Performance Studies of Activated Charcoal Based Electrical Double

Energy Fuels 2010, 24, 6644–6652 Published on Web 11/10/2010

: DOI:10.1021/ef1010447

Performance Studies of Activated Charcoal Based Electrical Double Layer Capacitors with Ionic Liquid Gel Polymer Electrolytes G. P. Pandey, S. A. Hashmi,* and Yogesh Kumar Department of Physics & Astrophysics, University of Delhi, Delhi-110007, India Received August 9, 2010. Revised Manuscript Received October 22, 2010

Symmetric electrical double layer capacitors (EDLCs) with ionic liquid based magnesium ion conducting gel polymer electrolytes and activated charcoal (AC) electrodes have been constructed and characterized. Comparative studies have been performed employing three different electrolytes to fabricate EDLCs consisting of ionic liquid 1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate (EMITf) or a solution of magnesium trifluoromethanesulfonate [Mg-triflate or Mg(Tf)2] in EMITf or solution of Mg-triflate in the mixture of EMITf and ethylene carbonate (EC)-propylene carbonate (PC) cosolvent, immobilized with poly(vinylidenefluoride-hexafluoropropylene) (PVdF-HFP) matrix. The performance characteristics of EDLCs based on impedance spectroscopy, cyclic voltammetry, and constant current charge-discharge technique are presented. The incorporation of magnesium salt and EC-PC cosolvent in gel polymer electrolytes play a significant role in the dramatic improvement of the EDLCs’ characteristics. The optimum capacitance value of 136 F g-1 of AC, which correspond to the specific energy of ∼18.8 Wh kg-1 and power density of ∼6.67 kW kg-1 have been achieved. The EDLCs show almost stable cyclic performance, after initial fading in capacitance, for 5000 charge-discharge cycles.

high charge storage and high power capability of EDLCs, high specific surface area carbons are essentially required.13 However, recent studies suggested that the maximum cell capacitance is influenced by the relationship between the ion size, its solvation shell, and the pore size of the carbon electrodes.14-18 The carbon materials can be activated chemically and physically to modify their various properties such as porosity, microstructure, surface area, etc. Different methods have been developed including chemical activation using molten NaOH and KOH19,20 or hot acids (HNO3, H2SO4, etc.)20,21 and physical activation in air or CO2.22,23 The properties of activated carbons strongly depend on the activation process. EDLCs are well reported with various aqueous (e.g., H2SO4, KOH) and organic (e.g., acetonitrile, propylene carbonate) electrolytes, which are associated with the similar problems as the cases of liquid electrolyte based rechargeable batteries, such as leakage, bulky design, limited transportability, low specific

1. Introduction The electrical double layer capacitors (EDLCs) are an important class of supercapacitors/ultracapacitors, which are unique electrical energy storage devices, useful as power sources for various low and high energy density applications including electronic/biomedical equipment, computer memory back-ups, spaceships, hybrid electric vehicles, etc.1-3 The high specific capacitance, high power density, enough specific energy comparable to lead-acid batteries, and long chargedischarge cycle life are some important properties of EDLCs.4-6 The electric charges are stored in the electric double layer formed on the phase boundary between a polarizing porous electrode and ion conducting aqueous/ nonaqueous electrolyte or polymer based electrolytes.4-6 A number of reports are available in the literature on the EDLCs based on activated carbons4-9 and single-walled/multiwalled carbon nanotubes as electrodes.6,10-12 To obtain reversible

(13) Gamby, J.; Taberna, P. L.; Simon, P.; Fauvarque, J. F.; Chesneau, M. J. Power Sources 2001, 101, 109–116. (14) Largeot, C.; Portet, C.; Chmiola, J.; Taberna, P.-L.; Gogotsi, Y.; Simon, P. J. Am. Chem. Soc. 2008, 130, 2730–2731. (15) Huang, J.; Sumpter, B. G.; Meunier, V. Chem.;Eur. J. 2008, 14, 6614–6626. (16) Lin, R.; Taberna, P. L.; Chmiola, J.; Guay, D.; Gogotsi, Y.; Simon, P. J. Electrochem. Soc. 2009, 156, A7–A12. (17) Lin, R.; Huang, P.; Segalini, J.; Largeot, C.; Taberna, P. L.; Chmiola, J.; Gogotsi, Y.; Simon, P. Electrochim. Acta 2009, 54, 7025– 7032. (18) Mysyk, R.; Raymundo-Pinero, E.; Beguin, F. Electrochem. Commun. 2009, 11, 554–556. (19) Illan-Gomez, M. J.; Garcia-Garcia, A.; Salinas-Martinez de Lecea, C.; Linares-Solano, A. Energy Fuels 1996, 10, 1108–1114. (20) Marsh, H.; Rodriguez-Reinoso, F. Activated Carbon; Elsevier Science & Technology Books: Amsterdam, The Netherlands, 2006. (21) Nian, Y.-R.; Teng, H. J. Electrochem. Soc. 2002, 149, A1008– A1014. (22) Rodriguez-Reinoso, F.; Molina-Sabio, M. Carbon 1992, 30, 1111–1118. (23) Navarro, M. V.; Seaton, N. A.; Mastral, A. M.; Murillo, R. Carbon 2006, 44, 2281–2288.

*To whom correspondence should be addressed. Telephone: þ91-1124514204. Fax: þ91-11-27667061. E-mail: [email protected]. (1) Conway, B. E. Electrochemical Supercapacitors: Scientific, Fundamentals and Technological Applications; Kluwer Academic/Plenum Publisher: New York, 1999. (2) K€ otz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483–2498. (3) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845–854. (4) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937–950. (5) Hashmi, S. A. Natl. Acad. Sci. Lett. 2004, 27, 27–46. (6) Frackowiak, E. Phys. Chem. Chem. Phys. 2007, 7, 1774–1785. (7) Qu, D.; Shi, H. J. Power Sources 1998, 74, 99–107. (8) Hashmi, S. A.; Latham, R. J.; Linford, R. G.; Schlindwein, W. S. J. Chem. Soc., Faraday Trans. 1997, 93, 4177–4182. (9) Taberna, P. L.; Simon, P.; Fauvarque, J. F. J. Electrochem. Soc. 2003, 150, A292–A300. (10) Ruch, P. W.; Hardwick, L. J.; Hahn, M.; Foelske, A.; Kotz, R.; Wokaun, A. Carbon 2009, 47, 38–52. (11) Honda, Y.; Ono, T.; Takeshige, M.; Morihara, N.; Shiozaki, H.; Kitamura, T.; Yoshikawa, K.; Morita, M.; Yamagata, M.; Ishikawa, M. Electrochem. Solid-State Lett. 2009, 12, A45–A49. (12) Pandey, G. P.; Hashmi, S. A.; Kumar, Y. J. Electrochem. Soc. 2010, 157, A105–A114. r 2010 American Chemical Society

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1,5

energy, and low power density. The lower value of the electrochemical potential window (limited to ∼1.2 V) is the limitation of aqueous electrolytes, whereas the flammability, toxicity, and environment hazards are the basic problems of organic (nonaqueous) electrolytes.3 Such problems can be partially avoided by substituting liquid electrolytes with the gel polymer electrolytes, which possess additional properties of high ionic conductivity, mechanical and thermal stability, wide enough electrochemical potential window, and excellent interfacial stability.24,25 The gel polymer electrolytes, in which liquid electrolytes (mixture of aprotic polar organic solvents and salts) are immobilized in the host polymer matrix, show the combined effect of high ionic conductivity of the liquid electrolyte and the mechanical properties of the polymer. A variety of polymers, including polyethylene oxide, polyvinyl alcohol, polyacrylonitrile, poly(methyl methacrylate), poly(vinylidene fluoride), etc., are used as host matrixes.24,25 However, the poor thermal and electrochemical properties of the entrapped liquid electrolytes limit the performance characteristic of gel polymer electrolytes. The volatile nature of organic solvents limits the thermal stability, whereas their relatively narrow electrochemical potential windows limit the electrochemical stability range of the gel polymer electrolytes.25 Recently, room temperature ionic liquids (RTILs) have been found to be attractive materials to substitute the aprotic solvents due to their unique properties like almost zero vapor pressure, nonvolatility, chemical and thermal stability, and wider electrochemical potential window, which lead them to be used in various electrochemical applications including rechargeable batteries, supercapacitors, fuel cells, etc.26,27 Recently, some ionic liquids based EDLCs are reported, which show excellent performance with different carbon electrodes.28-31 More recently, the development of ionic liquid based gel polymer electrolytes has also attracted considerable attention, which offers excellent thermal and electrochemical properties.32-37 The gel polymer electrolytes are mostly reported for lithium ion conducting systems for their application in lithium batteries. Some proton, sodium, and magnesium ion conducting gel polymer systems have also been reported, offering similar

potential applicability in electrochemical power storage devices, including EDLCs.5,8,35 Particularly, magnesium ion conducting gel electrolytes have attracted recent attention worldwide for their application in the development of rechargeable magnesium batteries as well as EDLCs.12,38-40 The present paper reports the fabrication and physical characteristics of EDLCs constructed using activated charcoal powder electrodes and the ionic liquid based Mg2þ ion conducting gel polymer electrolytes. The divalent Mg2þ ion conducting gel electrolytes were deliberately used in the fabrication of EDLCs, as they offer electrochemical performance equivalent to any other lithium based systems and these provide more charges for storage at electrodeelectrolyte interfaces. The performance characteristics of the EDLCs were evaluated by impedance measurements, cyclic voltammetry, and galvanostatic charge-discharge tests for numerous cycles. The experimental results indicate that the activated charcoal electrodes exhibit excellent capacitive performance with Mg2þ ion conducting gel polymer electrolytes. A noteworthy enhancement in the specific capacitance and specific energy of EDLCs has been observed due to partial substitution of ionic liquid with EC-PC cosolvent in gel polymer electrolyte. 2. Experimental Section 2.1. Preparation of Electrodes. Activated charcoal powder, sodium hydroxide (NaOH), and sulfuric acid (H2SO4) were purchased from Merck (India). Poly(vinylidenefluoride-cohexafluoropropylene) (PVdF-HFP, average MW ∼400 000) and graphite powder were obtained from Sigma-Aldrich. The high-density graphite sheets (250 μm thick, approximately 0.027 g cm-2, FMI Composite Ltd.) were used as current collectors. Activated charcoal powder was chemically treated with NaOH before use, as reported in the literature.20,41 For this purpose, the charcoal powder was mixed with concentrated aqueous solution of NaOH and refluxed at 150 °C for 4 h. The resulting material was extensively washed in distilled water until the pH of outlet water was found to be 7.0. Acid exposure was also performed through H2SO4 refluxing at 80 °C for 2 h, and material was extensively washed again. Finally, the activated charcoal was vacuum-dried at 100 °C. To prepare the electrode, a slurry of activated charcoal, graphite powder, and binder PVdF-HFP with the ratio 75:15:10 was prepared in acetone by thoroughly mixing them in a mortar and pestle. This slurry was spin coated on the flexible graphite sheet using a spin coater (Apex Instruments, India). The thickness of the carbon electrode on the graphite sheet was kept at ∼200 μm. The area of the electrodes used to fabricate the EDLCs was ∼2 cm2, and the mass of the carbon composite in the electrodes was ∼4 mg cm-2. The prepared electrodes were vacuum-dried overnight at 100 °C before using them to fabricate EDLCs. 2.2. Preparation of Gel Polymer Electrolytes. The ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMITf), magnesium trifluoromethanesulfonate [Mg-triflate or Mg(Tf)2], propylene carbonate (PC), and ethylene carbonate (EC) were obtained from Sigma-Aldrich. The EMITf and Mg(Tf)2 were vacuum-dried at ∼80 °C overnight before use. A solution cast method was used to prepare gel polymer electrolyte films. The details of the preparation method were published by us elsewhere.35

(24) Song, J. Y.; Wang, Y. Y.; Wan, C. C. J. Power Sources 1999, 77, 183–197. (25) Agrawal, R. C.; Pandey, G. P. J. Phys. D: Appl. Phys. 2008, 41, 223001-18. (26) Ohno, H., Ed. Electrochemical Aspects of Ionic Liquids; Wiley Interscience: Hoboken, NJ, 2005. (27) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Nat. Mater. 2009, 8, 621–629. (28) Lewandowski, A.; Galinski, M. J. Phys. Chem. Solids 2004, 65, 281–286. (29) Xu, B.; Wu, F.; Chen, R.; Cao, G.; Chen, S.; Wang, G.; Yang, Y. J. Power Sources 2006, 158, 773–778. (30) Balducci, A.; Dugas, R.; Tabernaa, P. L.; Simon, P.; Plee, D.; Mastragostino, M.; Passerini, S. J. Power Sources 2007, 165, 922–927. (31) Lu, W.; Qu, L.; Henry, K.; Dai, L. J. Power Sources 2009, 189, 1270–1277. (32) Lewandowski, A.; Swiderska, A. Solid State Ionics 2003, 161, 243–249. (33) Ye, H.; Huang, J.; Xu, J. J.; Khalfan, A.; Greenbaum, S. G. J. Electrochem. Soc. 2007, 154, A1048–A1057. (34) Sirisopanaporn, C.; Fernicola, A.; Scrosati, B. J. Power Sources 2009, 186, 490–495. (35) Pandey, G. P.; Hashmi, S. A. J. Power Sources 2009, 187, 627– 634. (36) Fernicola, A.; Weise, F. C.; Greenbaum, S. G.; Kagimoto, J.; Scrosati, B.; Soletoa, A. J. Electrochem. Soc. 2009, 156, A514–A520. (37) Lu, W.; Henry, K.; Turchi, C.; Pellegrino, J. J. Electrochem. Soc. 2008, 155, A361–A367.

(38) Morita, M.; Yoshimoto, N.; Yakushiji, S.; Ishikawa, M. Electrochem. Solid-State Lett. 2001, 4, A177–A179. (39) Pandey, G. P.; Agrawal, R. C.; Hashmi, S. A. J. Power Sources 2009, 190, 563–572. (40) Kumar, G. G.; Munichandraiah, N. J. Power Sources 2000, 91, 157–160. (41) Fonseca, D. A.; Gutierrez, H. R.; Lueking, A. D. Microporous Mesoporous Mater. 2008, 113, 178–186.

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Figure 1. Schematic representation of two processes of EDLCs’ fabrication with (a) gel polymer electrolyte thick film and (b) spin coated gel electrolyte.

The electrical conductivity of the polymer films was obtained from the complex-impedance measurements on the cell SS|gel polymer electrolyte|SS for a frequency range varying from 1 Hz to 100 kHz with a signal level of 20 mV (SS stands for stainless steel). The impedance measurements were carried out using an LCR meter (model 3522-50, HIOKI, Japan). The linear sweep cyclic voltammetry was performed to study the electrochemical stability of the gel polymer electrolytes using an electrochemical analyzer (model 608C, CH Instruments). The EDLCs were characterized using various electrochemical techniques like ac impedance spectroscopy, cyclic voltammetry, galvanostatic charge-discharge techniques. The impedance measurements were carried out using the LCR meter mentioned above in the frequency range from 1 mHz to 100 kHz. The value of overall capacitance of the EDLC cells was evaluated using the expression -1 C ¼ ð1Þ ωZ00

However, a brief description of preparation method is given as follows. The liquid electrolyte was prepared by dissolving 0.3 M Mg(Tf)2 in ionic liquid. The polymer PVdF-HFP was separately dissolved in acetone. Ionic liquid and liquid electrolyte (Mg(Tf)2/ EMITf) were separately mixed with the PVdF-HFP/acetone solution to form the films of EMITf/PVdF-HFP blend and Mg(Tf)2/ EMITf/PVdF-HFP gel electrolyte. The mixtures were thoroughly stirred magnetically for 4-5 h. The weight ratio of the ionic liquid or liquid electrolyte to PVdF-HFP was kept at 80:20 (w/w). The gel polymer electrolyte films of the composition 0.3 M Mg(Tf)2 in EMITf/PVdF-HFP/EC-PC mixture (1:1 v/v) with the ratio 65:20:15 w/w were also prepared. All the compositions were casted over glass Petri dishes, and acetone was allowed to evaporate slowly. Finally, semitransparent free-standing gel films (thickness ∼200-250 μm) were obtained. These films were stored in a dry chamber for characterization and device fabrication. The following three compositions of gel electrolytes were used as separators/ electrolytes in EDLC cells: (i) EMITf/PVdF-HFP polymeric blend of ratio 80:20, w/w (GPE-1); (ii) 0.3 M Mg(Tf)2 in EMITf/ PVdF-HFP with ratio 80:20, w/w (GPE-2); and (iii) 0.3 M Mg(Tf)2 in EMITf/PVdF-HFP/ (EC-PC mixture) with ratio 65:20:15, w/w (GPE-3). 2.3. Fabrication of EDLC Cells. EDLC cells were fabricated using the following two processes as schematically shown in Figure 1a,b: (i) In process one, the piece of free-standing gel electrolyte film was placed on one electrode surface and the other symmetrical electrode was placed over the gel film (Figure 1a). (ii) The second approach was adopted in which the acetone solution of gel polymer electrolyte was spin coated on the electrode surface and the acetone was allowed to evaporate. The two such systems were placed over each other and pressed slightly to form the complete cell (Figure 1b). As shown in Figure 1a, a large proportion of the pores of the electrodes has no access of the electrolyte film to form capacitive interfaces and are left unutilized. Whereas, in the spin coating process, the electrolyte is filled in the pores of carbon electrodes to a larger extent, and the larger surface area of the electrode is utilized to form interfacial contacts between the electrode and gel electrolyte after evaporation of the acetone (Figure 1b). Different EDLC cells were constructed using three types of gel polymer electrolytes with the activated charcoal (AC) electrodes following the above two processes given below for their comparative studies: cell-1a : ACjGPE-1ðFÞjAC; cell-1b : ACjGPE-1ðSCÞjAC cell-2a : ACjGPE-2ðFÞjAC;

cell-2b : ACjGPE-2ðSCÞjAC

cell-3a : ACjGPE-3ðFÞjAC;

cell-3b : ACjGPE-3ðSCÞjAC

where ω is the angular frequency and Z00 is the imaginary part of the total complex impedance Z. The single electrode specific capacitance values were evaluated by multiplying the overall cell capacitance by a factor of 2 and dividing by the mass of the single electrode material. The cyclic voltammetry was performed with the help of an electrochemical analyzer, mentioned above. The capacitance values were evaluated using the relation: i C ¼ ð2Þ s where i is the current and s is the scan rate (ΔV/Δt). The charge-discharge characteristics of the capacitor cells were evaluated at constant currents using a charge-discharge unit (model BT2000, Arbin Instrument). The discharge capacitance C was evaluated from the linear part of the discharge curves using the relation iΔt Cd ¼ ð3Þ ΔV where i is the constant current and Δt is the time interval for the voltage change of ΔV.

3. Results and Discussion 3.1. Characteristics of Gel Polymer Electrolytes. The gel polymer electrolyte films GPE-1 and GPE-2 possess excellent mechanical, thermal, and electrochemical stabilities with high room temperature conductivity of the order of 10-3 S cm-1. We have recently reported the detailed studies on these gel systems.35 Recently, few workers adopted a new approach to improve upon the performance characteristics of ionic liquid based gel polymer electrolytes by substituting some proportion of ionic liquid with PC or EC-PC cosolvent.33,34 Such modifications lead to the dramatic improvement in their ionic conductivity, ion transport kinetics, and electrode/electrolyte interfacial stability.33,34 A similar approach has been adopted by us to prepare the gel polymer electrolyte GPE-3, mentioned above, with the

where GPEs (F) stand for films and GPEs (SC) for spin coated gel electrolytes. The thickness of the electrolyte/separator was almost same in both the processes. 2.4. Instrumentation. The N2 adsorption-desorption experiment for the measurement of specific surface area and other porosity parameters of the electrode materials were measured using a surface area and pore size analyzer (Micromeritics). The powder samples were given a pretreatment by heating at ∼300 °C for 1 h in nitrogen atmosphere. 6646

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expectation to obtain high ionic conductivity and a flexible interface between the electrode and electrolyte for fast charge (or ion) transfer process. The electrolyte GPE-1 (i.e., EMITf/PVdF-HFP blend film) offers substantially good conductivity (σ = 6.3  10-3 S cm-1 at 20 °C), comparable with that of the pure ionic liquid EMITf, having appreciable dimensional stability. On addition of Mg(Tf)2 to the EMITf/PVdF-HFP blend, a slight decrease in conductivity (σ = 3.4  10-3 S cm-1 at 20 °C) is observed in the gel electrolyte film (GPE-2), which is due to a possible increase in viscosity of the ionic liquid due to addition of Mg-salt. Similar observation has been reported for lithium salt solutions in ionic liquid and other ionic liquid based gel polymer electrolyte systems in the literature.33,42-44 Probably, structural changes in the ionic liquid upon addition of the Mg-salt lead to a less open structure that causes higher density and viscosity values.42 On substitution of the EC-PC mixture in ionic liquid/ Mg-salt solution, the gel electrolyte film GPE-3 shows enhancement in conductivity close to the order of 10-2 S cm-1 at room temperature, which is high enough from the EDLC application point of view. The increase in ionic conductivity of the electrolyte GPE-3 is due to the high dielectric constant and relatively low viscosity of the EC-PC mixture introduced in the system. The high dielectric constant assists in shielding the cation-anion interactions, which in turn enhances the ionic dissociation of the Mg(Tf)2 salt. Hence, the number of the free ions would increase, which would contribute to the conduction, significantly.34 In addition, the lower value of the viscosity of the EC-PC cosolvent helps in enhancing the ionic mobility.45 Further, the GPE-3 films are relatively flexible, which would help in getting proper electrode-electrolyte contacts. The temperature dependent electrical conductivity of the electrolyte GPE-3 has been studied in the present work. The “σ vs 1/T” plot, as shown in Figure 2, represents non-Arrhenius Vogel-TammenFulcher (VTF) behavior as expressed by the following equation:   -B - 1=2 exp ð4Þ σ ¼ AT T - T0

Figure 2. Variation of conductivity of gel polymer electrolyte GPE-3 as a function of temperature.

Figure 3. Cyclic voltammogram of gel polymer electrolyte GPE-3 with glassy carbon electrodes at room temperature (20 °C) recorded at the scan rate 5 mV s-1.

where the parameter B is associated with rate at which viscosity changes with the temperature, A is a constant (pre-exponential factor, i.e., the conductivity at infinitely high temperature), and T0 is close to the glass transition temperature. These fitting parameters have been evaluated using nonlinear least-squares (NLLS) fitting of the data and found to be B = 23 K, A = 0.55 S cm-1 K1/2, and T0 = 278 K. The electrochemical stability, i.e., working voltage range for electrolytes, is an important parameter to be evaluated from their application point of view in electrochemical devices. The working voltage range of optimized gel electrolyte GPE-3 is evaluated by cyclic voltammetry using glassy carbon (GC) electrodes. Figure 3 shows the cyclic voltammogram of the cell GC|GPE-3|GC, recorded with a scan rate of 5 mV s-1

Figure 4. Nitrogen adsorption-desorption isotherms at 77 K of chemically treated charcoal powder. The pore size distribution from BJH analysis, applied to the desorption branch, is shown in the inset.

(42) Monteiro, M. J.; Bazito, F. F. C.; Siqueira, L. J. A.; Ribeiro, M. C. C.; Torresi, R. M. J. Phys. Chem. B 2008, 112, 2102–2109. (43) Fernicola, A.; Croce, F.; Scrosati, B.; Watanabe, T.; Ohno, H. J. Power Sources 2007, 174, 342–348. (44) Xu, J. J.; Ye, H.; Huang, J. Electrochem. Commun. 2005, 7, 1309– 1317. (45) Yu, B.; Zhou, F.; Wang, C.; Liu, W. Eur. Polym. J. 2007, 43, 2699–2707.

at room temperature. The working voltage range has been found in the range from -2 to 2 V, which is substantially enough from the device applications point of view. 3.2. Characteristics of Activated Charcoal Electrodes. N2 adsorption-desorption isotherm and pore size distribution of the chemically treated activated charcoal powder are 6647

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Figure 5. Parts A, B, and C show the complex impedance plots of different EDLC cells with (a) gel electrolyte films and (b) spin coated electrolytes on porous AC electrodes, recorded at room temperature in the frequency range from 100 kHz to 10 mHz. Part D represents the possible modified Randle’s equivalent circuit of EDLC cells. Table 1. Specific Surface Area and Porosity Parameters of Chemically Activated Charcoal Powder, Evaluated from N2-Adsorption Isotherm BET specific surface area (m2 g-1)

micropore area (m2 g-1)

micropore volume (cm3 g-1)

total pore volume (cm3 g-1)

mesopore volume (cm3 g-1)

average pore size (nm)

875

577

0.303

0.528

0.226

2.43

shown in Figure 4. The material shows the type-I isotherm according to the IUPAC classification. The steep rising behavior of the isotherm, which indicates the major uptake of nitrogen, has been observed at low relative pressure (P/P0 < 0.005), followed by a gradual increase in adsorption for the entire range of P/P0. The steep rising pattern (below P/P0 = 0.005) indicates that the activated charcoal powder contains a large proportion of microporosity. The gradual increase in adsorption (beyond P/P0 = 0.01) is indicative of the presence of mesopores also. The pore size distribution of the activated charcoal powder, calculated using the BarrettJoyner-Halenda (BJH) method with desorption data, is shown in the inset of Figure 4. This shows that the electrode material contains the mesopores of the pore size between 3 and 4 nm along with micropores as observed from the t-plot study. A quantitative estimate of the BET surface area, micro- and mesopore volumes, and average pore size are given in Table 1. The total pore volume has been estimated at the relative pressure of ∼0.95. The mesopore volume has been calculated by subtracting the micropore

volume from the total pore volume. The presence of micropores and mesopores in the electrodes leads to the enhanced performance characteristics of EDLCs. 3.3. Characterizations of EDLCs. 3.3.1. Impedance Spectroscopic Studies. Figure 5A-C shows the impedance spectra of different EDLC cells fabricated with (a) gel polymer electrolyte films and (b) spin coated gel electrolyte on activated charcoal (AC) electrodes. The steep rising nature of the value of Z00 with respect to Z0 in complex impedance plots indicates the capacitive behavior, close to ideal capacitive characteristics, of all the EDLC cells with all the three gel electrolytes, incorporated following both the processes. The ideal impedance behavior for a pure capacitor is a straight line parallel to the imaginary axis (Z00 ). However, in practical capacitors, the steep rising capacitive impedance response is observed in the low frequency region accompanied with high frequency features owing to the bulk and interfacial properties of the capacitor cells. Various electrical parameters associated with bulk properties of electrolytes and electrode-electrolyte interfaces of EDLCs can be 6648

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Table 2. Electrical Parameters of Different EDLC Cells from Impedance Analysis C (mF cm-2)a 2

2

c

2

C (F g-1)b

EDLC cells

electrolytes

Rb (Ω cm )

Rct (Ω cm )

R (Ω cm )

10 mHz

2 mHz

10 mHz

2 mHz

cell-1a cell-1b cell-2a cell-2b cell-3a cell-3b

GPE-1, film GPE-1, spin coated GPE-2, film GPE-2, spin coated GPE-3, film GPE-3, spin coated

5.5-8.2 5.25-7.5 5.4-8.1 4.6-6.3 5.4-7.1 2.1-4.0

2.6-3.5 2.8-3.0 0.9-2.1 0.6-1.3 0.5-1.5 0.5-1.1

75-90 50-60 80-95 35-45 60-80 20-30

49-66 104-131 53-73 127-130 84-91 193-225

62-83 117-145 71-90 140-149 94-102 206-239

33-44 69-87 35-48 84-87 56-60 128-150

41-55 78-96 47-59 93-100 62-68 137-160

a

Overall capacitance of the cell. b Single electrode capacitance. c At 10 mHz.

evaluated using impedance measurements separately in different frequency ranges. The values of bulk resistance (Rb), interfacial charge transfer resistance (Rct), total cell resistance (R), and the capacitance values (evaluated using eq 1 at 10 and 2 mHz) of the different capacitor cells are listed in Table 2. A substantial improvement in the capacitance values is observed, when gel polymer electrolytes are spin coated on AC electrodes for cell fabrication. This is expected as the spin coated electrolytes form larger interfacial area (i.e., larger capacitive interfaces) with AC electrodes as compared to the film form of gel polymer electrolytes. The proper electrode-electrolyte contacts due to spin coating of electrolytes lead to the significant lowering in the resistance values (Table 2). Further, the cell-1 with the ionic liquid/polymer blend (GPE-1) electrolytes incorporated in the form of film and spin coating on AC electrodes offer the capacitance values of 62-83 and 117-145 mF cm-2 (or the specific capacitance 41-55 and 78-96 F g-1), respectively, measured at 2 mHz (Figure 5A, Table 2). A significant enhancement in the capacitance values is observed, when Mg2þ ion conducting gel polymer electrolyte Mg(Tf)2/EMITf/PVdF-HFP is introduced to form EDLCs (cell-2). This shows the effect of salt addition in ionic liquid based gel polymer electrolytes. As discussed in section 3.2, the activated charcoal electrodes possess a large amount of micropores (sizes