. Cesium hexafluoroarsenate was prepared by precipitation from an aqueous solution containing an equimolar mixture of nearly saturated CsBr 共Aldrich兲 and LiAsF6 共Aldrich兲. The CsAsF6 was recrystallized from methanol and then dried under reduced pressure at 90°C. Acetonitrile 共99.9%兲, N,N-dimethylformamide 共99.9%兲, dimethylsulfoxide 共anhydrous 99.9%兲, acetone 共99.5%兲, ethanol共HPLC/spectrophotometric grade兲, methanol 共99.8%兲, and n-propanol 共anhydrous 99.7%兲 were used as received from Aldrich Chemical Co. Toluene 共Aldrich Chemical Co.兲 was purified by distillation over sodium under a nitrogen atmosphere. Voltammetric and electrochemical impedance spectroscopy experiments were performed on a potentiostat/galvanostat model 283 and Frequency Response Detector model 1025 共EG&G Instruments兲 with a three-electrode cell. A gold disk with a diameter of 1.5 mm 共Bioanalytical Systems, Inc.兲 was used as the working electrode. Prior to the experiment, the electrode was polished with fine carborundum paper and then with a 0.5 m alumina slurry. Subsequently, the electrode was sonicated in water to remove traces of alumina from the gold surface, washed with water, and dried. A silver wire immersed in 0.010 M silver perchlorate and 0.09 M tetra共n-butyl兲ammonium perchlorate in acetonitrile that was separated from the working electrode by a ceramic tip 共Bioanalytical Systems Inc.兲 served as the reference electrode. The counter electrode was a platinum tab with an area of about 0.5 cm2. Simultaneous voltammetric and piezoelectric microgravimetry experiments were carried out with a home-built potentiostat and an electrochemical quartz crystal microbalance, EQCM 5510 共Institute of Physical Chemistry, Warsaw, Poland兲. Plano-convex quartz crystals were used. The 14 mm diameter AT-cut, plano-convex quartz crystals with a 5 MHz resonant frequency were obtained from Omig 共Warsaw, Poland兲. A 100 nm gold film, which was vacuum deposited on the quartz crystal, served as the working electrode. The projected region of this Au electrode was 5 mm in diameter. The area of the circuit center spot and two contacting radial strips was 0.24 cm2. Unpolished quartz crystals were used for better adherence of the film. The sensitivity of the mass measurement calculated from the Saurbrey equation was 17.7 ng Hz−1 cm−2. The C60 /Pd film was prepared by electroreduction of an acetonitrile/toluene 共1:4, v:v兲 solution that contained the fullerene and palladium acetate as well as the supporting electrolyte, 0.10 M tetra共n-butyl兲ammonium perchlorate. Films were grown under cyclic voltammetry conditions at a potential sweep rate of 50 mV/s. The electrochemical properties of the film were studied in a solution of the chosen solvent containing only the supporting electrolyte. In this case, the electrode covered with the film was removed from the growth solution, rinsed several times with an acetonitrile/toluene 共1:4, v:v兲, and then placed in an acetonitrile solution containing 0.10 M of supporting electrolyte. The modified electrode was allowed to equilibrate for 10 min while degassing with argon in a fresh solution before electrochemical measurements were performed.
Results and Discussion The properties of the C60 /Pd films depend on the composition of growth solution. Figure 1 shows the effect on the concentration of palladium共II兲 acetate on the redox properties of films deposited on the electrode surface. An increase in the palladium acetate concentration in the growth solution results in a decrease of the charges associated with reduction and reoxidation of the film at potentials more negative than −700 mV 共peaks R1 and O1兲. In solution with a palladium acetate concentration higher than about 8.9 mM, the peaks R1 and O1 almost completely disappear. The composition of the growth solution also influences the capacitance at potentials less negative than −700 mV. For the films formed in a solution contain-
Figure 1. Cyclic voltammograms of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 100 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 共a兲 3.35 mM Pd共ac兲2, 共b兲 6.68 mM Pd共ac兲2, and 共c兲 9.81 mM Pd共ac兲2.
ing a large excess of palladium acetate, a large increase in the capacitance at potentials less negative than −700 mV is observed. The C60 /Pd film is formed in the potential range where both fullerene and the palladium ions are reduced.51 Consequently, a parallel deposition of palladium nanoparticles and polymer on the electrode surface can take place.56 The increase of the amount of palladium nanoparticles in the polymeric phase with the increase of the palladium acetate concentration in the growth solution results in changes in the conductivity of the film. C60 /Pd films formed in solutions containing a large excess of palladium共II兲 acetate 共higher than 7 mM兲 exhibit high electronic conductivity at potentials less negative than the potential of film reduction.56 The conductivity of the C60 /Pd film formed in solution with lower concentration of Pd共ac兲2 depends on its oxidation state. For this film, the conductivity increases in the potential range of the film reduction. Such a behavior resembles conductivity properties of typical conducting polymers such as polyaniline, polypyrrole, or polythiophene.61,62 Therefore, potential applications of the following two types of capacitors are conceivable: 共i兲 Films formed from solutions with a 关Pd兴/关C60兴 ratio lower than 20:1 which exhibit conductivity in the potential range of film reduction. As shown below, the electrochemical properties of these films indicate that the faradaic process of reduction of the C60 moieties gives rise to pseudocapacitance. 共ii兲 Films formed from solutions with a 关Pd兴/关C60兴 ratio higher than 25:1 which exhibits conductivity also at potentials less negative that the potentials for film reduction. These films can be considered as double-layer capacitors, as the following data demonstrates. Capacity performance of the C60 /Pd pseudo-capacitors.— Figure 2 shows cyclic voltammograms of the C60 /Pd film in acetonitrile containing tera共n-butyl兲ammonium perchlorate as a supporting electrolyte recorded at different sweep rates. The film was cycled between −800 and −1500 mV without noticeable change in the shape of voltammograms. Voltammograms show almost pseudorectangular
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
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Figure 3. 共a兲 Multicyclic voltammograms and 共b兲 curves of the frequency change vs potential 共b兲 simultaneously recorded at the same Au/quartz electrode in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 3.6 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s.
Figure 2. 共a兲 Cyclic voltammograms of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 共1兲 20, 共2兲 50, 共3兲100, and 共4兲 200 mV/s. 共b兲 Dependence of the pseudocapacitive current on the sweep rate. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2.
cathodic and anodic profiles that are the mirror image of one another, characteristic behavior for an ideal capacitor. The departure from the ideal rectangular shape is related to the faradaic process of film reduction and reoxidation. The current at negative potentials includes capacitive and faradaic contributions. Both components depend linearly on the sweep rate. Therefore, the linear dependence of the total current on the sweep rate is expected. Such relation is shown in panel b of Fig. 2b. The specific capacitance Cs of the C60 /Pd film can be calculated according to the following equation Cs =
i 共dv /dt兲m
Fig. 4 for the first ten cycles. The amount of C60 /Pd film deposited on the electrode in each cycle decreases with the increase in the scan number. This effect can be attributed to the partial inhibition of the polymer formation on the electrode covered with the C60 /Pd film. In Fig. 5, the effect of the C60 /Pd film thickness on its voltammetric response in acetonitrile containing tera共n-butyl兲ammonium perchlorate is shown. From the slope of the i-m linear relationship, predicted by Eq. 1, the specific capacitance of the film is 170 F g−1. The capacity performance of the C60 /Pd film depends on the supporting electrolyte and solvent. The pseudocapacitance is
关1兴
where i is the pseudocapacitive current, dv /dt is the sweep rate, and m is the mass of electroactive material deposited on the electrode surface. The amount of C60 /Pd deposited on the electrode surface can be calculated on the base of results of the electrochemical quartz crystal microbalance 共EQCM兲 study. The frequency changes of the quartz crystal during the C60 /Pd film deposition are shown in Fig. 3. The frequency decreases in the cathodic cycle in the potential range of palladium cation reduction. A small increase of frequency in the positive scan direction is related to the film oxidation followed by egress of the supporting electrolyte cations from the polymer to the solution. The changes in mass of the electrode as a function of the scan number is shown in
Figure 4. Dependence of the mass of Au/quartz electrode on the cycle number during C60 /Pd film deposition in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 3.6 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s.
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
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Figure 6. Cyclic voltammograms of C60 /Pd films in acetonitrile containing 共1兲 0.10 M tetra共n-hexyl兲ammonium perchlorate, 共2兲 0.10 M tetra共n-butyl兲ammonium perchlorate, 共3兲 0.10 M tetra共ethyl兲ammonium perchlorate, and 共4兲 0.07 M tetra共methyl兲ammonium perchlorate. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2. Figure 5. 共a兲 Cyclic voltammograms of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2. The amount of C60 /Pd was 共1兲 11.5, 共2兲 21, 共3兲 28, 共4兲 41, and 共5兲 62 g/cm2. 共b兲 Dependence of the pseudocapacitive current at −1200 mV on the mass of C60 /Pd film deposited on the electrode.
strongly affected by the cation of the supporting electrolyte. The anion does not influence the capacitive currents in the negative potential range. The effect of the cation of supporting electrolyte on the pseudocapacitance of C60 /Pd film in acetonitrile is shown in Fig. 6. The decrease of the size of tetra共alkyl兲ammonium ion results in the increase of the pseudocapacitance. The specific capacitance can be calculated by the integration of cyclic voltammograms according to the equation
Ci =
冕
The effect of solvent on the capacitive performance of the C60 /Pd films was also investigated. The C60 /Pd films are electrochemically active both in protic and aprotic solvents. The voltammeric characteristics of good capacitor were observed in acetonitrile, N,N-dimethylformamide, acetone, and dimethylsulfoxide as shown in Fig. 8. For these solvents, voltammograms show pseudorectangular cathodic and anodic profiles that are of mirror-image shape. The specific capacitance depends only slightly on the solvents and changes in the range from 165 F/g for acetonitrile to 105 F/g for acetone. For dichloromethane, 1,2-dichloroethane, propylene carbonate, and benzonitrile, cyclic voltammograms exhibit less reversibility in the potential range of polymer reduction. Similar behavior is observed for most of the protic solvents 共Fig. 9兲. However, reversible behavior with a pseudorectangular shape of the voltammograms was observed for methanol and ethanol.
Table I. Specific capacitance of the C60 /Pd films in acetonitrile solutions of different tetra(n-alkyl) ammonium perchlorates.
idt
⌬Em
关2兴
where ⌬E is the potential range of integration. The values of specific capacitance obtained for different tetra共alkyl兲ammonium cations are collected in Table I. Similar studies have been done in acetonitrile solutions containing different alkali metal cation salts. Voltammograms recorded for two different potential ranges are shown in Fig. 7. The shapes of these voltammograms depend on the alkali metal cation. The specific capacitance of the C60 /Pd film is also affected by the alkali metal cations 共Table I兲. Values of the specific capacitance for alkali metal cations are comparable with those obtained for tetra共alkyl兲ammonium cations. The highest value 共375 F/g兲 was obtained for the large Cs+ cations. These cations are weakly solvated.63 They are probably transported to the polymeric phase without their solvation shell during the film reduction process. Smaller lithium and potassium cations are more strongly solvated by acetonitrile. The solvation results in the increase of effective radius of the alkali metal cation and lower values of polymer capacitance.
a
Supporting electrolyte
Specific capacitance 共F g−1兲
共n-Hx兲4NClO4 共n-Bu兲4NClO4 共Et兲4NClO4 共Me兲4NClO4 LiClO4 KPF6 CsAsF6
135a 200a 255a 295a 300a 289a 375a
21b 25b 23b
Polymer film electrochemically synthesized in 0.10 M tetra共nbutyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2, in an acetonitrile-toluene 共1:4, v:v兲 mixture. Specific capacitance was determined by the integration of voltammograms in the negative potential range. b Polymer film, rich in Pd nanoparticles, electrochemically synthesized in 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 9.15 mM Pd共ac兲2, in an acetonitrile-toluene 共1:4, v:v兲 mixture. Specific capacitance was determined by integration of voltammograms in the potential range from −500 to −700 mV.
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
Figure 7. Cyclic voltammograms of C60 /Pd films in acetonitrile containing 共a兲 0.10 M LiClO4, 共b兲 0.10 M KPF6, and 共c兲 0.10 M CsAsF6. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲, containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2.
Electrochemical impedance spectroscopy has been extensively used to study the redox processes of conducting polymers and their capacitive properties.64-68 The polymer/electrolyte interface can be represented by the following equivalent circuit69
where R1 is the resistance of the ionic conductivity of the electrolyte, Cdl is the double-layer capacitance of the external polymer/ electrolyte interface, Rct is a charge-transfer resistance related to the process of polymer oxidation or reduction, ZW is the Warburg impedance which is related to the transport of counter ions during the process of polymer oxidation or reduction, and CL the capacitance of the internal polymer chain/solution interface in the polymer microphores. The Warburg impedance is given by the equation ZW =
AW
冑 j
关3兴
where = 2f and AW is expressed as follows AW =
冑
3RL CL
关4兴
In this equation RL represents the resistance related to the iontransport process.
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Figure 8. Cyclic voltammograms of C60 /Pd films in 共a兲 acetonitrile, 共b兲 N,N-dimethylformamide, 共c兲 acetone, and 共d兲 dimethylsulfoxide containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2.
Figure 10 presents Nyquist plots for an electrode covered with C60 /Pd film in acetonitrile containing 0.1 M tetra共n-butyl兲ammonium perchlorate as a function of potential. At potentials less negative than about −700 mV, the polymer is an electronic insulator 共oxidized form兲. The contribution of diffusion process ZW and capacitance CL can be neglected in this case. In this potential region, the charge-transfer resistance, Rct, is very large, and the impedance response is dominated by the charge-transfer process of the C60 /Pd film. The electrode exhibits the double-layer characteristics of the polymer solution interface 共Fig. 10b兲. At potentials more negative than −700 mV, the impedance response of an electrode coated with a C60 /Pd film changes. Panel c of Fig. 10 shows the Nyquist plots obtained in a high frequency range. Before the semicircle becomes closed, the imaginary part of impedance increases. In this frequency range, the phase angle is close to 40 deg. This short, linear part of the Nyquist plot represents the process of counter ion diffusion inside the polymeric film. At low frequencies, the impedance depends on the capacitance of the polymer. The imaginary part of impedance 共Z⬙兲 increases rapidly with a decrease in frequency 共Fig. 10b兲. The departure of the Z⬘–Z⬙ response from the ideal capacitive behavior may be related to the porous structure of the film and its nonuniform thickness. The shapes of the experimental impedance plots at different polarization potentials were consistent with the curves simulated for the equivalent circuit. In panels a and b of Fig. 10, the experimental and calculated Z⬘–Z⬙ plots are compared. The values used to generate the simulated plots are collected in Table II. The Rct values decrease with the shift of potential toward more negative values. Such behavior indicates that the reduced C60 /Pd polymer is a good electronic conductor. The capacitance CL increases in the potential range of film reduction. At negative potentials CL reaches its limiting value corresponding to the specific capacitance of 140 F/g. This
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Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲 Table II. Faradaic impedance data for Au electrodes „1.5 mm diam… covered with C60 /Pd film formed under cyclic voltammetry conditions in acetonitrile-toluene (1:4, v:v) containing 0.10 M tetra (n-butyl) ammonium perchlorate, 4.65 mM Pd„ac…2, and 0.27 mM C60. E 共mV兲 −550 −750 −800 −850 −900 −950 −1000 −1050 −1100
R 共⍀兲
Cdl 共F兲
Rct 共⍀兲
CL 共F兲
RL 共⍀兲
320 323 332 330 315 320 310 310 314
1.20 0.37 0.60 1.10 2.40 3.60 5.59 6.55 6.20
— 275 225 160 93 72 65 60 52
0.55 7.71 19.4 40.6 61.7 62.2 63.1 61.2 61.3
— 29.8 12.0 8.8 8.2 7.5 6.0 5.1 5.5
value is lower than the value of specific capacitance obtained from cyclic voltammetry 共Table I兲. However, the film used in the faradaic impedance study was electrodeposited from a solution containing a higher concentration of palladium共II兲 acetate. Moreover, the voltammetric current in the negative potential range includes both capacitive 共iCL兲 and noncapacitive 共iF兲 contributions due to the faradaic process of film reduction.
Figure 9. Cyclic voltammograms of C60 /Pd films in 共a兲 H2O containing 0.10 M LiClO4, 共b兲 methanol containing 0.10 M tetra共n-butyl兲ammonium perchlorate, and 共c兲 n-propanol containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 50 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 3.6 mM Pd共ac兲2.
Capacitive performance of the C60 /Pd film containing metallic palladium nanoparticles.— Figure 11a shows the voltammetric behavior of a C60 /Pd polymer in acetonitrile containing different tetra共alkyl兲ammonium perchlorates. The solid phase was formed under cyclic voltammetry conditions in a solution containing a high concentration of palladium共II兲 acetate. In this case, the film contains metallic palladium nanoparticles that participate in the chargetransfer process.56 The film is conductive in the potential range less negative than potential required for film reduction. In the negative potential range, current related to the reduction of fullerene moieties is observed. The charge of this process strongly depends on
Figure 10. Nyquist plots of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate at 共a兲 −500 and 共b兲 −800 mV. Solid lines represent simulated data. 共c兲 Z⬘Z⬙ dependences in the frequency range of semicircle formation at −750, −800, −850, −900, and −1100 mV. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.10 M tetra共n-butyl兲ammonium perchlorate, 0.27 mM C60, and 4.65 mM Pd共ac兲2.
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
Figure 11. Cyclic voltammograms of C60 /Pd films in acetonitrile containing 共1兲 0.10 M tetra共n-hexyl兲ammonium perchlorate, 共2兲 0.10 M tetra共n-butyl兲ammonium perchlorate, and 共3兲 0.10 M tetra共ethyl兲ammonium perchlorate in potential range 共a兲 from −100 to −1600 mV and 共b兲 from −100 to −775 mV. The sweep rate was 100 mV/s. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 8.46 mM Pd共ac兲2, and 0.10 M tetra共nbutyl兲ammonium perchlorate.
the size of tetra共alkyl兲ammonium cations. For large tetra共n-hexyl兲ammonium cations, almost no reduction of the polymer was observed. At potentials less negative than the potential of film reduction, the capacitive current of the film is much higher than the capacitive current of the C60 /Pd polymer formed in solution with a low concentration of palladium共II兲 acetate. The capacitive current in this potential range is almost independent of the size of the cation in the supporting electrolyte 共Fig. 11b兲. Similar to the behavior observed for the film formed in a solution containing a low concentration of Pd共ac兲2 共Fig. 2兲, the capacitive current recorded at potentials less negative than −700 mV linearly depends on the sweep rate. In Fig. 12, the effect of the sweep rate on the voltammetric response of the C60 /Pd film formed in a solution of high ratio of palladium acetate to C60 兵关Pd共ac兲2兴/关C60兴 = 39:1其 is shown. The capacitive current depends linearly on the sweep rate. From the slope of this current–sweep rate relation the specific capacitance was determined. Similar studies of the effects of sweep rates on the voltammeric responses of C60 /Pd films formed from solutions with a high concentration of palladium acetate were done for acetonitrile containing other tetra共alkyl兲ammonium perchlorates. The values of the specific capacitances obtained in acetonitrile with different supporting electrolytes are collected in Table I. The mass of the film deposited on the electrode surface was
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Figure 12. 共a兲 Cyclic voltammograms of C60 /Pd films in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The sweep rate was 共1兲 20, 共2兲 50, 共3兲 100, and 共4兲 200 mV/s. 共b兲 Dependence of the capacitive current on the sweep rate. The C60 /Pd films were grown under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 8.46 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate.
determined from the frequency changes of an electrochemical quartz crystal microbalance. The dependence of the mass of the film deposited as a function of scan number is shown in Fig. 13. The higher mass changes of the electrode during potential cycling as compared to that shown in Fig. 4 for the lower ratio of concentration of palladium共II兲 acetate to the concentration of fullerene can be attributed to the high amount of metallic palladium phase deposition due to the higher concentration of Pd共ac兲2 employed. The capacitance of the film was calculated by the integration of the current density–voltage 共i–E兲 curves. Values of the specific capacitance obtained for this film at potentials less negative than −700 mV are summarized in Table I. The values of Cs are almost independent of the nature of the supporting electrolyte. These values are also much smaller than the values of specific pseudocapacitance obtained for the films that do not contain palladium particles. However, it has to be taken into account that the density of material deposited on the electrode surface from a solution containing a large concentration of palladium acetate is very high. The amount of C60 /Pd polymer which gives rise to the capacitance is small in comparison to the metallic palladium phase. Figure 14 presents the results of an electrochemical impedance spectroscopy study of C60 /Pd films formed in solutions containing a relatively large concentration of Pd共ac兲2. At high frequencies, the film behaves like a resistance R. At low frequencies, the imaginary part of impedance sharply increases and the plot tends to be a vertical line. However, departure from the ideal capacitive behavior is observed. In the case of the Nyquist plot presented in Fig. 14c, a small semicircle at high frequencies is recorded. Cyclic voltammograms recorded for this layer show traces of faradaic current at
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Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲 The equivalent circuit of an electrode covered by a C60 /Pd film containing palladium nanoparticles can be represented by resistance and capacitance that are functions of the pulsation 68
The impedance of such a system is defined by the following equation Z共兲 =
1 j ⫻ C共兲
关5兴
The capacitance C共兲 can be written under its complex form C共兲 = C⬘共兲 − jC⬙共兲
关6兴
The real C⬘共兲 and imaginary C⬙共兲 part of capacitance are expressed by the following equations70 Figure 13. Dependence of the mass of Au/quartz electrode on the cycle number during C60 /Pd film deposition in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 8.46 mM Pd共ac兲2, and 0.10 M tetra共nbutyl兲ammonium perchlorate. The sweep rate was 50 mV/s.
potentials more negative than about −800 mV 共Fig. 14a兲. This faradaic process is responsible for the semicircle in the Z⬘–Z⬙ plot.
C⬘共兲 =
− Z⬙共兲 兩Z共兲兩2
关7兴
C⬙共兲 =
Z⬘共兲 兩Z共兲兩2
关8兴
The changes in the real and imaginary parts of the capacitance are shown in Fig. 15 as functions of frequency. These plots were constructed on the basis of impedance data obtained for the film formed in solutions containing 0.27 mM C60 and 9.13 mM Pd共ac兲2 共Fig. 14兲. The specific capacitance of the film calculated from the limiting value of C⬘共兲 obtained for low frequencies at potential −550 mV is 17 F/g. This value is slightly lower than the specific capacitance obtained from cyclic voltammetry 共Fig. 15a兲, which is 25 F/g. A higher limiting value of the real part of the capacitance was obtained for impedance data collected at −1100 mV. In this case, the higher value of C⬘共兲 in the low-frequency range is related to the small contribution of the pseudocapacitance due to the faradaic process of the film reduction. Figure 15b shows the evolution of the C⬙共兲 component on frequency. The imaginary part of capacitance goes through a maximum. From the frequency of the C⬙共兲 maximum, a time constant o was calculated. This time constant is associated with the dielectric relaxation time. Small values of o indicate that the system passes the frontier between resistive and capacitive behavior relatively quickly. Conclusions
Figure 14. 共a兲 Cyclic voltammogram and Nyquist diagrams obtained at 共b兲 −550 mV and 共c兲 −1100 mV for C60 /Pd film in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate. The film was formed under cyclic voltammetry conditions in acetonitrile-toluene 共1:4, v:v兲 containing 0.27 mM C60, 9.13 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate.
The redox-active C60 /Pd films are shown to be very promising electrode materials for production of supercapacitors. Depending on the conditions of film formation, two kinds of capacitors are formed. Films formed from the solutions with a low ratio of 关Pd共ac兲2兴/关C60兴 exhibit large capacitance due to the contribution from the faradaic pseudocapacitance. These capacitors function in the negative potential range where reduction of the fullerene occurs. They do not display capacitive behavior in the positive potential region. The microporous structure of the film allows for swelling of the polymer with solvent and transport of ions between solution and polymeric phase. The capacitance of this polymer is strongly affected by the solvent and supporting electrolyte. The C60 /Pd film is electrochemically active both in protic and aprotic media. The highest values of specific capacitance were found for acetonitrile. In this solvent, voltammograms show pseudorectangular cathodic and anodic profiles that are mirror images of one another, characteristic behavior for an ideal capacitor. The current linearly depends on the sweep rate. The sizes of the cations in the supporting electrolyte influence the value of the specific capacitance of these C60 /Pd films. For
Journal of The Electrochemical Society, 154 共4兲 K1-K10 共2007兲
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relatively high specific capacitance of the C60 /Pd films, there are additional advantages making them desirable for building charge storage devices. The C60 /Pd polymers can be easily formed either by electrochemical51,53-55 or chemical methods in solution.71-73 The electrochemical properties of the film can be tuned by changing the composition of the grown solution. They exhibit high conductivity in the reduced state. They can also be used in nonaqueous solvents and the range of cell voltage can be extended. In many organic solvents, the C60 /Pd films exhibit a high potential range of stability. Finally, in contrast to other electroactive polymeric materials used in literature,4-37 the C60 /Pd films are electrochemically active in the negative potential range, thus fulfilling the n-doping anode electroactive material needs. Films formed from solutions with a high ratio of concentration of Pd共ac兲2 to the concentration of C60 exhibit lower permeability to the ions of the supporting electrolyte. In this case, the charge related to the process of polymer reduction and therefore the capacitance of the polymer in negative potential range decreases. However the polymer becomes conductive in the whole potential range with increased stability. Acknowledgment Support from the Polish State Committee for Scientific Research 共grant 3T09A04626 to K.W.兲, National Science Foundation 共grants CHE0413857 to A.L.B. and 0453464 to F.D.兲, and the Petroleum Research Fund administered by the American Chemical Society is gratefully acknowledged. University of California, Davis assisted in meeting the publication costs of this article.
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Figure 15. Dependence of the 共a兲 real part of capacitance, 共b兲 imaginary part of capacitance, and 共c兲 phase angle on frequency for C60 /Pd film in acetonitrile containing 0.10 M tetra共n-butyl兲ammonium perchlorate at −550 mV. The film was formed under cyclic voltammetry conditions in acetonitriletoluene 共1:4, v:v兲 containing 0.27 mM C60, 9.13 mM Pd共ac兲2, and 0.10 M tetra共n-butyl兲ammonium perchlorate.
12. 13. 14. 15. 16. 17.
tetra共methyl兲ammonium ions a specific capacitance of ca. 295 F g−1 was found. This value is in the range of the values of specific capacitance obtained for other typical polymers. High values of specific capacitance of C60 /Pd films were also obtained for acetonitrile solutions containing alkali metal cations. For example, the pseudocapacitance of the C60 /Pd film is equal to 375 F g−1 in a solution containing Cs+ ions. A simulation of the measured impedance behavior of these systems was possible using an equivalent circuit model, which takes into account the double-layer capacitance, the process of electrochemical reduction of the C60 moieties, the Warburg impedance of the counter ions’ migration into the polymeric phase during its reduction, and the capacitance of the polymer micropores. The impedance measurements revealed that both double-layer capacitance and pseudocapacitance contribute to the large specific capacitance of the C60 /Pd materials. Apart from the
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