Asymmetric supercapacitor containing poly(3-methyl thiophene)-multiwalled carbon nanotubes nanocomposites and activated carbon

Electrochimica Acta 94 (2013) 182–191

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Asymmetric supercapacitor containing poly(3-methyl thiophene)-multiwalled carbon nanotubes nanocomposites and activated carbon P. Sivaraman a,b , Arup R. Bhattacharrya b , Sarada P. Mishra a , Avinash P. Thakur a , K. Shashidhara a , Asit B. Samui a,∗ a b

Naval Materials Research Laboratory, Shil Badlapur Road, Anand Nagar, Ambernath 421 506, Thane, Maharashtra, India Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400 072, India

a r t i c l e

i n f o

Article history: Received 22 August 2012 Received in revised form 10 January 2013 Accepted 12 January 2013 Available online 31 January 2013 Keywords: Poly(3-methyl thiophene) Multiwalled carbon nanotubes Nanocomposites Supercapacitor Specific capacitance

a b s t r a c t Poly(3-methyl thiophene) (PMT) and multiwalled carbon nanotubes (MWCNT) based nanocomposites have been prepared chemically at various wt. ratios. Nanocomposites are characterized by TEM, TGA, XRD and Raman spectroscopy. The morphology of nanocomposites shows fine wrapping of PMT over MWCNT. Electrodes containing PMT nanocomposites have been prepared and p-doping of nanocomposites studied in electrolyte containing 1 M tetraethylene ammonium tetra fluroborate in propylene carbonate. Specific capacitance of the nanocomposites increases with increase in PMT loading and a maximum specific capacitance of 296 F g−1 has been obtained for the nanocomposite with PMT/MWCNT wt. ratio of 87.5/12.5. Non aqueous based asymmetric supercapacitors containing PMT nanocomposites as the positive electrode and activated carbon (AC) as the negative electrode have been fabricated. Supercapacitor unit cells have been constructed at the optimized mass ratio of the electrodes so as to obtain maximum total specific capacitance and specific energy. Highest specific capacitance obtained for PMT nanocomposite-AC system is 38.5 F g−1 of the total active material in the supercapacitor. The complex capacitance of the supercapacitors derived from electrochemical impedance spectroscopy reveals that supercapacitor containing nanocomposites with higher concentration of MWCNT has shorter relaxation time constant ( 0 ) leading to higher rate capability. Continuous charge–discharge cycling studies indicate that PMT nanocomposite–AC supercapacitor have better stability than bulk PMT–AC supercapacitor. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Supercapacitors are electrochemical devices which are used for pulse power applications. They find applications in electric vehicles, uninterrupted power supplies, memory backups, mobiles, cameras, etc. [1–3]. Based on electrode material, supercapacitors can be classified into electric double layer capacitors (EDLC) and pseudocapacitors [4,5]. In EDLCs, charge separation takes place at the electrode/electrolyte interface and the energy storage mechanism is electrostatic in nature. The commonly used EDLC materials are activated carbon (AC), carbon nanotubes (CNT), graphene and carbon xerosols [6–11]. In pseudocapacitors, fast reversible faradaic reaction takes place at the electrode/electrolyte interface. Conducting polymers and metal oxides are commonly used pseudocapacitance materials [12–17]. In conducting polymers, the whole bulk of the polymer can be utilized for energy storage and hence, generally conducting polymers have higher specific capacitance [15,16]. Besides these, conducting polymers have several other advantages

∗ Corresponding author. Tel.: +91 251 2623036; fax: +91 251 2623004. E-mail addresses: asit [email protected], [email protected] (A.B. Samui). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.01.123

like low cost, environmentally friendly, high conductivity in doped state, wide voltage window, high storage capacity and adjustable redox activity through chemical modification [17]. Conducting polymers in n-doped state exhibit lower conductivity and poorer stability than p-doped state [18–22]. Hence asymmetric supercapacitors were developed, where n-doped polymer is replaced by activated carbon or lithium intercalating compounds in order to improve stability and cycling performance compared to symmetric supercapacitors [5,6]. Poly(3-methyl thiophene) (PMT) is an important conducting polymer which is studied as the electrode materials in both symmetric as well as asymmetric supercapacitors [23–31]. Mastragostino and group compared PMT based symmetric supercapacitors with asymmetric supercapacitor (PMT-AC) and achieved specific capacitance of 240 F g−1 for PMT and 39 F g−1 for total active material [23–26]. Laforgue et al. fabricated 3 V-prototype asymmetric supercapacitor containing PMT and AC respectively. The device exhibited specific capacitance of 35 F g−1 for the total composite material while the specific capacitance of PMT was 230 F g−1 [27]. Arbizzani et al. studied PMT with ionic liquids and reported specific capacitance of 250 F g−1 at 60 ◦ C [28]. Hybrid supercapacitor containing poly(ethylene oxide) based gel electrolyte, with PMT as the

P. Sivaraman et al. / Electrochimica Acta 94 (2013) 182–191

positive electrode was also reported and the supercapacitor exhibited specific capacitance around 18 F g−1 of total active material [29]. Hashmi et al. used different gel polymer electrolytes for PMT–PMT and PMT–polypyrrole (PPY) supercapacitors and reported discharge capacitance of 8–15 mF cm−2 [30]. PMT was electropolymerized on porous poly(vinylidine fluoride) (PVDF) membrane and the composites membrane exhibited discharge capacitance of 82 F g−1 [31]. Generally in conducting polymers, most of the pseudocapacitance are obtained from the surface redox reactions and therefore only a very thin surface layer undergo faradaic reaction [5,31]. Moreover during doping and undoing, conducting polymers undergo volume change and hence are bound to undergo mechanical degradation which results in capacitance decay. These drawbacks can be minimized either by controlling polymer morphologies like nanorod formation or by nanocomposite formation using carbon nanotubes (CNTs), graphene [32–39]. It is demonstrated that nanocomposites of conducting polymers have enhanced specific capacitance, cycling stability and conductivity. Reports on PMT nanocomposite electrode materials for supercapacitor application are scantly. Xiao et al. utilized PMT-multiwalled carbon nanotubes (MWCNT) nanocomposites as the positive electrode in supercapacitor in voltage window of 0–1 V [40]. Kim et al. used PMT-MWCNT nanocomposites as the cathode material in lithium metal polymer battery [41]. Zinc ion doped PMT-MWCNT composites were studied for aqueous electrolyte based supercapacitor by Karthikeyan et al. [42]. However, detailed analysis of PMT-MWCNT nanocomposites for non aqueous supercapacitor has not been reported earlier. In this paper, PMT-MWCNT nanocomposites were prepared by in situ chemical method and their physical and electrochemical properties were studied. Also, we report electrochemical performance of hybrid supercapacitor based on PMT-MWCNT nanocomposites and AC, containing optimized mass ratio of the electrodes. 2. Experimental 2.1. Materials 3-Methyl thiophene was obtained from Acros Chemicals, India. Ferric chloride, chloroform, methanol and hydrazine hydrate (SdFine Chemicals, India), MWCNT Grade:Nanocyl 3100 (Nanocyl, Begium, surface area: 300 m2 g−1 ), electrolytic grade propylene carbonate (PC) (Merck) were used without future purification. Activated carbon used in this study was obtained from PICA, France (grade: BP 10, surface area: 1900 m2 g−1 ). Tetra ethylene ammonium tetrafluroborate (TEABF4 ) (Alfa Aser) was thoroughly dried before electrolyte preparation. Carbon paper and conducting carbon (CC) (grade: Vulcan XC-72) were purchased from Toray and Carbot Corp, Japan respectively. PTFE suspension was obtained Hindustan Flurocarbons, India. 2.2. Synthesis of PMT-MWCNT nanocomposites PMT and PMT-MWCNT nanocomposites were prepared following the procedure as per the reference [43]. In a typical process,

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3-methyl thiophene (2.0 g, 11.88 mmol) was taken with CHCl3 (120 mL) to which appropriate quantity of MWCNT added and sonicated for 30 min. To this mixture, dispersion of anhydrous FeCl3 (5.74 g, 35.38 mmol) in CHCl3 (60 mL) was added drop wise over 6 h. The reaction mixture was carried out at room temperature under stirring for additional 18 h and then quenched with methanol. The precipitate obtained was filtered and washed with methanol. The residue was stirred in 5% methanolic solution of hydrazine hydrate for 24 h to dedope PMT. PMT-MWCNT nanocomposites were filtered, washed several times with methanol and Soxhlet extracted with methanol. Different PMT nanocomposites samples were synthesized by varying PMT to MWCNT wt. ratio viz. 87.5/12.5, 75/25, 50/50, 37.5/63.5, 25/75 and 12.5/87.5 and nomenclatured as PMT-X, where X denotes the wt. ratio of PMT present in the nanocomposite. 2.3. Preparation of electrode PMT, PMT nanocomposites or AC was mixed with CC in isopropyl alcohol and then PTFE (≤5%) was added and mixed in a mortar and pestle to yield a paste. The paste was applied uniformly on to a carbon paper. The composition of PMT, PMT nanocomposites, AC and MWCNT electrodes is given in the Table 1. The electrodes were dried at 60 ◦ C under vacuum before using for electrochemical characterization. Electrodes of size 2 cm × 2 cm were prepared and loading of active material was 0.625 mg cm−2 . The coating thickness was found to be 40 ± 5 ␮m. 2.4. Unit cell fabrication Unit cell was prepared by using two electrodes of size 2 cm × 2 cm separated by capacitor grade paper. Positive electrode contained PMT nanocomposite material while negative electrode was AC. Amount of PMT in the positive electrode was kept constant (0.625 mg cm−2 ) while AC was varied according to mass ratio of positive to negative electrode ( max ). Consequently, the coating thickness of the negative electrodes varies from 20 ± 5 to 70 ± 5 ␮m depending upon the electrode loading. A silver wire (quasi reference electrode) was kept near to the electrodes so that it was equidistant from both the electrodes. The assembly was kept for drying in vacuum oven (attached to Mbraun glove box) at 60 ◦ C for 6 h. It was sealed with plastic coated aluminum foil after addition of 1 M TEABF4 in PC. Sealing of unit cell was carried out inside the glove box which was kept in nitrogen atmosphere. 2.5. Characterization Thermal analysis of the PMT-MWCNT nanocomposites was carried out using a Thermogravimetric analyzer (TA Instruments: Q500). Raman spectroscopy of the nanocomposites was performed using Jobin Yovonspectometer (HR 800 micro-Raman) on powder samples of PMT-MWCNT nanocomposites with incident laser excitation wavelength of 514 nm. TEM analysis of nanocomposites was carried out using a Philips CM 200 microscope, operated at 200 kV. Dilute suspension of the nanocomposite was sonicated in isopropyl alcohol and then drop cast using a capillary on a copper grid, for TEM analysis. XRD analysis was carried out using

Table 1 Composition of PMT, PMT nanocomposite, AC and MWCNT electrodes. Electrode composition (wt. ratio)

PMT electrode

PMT nanocomposite electrode

AC electrode

MWCNT electrode

PMT PMT nanocomposites AC MWCNT CC PTFE

1 – – – 1 0.05

– 1 – – 1 0.05

– – 1 – 0.2 0.05

– – – 1 – 0.05

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Fig. 1. TEM of PMT nanocomposites. (a) MWCNT, (b) PMT25, (c)PMT50 and (d) PMT 87.5.

Philips X’Pert X-ray diffractometer. Electrochemical characterization of the nanocomposites was carried out using an Eco Chimie potentiostat-galvanostat (PGSTAT30). Both three electrode as well as two electrode (unit cell) methods were employed for characterization of electrodes. 3. Results and discussion

of polymerization, may act as the dispersing agents for MWCNT in the reaction medium and the strong interaction between the thiophene ring and MWCNT facilitates growth of PMT around the MWCNTs. Such kind of decoration of pseudocapacitive materials over electric double layer capacitive material is advantages than physically mixing the two components [5,45], since there is most intimate contact between the two components.

3.1. TEM of nanocomposites

3.2. TGA of PMT-MWCNT nanocomposites

TEM micrographs of the PMT nanocomposites are shown in the Fig. 1. The micrographs of the nanocomposites reveal wrapping of PMT around MWCNT. The thickness of the PMT layer around MWCNT decreases with increase in MWCNT concentration in the nanocomposite. During in situ preparation of nanocomposites, the monomer concentration was kept constant while that of MWCNT increased. Obviously, as the heterogonous surface available during polymerization increases, the wrapping layer thickness wrapping around the MWCNT decreases. From the TEM micrograph, it can be inferred that PMT coating takes place around individual MWCNT and this indicates that the MWCNTs are well dispersed in the reaction medium during the polymerization. Kim et al. showed that oligomers from thiophene monomer act as dispersing agent for carbon nanotubes in non aqueous reaction medium [44]. Authors showed that the dispersion is facilitated by the strong adsorption of thiophene ring to CNT surface caused by charger-transfer from the CNT to the strongly electronegative sulfur atom of the monomer. In the present case also, methyl thiophene monomer or oligomer, which forms during initial stages

Fig. 2 shows the thermograms of PMT nanocomposites at different PMT concentration carried out in air. Pure MWCNT shows one step degradation process with initial degradation temperature of 540 ◦ C and after that it starts to oxidize into CO2 . The low ash content of MWCNT indicates that the purity of MWCNT used for the nanocomposite preparation is above 95%. Pure PMT also shows one step degradation process with initial degradation temperature of 350 ◦ C. The nanocomposites are characterized by two step degradation profile corresponding to degradation of polymer followed by oxidation of MWCNT. Thus thermal stability of the nanocomposites shows increasing trend with increase in the MWCNT content. At higher concentration of MWCNT, the thickness of the coating decreases and this increases the stability of PMT. 3.3. Raman spectroscopy Raman spectroscopy was carried out for the nanocomposites to study the interaction between PMT and MWCNT. Fig. 3 shows the Raman spectra of nanocomposites at different concentration

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Intensity /arbitrary Units

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a b c d e f g

10

15

20

25

30

35

40

45

2θ /° Fig. 2. TGA of PMT nanocomposites. (a) MWCNT, (b) PMT12.5, (c) PMT25, (d) PMT50, (e) PMT75, (f) PMT87.5 and (g) PMT.

of PMT. Spectrum of pure MWCNT shows characteristic D band at 1342 cm−1 and G band at 1586 cm−1 [46]. The spectrum of pure PMT shows peak at 1454 cm−1 (ring stretching of symmetric C˛ –Cˇ ), 1354 cm−1 (ring symmetric stretching Cˇ –Cˇ ), 1205 cm−1 (symmetric stretching of interring C˛ –C˛ ) and 1186 cm−1 (Cˇ –H bending) [47]. The interaction between MWCNT and PMT is evident from the shift in the peak from 1454 cm−1 to 1446 cm−1 in the nanocomposites. The peaks at 1342 cm−1 for MWCNT and 1354 cm−1 for PMT merge to give a broader peak at 1357 cm−1 in the nanocomposites. The intensity of peaks corresponding to D and G band of MWCNT reduces with decrease in MWCNT content in the nanocomposites. The shifting and broadening of the peak is attributed to the ␲–␲ interaction between thiophene ring of PMT and six member ring of MWCNT [46]. 3.4. X-ray diffraction Fig. 4 shows the XRD pattern of the PMT nanocomposites at various compositions. Pure MWCNT shows a strong peaks at 2 = 27◦ (0 0 2) and 43◦ (1 0 1) [48]. These peaks can be attributed to the distance between the concentric layers of the MWCNT. Pure PMT shows a broad amorphous peak at 2 = 24◦ . XRD pattern of nanocomposites shows broad peak at 2 = 24–27◦ , due to mixing of the peaks of MWCNT and PMT.

Fig. 4. XRD of PMT nanocomposites. (a) MWCNT, (b) PMT12.5, (c) PMT25, (d) PMT50, (e) PMT75, (f) PMT87.5 and (g) PMT.

3.5. Electrochemical characterization 3.5.1. Cyclic voltammetry (CV) studies of PMT nanocomposites CV of PMT nanocomposites electrodes was carried out using three-electrode method at scan rate of 10 mV s−1 and profiles are shown in Fig. 5. The CV profiles show p-doping of PMT nanocomposites at different concentrations of PMT. Pure MWCNT shows a rectangular profile which indicates that capacitance is predominantly due to electric double layer while that of pure PMT indicates predominantly pseudocapacitance behaviour with redox peaks at 0.52/0.17 vs Ag/Ag+ . With increase in PMT loading in the nanocomposites, the cathodic and anodic current increases implying increased capacitance. The redox peaks of nanocomposites at lower concentration of PMT show sharp redox peaks while for higher concentration the peak gets broadened and they also get shifts to higher potential. This is due to the increase thickness of the PMT coating over MWCNT as observed from the TEM micrographs. Fig. 6 shows the charging–discharging profile of PMT nanocomposites carried out at a current density of 0.625 mA cm−2 (1 A g−1 ) in the voltage range of 0–0.85 V vs Ag/Ag+ . The specific capacitance of the nanocomposites was calculated using Eq. (1) C=

Fig. 3. Raman spectroscopy of PMT nanocomposites.

It Vm

(1)

Fig. 5. CV of PMT-MWCNT nanocomposites during p-doping. (a) MWCNT, (b) PMT12.5, (c) PMT25, (d) PMT37.5, (e) PMT50, (f) PMT75, (g) PMT87.5 and (h) PMT.

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Specific capacitance/Fg

-1

350 300 250 200 150 100 50 0 0

20

40

60

80

100

Wt. % Fig. 6. Galvanostatic charge–discharge curves of PMT nanocomposites during pdoping. (a) MWCNT, (b) PMT12.5, (c) PMT25, (d) PMT37.5, (e) PMT50, (f) PMT75, (g) PMT87.5 and (h) PMT.

where I is the current, t is the time of discharge, V is the potential difference and m is the mass of electrode. Specific capacitance was calculated from the linear portion of discharge curve after IR drop. Specific capacitance based on the total electrode mass and also based only on nanocomposites in the electrode is given in the Table 2. The specific capacitance of CC is found to be around 10–12 F g−1 and is not considered for the specific capacitance calculation of the nanocomposites. Specific capacitance values based only on nanocomposites is taken further analysis. The specific capacitances value reported here for pure PMT is close to that reported in the literature [23–27]. The specific capacitance values for nanocomposites are more than just the arithmetic sum of the specific capacitance of MWCNT and PMT. This is evident from the Fig. 7, which shows that the specific capacitance of the nanocomposites falls above the arithmetic mean line. The observed effect on the specific capacitance of nanocomposites can be attributed to the morphology of thin wrapping of polymer over MWCNT. Thin wrapping of polymer over MWCNT increases the surface area of the polymer and thereby increases the accessibility of ions to most of the polymer chains. In conducting polymers, doping and dedoping (charging and discharging) involves intercalation and depletion of ions in the matrix of the polymer in order to maintain the electroneutrality [15,16]. In the bulk polymer, intercalation of ions into the inner most polymer chain is very difficult due to low ionic diffusion and hence the specific capacitance obtained for the conducting polymer is far less than the theoretical value [34]. On the other hand, morphology of thin coating of polymer over MWCNT increases surface area and thereby enhances the accessibility of ions to most of the polymer chains. Huang et al. demonstrated Table 2 Specific capacitance of PMT, PMT nanocomposites and MWCNT based on total electrode mass and PMT nanocomposite present in the electrode. Electrode material

Specific capacitance based on total electrode mass (F g−1 )

Specific capacitance based on PMT nanocomposite (F g−1 )

MWCNT PMT12.5 PMT25 PMT37.5 PMT50 PMT75 PMT87.5 PMT

23.9 34.5 46.6 66.7 90.3 107.4 144.1 113.1

25.1 70.7 95.6 136.7 185.3 220.2 295.6 231.8

Fig. 7. Specific capacitance of PMT nanocomposites as a function of PMT concentration. Red line represents the arithmetic mean line. Blue line represents specific capacitance. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

that thin polyaniline (PANI) layer around CNT offers low resistance to ionic diffusion and accessibility of ions to most of the active cite of PANI increases which, in turn, increases the specific capacitance of PANI [34]. Frackowaik et al. reported synergic effect between different types of CNT and PPY [49]. Lota et al. showed that poly(3,4-ethylene dioxythiophene) (PEDOT)-MWCNT nanocomposites exhibited higher capacitance due to the synergic effect of two components with an efficient energy extraction from PEDOT [50]. PANI/single walled carbon nanotubes (SWCNT) nanocomposites exhibited specific capacitance of 485 F g−1 for 73% PANI deposition on SWCNT while PANI and SWCNT showed specific capacitance of 234 and 34 F g−1 respectively [35]. 3.5.2. Unit cell characterization Unit cell of asymmetric supercapacitors have been assembled with PMT nanocomposites as the positive electrode and AC as the negative electrode. The challenge in the asymmetric supercapacitors is the optimization of electrode mass ratio so as to get maximum specific capacitance and energy density of the device while maintaining equal voltage swing between the electrodes in order to get enhanced cycle life. Recently, Snook et al. proposed a series of mathematical formula for the optimization of electrode materials for asymmetric supercapacitors, for obtaining maximum specific capacitance and energy density with extended cycle life [51]. Cell capacitance (CT ) of asymmetric supercapacitor is given by 1 1 1 = + CT CP CC

(2)

where CP and CC are the capacitance of positive (PMT nanocomposites) and negative (AC) electrodes respectively. The capacitance of electrode material in term of specific capacitance of the individual electrode mass is given by Eqs. (3) and (4) CP = mP SP

(3)

and CC = mC SC

(4)

where mP and SP are mass and specific capacitance of positive electrode material respectively, while mC and SC are mass and specific capacitance of negative electrode material respectively. CT in terms

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187

Cell potential/ V -1.5 0.010

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

PMT25

AC

I/A

0.005

PMT25-AC

0.000

-0.005

-0.010 -1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

Electrode potential vs. (Ag wire)/ V Fig. 8. CV of positive electrode, negative electrode and unit cell of PMT25-AC supercapacitor.

of specific capacitance of total active material (ST ) in both the electrode is given by CT = (mP + mC )ST

(5)

ST in terms of specific capacitance of individual electrode material is given by the Eq. (5) ST =

SP SC /(SC + SP ) 1 + (1/)

(6)

where mP = mC

(7)

The electrode mass ratio at which maximum ST can be obtained is expressed by



max =

SC SP

(8)

All the unit cells have been prepared by fixing active electrode material ratio at  max . Table 3 gives  max calculated based on specific capacitance of PMT nanocomposites at different concentration of PMT and specific capacitance of AC (100 F g−1 ). 3.5.2.1. CV of the unit cells. In order to get maximum specific capacitance and energy density from the supercapacitors, all the unit cells have been fabricated by maintaining the positive to negative electrode mass ratio at  max . To find out, the potential sweep of individual electrodes of the cell, CV was carried out for AC and PMT nanocomposite electrodes separately with reference to silver wire, which acts as the quasi reference electrode. Fig. 8 shows the CVs of AC electrode, PMT25 electrode and PMT25-AC unit cell.

Fig. 9. CV of PMT nanocomposite-AC unit cells. (a) PMT12.5-AC, (b) PMT25-AC, (c) PMT50-AC, (d) PMT75-AC, (e) PMT87.5-AC and (f) PMT-AC.

AC exhibits rectangular shape CV due to the predominant electric double layer capacitive behavior even though some amount of pseudocapacitance coexists. On the other hand, as mentioned earlier, PMT nanocomposite exhibit CV, which is potential dependant due to their predominant pseudocapacitance behavior in addition to minor electric double layer capacitance [1]. Hence, the resultant CV profile of the unit cell is a cumulative effect of these two capacitive behaviours. Below 1 V of the cell, contribution by PMT nanocomposite electrode is comparatively less. The current response increases once the oxidative potential of PMT reaches and cell attains almost a plateau current value above 1 V. As the unit cells are constructed with electrode mass ratio  max , when the unit cell is cycled from 0 to 2.5 V, the potential sweep of negative electrode varies from 0 to −1.25 V vs ref while the positive electrode varies from 0 to 1.25 vs ref. The CV of supercapacitors carried out at scan rate of 25 mV s−1 is shown in the Fig. 9. The total specific capacitance of the cells (ST ) has been calculated using the Eq. (9) [52]: ST =

Q1 + Q2 2mV

(9)

where Q1 and Q2 are indicative of the sums of anodic and cathodic voltammetric charges during oxidation and reduction of the scans respectively, m is the mass of active material in both the electrodes of the cell and V is the potential difference. Q1 and Q2 values are obtained by integrating the CV curve between 1 and 2.5 V (V = 1.5 V) since in this voltage range, the galvanostatic charge–discharge curve of the supercapacitors exhibits linear profile (refer the charge–discharge section) and continuous charge–discharge cycles are carried out in this range. The values of ST obtained from CV are given in the Table 3 (STb ). The specific capacitance of PMT87.5-AC

Table 3 Theoretical and experimental specific capacitance (ST) of unit cells at different concentration of PMT. Unit cell

SP (F g−1 )

SC (F g−1 )

 max

ST a (F g−1 )

ST b (F g−1 )

ST c (F g−1 )

Emax d (Wh kg−1 )

PMT12.5-AC PMT25-AC PMT50-AC PMT75-AC PMT87.5-AC PMT-AC

70.7 95.6 185.3 220.2 295.6 231.8

100 100 100 100 100 100

1.19 1.02 0.74 0.67 0.58 0.66

20.1 24.5 28.9 32.4 37.4 34.9

18.6 23.8 27.7 31.8 36.7 32.9

19.8 25.9 30.1 33.5 38.5 34.1

17.2 22.5 26.1 29.1 33.4 29.6

a b c d

Theoretical value calculated from the Eq. (5). Experimental values calculated from the CV at the scan rate of 25 mV s−1 . Experimental values calculated from the charge–discharge curve. Calculated based on experimental values of STc .

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3.0

a

b

c d f

e

Cell potential / V

2.5

2.0

1.5

1.0 0

100

200

300

Time /s Fig. 12. Galvanostatic charge–discharge profile of PMT nanocomposite-AC unit cells. (a) PMT12.5-AC, (b) PMT25-AC, (c) PMT50-AC, (d) PMT75-AC, (e) PMT87.5-AC and (f) PMT-AC.

Fig. 10. CV of PMT87.5-AC (A) and PMT25-AC (B) supercapacitors at different scan rates. (a) 10 mV s−1 , (b) 2 5 mV s−1 , (c) 50 mV s−1 , (d) 100 mV s−1 and (e) 250 mV s−1 .

unit cell is found to 36.7 F g−1 for the total weight of the active material in the unit cell. Unit cells were subjected to different scan rates to study the rate capability of the supercapacitors. Fig. 10 shows the CVs for PMT87.5-AC and PMT25-AC at different scan rates. The change in the specific capacitance with scan rate for supercapacitors is shown in Fig. 11. PMT87.5-AC unit cell shows larger distortion in the CV profile, especially at higher scan rates, than that of PMT25-AC unit cell compared to their respective CVs at lower scan rates. Supercapacitor that has low internal resistance, change the current polarity fast when the direction of scan is reversed and they have negligible change in the CV profile as the scan rate is increased. On the other

hand, supercapacitor with high internal resistance, the change in current polarity is slow when the direction of scan is reversed and also results in large distortion in the CV profile. At lower scan rates, the concentration gradient at the electrode/electrolyte interface is low. Hence, insertion and de-insertion of ions in the polymer matrix (positive electrode) and electric double layer formation and neutralization on the activated carbon during charge and discharge of the unit cell is kinetically facile and results in enhanced specific capacitance. However, at higher scan rates, the concentration gradient at the electrode/electrolyte interface increases, inducing a lag in charging and discharging, which in turn distorts of CV profile [53] and also decreases specific capacitance. Fig. 11 also shows that the unit cell containing nanocomposite with lower MWCNT content has steeper decrease in specific capacitance. This indicates that the unit cells with higher content of MWCNT in the positive electrode have enhanced rate capability. Increasing the concentration of MWCNT in the polymer matrix facilitates charge transport in the electrode and hence might have low internal resistance. 3.5.2.2. Charge–discharge of the unit cell. Galvanostatic charge–discharge of the unit cells was carried out at a current density of 1.25 mA cm−2 and the results are shown in Fig. 12. The curves show symmetric charge–discharge profile in the potential range of 1–2.5 V. Specific capacitance of the unit cell was calculated using the Eq. (1), here, I is discharge current, t is discharge time, V is voltage and m is total mass of the active material in both the electrodes of the unit cell. The specific capacitance values obtained for the supercapacitors are given in Table 3 (STc ). Maximum specific energy of the unit cells based on the specific capacitance (ST ) can be calculated by Eq. (10) [51]: Emax =

2 1/2S T Vm 3.6

(10)

where Vm is the upper voltage limit of the cell. Emax values for the unit cell are given in Table 3. The values obtained here is lesser than that reported value. The upper cell voltage studied in the present case is 2.5 V even though supercapacitors can be operated with the maximum voltage up to 3 V for electrolyte containing TEABF4 in PC [27]. It should be noted that energy stored in the capacitor changes steeply even for a modest change in the voltage as it depends upon the square of Vm . Fig. 11. Specific capacitance of the supercapacitors vs scan rate. (a) PMT12.5-AC, (b) PMT25-AC, (c) PMT50-AC, (d) PMT75-AC, (e) PMT87.5-AC and (f) PMT-AC.

3.5.2.3. EIS of the unit cells. EIS studies were carried out for the supercapacitors at cell potential of 1.5 V and the results are shown

P. Sivaraman et al. / Electrochimica Acta 94 (2013) 182–191

-250

-150

PMT12.5-AC

30

Z"/ohm

-50

PMT25-AC PMT50-AC PMT75-AC

-10

-100

40

C'/ Fg-1

-200

Z"/ohm

a

PMT12.5-AC PMT25-AC PMT50-AC PMT75-AC PMT87.5-AC PMT-AC

189

20

PMT87.5-AC PMT-AC

10

-5

0 1.0E-02

0 0

5

Z'/ ohm

1.0E-01

10

1.0E+00 1.0E+01 1.0E+02 Frequency/ Hz

0

0

50

100

150

200

250 16

b

Z'/ ohm

PMT12.5-AC

Fig. 13. Impedance spectroscopy of supercapacitors. Inset shows the spectra at high frequency end.

PMT25-AC

12

in the Fig. 13. At high frequency end, where the spectra intercepts real axis denote the solution resistance. Generally impedance spectra contain a small semi circle at high frequency end followed by 45◦ slope in midfrequeny region which is due to the Warburg diffusion resistance, and a vertical spike at low frequency [3]. Presence of semicircle shifts the capacitive behavior along the real axis and increases the internal resistance of unit cell [54]. Absence of semi-circle in the figure suggests that electrode have low contact resistance between the current collector and electrode material have low resistance. The solution resistance of the unit cells is found to be 2.3 ± 0.5  (inset in Fig. 13). In order to understand the resistance associated during charging of the supercapacitors containing different wt. ratio of PMT and MWCNT in the positive electrode, the complex impedance obtained from spectra is resolved into real capacitance and loss capacitance [55,56]. The complex impedance of the supercapacitor can be expressed as Z(ω) = Z  (ω) + iZ  (ω) Z

(11)

Z

and are real and imaginary part of frequency depenwhere dant impedance of the supercapacitor respectively. The complex capacitance of the supercapacitor can be expressed as C(ω) = C  (ω) − iC  (ω)

(12)

where C =

−Z  (ω) ω|Z(ω)|2

(13)

C  =

Z  (ω) ω|Z(ω)|2

(14)

C represents the real part of frequency dependant capacitance and corresponds to the capacitance which is measured by galvanostatic charge–discharge at very low frequency while C represents the loss part of the frequency dependant capacitance and corresponds to the irreversible energy dissipation which may be due to dielectric loss of the medium [57]. Fig. 14a shows the real capacitance of the supercapacitors as a function of frequency and the maximum specific capacitance obtained at 10 mHz is given the Table 4. The loss capacitance of the supercapacitors show maximum at frequency, f0 and 1/f0 denote time constant  0 , which represents dielectric relaxation time. This time constant corresponds to the factor of merit of the supercapacitor [55]. The time constant denotes the time at which the supercapacitor shifts from resistive behavior to capacitive behavior. Fig. 14b shows the loss capacitance of the supercapacitors as a function of frequency. The peak

C"/Fg-1

PMT50-AC PMT75-AC PMT87.5-AC

8

PMT-AC

4

0 1.0E-02

1.0E-01 1.0E+00 1.0E+01 1.0E+02

Frequency/Hz Fig. 14. Real (a) and imaginary (b) capacitance of supercapacitors.

loss capacitance and  0 of the supercapacitors are also given in Table 4. Supercapacitors having higher capacitance exhibit higher loss capacitance. It is important to note that supercapacitors containing nanocomposite with higher MWCNT content show lower  0 and this implies that the MWCNT facilitates charging process. The coating thickness of PMT around MWCNT reduces with increase in MWCNT concentration in the nanocomposites and hence, diffusion of ions into the thinner layer of PMT must be easier and get doped quicker. This in turn results in smaller  0 than the nanocomposites with thicker layer of PMT around MWCNT. Supercapacitors charge fast when the internal resistance of the cell is small and the results also indicate that supercapacitors with higher MWCNT, exhibit lower internal resistance. This corroborates the CV results which exhibit better rate capability for supercapacitor containing higher MWCNT content in the electrode than that of supercapacitor containing pure PMT or lower concentration of MWCNT. 3.5.2.4. Cycling studies. Continuous charge–discharge was carried out for the supercapacitors at constant current of 1.25 mA cm−2 for 1000 cycles. Fig. 15 shows the change in specific capacitance

Table 4 Real capacitance, loss capacitance and time constant of supercapacitors. Unit cell

Real capacitance (F g−1 )

Loss capacitance (F g−1 )

 0 (s)

PMT12.5-AC PMT25-AC PMT50-AC PMT75-AC PMT87.5-AC PMT-AC

17.9 20.1 24.9 27.6 34.1 30.3

6.1 7.6 9.3 10.9 12.9 13.8

17.0 23.6 25.8 32.8 35.6 49.3

190

P. Sivaraman et al. / Electrochimica Acta 94 (2013) 182–191

higher stability during continuous charge–discharge cycling and the result is attributed to uniform wrapping of PMT around MWCNT and higher conductivity of the electrode material and equal swing in the voltage of the positive and negative electrodes of the cell.

Specific capacitance/F g-1

40 35

PMT87.5-AC

30

PMT-AC

25

PMT50-AC

Acknowledgements

PMT25-AC

The authors express their sincere thanks to Dr. R.S. Hastak, Director, NMRL Ambernath, for his encouragement and permission to publish this article. We would also like to acknowledge Sophisticated Analytical Instrumentation Facility (SAIF), IIT Bombay for Raman Spectroscopy and TEM experiments.

20 15 10 5

References

0 0

200

400

600

800

1000

1200

No of Cycles Fig. 15. Continuous charge–discharge cycling of supercapacitors.

of PMT-AC, PMT87.5-AC, PMT50-AC and PMT25-AC supercapacitors. PMT-AC supercapacitor shows sharp decrease in specific capacitance during initial 100 cycles and thereafter it decreases monotonically. The decrease in specific capacitance is found to be around 20%. For supercapacitors containing nanocomposites, both initial drop (initial 100 cycles) and the subsequent decrease in specific capacitance found to be lower compared to pure PMT supercapacitor. The decrease in specific capacitance is found to around 15% for the supercapacitors with nanocomposite electrodes which implies that the nanocomposite based supercapacitors have better stability during continuous charge discharge. In pure PMT, the possibility of charge trapping is more and they in turn promote faster degradation [58]. Moreover, insertion and de-insertion of ions in the matrix of conducting polymer involves volumetric changes and causes cycling degradation during cycling. However in nanocomposites, the volumetric changes and thereby mechanical strain on the electrodes are greatly reduced since conducting polymer is finely coated on the MWCNT and it also offer shorter diffusion length so as to enhance effective utilization of electrode material [5]. Additionally MWCNT present in the electrodes allows efficient charge transport and reduces the possibility of charge trapping inside the electrode and there by reduces the polymer degradation. 4. Conclusions PMT-MWCNT nanocomposites with varying concentration of PMT have been prepared by chemical method. TEM analysis reveals that the PMT get wrapped around MWCNT and thickness of wrapping around MWCNT depends upon the concentration of PMT in the nanocomposites. TGA analysis shows that nanocomposites have higher thermal stability than pure PMT. Strong interaction between MWCNT and PMT is evidenced by Raman spectroscopy and XRD. PMT87.5 nanocomposite shows specific capacitance of 296 F g−1 . The increase in specific capacitance calculated for PMT87.5 nanocomposite is found to be 68% more than that of pure PMT. Unit cell of supercapacitors have been fabricated using PMT nanocomposites as the positive electrode and AC as the negative electrodes by fixing their mass ratios at  max. The maximum specific capacitance obtained for unit cell PMT87.5-AC is found to be 38.5 F g−1 . Impedance spectra of the supercapacitors have been resolved into real and imaginary capacitance and the analysis reveals that the nanocomposite containing higher concentration of MWCNT can be charged faster. PMT nanocomposites exhibits

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