t in

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of suryingwith

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charge and discharge, to obtain a large amount of electric charge. A terface must be realized. Therefore, an appropriate preparation pro-

l Soc00 Ttypical supercapacitor has two electrodes, made of high surface areacarbon, and an aqueous or nonaqueous electrolyte with a porousseparator between them. In most commercial supercapacitors, tetra-ethylammonium tetraflouroborate in acetonitrile or propylene car-bonate is used as the organic electrolyte,1-5 while in others sulfuricacid or potassium hydroxide is used as the aqueous electrolyte.1,2,6Many research efforts and studies in this field have been addressedto the development of new combinations of nonaqueous organicelectrolytes and solvents, and less attention has been paid to thedevelopment of solid polymer electrolytes as alternative materials tononaqueous electrolytes. The introduction of polymer electrolytes insupercapacitors could provide several advantages in the realizationof devices, such as a more flexible structure, a more compact geom-etry, and easier packaging.7-9 Polymer electrolytes, which could pro-vide these characteristics, are perfluorosulfonate ionomers likeNafion. Nafion has high ion conductivity (.5 3 1022 Scm21) atroom temperature, low volatility, and remarkable stability. Theseproperties make it a promising electrolyte for the realization of anall-solid double-layer capacitor with appreciable performances. Todate, most known industrial applications of perfluorosulfonate elec-trolyte membranes are those in chloroalkali and electrodialysisprocesses.10 More recently, these electrolytes have been strongly

11-14

cedure requires a well-mixed dispersion of carbon powder and poly-mer electrolyte precursor ~Nafion in solution! before layerformation. The layer preparation is realized by spraying the well-dispersed ink on the conductive substrate. The all-solid supercapaci-tors are realized by coupling two electrodes with a Nafion electro-lyte membrane. Clearly, the composition of the carbon layer and thepreparation procedure of the carbon/Nafion electrodes must be opti-mized.

In this paper, the preparation procedure of electrodes based onthe spraying method is presented and discussed, and the effects ofpolymer electrolyte loading in electrodes on supercapacitor perfor-mances are evaluated by electrochemical characterizations.

Experimental

Preparation of electrodes.All the electrodes used in the EDLCexperiments were prepared by the spraying method. The followingmaterials were used in the electrode preparation: Norit SA Superactivated carbon ~Brunauer, Emmett, and Teller method surface area1150 m2/g!, 5% Nafion 1100 DuPont ionomer solution, AvCarb1071 HCB carbon cloth, and N,N-dimethylacetamide ~DMA!. Theactivated carbon was generously furnished by Norit Italia SpA,Influence of Nafion Contenof Carbon SupercapacitorsF. Lufrano,z P. Staiti, and M. MinutoConsiglio Nazionale delle Ricerche-Istituto di Te98126 S. Lucia-Messina, Italy

The effects of Nafion loading in electrodes on the performancebased on carbon material were prepared with Nafion loading vaby electrochemical impedance spectroscopy. The capacitorscapacitance performances. A maximum specific capacitance owith 10% Nafion loading in the electrode. Impedance spectrNafion electrolyte and of the carbon/Nafion layer of the electra resistance about 50% higher with respect to that with 10%the former electrode is only 20% lower. The similar performanconventional devices, based on liquid electrolytes, is likely dbound water surrounding the Nafion electrolyte in the electrocycles, in cyclic voltammetry mode, showed variations of ,3 2003 The Electrochemical Society. @DOI: 10.1149/1.1626

Manuscript submitted April 26, 2003; revised manuscript rece

Electrochemical double-layer capacitors are energy storage de-vices that can be used for applications that require higher powercapabilities than rechargeable batteries and higher specific energiesthan conventional capacitors.1,2 The storage mechanism in superca-pacitors consists mainly of two types of processes, a purely capaci-tive and a pseudocapacitive process. The former is based on theelectric charge separation at the electrode/electrolyte interface~double layer!, the latter on electrochemical reactions occurring onthe electrodes ~faradaic process!. In double-layer capacitors, the ca-pacitance performance exhibited by the devices is strongly depen-dent on the nature of the electrode/electrolyte interface. Generally,the larger the specific surface area of carbon in the electrodes, thehigher the capability of accumulation of electric charges at the in-terface, and thus the higher the capacitance. However, high surfacearea is not a sufficient condition to achieve high capacitance; thecarbon must also contain a large fraction of mesopores. The charge~discharge! mechanism in an electrochemical capacitor must involvean easy access of electrolyte into the carbon pores, possible only inthe presence of macro- and mesopores, which permit a high rate of

Journal of The Electrochemica0013-4651/2003/151~1!/A64/5/$7.

A64investigated for fuel cell applications. Nafion is a well-known

z E-mail: lufrano@itae.cnr.itElectrodes on Performance

ie Avanzate per lEnergia Nicola Giordano,

percapacitors have been investigated in this work. Electrodesfrom 10 to 50 wt %, and the optimal loading was investigatedlower Nafion content had higher conductivities and betterF/g ~carbon of the electrode! was achieved for the capacitory was also used to evaluate separately the resistance of theThe results showed that the electrode with 50% Nafion gives, even if the specific capacitance of the supercapacitor usingibited by these all-solid supercapacitors compared to those ofthe high conductivity of Nafion and the contribution of thelife cycling test carried out on a supercapacitor for 20,000

capacitance.ll rights reserved.

June 16, 2003. Available electronically December 9, 2003.

ion-exchange membrane produced by DuPont, characterized by verystable perfluorinated polymer chains, with outstanding chemical andelectrochemical properties. The Nafion membrane, which containssulfonate groups, exhibits a proton conductivity comparable to thatof dilute acid solutions ~e.g., 0.1 M H2SO4) and lower than that ofmore concentrated liquid electrolytes ~e.g., 3 M H2SO4).15 There-fore, it could also be utilized as an alternative to replace the tradi-tional electrolytes in electric double-layer capacitors ~EDLCs!.Moreover, as the Nafion is both electrolyte and binder, it can be usedto enhance the capability of electric charge separation and to impartmechanical stability to the electrodes. These claimed advantageshave been preliminarily considered and evaluated in a previous pa-per, which reported the results obtained by a new type of all-solidEDLC that utilized Nafion as the electrolyte.16 In particular, withthis supercapacitor, a capacitance performance of about 70% of thatmeasured for a capacitor that utilized similar electrodes with a 1 Msolution of sulfuric acid as electrolyte was obtained.

To prepare efficient electrodes with solid polymer electrolytes, ahomogeneous carbon/Nafion layer and large electrode/electrolyte in-

iety, 151 ~1! A64-A68 ~2004!he Electrochemical Society, Inc.Ravenna, Italy, the AvCarb 1071 HCB carbon cloth was purchasedfrom Ballard Material Products, Lowell, MA, and both the Nafionsolution and DMA solvent were purchased from Aldrich. As a firststep, an ink containing activated carbon, DMA, and Nafion solution

l Socwas prepared with a carbon/DMA ratio of 1:10 and (carbon1 Nafion)/solvent ratio of 0.08 6 0.01. The ink was blended in anultrasonic bath for 30 min, then sprayed with an air spray gun ~0.5mm orifice nozzle! on the carbon cloth substrate, and dried. Thedesired amount of carbon/Nafion loading is obtained by repeatedspraying and drying steps. The carbon cloth in these electrodes actsas a mechanical support and an electronic current collector. Theso-prepared carbon/Nafion electrodes were dried and pressed at70C and then thermally treated for 1 h at 120C and for 20 min at160C to impart mechanical strength. After the thermal treatment,the electrodes were rinsed several times in water and then chemi-cally treated in 1 M H2SO4 solution to ensure that all the metalliccations, which were attracted to the sulfonate groups during thepreparation process, were completely exchanged with protons. Theelectrodes were finally washed in warm distilled water to eliminatethe free sulfuric acid adsorbed in the porous structure. To study theeffect of Nafion loading on supercapacitor performance, electrodeswith 10, 20, 30, and 50 wt % Nafion ~in the dry carbon layer! wereprepared. The carbon loading in the active layer was 8.26 0.9 mg/cm2 for all prepared electrodes. By visual evaluation, theelectrodes prepared by this spraying technique were characterizedby a uniform distribution of the carbon/Nafion agglomerates in thelayer, and they showed a good mechanical strength on handling.

Polymer electrolyte membrane.Samples of Nafion 115 mem-brane produced by DuPont were utilized in all the experiments as anelectrolyte separator between the electrodes. The water-swelledmembrane was approximately 160 mm thick. It was purified in 3 wt% hydrogen peroxide for about 1 h at 70C to remove organic im-purities. It was then treated in 1 M sulfuric acid solution at 70-80Cto remove metallic impurities. The membrane was finally rinsed indistilled water.

Membrane and electrode assembly.The membrane electrodeassembly ~MEA! was obtained by contacting a membrane and twoelectrodes face-to-face. The contact was realized by a hot-pressingprocedure carried out at 100 kg/cm2 and 130C for 10 min. The dryassembly obtained after hot-pressing was rehydrated by immersionin 1 M H2SO4 solution and subsequently washed in warm distilledwater. The different humidified assemblies were inserted in the testcell for the electrochemical characterization. In the text they arenamed NSN10, NSN20, NSN30, and NSN50, being formed respec-tively of pairs of electrodes with 10, 20, 30, and 50% Nafion in thecarbon composite layer. The same names are also used to identifythe respective capacitors. The assemblies exhibit good mechanicalcharacteristics, and they do not require particular precautions in han-dling during insertion in the test cell.

Test cell and electrochemical characterizations.All the electro-chemical tests were performed at room temperature using a test cellcomposed of two graphite end plates connected with some specificanalytical instruments. Insulating adhesive rubber gaskets were fixedon both the internal faces of the two graphite end plates to preventlateral short circuits, to delimit the central region where the assem-bly was positioned, and to seal the assembly in the cell to avoiddrying of the electrolyte during the test. Conductivity data wereobtained by measuring the internal resistance of the assemblies andsometimes of the electrodes with an impedance analyzer and a uni-versal bridge LCR meter at 1 kHz.

The electrochemical characteristics were evaluated by cyclic vol-tammetry ~CV! using AMEL equipment composed of a high powerpotentiostat, model 2055, an integrator, model 731, and a functiongenerator, model 568. The CV measurements were carried out inpotentiodynamic mode at different scan rates in a range from 10 to40 mVs21 and in the voltage window 0-1 V. At least 100 cycleswere made at a constant scan rate for each capacitor before register-

Journal of The Electrochemicaing the experimental results shown and discussed in the next section.A life cycling test in CV mode at a scan rate of 40 mVs21 was

carried out on capacitor NSN30 for more than 20,000 cycles toevaluate the cycling stability.Electrochemical impedance spectroscopy ~EIS! measurementswere performed at ambient temperature using a test cell in superca-pacitor configuration. The electrochemical cell was connected to apotentiostat ~PGSTAT30, Autolab/Eco Chimie NL! with a frequencyresponse analyzer ~FRA2! module interfaced to a PC. Electrochemi-cal impedance software ~by Autolab! was used to carry out the im-pedance measurements between 10 MHz and 1 mHz. The amplitudeof the sinusoidal voltage used in the tests was 10 mV. All the elec-trochemical characterizations and EIS measurements were carriedout on 4 cm2 MEAs.

Results and DiscussionThe CV curves carried out at 10 mVs21 on the different capaci-

tors are shown in Fig. 1. The current plots are normalized for theweight of carbon in the electrode. From the shape of the curves, it isevident that redox effects in the potential range from 0 to 1 V are notpresent; i.e., a purely capacitive storage mechanism occurs. As re-ported in the literature, sometimes redox processes, determined bysurface functional groups or low level impurities, can be present inhigh surface carbons.17,18 A comparison of the curves shows verylittle variation in charge/discharge current from the supercapacitorswith Nafion loading in electrodes varying from 10 to 30%, while alower current is obtained by the capacitor with 50% Nafion. Theanalysis of the curves shows a quasi-rectangular ideal shape. Fromthe rectangle drawn around the curve of the sample at 10% Nafion,it is possible to identify the regions where larger deviations fromideal behavior are present. The main causes of these deviations arelikely due to the distributed resistance in the porous electrodes,which appears after voltage inversion when the electric currentshould instantaneously change sign. The influences of electrolyteand distributed resistance in the different carbon electrodes havebeen analyzed and discussed by interpreting the impedance behaviorof complete supercapacitors.

Nyquist impedance plots, related to the tests carried out on thedifferent samples in capacitor configuration, are reported in Fig. 2.The inset in the figure reports the plots at high frequencies of thecapacitors having electrodes with Nafion loading ranging from 10 to50%. The curves show a slope of 45 in the region of higher fre-quency, which is a consequence of the distributed resistance andcapacitance of typical porous electrodes. At lower frequencies, theplots assume a shape close to that of an ideal capacitor, with analmost vertical line. The resistances of the different capacitors,

Figure 1. CV curves of capacitors with 10, 20, 30, and 50 wt % Nafionloading in electrode. The current is normalized for the weight of activecarbon material in the electrode. Voltage sweep rate: 10 mVs21.

iety, 151 ~1! A64-A68 ~2004! A65evaluated from the Nyquist plots, indicate that the capacitors withlower Nafion content are less resistive. However, at low frequencies,a clear difference is not evident because the data points in the plots~at the same frequencies! are very close.

l SocIn Fig. 3, the specific capacitance as a function of the frequencyfrom about 6 kHz to 2.33 mHz is shown for the different capacitors.The values of specific capacitance are obtained using the imaginarycomponent of the impedance (Z9) in the following expression: C5 21/(2p f Z9). The values of capacitance ~F/g! are normalized forthe weight of the active carbon material in the electrodes. The trendsof the capacitances indicate clearly that the capacitor with lowerNafion content displays better performance, even if the differencesof capacitance showed by other capacitors are not large ~less than20%!. Electrodes with Nafion loading lower than 10% were notprepared because they are mechanically less stable and difficult tohandle. Thus, any small improvements in capacitance performancethat could be obtained are not justified by the realization of elec-trodes with worse mechanical characteristics. At higher frequencies~.100 Hz!, because of the prevailing influence of the electrolyteresistance, the behavior of the supercapacitors is like that of an idealresistor, and the capacitance is very low. In frequencies from 100 Hzto 100 mHz, the supercapacitors show a transition behavior from

Figure 2. Nyquist plots of capacitors with 10, 20, 30, and 50 wt % Nafionloading in electrodes. The inset shows the high frequency region of imped-ance.

Journal of The ElectrochemicaA66Figure 3. Specific capacitance ~F/g! for the weight of active carbon materialin the electrodes as a function of the frequency.resistor to capacitor typical of an EDLC based on porous carbonmaterial. At frequencies ,10 mHz, the capacitances approach a pla-teau, and at this time scale, the electric signal reaches the maximumpossible penetration in the pores of the carbon.

To further show the characteristics of the different capacitors, thecapacitance data were reported in F/g ~active carbon material forelectrode! and F/cm2 as a function of the Nafion content in theelectrodes ~Fig. 4!. The capacitance data in terms of F/g decreasewith Nafion loading, whereas expressed in F/cm2 they do not reveala definite trend; they depend only on the carbon loading of theelectrodes.

To better show the effect of Nafion content of electrodes oncapacitor performance, the variations of thickness (Dl), resistance(DR), and time constant (Dt) vs. Nafion content are reported in Fig.5. The percentage variations were calculated taking the electrodewith the 10% Nafion as a reference. Therefore, at constant carbonloading, the increase in the Nafion loading increases the thickness ofthe electrodes. The electrode resistance and the time constant of thecapacitors are calculated taking into account electrode thickness andcapacitance ~see Fig. 5!. The values of the electrode thicknesses areobtained as average values between that obtained from direct mea-surement and that calculated as a proportional increase in totalweight of electrodes, i.e., as a sum of the weight of carbon andNafion. The Nafion loading of the carbon/Nafion layer influences theresistance of the different capacitors as evidenced by the Nyquist

Figure 4. Capacitance in F/g ~weight of carbon in electrode! and in F/cm2 asa function of Nafion content in electrode.

iety, 151 ~1! A64-A68 ~2004!Figure 5. Percentage of variation of thickness (Dl), resistance (DR), andtime constant (Dt) vs. Nafion content in carbon electrodes with respect to theelectrode with 10% of polymer electrolyte.

l Socplots ~Fig. 2! as well as the time constant of the capacitors (RC),which that increases from 0.35 to 0.53 s with the Nafion content inthe electrodes.

As a further step, the different contributions to the total resis-tance values are represented in Fig. 6. In this figure, we have con-sidered the Nafion 115 membrane to have an absolute value of re-sistance of 0.17 Vcm2 ~0.095 Scm21! at room temperature,according to the literature.19-22 This value corresponds to that of ahighly swelled membrane, free from contact resistances or otherpossible problems of the resistance measurement. In Fig. 6, the re-sistances of the different capacitors are displayed, as contributionsof both membrane ~Nafion 115! and electrode resistances, as a func-tion of the thickness of the carbon layer of the electrodes. On twosamples of electrode ~20 and 50 wt % Nafion!, an additional mea-surement of resistance was carried out by a bridge LCR meter. Goodagreement of the resistance values measured with the different meth-ods has been obtained. Figure 6 also reports the range of fluctuationof resistance values as a function of the thickness of the variouselectrodes with different Nafion loading. Almost all values obtainedshow some oscillation; we believe that the data also may be influ-enced by material composition or loading. The resistance increaseswith the electrode thickness. The calculated resistivity of the elec-trodes varies between 2 to 4 Vcm as the Nafion loading increasesfrom 10 to 50%. These values of resistivity, as well as the thick-nesses of the electrodes, are less accurate than the measurement ofsimple resistance because many errors can occur in the evaluation ofthe thickness. For example, the carbon cloth substrate changes thick-ness with pressure. It is useful to specify that measured values in-clude the contact and carbon cloth resistance.

The real impedance ~resistance! as a function of the frequencyfor the capacitors with different amounts of Nafion loading is re-ported in Fig. 7. The resistance behavior of these supercapacitors isgreatly influenced by the porous nature of carbon electrodes; e.g., anadditional distributed resistance is generated along the thickness ofthe electrodes which may be due to the low accessibility of ions ofthe electrolyte in the smaller pores and could include the resistancesof the interface, i.e., due to carbon/Nafion and/or carbon/carbon con-tact. The increase in resistance with decreasing frequency, from a

Figure 6. Resistance ~Vcm2! of the different capacitors. Abscissa reportsthe sum of the two carbon/Nafion layer thicknesses for each capacitor with-out carbon cloth substrate. Measurements were carried out using universalbridge ~B LCR! and impedance ~Imp! on the complete capacitors ~D, h! andon the electrodes ~s!. The horizontal line at 0.17 Vcm2 represents theNafion 115 resistance at room temperature.

Journal of The Electrochemicaphysical point of view, can be seen as the difficulty of penetration ofthe electric signal into the deeper pores ~filled with electrolyte!and/or in the smaller particles. We can imagine that at very lowfrequencies ~mHz range!, some regions of electrode with a lowamount of electrolyte or with poor particle contact do not take partin the charge separation in the electric double layer. A differentexplanation may be that the deeper carbon pores, not containingpolymer electrolyte but filled with water, give, in the proximity ofthe electrolyte, a minor contribution to the ion conductivity. In Fig.7, the resistance of the capacitor with 50% Nafion is lower thanthose of all other samples at lower frequencies ~,10 mHz!, while anopposite trend is observed at higher frequencies ~see inset in Fig. 2!.These observations could explain the small differences in capaci-tance ~in the range from 90 to 110 F/g! shown by capacitors withdifferent Nafion content, whereas in the literature larger changes inspecific capacitance occurred by using increasingly nonconductivepolymer binders in the electrodes. Accordingly, Osaka23 showed thatin the range 10-50 wt % polyvinylidene fluoride gel electrolyte, thecapacitance of electrodes may vary up to 80%. Similarly, Richner24found that by increasing the carboxymethyl cellulose binder in elec-trodes from 5 to 15 wt %, a decrease in capacitance of about 30%was observed. In this study, with Nafion loading varying from 10 to50%, a more restricted variation of specific capacitance is obtained;accordingly, the polymer electrolyte appears to have a beneficialeffect on the performance of the supercapacitor.

The highest attained performance is about 110 F/g ~active carbonmaterial for electrode! for 1150 m2/g surface area carbon. This valueis not far from that obtainable with capacitors based on a liquidelectrolyte.

Many studies1,25,26 have shown that the carbon microporosityhinders the achievement of high specific capacitance due to the dif-ficulty of penetration of the electrolyte into the smaller pores. For anorganic electrolyte, for which the size of ions is much larger thanthose in aqueous electrolytes, only access to meso- and macroporesmay occur. For Nafion, the formation of self-aggregates of micelleswith large size27 and the hydrophobic nature of their surfaces28,29further hinder the accessibility. Nafion is very likely covering onlythe macropores and the external surface of the carbon. Nevertheless,because the capacitance of Nafion-based supercapacitors is compa-rable to that obtainable with aqueous electrolytes, one can envisagethat the polymer electrolyte extends its ionic conductive effect intothe small pores of carbon through the surrounding water. In practice,the water could behave as a proton carrier between the bulk electro-lyte and the internal surface of carbon pores. Further studies arenecessary to better understand the high rate of proton diffusion andthe ion conduction mechanism occurring in the pores of carbon,which are not in direct contact with the solid electrolyte.

Finally, to verify the reliability and the electrochemical stability

Figure 7. Real impedance vs. frequency for the supercapacitors with differ-ent amounts of Nafion loading in the electrodes.

iety, 151 ~1! A64-A68 ~2004! A67of these supercapacitors, a life cycling test was carried out in CVmode at a scan rate of 40 mVs21 on the capacitor NS30N. It gavea very stable performance for more than 20,000 cycles. Three rep-resentative curves, recorded during the 100th, 10,000th, and

20,000th cycle, are shown in Fig. 8. During the life cycling test,periodic integration measurements were carried out on the dischargeprocess to evaluate the performance stability. A maximum variationof 3% of the specific capacitance values was evaluated by compar-ing numerous measurements. These results demonstrate that super-capacitors that use the carbon-Nafion composite in the active layerof the electrodes exhibit advantages such as high specific capaci-tance as well as excellent mechanical and electrochemical stability.

ConclusionsElectrodes based on carbon and Nafion have been prepared by

the spraying technique. This procedure allows one to obtain elec-trodes with a uniform and homogeneous distribution of carbon/Nafion layer and excellent mechanical characteristics. Electrodeswith 10, 20, 30, and 50 wt % Nafion loading in the carbon layerhave been assembled with Nafion 115 membranes and tested in asupercapacitor configuration. CV measurements have proved thatthe amount of Nafion content, in the range from 10 to 30%, has littleinfluence on specific capacitance; however, the capacitor with 50%Nafion in the electrodes exhibits lower capacitance performance dueto its higher internal resistance. EIS analysis has shown the highestspecific capacitance ~110 F/g! for the capacitor with 10% Nafionloading; a lower performance is exhibited by the capacitor with 50%Nafion ~90 F/g!. This is attributable to the high resistance and lowaccessibility of the electrolyte into the carbon pores. However, only20% of capacitance was lost despite an increase in resistance ofabout 50% for this capacitor. These results demonstrate that a lowresistance favors the achievement of high specific capacitance. Theuse of Nafion electrolyte ~also as a binder! in carbon electrodes hasa beneficial effect, attaining high conductivity as well as high ca-pacitance performance. As an interpretation of these results, onesupposes that the bound water surrounding the Nafion clusters actsas a carrier for proton transport from bulk electrolyte to the smallercarbon pores.

The interesting capacitance performance of supercapacitors isalso supported by excellent reliability and electrochemical stability,

Figure 8. Representative CV curves recorded during life cycling test onsupercapacitor with 30% Nafion loading in the electrodes ~NSN30!. Voltagesweep rate: 40 mVs21.AcknowledgmentsThe authors acknowledge Professor P. Antonucci ~University of

Reggio Calabria! for helpful discussions and comments, Dr. A. S.Arico` of Consiglio Nazionale delle Ricerche ~CNR-ITAE!-Istituto diTechnologia Avanzate per lEnergia, for his help in impedance mea-surements, and CNR for financial support.

CNR-ITAE, Istituto di Technologie Avanzate per lEnergia Nicola Gior-dano assisted in meeting the publication costs of this article.

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trochem., 23, 86 ~1993!.29. R. S. McLean, M. Doyle, and B. B. Sauer, Macromolecules, 33, 6541 ~2001!.as demonstrated by the life cycling test for a 30% Nafion capacitor.The life cycling test carried out for more than 20,000 cycles ~CVmode at 40 mVs21! demonstrated stable capacitance performance.A 3% maximum variation in specific capacitance values was regis-tered during the whole period of the cycling test.

Journal of The Electrochemical Society, 151 ~1! A64-A68 ~2004!A68