Development and fabrication of a 1.5 F – 5 V solid state

Pietro Staiti and Francesco Lufrano

CNR-ITAE, ISTITUTO DI TECNOLOGIE AVANZATE PER L’ENERGIA “NICOLA GIORDANO”

Via Salita S. Lucia sopra Contesse n. 5

Messina, Italy

Phone: 0039 090 624226 / fax: 0039 090 624247

E-mail: [email protected]

Acknowledgments

The authors acknowledge the Consiglio Nazionale delle Ricerche that has financially supported the research project for the development of this type of supercapacitor.

Keywords

Supercapacitor, energy storage, solid , acidic electrolyte.

Abstract

A five cells supercapacitor prototype with special electrolyte is designed and fabricated at the Institute CNR-ITAE of Messina. It has a nominal capacitance of 1.5 F and a maximum voltage of 5 V. The of prototype are formed of high surface area carbon material and ionomer. Nafion is used as an electrolyte membrane separator between the electrodes of each single cell and as a binder/ conductor in the electrodes. The fabricated prototype achieves specific capacitance of 114 F/g (referred to the weight of active carbon materials for single ), that is comparable to the specific capacitance previously obtained from a smaller scale single cell of same type of supercapacitor. A power density of 1.4 kW/l and a RC-time constant of 0.3 s have been calculated for the device.

Introduction

Electric double layer capacitors (EDLCs) or (SCs) are promising energy storage devices, which are being considered for applications such as electric vehicles, uninterruptible power systems, and computer memory protection. Their main characteristic is the 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 in the electrodes. In fact, in first approximation the capacitance of supercapacitors is proportional at the electrode/electrolyte interface and then dependent from surface area of carbon material. 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 is composed of two electrodes, made of high surface area activated carbon material, and an aqueous or non-aqueous electrolyte impregnated in a porous separator that is stacked between the electrodes. Usually, the aqueous electrolyte is a low cost and high ionic conductivity component that provides a high supercapacitor power density. Instead the nonaqueous electrolyte provides a higher achievable voltage leading to a higher supercapacitor energy density. Both type of devices have been used in the EDLC manufacturing respectively for pulse power application or stable current operation systems. 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 of the auxiliary components and no leakages of dangerous liquid from the device are possible. Thus this device has a high level of safety. Our recent researches on this type of SC, in single-cell configuration, achieved interesting results in terms of electrochemical performances 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 (referred to the weight of activated carbon material in the electrode) [3]. Moreover, the internal resistance of the capacitor with Nafion electrolyte was comparable to that measured with a 1 M H2SO4 solution [2, 3]. The solid polymer electrolyte used in the experiments (Nafion, from Du Pont) 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], phosphoric acid and polyvinyl alcohol [9], and TEABF4 with propylene carbonate (or ethylene carbonate) and poly(vinylidene fluoride-hexafluoropropylene) [10]. Such gel-like 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 bound to the backbone, are homogeneous materials with outstanding mechanical strength and exhibit high ionic conductivity in their highly hydrated form. The EDLC prototype presented in this paper has been fabricated by stacking five cells and was charged 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 multi-cell stack as well as from a small to a larger geometrical area. The paper discusses and attempts to explain the problems encountered in scaling-up process. Moreover, the performance of the electrodes is compared to that of electrodes previously studied in the small 4 cm2 single cell.

Experimental

Preparation of electrodes The electrodes were cut from a larger electrode prepared by a casting method consisting in 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 (DMAc) solvent. The activated carbon powder (Norit A Supra Eur) used in the ink preparation has been furnished by Norit Italia S.p.A. (Ravenna, Italy) and had a surface area of 1500 m2/g. The graphite fibers were cut in length 400- 500 µm with a high speed grinder from a carbon fabric (Avcarb 1071) furnished by Ballard Material Products Inc. (MA, USA). 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 (a Du Pont product) intermixed with the carbon in the electrode is acting as the binder of carbon materials (powder and fibers) and as the ion conductor. The composite electrode film was obtained after drying at 70 °C for 5-6 hours the ink cast onto the glass plate. Further thermal treatments at 120 °C for 1 hour 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 1M H2SO4 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, till neutral pH, was made to eliminate all the free sulphuric acid adsorbed in the porous structure. The final electrode composition was 65 wt.% of activated carbon powder, 30 wt. % of Nafion and 5 wt. % of graphite fibers. The carbon loading was 8.2 ± 0.9 mg/cm2 and the electrode thickness 150 ± 30 µm. 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 Du Pont was utilized as electrolytic separator between the electrodes. The water-swelled membrane had an approximate thickness of 160 µm. It was purified by hydrogen peroxide at 3 wt.% for about 1 hour at 70 °C, to remove organic impurities. After three-four washings in pure water to remove H2O2 and traces of soluble organic impurity, a treatment in 1 M sulphuric 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 till the complete elimination of free sulphuric acid.

Membrane and electrodes assembly The membrane and electrode assemblies (MEAs), which were inserted in the prototype, were pre- formed 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 re-hydrated by immersion in 1M H2SO4 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.

Titanium & aluminum External plate Monopolar plate Electrodes Membrane

Bipolar plate

Monopolar plate

Fig.1. Schematic representation of the supercapacitor 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 µm thick (AvCarb P50T by Ballard Material Products). Two end monopolar plates of the same Ballard 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 µm and were externally strengthened by thicker aluminum plates (thickness 2 mm). The use of the metallic current collectors allowed electrical wire connection with testing equipment. The aluminum plate helped the maintaining of titanium foil flatness also during and after the sealing of the device. The prototype was fastened on three sides with an “U” shaped iron that was electrically insulated from the bottoms of the current collectors by a thin PTFE 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 respectively in Fig.1 and Fig. 2.

Fig.2. View of the supercapacitor prototype

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. Cyclic voltammetry (CV) and d.c. galvanostatic measurements were carried out with AMEL equipment composed of a high power potentiostat mod. 2055, an integrator mod. 731 and a function generator mod. 568. Cyclic voltammetry 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 Volt. 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 maintaining voltage variation from 0 to 5 V. For each of these currents, the supercapacitor is charged and discharged in a continuous manner and for several cycles. The amount of charge (coulomb) delivered by the capacitor during the discharging 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 (EIS) measurements were carried out on the discharged supercapacitor at ambient temperature using a Potentiostat (PGSTAT30 Autolab/Eco chimie NL) with a frequency response analyser (FRA2) module interfaced to a PC. An electrochemical impedance software (by Autolab) 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

Cyclic voltammetry measurements were carried out to evaluate the electrochemical stability of the prototype in the voltage range from 0 to 5V. Cyclic voltammograms were 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 behaviour of the system, i.e. only storage of electrostatic charges occurs at the interface electrode/electrolyte during the charging/discharging process in the indicated voltage range. In other words, faradaic processes (reversible or irreversible red-ox reactions) 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 cyclic voltammetry tests were divided by the respective voltage scan rates and reported as capacitance values in Fig. 3.

2.5

2

1.5

1

0.5

0

-0.5

Capacitance, F Capacitance, -1 100 mV/sec -1.5 200 mV/sec

-2 400 mV/sec

-2.5 012345 Supercapacitor Voltage, V

Fig. 3. Capacitance Vs. potential curves at various voltage scan rates.

The figure shows the capacitance of supercapacitor as a function of the voltage at the different voltage scan rates. The voltammograms highlights a clear slight dependence of capacitance by the voltage scan rate with a decreasing of the former upon increasing the latter. This observed variation could have different explanations although two seem more probable. The first one could be that the internal resistance of 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 (low local ionic conductivity) contained in the narrow pores of electrodes that are more distant from bulk of the electrolyte. Thus, because it is known that the capacitance of the supercapacitor is influenced from the electrical series resistance, this latter should be kept as low as possible in the device by using appropriate building materials (bipolar plates, current collectors), 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 will be reported during the discussion about the impedance behaviour of device. Fig. 4 shows dc galvanostatic discharge curves of the prototype 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. 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 of 1.12 F at 200 mA. This variation of capacitance is comparable to the previously determined on small size single cell supercapacitor [3].

5 200 mA

150 mA 4 100 mA

3 50 mA

Voltage, V 2

1

0 0 20 40 60 80 100 120 140 160

time, s Fig. 4. Discharge curves from 5 to 0 Volt at different currents.

The influence of series resistance on prototype performance has been analyzed and discussed by taking into account the impedance spectra. Fig. 5 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 Ω. A resistance of 0.64 Ω·cm2 for the single cell has been calculated taking into account the geometric area of the electrodes (16 cm2) and the number of cells (five) 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 parasitic 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 the not uniform or insufficient pressure exerted by the external plates on the inner components and, the latter to a not appropriate pore size distribution in the carbon material. Referring to the latter aspect, in fact, Yoon [11] found that using a carbon material with an well-ordered pore structure with uniform mesopores of 2.3 nm a 3.3 times smaller electric series resistance (ESR) 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 (calculated by P=V2/4R) of device was 31 Watt corresponding, when the volume of the prototype is taken into account, a maximum power density at 1.4 kW/l.

20

15

1

Z'', Ohm 10 0.5

5 0 00.51

0

0 5 10 15 20 Z', Ohm

Fig.5. Nyquist plot of the protoype. The inset shows the high frequency region of impedance.

The energy density calculated for 1.5 F device capacitance is 0.23 Wh/l. From the electric serial resistance (R) and the capacitance (C) 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. Smaller is the time constant higher is the charging and discharging rate. The RC time constant of 0.3 sec calculated for our devise corresponds to a quick charge-discharge of the supercapacitor. In current literature, few examples of stacked-prototype of supercapacitors have been reported, and this makes difficult a comparison of actual performance with that of our device. Moreover, these stacked-prototypes use aqueous electrolyte solutions, while organic electrolytes, which have low specific conductivity (about 20 mS/cm), are not considered suitable for the fabrication of multi-cell capacitors. On the other hand, the aqueous electrolytes, that shows specific conductivity so high as 800 mS/cm, are penalized by a lower working voltage (1 V) compared to that of a nonaqueous electrolyte (2.3-2.7 V) [12]. Thus, the higher voltage permits to obtain a much higher energy density with supercapacitors using organic electrolytes, while the high ionic conductivity is favoring a higher power density of aqueous supercapacitors. A prototype of multi-cell module was reported from M. Hahn et al. [12], and was based on electrodes made by activated glassy carbon. Their 0.4 F-24 V device, which used a 3 M H2SO4 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 [14] showed that a high power electric double layer capacitor of 470 F-15 V were realized with activated carbon electrodes and 30% solution of sulfuric acid. The device had about 2.5 Wh/l and 2.3 kW/l of 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, the fabrication, and the results for a solid state supercapacitor in multi-cell 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: a) higher flexibility in cell design, b) reduced corrosion, c) increased safety in handling, d) probably, longer lifetime.

2

1.8

1.6

1.4

1.2

1

0.8

0.6 Capacitance cell, F cell, Capacitance 0.4

0.2

0 0.001 0.01 0.1 1 10 100 1000

Frequency, Hz Fig. 6. Capacitance as a function of frequency of the prototype.

In Fig. 6 the specific capacitance as a function of frequency (f) from 1000 Hz to 5 mHz is shown for the device. The capacitance is calculated from imaginary component (Z”) of impedance by the expression C = - 1/(2π·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 (5 mHz). A maximum capacitance of 1.55 F is recorded at 5 mHz for the supercapacitor. From this value the energy of 19.4 Joule is calculated for the supercapacitor, which corresponds an energy density of about 0.23 Wh/l. Moreover, considering that the capacitance of the five cells prototype is of 1.5 F, the capacitance for unit area of a single cell is 0.47 F/cm2. By this, taking into account the surface density of carbon (0.0082 g/cm2 for each electrode) a single cell capacitance of 28.6 F/g is calculated. As normally, the capacitances of a supercapacitor are referred to the weight of the single electrode, the above value must be multiply for a factor four. Therefore, we obtain a value of capacitance 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 (0.23 Wh/l) 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 mono-polar plates [19, 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 multi-cell prototype of supercapacitor based on high surface area activated carbon material and solid polymer electrolyte has been successfully fabricated. The prototype, realized by stacking five identical single-cells, is electrochemically stable in the range of potential between 0 and 5 V. Values of maximum capacitance and resistance respectively of 1.55 F and 0.2 Ω are obtained for the device by impedance analysis. The electrical series resistance for the multi-cell prototype, reported to a single cell, is higher than that previously measured in single-cell capacitor. This is likely due to larger contact resistances between the different components and/or to the not fully optimised structure of electrodes. Nevertheless, a RC-time constant of 0.3 seconds 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. Moreover, specific capacitance of 114 F/g (referred to weight of active carbon materials of the single electrode) for the prototype has been demonstrated, which is comparable to that previously reported for the single cell supercapacitor, confirming the optimal scale-up design of this multi-cell prototype.

References

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