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Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲 A617 0013-4651/2005/152共3兲/A617/5/$7.00 © The Electrochemical Society, Inc. Design, Fabrication, and Evaluation of a 1.5 F and 5 V Prototype of Solid-State Electrochemical Supercapacitor P. Staitiz and F. Lufrano CNR-ITAE, Istituto di Tecnologie Avanzate per l’Energia ‘‘Nicola Giordano,’’ 98126 S. Lucia Messina, Italy The scale-up from a small single cell to a larger stack prototype of a solid-state electrochemical supercapacitor based on polymer electrolyte and activated carbon is demonstrated. The prototype is composed of five single cells stacked in series, has a nominal capacitance of 1.5 F, and a maximum voltage of 5 V. The electrodes of the prototype were prepared using high-surface-area carbon material 共Norit A Supra Eur兲 and Nafion ionomer. Nafion was used as the electrolyte membrane separator between the electrodes of each single cell and as the binder/ion conductor in the electrodes. The fabricated prototype showed a higher series resistance compared to that estimated in our previous study of a small size single supercapacitor. However, the prototype achieved a specific capacitance of 114 F/g 共referred to the weight of active carbon materials for a single electrode兲, which is comparable to the specific capacitance of the previously reported 4 cm2 single-cell supercapacitor. Moreover, an appreciable power density of 1.4 kW/L and a RC-time constant of 0.3 s have been calculated for the device. © 2005 The Electrochemical Society. 关DOI: 10.1149/1.1859614兴 All rights reserved. Manuscript submitted July 12, 2004; revised manuscript received August 5, 2004. Available electronically January 31, 2005. Electric double-layer capacitors 共EDLCs兲 or supercapacitors 共SCs兲 are promising energy storage devices being considered for applications in areas such as electric vehicles, uninterruptible power systems, and computer memory protection. Their main characteristic is their excellent high-rate charging-discharging ability that makes these devices, referring to this specific aspect, more efficient than batteries and fuel cells. High-surface-area carbons are the active materials commonly used for supercapacitor electrodes. The capacitance of the electric double layer of charges at the electrode/ electrolyte interface is proportional to the available surface area of carbon composite electrodes. Other parameters such as electrolyte accessibility in the carbon pores and electric conductivity of the entire device are important figures of merit too. A typical supercapacitor has two electrodes, made of highsurface-area activated carbon materials and, an aqueous or nonaqueous electrolyte impregnated in a porous separator stacked between the two electrodes. An aqueous electrolyte is a low-cost and high ionic conductivity component that provides a high supercapacitor power density. A nonaqueous electrolyte provides a high achievable voltage leading to a high supercapacitor energy density. Both types of devices have been used in the EDLC manufacturing for pulse power application or stable current operation systems, respectively. This paper describes the performance of a carbon and Nafion polymer-based supercapacitor developed for pulse power applications. Water is the solvating agent of ionic species and the charge carrier in the solid polymer electrolyte; therefore, the device may be considered an aqueous type of supercapacitor. Differently from commonly used acidic aqueous solutions, the acid electrolyte in solid polymer form shows reduced corrosion of the auxiliary components and no leakage of dangerous liquid from the device is possible. Thus, this device has a high level of safety. Our recent research on this type of supercapacitor, in single-cell configuration, achieved interesting results in terms of electrochemical performance and stability.1-5 From the point of view of electrochemical performance, supercapacitor based on a Nafion polymer electrolyte and on electrodes containing a Norit activated carbon material gave a specific capacitance as high as 130 F/g 共referred to the weight of activated carbon material in the electrode兲.3 Moreover, the internal resistance of the capacitor with Nafion electrolyte was comparable to that measured with a 1 M H2 SO4 solution.2,3 The solid polymer electrolyte used in the experiments 共Nafion, from DuPont兲 is different than other gel-like materials tested as supercapacitor electrolyte.6-9 The latter includes materials obtained by mixing liquid electrolyte with inert supports, for instance, phosphoric acid with nylon,6 phosphoric acid-doped silica gel and polymers,7 perchloric acid-doped silica,8 z E-mail: [email protected] phosphoric acid and polyvinyl alcohol,9 and TEABF4 with propylene carbonate 共or ethylene carbonate兲 and poly共vinylidene fluoride-hexafluoropropylene兲.10 Such gel-like electrolytes exhibited insufficient mechanical strength, lower ion conductivity, and lower dimension stability in solvents than solid polymer electrolyte. Solid polymer electrolytes, like Nafion used in this work, consist of an inert hydrophobic backbone, and highly hydrophilic terminal functional groups attached to the backbone are homogeneous materials with outstanding mechanical strength, exhibiting high ionic conductivity in their highly hydrated form. The EDLC prototype presented in this paper has been fabricated by stacking five cells and can be charged and operated up to 5 V. The goal of our design and prototype fabrication is to highlight the problems arising from the scale-up process from a single cell to a multicell stack as well as from a smaller to a larger geometric area. The paper discusses and attempts to explain the problems encountered in the scaling-up process. Moreover, the performance of the electrodes are compared to that of electrodes previously studied in the small, 4 cm2 single cell. Experimental Preparation of electrodes.—The electrodes for the supercapacitor prototype were cut from a larger electrode prepared by a casting method consisting of spreading onto a glass plate with a film applicator the ink containing the activated carbon, the Nafion ionomer electrolyte, the graphite fibers, and the N,N-dimethylacetamide 共DMAc兲 solvent. The activated carbon powder 共Norit A Supra Eur兲 used in the ink preparation has been furnished by Norit Italia S.p.A. 共Ravenna, Italy兲 and had a surface area of 1500 m2/g. The graphite fibers were cut 400-500 m in length with a high-speed grinder from a carbon fabric 共Avcarb 1071兲 furnished by Ballard Material Products, Inc. 共MA, USA兲. The presence of graphite fibers in the final composite electrode allowed the formation of an electrically more conductive and mechanically stronger composite film. The Nafion ionomer 共a DuPont product兲 intermixed with the carbon in the electrode is acting as the binder of carbon materials 共powder and fibers兲 and as the ion conductor. The composite electrode film was obtained after drying the ink cast onto the glass plate at 70°C for 5-6 h. Further thermal treatments at 120°C for 1 h and at 160°C for 20 min were made to produce an electrode with adequate mechanical strength and insolubility of polymer binder in water. Subsequently, the electrode was rinsed several times in distilled water and then chemically treated in 1 M H2 SO4 solution to obtain the material free from contaminant ionic species eventually attracted by sulfonic groups of Nafion during the preparation process. Further washing of electrode in warm distilled water, until neutral pH, was made to eliminate all the free sulfuric acid adsorbed in the porous structure. The final electrode composition was 65 wt % of activated carbon powder, 30 wt % Nafion, and 5 wt % graphite fibers. The carbon A618 Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲 Figure 1. Schematic representation of the supercapacitor prototype. loading was 8.2 ⫾ 0.9 mg/cm2 and the electrode thickness 150 ⫾ 30 m. The electrodes prepared with the casting technique showed uniform distribution of the materials and exhibited excellent mechanical strength. Polymer electrolyte membrane.—The Nafion 115 membrane produced by DuPont was utilized as electrolytic separator between the electrodes. The water-swelled membrane had an approximate thickness of 160 m. It was purified by hydrogen peroxide at 3 wt % for about 1 h at 70°C to remove organic impurities. After three-four washings in pure water to remove H2 O2 and traces of soluble organic impurity, a treatment in 1 M sulfuric acid solution at 70-80°C was carried out to exchange with protons the eventual metallic ionic species attracted on the sulfonic groups of Nafion. The membrane was finally rinsed several times in warm distilled water until the free sulfuric acid was completely eliminated. Membrane and electrodes assembly.—The membrane electrode assemblies 共MEAs兲, which were inserted in the prototype, were preformed out the device by contacting face-to-face the membrane and two electrodes. Each MEA was realized by a hot-pressing procedure carried out at 100 kg/cm2 and 130°C for 10 min. The assembly, after the hot-pressing process, was rehydrated by immersion in 1 M H2 SO4 solution first and then rinsed in warm distilled water. All the assemblies exhibit good mechanical characteristics, and they did not need particular precaution in handling during the stacking of prototype. Preparation of the supercapacitor.—A supercapacitor prototype was fabricated by stacking five well-humidified 16 cm2 MEAs. The assemblies were separated by four bipolar plates of carbon fiber paper 150 m thick 共AvCarb P50T by Ballard Material Products兲. Two end monopolar plates of the same carbon fiber paper interfaced the external cells of stack with the metallic current collectors. These latter were formed of titanium foils having a thickness of 250 m and were externally strengthened by thicker aluminum plates 共thickness 2 mm兲. The use of the metallic current collectors allowed electrical wire connection with testing equipment. The aluminum plate helped the maintainance of titanium foil flatness also during and after the sealing of the device. The prototype was fastened on three sides with a ‘‘U’’-shaped iron that was electrically insulated from the bottoms of the current collectors by a thin poly共tetrafluoroethylene兲 共PTFE兲 foil. The remaining side of the prototype was sealed with silicon rubber. The so-formed case, containing the active elements of the supercapacitor, is completely sealed and thus the drying of the Nafion electrolyte is not allowed. The weight and the volume of the full supercapacitor device were 55 g and 22.5 cm3, respectively. The schematic view and the photograph of the named ITAECAP-1 prototype are shown in Fig. 1 and 2, respectively. Evaluation of the supercapacitor.—Cyclic voltammetry, dc galvanostatic charge/discharge tests, and ac impedance analysis were carried out on the prototype to evaluate its electrochemical and conductivity performances. Figure 2. The 1.5 F-5 V ITAECAP-1 prototype. Cyclic voltammetry 共CV兲 and dc galvanostatic measurements were carried out with AMEL equipment composed of a high-power potentiostat model 2055, an integrator model 731, and a function generator model 568. CV tests were carried in potentiodynamic mode at prefixed voltage scan rates using a range from 100 to 400 mV s⫺1 and using a voltage range between 0 and 5 V. At least 100 cycles have been performed at each constant scan rate before collecting the results, which are shown and discussed in the next section of this paper. Galvanostatic charge-discharge measurements were carried out injecting currents of 50, 100, 150, and 200 mA and applying a voltage varying from 0 to 5 V. For each of these currents, the supercapacitor is charged from 0 to 5 V and discharged from 5 to 0 V in a continuous manner and for several cycles. The amount of charge 共coulomb兲 delivered by the capacitor during the discharge process from 5 to 0 V was measured by an electrical integrator and used to determine the capacitance of the prototype device. The electrochemical impedance spectroscopy 共EIS兲 measurements were carried out on the discharged supercapacitor at ambient temperature using a potentiostat 共PGSTAT30 Autolab/Eco chimie NL兲 with a frequency response analyser 共FRA2兲 module interfaced to a PC. An electrochemical impedance software 共by Autolab兲 was used to carry out the impedance measurements between 10 MHz and 1 mHz. The amplitude of the sinusoidal voltage used in the tests was 10 mV. Results and Discussion CV measurements were carried out to evaluate the electrochemical stability of the electrodes and of the electrolyte by applying a potential varying from 0 to 5 V in which the prototype operates. Figure 3 shows CVs recorded at fixed voltage scan rates of 100, 200, and 400 mV/s in the voltage range from 0 to 5 V. The absence of any current peaks along the profile of the voltammograms is indicative of pure capacitive behavior of the system, i.e., during the charging/ discharging phases in the range 0-5 V; therefore, only the storage of electric charges occurs at the electrode/electrolyte interface. In other words, faradaic processes 共reversible or irreversible redox reactions兲 are absent or negligible in the studied range of voltage; thus, the system can be considered electrochemically stable. To obtain a preliminary evaluation of the capacitance of the prototype the values of current collected during the CV tests were divided by the respective voltage scan rates and reported as capacitance values in Fig. 4. The figure shows the capacitance of supercapacitor as a function of the voltage at the different voltage scan rates. The voltammograms highlight a clear slight dependence of capacitance by the voltage scan rate with a decrease of the former upon increasing the latter. This observed variation could have different explanations, although Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲 Figure 3. CVs of the ITAECAP-1 prototype at various voltage scan rates: 共a兲 100, 共b兲 200, and 共c兲 400 mV/s. A619 Figure 5. Typical charge/discharge curve obtained at constant current of 200 mA. two seem more probable. The first one could be that the internal resistance of the device is somewhat high and at the highest voltage scan rate, when highest current is delivered by the capacitor, a more considerable voltage loss (⌬V) is originated. This voltage loss, which has a resistive nature, is a parasitic process that subtracts part of the capacitance of the device especially at higher current flows; therefore, it should be maintained as low as possible. Unfortunately, the voltage loss could never be totally eliminated. The second possible explanation is that the electric charges could have some difficulty to occupy homogeneously and rapidly all the available sites at the interface electrode/electrolyte due to their limited rate of orientation and migration in the electrolyte 共low local ionic conductivity兲 contained in the narrow pores of electrodes that are farther from the bulk of the electrolyte. Thus, because it is known that the capacitance of the supercapacitor is influenced by the electrical series resistance, this latter should be kept as low as possible in the device by using appropriate building materials 共bipolar plates, current collectors兲, by improving the properties of electrode and electrolyte materials, and by decreasing the contact resistances. A more in-depth analysis of the negative contributions to the ionic and electronic resistances of the prototype performance are reported during the discussion about the impedance behavior of device. Figure 5 shows a dc galvanostatic charge/discharge curve of the prototype at the current of ⫾200 mA as a function of time. The charge amount that was discharged from the supercapacitor at each prefixed electrical current was measured by a charge integrator and used to calculate the capacitance of the device. Figure 6 plots discharge vs. time curves at different discharge currents. It is clear in the figure that there is not proportionality between the increasing currents and the decreasing discharge times, and thus higher capacitances are obtained at lower discharge currents. The dependence of the capacitance of the device from the discharge current is also evidenced in Fig. 7. A decrease of more than 20% of the capacitance arises when the discharge current is increased by four times; thus, the capacitance is 1.45 F at 50 mA and 1.12 F at 200 mA. This variation of capacitance is comparable to that previously determined on small size single-cell supercapacitors.3 The influence of series resistance on prototype performance has been analyzed and discussed by taking into account the impedance spectra. Figure 8 shows the Nyquist plot of the supercapacitor. At high frequency, the supercapacitor behaves like a pure resistor with a minimum of resistance of about 0.2 ⍀. A resistance of 0.64 ⍀ cm2 for the single cell has been calculated taking into account the geometric area of the electrodes 共16 cm2兲 and the number of cells 共five兲 contained in the device. This value, excessively high for this system, is explained by the presence of some unexpected resistances, which add to those of well-humidified membranes and electric conductive components. We believe that these parassitic resistances are due to excessive contact resistance between the interfaces of the components and/or to the distributed resistance of the composite electrodes. The former could be due to nonuniform or insufficient pres- Figure 4. Capacitance vs. potential curves at various voltage scan rates as in Fig. 3. Figure 6. Discharge curves from 5 to 0 V at various currents: 共a兲 50, 共b兲 100, 共c兲 150, and 共d兲 200 mA. A620 Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲 Figure 9. Capacitance as a function of frequency of the prototype. Figure 7. Discharge capacitance of prototype as a function of current. sure exerted by the external plates on the inner components and the latter to an inappropriate pore size distribution in the carbon material. Referring to the latter aspect, Yoon et al.11 found that using a carbon material with a well-ordered pore structure with uniform mesopores of 2.3 nm, a 3.3 times smaller electric series resistance 共ESR兲 as compared to that observed with a conventional activated carbon can be obtained. Notwithstanding the high resistance found in our supercapacitor, the maximum deliverable power 共calculated by P ⫽ V 2 /4R) of the device was 31 W, corresponding, when the volume of the prototype is taken into account, to a maximum power density of 1.4 kW/L. The energy density calculated for 1.5 F device capacitance is 0.23 Wh/L. From the electric serial resistance 共R兲 and the capacitance 共C兲 of the device it is possible to calculate the time constant, , by the expression ⫽ RC. The time constant gives information on the rate of charge and discharge process. The smaller the time constant the higher the charge and discharge rate. The RC time constant of 0.3 s calculated for our device corresponds to a quick chargedischarge of the supercapacitor. Figure 8. Nyquist plot of the prototype. The inset shows the high-frequency region of impedance. In the current literature, few examples of stacked-prototype of supercapacitors have been reported, and this makes a comparison of actual performance with that of our device difficult. Moreover, these stacked prototypes use aqueous electrolyte solutions, while organic electrolytes, which have low specific conductivity 共about 20 mS/ cm兲, are not considered suitable for the fabrication of multicell capacitors. The aqueous electrolytes, that show specific conductivity as high as 800 mS/cm, are penalized by a lower working voltage 共1 V兲 compared to that of a nonaqueous electrolyte 共2.3-2.7 V兲.12 Thus, the higher voltage permits a much higher energy density to be obtained with supercapacitors using organic electrolytes, while the high ionic conductivity favors a higher power density of aqueous supercapacitors. A prototype of multicell module was reported from Hahn et al.12 based on electrodes made by activated glassy carbon. Their 0.4 F-24 V device, which used a 3 M H2 SO4 solution as electrolyte, was characterized by a maximum power density of 15 kW/L and by an energy density of about 0.09 Wh/L. In another paper, the same group reported for an experimental 0.8 F-5 V bipolar glassy carbon capacitor, stable resistance and capacitance performance up to 100,000 cycles.13 Similarly, Kibi et al.14 showed that a high-power electric double-layer capacitor of 470 F–15 V was realized with activated carbon electrodes and 30% solution of sulfuric acid. The device had about 2.5 Wh/L and 2.3 kW/L maximum energy density and power density, respectively. Compared with the results reported in the literature, lower energy and power density have been obtained by our prototype. However, here we are showing for the first time the design, fabrication, and the results for a solidstate supercapacitor in multicell module in which a Nafion 115 membrane is used as the electrolyte. We are remarking that the development of solid-state supercapacitors could give the following advantages: 共i兲 higher flexibility in cell design, (ii) reduced corrosion, (iii) increased safety in handling, and (i v ) probably, longer lifetime. In Fig. 9 the specific capacitance as a function of frequency 共f兲 from 1000 Hz to 5 mHz is shown for the device. The capacitance is calculated from an imaginary component (Z ⬙ ) of impedance by the expression C ⫽ ⫺1/(2 f Z ⬙ ). From the figure, it is evident that there is a continuous increase of capacitance by decreasing the frequency and a plateau in capacitance is never achieved. This means that the electrical signal is not yet reaching the deepest narrow pores of carbon in the electrodes even at very low frequency 共5 mHz兲. A maximum capacitance of 1.55 F is recorded at 5 mHz for the supercapacitor. From this value the energy of 19.4 J is calculated for the supercapacitor, which corresponds to an energy density of about 0.23 Wh/L. Moreover, considering that the capacitance of the five cell prototypes is 1.5 F, the capacitance for a unit area of a single cell is 0.47 F/cm2. By this, taking into account the surface density of carbon 共0.0082 g/cm2 for each electrode兲, a single cell capacitance of 28.6 F/g is calculated. As normally the capacitances of superca- Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲 pacitors are referred to the weight of the single electrode, this value must be multiplied by a factor of four. Therefore, we obtain a capacitance value of 114 F/g, which is comparable to those previously reported in the single-cell supercapacitor.3,4,15 This is an important result because it demonstrates that the capacitance performance in the scale-up from 4 cm2 single cell to the stack prototype is fully maintained. Capacitances of 100–120 F/g are generally considered typical values of well-designed practical supercapacitors using activated carbon materials and liquid electrolytes.16-18 The relatively low energy density 共0.23 Wh/L兲 of our device is influenced by an excessive volume of the external building components and by the dead volume inside the case of the device. Furthermore, it is likely that power and energy densities of this type of supercapacitor may be further improved in the future by decreasing the resistance of the full device and also by optimizing the carbon/Nafion loading, the composition, and the structure of electrodes. The former aspect could be improved by using adhesive paint to reduce contact resistance between the single cells and the bi- and monopolar plates19,20 and/or by balancing the pressure exerted on the single cells with appropriate deformable bipolar plates, and the latter aspect, by using electrodes with carbon having a more appropriate pore size distribution in order to increase the ion mobility and to have a faster charge separation process at the electrode/electrolyte interface. Conclusions A multicell prototype of supercapacitor based on high-surfacearea activated carbon material and solid polymer electrolyte has been successfully fabricated. The prototype, which was made by stacking five single cells, is electrochemically stable in the range of potential between 0 and 5 V. A maximum capacitance of 1.55 F and a resistance of 0.2 ⍀ have been obtained for the device by impedance analysis. The electrical series resistance for the multicell prototype was higher than that measured in previous experiments in single-cell capacitor, most likely due to larger contact resistances between the different components and/or the not fully optimized structure of electrodes. Nevertheless, an RC-time constant of 0.3 s is obtained for the supercapacitor. Values of energy and power density of 0.23 Wh/L and 1.4 kW/L, respectively, have been also calculated for the prototype. A621 A high specific capacitance of 114 F/g 共referred to weight of active carbon materials of the single electrode兲 for the fabricated prototype has been demonstrated, which is comparable to those previously reported for single-cell supercapacitors, confirming the optimal scale-up design of this multicell prototype. Instituto di Tecnologie Avanzate per l’Energia assisted in meeting the publication costs of this article. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. P. Staiti, M. 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