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Stretchable supercapacitors based on highly stretchable ionic liquid incorporated polymer electrolyte
Materials Chemistry and Physics 148 (2014) 48e56
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Stretchable supercapacitors based on highly stretchable ionic liquid incorporated polymer electrolyte P. Tamilarasan, S. Ramaprabhu* Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology Centre (NFMTC), Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A stretchable supercapacitor has been fabricated using stretchable electrolyte. Here ionic liquid incorporated polymer plays dual role as electrolyte and stretchable support. The developed device shows low equivalent series resistance. The device has specific capacitance of 83 F g1, at the specific current of 2.67 A g1. The energy density and power density of 25.7 Wh kg1 and 35.2 kW kg1, respectively.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2013 Received in revised form 26 June 2014 Accepted 9 July 2014 Available online 1 August 2014
Mechanical stability of electrolyte in all-solid-state supercapacitor attains immense attention as it addresses safety aspects. In this study, we have demonstrated, the fabrication of stretchable supercapacitor based on stretchable electrolyte and hydrogen exfoliated graphene electrode. We synthesized ionic liquid incorporated stretchable Poly(methyl methacrylate) electrolyte which plays dual role as electrolyte and stretchable support for electrode material. The molecular vibration studies show composite nature of the electrolyte. At least four-fold stretchability has been observed along with good ionic conductivity (0.78 mS cm1 at 28 C) for this polymer electrolyte. This stretchable supercapacitor shows a low equivalent series resistance (16 U) due to the compatibility at electrodeeelectrolyte interface. The performance of the device has been determined under strain as well. © 2014 Elsevier B.V. All rights reserved.
Keywords: Polymers Nanostructures Chemical synthesis Evaporation Mechanical properties Electrochemical techniques
1. Introduction All-solid-state supercapacitors have great advantage as it offers mechanical robustness. Several attempts are experimented towards a high performance, mechanically robust, electrochemically stable supercapacitors [1e3]. In this regards, solid state electrolytes,
* Corresponding author. E-mail address: [email protected] (S. Ramaprabhu). http://dx.doi.org/10.1016/j.matchemphys.2014.07.010 0254-0584/© 2014 Elsevier B.V. All rights reserved.
especially gel polymer electrolytes, attain more interest as they offer excellent mechanical stability and ionic conductivity [4e10]. Here, mobile phase is imbibed in host polymer, where polymer matrix gives mechanical stability and the mobile phase gives ionic conductivity to the electrolyte. In general, gel polymer electrolytes contain aqueous or organic solutions of suitable salts. Here, aqueous solutions limit the potential window of the electrolyte to the maximum of 1.23 V, whereas the flammability of organic solvents make the device non-robust and results in safety issues. Ionic liquids (ILs) attain more importance as electrolytes for energy
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storage applications due to their wide operating potential, high ionic conductivity, wide liquid range along with high thermal and electrochemical stability [11e13]. In addition, ILs are generally known as a unique mixture of 100% salt and 100% solvent, which accounts electrolyte depletion problem. Hence, conventional salt based electrolytes have been replaced by ionic liquids for superior performance. The scientific community have employed ionic liquid as mobile phase in gel polymer electrolyte. Recently, Ye et al., have reviewed the progress and development in various aspects of ionic liquid based polymer electrolytes [14]. Since, ionic liquid ions are made up of bulk organic moieties, the interaction with other organic molecules, like solvents, polymers, gases etc., is unique. In ionic liquid based gel polymer electrolyte, the ionic conductivity, electrochemical stability and mechanical properties, depend on the interaction between the host polymer and the mobile phase [15,16]. Several host polymers have been studies including, polyvinylidene fluoride (PVDF), polyvinylidene fluorideehexafluoropropylene (PVDFeHFP), polyacrylonitrile (PAN), polyethylene oxide (PEO) and poly (methyl methacrylate) (PMMA) [17]. Among them, PMMA and its co-polymers are well known for their mechanical strength with good electrochemical properties [5e8,18]. Duluard et al., made a detail investigation on PMMA based gel polymer electrolytes with ionic liquid as mobile phase [19]. In the recent past, graphene grabbed the interest of material science community, due to its unique physical, electrical and electrochemical properties, which promote graphene as a better choice for energy storage applications, especially in supercapacitors [20e22]. The electrodeeelectrolyte interface plays a major role in all-solid-state supercapacitors as it is one of the factor decides the equivalent series resistance (RESR) of the device. Hence, graphene based supercapacitors with mechanically robust, highly conductive electrolytes, which can make unique interface with electrode, are anticipated to have potential advantages. Graphene based all-solidstate supercapacitors with ionic liquid incorporated PVDFeHFP [23,24], PEO [24], PAN [25,26] electrolyte have been reported. But, much studies have not been done on ionic liquid incorporated PMMA as electrolyte for graphene based supercapacitor though it has good mechanical, thermal and electrochemical properties [19]. In this study, we have prepared [BMIM][TFSI] incorporated PMMA electrolyte (PMMA/[BMIM][TFSI]) and studied its properties. PMMA/[BMIM][TFSI] (1:2) electrolyte exhibits high ionic conductivity (0.78 mS cm1 at 28 C) and has good transparency in visible range (>98%). The electrolyte is a highly stretchable film with at least four-fold stretchability. From the inspiration drawn from the recent report on buckled single walled carbon nanotube based stretchable supercapacitors, [27e29] we fabricated stretchable supercapacitor device with highly stretchable polymer electrolyte (PMMA/[BMIM][TFSI]) and hydrogen exfoliated graphene (HEG) electrode and demonstrated. To the best of our knowledge, this is the first study on supercapacitor based on PMMA/[BMIM] [TFSI] electrolyte and graphene electrode. 2. Experimental section 2.1. Material synthesis PMMA/[BMIM][TFSI] was synthesized as follows: 300 mg of PMMA (M.W ¼ 550,000 g mol1) was dissolved in 10 ml of acetone by stirring at 1000 rpm for 30 min, which resulted in a transparent gel. Acetone has been chosen as solvent medium, since PMMA has good solubility and the solution has better miscibility with [BMIM] [TFSI] ionic liquid. Appropriate amount of [BMIM][TFSI] was added in to the gel under stirring. Further the solution was stirred for 2 h at 1000 rpm. A transparent gel was obtained which was casted on a bare or graphene coated glass surface at ambient conditions for
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24 h. Then the film was removed from the surface and dried under vacuum (400 mbar) and room temperature before use for further analysis. A solid state blending route is also employed to obtain PMMA/[BMIM][TFSI] blend, where PMMA powder was mechanically mixed with [BMIM][TFSI] using mortar and pestle. But the obtained blend was inhomogeneous at low IL to polymer ratios as IL interacts with polymer rapidly. Graphene was prepared from graphite by a two step topedown approach, reported elsewhere [30]. Briefly, graphite was oxidized using water-free mixture of conc. H2SO4, NaNO3 and KMnO4 (Hummers' method [31]) to result graphite oxide (GO). Graphene was obtained by thermal exfoliation of GO at 200 C under hydrogen atmosphere in a tubular furnace and labelled as HEG. The as-exfoliated HEG has the surface area as high as 443 m2 g1, with 1.64 103 S m1 of electrical conductivity with 14.6 C/O ratio [32]. 2.2. Fabrication of supercapacitor The stretchable supercapacitor has been fabricated as follows: HEG was dispersed in the solution of 5 wt % PMMA/[BMIM][TFSI] blend as a binder in acetone medium under ultrasonic irradiation. We chose acetone medium because PMMA/[BMIM][TFSI] blend is either insoluble or phase separated in most of the solvents, other than acetone. But, graphene with large amount of residual functional groups cannot be dispersed in acetone. Hence, PMMA/ [BMIM][TFSI] was first dissolved in acetone followed by the addition of required amount of graphene. We found a good dispersibility of graphene in this solution, which may be attributed to the deposition of PMMA/[BMIM][TFSI] blend on graphene surface. The dispersion was coated (~1 mg cm2) on a smooth glass surface. The PMMA/[BMIM][TFSI] electrolyte gel was poured on graphene film (50 mg cm2) and allowed to evaporate acetone for 24 h at ambient conditions. Then the electrodeeelectrolyte assembly (EEA) was removed from the surface. A thin layer of [BMIM][TFSI] was coated on electrolyte sides of two EEAs and the assemblies were joined as shown in Scheme 1. The device was dried for 1 h under vacuum (400 mbar) at room temperature in order to remove residual solvent molecules. Here, graphene film itself plays a role as current collector. In addition, electrolyte acts as support for electrode material, unlike the previous reports [27e29]. Stainless steel foils were used as electrical connections. This device was denoted as HEG e PMMA/[BMIM][TFSI] supercapacitor. 2.3. Characterization techniques The structural analysis of synthesized materials was done by X'Pert Pro PANalytical powder X-ray diffractometer. The morphological analysis was carried out by electron microscopy (FEI Quanta and Technai G-20). WITec alpha 300 Confocal Raman spectrometer, equipped with a Nd:YAG laser as the excitation source, has been employed to acquire Raman spectra while Fourier transform infrared spectra were acquired by PerkineElmer FT-IR spectrometer. The transmittance studies were carried our using JASCO corp., V-570 spectrophotometer. Electrochemical behaviour was studied using CH instrument (CHI608C). 3. Results and discussions 3.1. Morphological analysis The cross-sectional view of FESEM image of HEG e PMMA/ [BMIM][TFSI] supercapacitor (Fig. 1) reveals the graphene electrode thickness of ~150e200 mm. The cross-sectional view of roughly tore PMMA/[BMIM][TFSI] shows a smooth morphology. It is a consequence of microscopic elastic deformation of the film which gives
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Scheme 1. Fabrication of stretchable HEG e PMMA/[BMIM][TFSI] supercapacitor.
extraordinary mechanical stability. The incorporation of ionic liquid reduces the glass transition temperature of PMMA which makes it stretchable [19]. This can be confirmed by X-ray diffraction pattern and thermal analysis. The white spots embedded in the film may be nano-sized solid like short range ordering of ionic liquid. It is reported that ionic liquid ions makes short range ordering at the solid surfaces [33e35]. The morphological analysis clearly displays the unique, well connected interface between polymer electrolyte and graphene electrode produced by this method which can effectively reduce the RESR. The HRTEM image of HEG shows a highly wrinkled nature of graphene layers due to the rapid removal of functional
groups from the lattice. In energy storage applications the lattice defects (hexagons and pentagons) in graphene lattice plays a role as anchoring sites for charge storage which enhances the binding energy with ions. In addition the wrinkles present in graphene layers may assist the electrode to follow the expansion of electrolyte under strain and retain the electrodeeelectrolyte interface. 3.2. Powder X-ray diffraction analysis The X-ray diffractogram of PMMA (Fig. 2(a)) shows a peak at ~16 , which is a strong proof for the tactic nature of PMMA. It is
Fig. 1. FESEM image of HEG e PMMA/[BMIM][TFSI] supercapacitor.
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original shape within 2 min. Ionic liquids are identified as a good plasticizer for a wide range of polymers. Unlike other plasticizers, ions of IL interact with the polar side chain of PMMA which increases the mechanical stability and high stretchability. In addition, the digital image illustrates the transparency of the electrolyte also. The optical transmission spectrum (Fig. 3) of the dry PMMA/ [BMIM][TFSI] shows at least 95% transmittance in the complete visible region which is in good agreement with literature [19]. These properties promote PMMA/[BMIM][TFSI] as a unique electrolyte for stretchable electrochemical devices. 3.4. Molecular vibration spectra of PMMA/[BMIM][TFSI] electrolyte
Fig. 2. X-Ray diffraction pattern of (a) PMMA and (b) PMMA/[BMIM][TFSI].
clear from the XRD pattern of PMMA/[BMIM][TFSI] (Fig. 2(b)) that the tacticity of PMMA is reduced significantly due to the incorporation of ionic liquid. Here, it is anticipated that the ionic liquid ions are accommodated in the inter-chain space and disturb the orientation of building blocks. This reduces the glass transition temperature of tactic PMMA matrix and makes it stretchable. We observed a new peak at ~13 , corresponding to the inter-layer distance of ~0.7 nm, along with PMMA peaks. This may arise due to the short range ordering of IL in the polymer matrix which is seen in FESEM image as well [33e35]. 3.3. UVeVisible spectrum of PMMA/[BMIM][TFSI] electrolyte The electrolyte is a transparent, stretchable film with excellent mechanical retainability. The inset in Fig. 3 clearly demonstrates at least four-fold stretchability of the PMMA/[BMIM][TFSI]. We anticipated, as a conclusion of XRD analysis, ionic liquid ions may accommodate in the inter-chain space and reduce the interaction, which results in highly stretchable film. The given external force may elastically deform the polymer array to elongate it. As soon as the force is released, the film restores its shape with less than 150% of its initial length which is also displayed. The film retains its
The interaction between host polymer and mobile phase has been analysed by molecular vibration spectroscopy (Raman and FTIR spectroscopy). Table 1 shows the detailed assignments for the Raman and IR features of PMMA/[BMIM][TFSI]. The Raman spectrum of PMMA (in Fig. 4(a)) shows peaks corresponding to stretching and bending vibrations of CeH at 3000, 2950 cm1 and 1454 cm1, respectively, which has the corresponding features in IR spectrum also. In addition, the presence of stretching vibrations of CH2 (2950, 883 cm1), C]O (1727 cm1), CeO (1243 cm1), CeCOO (1243 cm1), CeOeCe (1131 cm1 in IR), CeC (1065 cm1), CeCeO (603 cm1) and bending vibration of CeH (1454 cm1) along with rocking modes of OeCH3(979 cm1), a-CH3(964 cm1 in IR) confirms the structure of PMMA [36,37]. The long polymeric chain results in the skeletal mode of v(CeC) (1065 cm1) vibrations. A combination band involving OeCH3 vibrations has been assigned to the feature present at 2845 cm1. The incorporation of ionic liquid into the polymer matrix introduces the bands, corresponding to the constituents. The in-plane (ip) stretching vibrations of imidazolium ring appear at 900 to 1600 cm1 region along with a peak at 3172 cm1 while the bending vibrations occur at low wavenumber region (600e800 cm1). The out-plane (op) asymmetric bending vibration of imidazolium ring is positioned at 654 cm1. The NeCH2 and NeCH3 stretching modes (600e1600 cm1 region) confirms that the side chains are attached with the nitrogen atom of the imidazolium ring. The butyl chain produces the d(CeCeCeCeH) and v(CeCeCeC) vibrations. The sulfonylimide (1392 cm1), symmetric (1142, 603 cm1) and asymmetric (1339, 1243 cm1) vibrations of SO2, stretching (1243 cm1) and bending (746 cm1) vibrations of CF3 confirms the structure of trifluoromethylsulfonyl imide anion. The appearance of symmetric stretching mode of SO3, from free triflates (1026 cm1) reveals the non-aggregation of electrolyte ions [38]. Fig. 4(b) clearly shows that the v(CeC) mode (1065 cm1) is strengthened due to the incorporation of ionic liquid. The v(CeO) feature is superimposed with vs(SO2) and produces a strong signal centred at 1177 cm1 [39,40]. Table 1 and Fig. 4 clearly show that all the existing peaks are assigned to either PMMA or [BMIM][TFSI] and no new vibrational signal is produced. This detailed analysis suggests that there is no chemical bonding between host polymer and ionic liquid and ions simply stay at inter-chain or inter-side chain space. This indeed reduces the interaction between polymer chains and makes it stretchable. In addition, the presence of foreign moieties (ionic liquid) disturbs the tacticity of PMMA which is confirmed by XRD pattern. 3.5. Electrochemical analysis
Fig. 3. UVeVisible spectrum of PMMA/[BMIM][TFSI]. Inset: digital image of demonstration of transparency and stretchability.
The electrochemical analysis has been carried out using a more practical two electrode supercapacitor setup at ambient conditions. The ionic conductivity of PMMA/[BMIM][TFSI] is determined by AC impedance analysis where stainless steel foil has been used as
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Table 1 Spectral assignments for molecular vibration spectra of PMMA and PMMA/[BMIM] [TFSI]. Wavenumber (cm1)
PMMA [36,37]
[BMIM][TFSI] [39,40]
vs(ring HCeCH), v(CeH of NeCHeN), vs(ring ip) va(CH3eNeCH) va(CeH) vs(CeH) &va(CH2) vs(terminal CH3HCH) e
Raman
IR
3172
3159
e
3113 3000 2950 2879 2845
3120 3003 2960 2880 e
1727 1568 e 1454
1726 1571 1486 1453
e va(CeH) vs(CeH) &va(CH2) e Combination band involving OeCH3 v(C]O)from (CeCOO) e d(CH2) da(CeH)
1421
e
e
1392
1385
e
1339
1348
e
e 1243 1142 e 1065 1026
1330 1276, 1228 1177 1131 1052 e
v(CeO) va(CeOeCe) v(CeC)skeletal mode e
979 e 883 & 817 e 746
988 964 844 787 739
OeCH3 rock aeCH3 rock v(CH2) e e
e 603
654 613
e v(CeCOO), vs(CeCeO)
v(CeO), v(CeCOO)
e va(ring ip), v(CH3(N)), v(CH2(N)) d(CH2) ds(CCH HCC), ds(CH3(N)HCH), terminal ds(CH3 HCH) va(ring ip), v(CeC), v((N)CH2), v(CH3(N)CN) va(ring ip), v(CeC), v((N)CH2), v(CH3(N)CN), sulfonamide from anion vs(ring ip), v(CH2(N)), va(SO2), v(CeC), v(CeN of NeCH3) v(CeCeCeC), va(ring ip) va(SO2), vs(CF3) vs(SO2) e v(CeC) vs(CeF), vs(ring ip), v(CH3(N)), v(CH2(N)) vs (SO3) from free triflates anion va(ring ip), v(CeC), e v(CH2) d(CeCeCeCeH) 2 conformers of [TFSI], ds(ring HCCH), ds(CF3) v(CH3(N)), v(CH2(N)), da(ring op) ds(ring ip), ds(SO2), v(CH2(N)), v(CH3(N)CN)
current collectors. Un-doped PMMA has the ionic conductivity in the order of 1016 S cm1. It is anticipated that, ionic liquid ions are simply stored in the inter-chain space by a weak hydrogen bonding between the hydrogen from host polymer and highly electro
Fig. 4. (a) Raman spectrum and (b) FTIR spectrum of PMMA/[BMIM][TFSI].
negative nitrogen in IL cation. Storage of IL in polymer matrix has a threshold, known as percolation threshold. Hence, the ionic conductivity should depends on the IL concentration, not surprisingly. Fig. 5 shows significant improvement in ionic conductivity of the polymer due to the incorporation of IL. However, the improvement in ionic conductivity and stretchability are not much impressive at low ionic liquid concentration. We observed a considerable raise in ionic conductivity above 1:1 polymer to IL ratio. This may be attributed to the polymer-IL interaction. Briefly, IL ions are immobilised in the inter-chain space of polymer matrix by weak hydrogen bonding between the hydrogen from host polymer and highly electro negative nitrogen in IL cation. The amount of ionic liquid has a threshold limit known as percolation threshold. Above this limit, the excess IL ions make long-range connectivity in the GPE system and move freely under the applied potential. In polymer electrolytes, the segmental motion or alignment of polar moieties of host polymer also contributes in ionic conductivity along with the mobile phase. PMMA is a polar material at above the glass transition temperature (Tg) [41]. Puyu et al. have reported that [BMIM][PF6] ionic liquid can play a role as plasticizer for PMMA to attain a low Tg and high thermal stability along with good compatibility [42]. The low Tg facilitates improvement in segmental motion or alignment of polar moieties and thus high ionic conductivity. We have studied different compositions up to 1:2 ratios of PMMA to IL. PMMA/[BMIM][TFSI] (1:2) has the ionic conductivity of 0.78 mS cm1 at 28 C, which is comparable to that of PMMA/ LiClO4/EC/PC electrolyte, along with remarkable stretchability [38]. The polymer electrolytes, up to 1:1 composition, are flexible, but hard to stretch. Further addition of IL, gradually decreases the hardness. The compositions above 1:2.5 result in a transparent, sticky, mechanically unstable gel. Since IL plays a role as plasticizer, higher composition reduces the polymer chain interaction and thus the mechanical stability. The inset in Fig. 5 shows ionic conductivity of PMMA/[BMIM][TFSI] (1:2) as a function of temperature which suggests the temperature assisted improvement in ionic transport above 310 K. At low temperatures, ionic liquid produces solid-like long range ordered particles in the polymer matrix which is confirmed by FESEM image. As temperature increases, immobilized ions get high kinetic energy and tend to move freely. Further increase in temperature increases the kinetic energy ions and shows a linear behaviour. The performance of the device has been reported in terms of mass of electrode materials. The device was activated to stabilize
Fig. 5. Ionic conductivity as a function of composition. Inset: Variation of ionic conductivity with temperature.
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Fig. 6. Cyclic voltammograms of HEG e PMMA/[BMIM][TFSI] supercapacitor. Fig. 8. Discharge behaviour of HEG e PMMA/[BMIM][TFSI] supercapacitor at various specific currents.
the performance by scanning 100 cycles at 100 mV s1 scan rate. Specific capacitance (Cs) was calculated using the equation-
Cs ¼ 2
I m dV dt
(1)
where, I is the current, dV/dt is the potential scan rate and m is the mass of electrode material at each electrode. A factor of two is multiplied due to the series capacitance formed in two electrode system [43]. The cyclic voltammograms (CV) of HEG e PMMA/[BMIM][TFSI] stretchable supercapacitor is shown in Fig. 6 at various scan rates, which clearly shows that the device is capable of operating at least up to 3 V, which is less than that of neat [BMIM][TFSI] (3.5 V). The CV deviates from the rectangular shape above 3 V and shows rapid increase in current. We expect irreversible electrochemical reaction (like a pseudocapacitor) of PMMA at this potential. The CV shows no oxidation or reduction peak, which ensures the complete removal of solvent molecules. The rectangular shape of the CV ensures the better supercapacitive behaviour. However, at high scan rates the CV deviates much from the ideal response. Here, specific capacitance has been calculated from Equation (1). The device is highly flexible and stretchable with at least twofold stretchability. The performance of the device under strain
Fig. 7. Discharge behaviour of HEG e PMMA/[BMIM][TFSI] supercapacitor under strain. Inset: change in specific capacitance with strain cycles.
and the effect of strain cycles have been determined by chargeedischarge cycles. Fig. 7 shows that the device losses 40% of its specific capacitance under strain (30%). This may be attributed to the discontinuities introduced in graphene network by the elongation of polymer which affects the electrical conductivity of electrode. However the device retains at least 90% of its initial capacitance when the strain is released (inset in Fig. 7). It has to be noted that the subsequent strain cycles did not result much change in capacitance. The chargeedischarge behaviour of the device is shown in Fig. 8 at various specific current. Specific capacitance (Cs) of the device has been measured employing Equation (1). Here, I is the specific current and dV/dt is the slop of the straight line fit of discharge curves from Vmax to Vmax/2 [43]. The intercept of linear fit with potential axis gives the potential drop (Vdrop) due to Equivalent series resistance (RESR ¼ Vdrop/I). The performance of the supercapacitor with chargeedischarge cycles is reported at different current with at least 96% coulombic efficiency. The device shows specific capacitance of 83 F g1 at 2.67 A g1 which is almost 60% higher than the previous reports [27e29]. The increase in scan rate or current affects capacitance (Fig. 9). The specific capacitance of the device was ~220 F g1 at 5 mV s1 scan rate. However, a slight raise in scan rate results a large fade in capacitance (Fig. 9(b)). This may be attributed to the following reasons, first, the finite mobility of the massive electrolyte ions and second, the ionic liquid ions play a role as plasticizer by
Fig. 9. (a) Specific capacitance as a function of specific current and (b) scan rate.
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accommodating the inter-chain space of PMMA matrix, which effectively traps the ions from moving under applied potential. At very low scan rates, the ions find sufficient time to get out of the trap. At high scan rates, the polarity of the device may reverted before the ion move significant distance in the polymer matrix which drives the ion in opposite direction. This suggests that this electrolyte can perform two-fold better in slow chargingedischarging applications. Capacitance falls with increase in current due to the rapid motion of massive ions which results in poor accommodation of ions inside the pores of electrode (Fig. 9(a)). At high currents, the ions get insufficient time to diffuse into the pores of electrode which results in a simple polarization followed by the accumulation of charges at the electrodeeelectrolyte interface. An electrochemical device should have long life time. The cyclic stability of the device with repeated chargeedischarge cycles has been determined with a newly fabricated cell. Fig. 10 shows at least 88% of its initial performance even after 1000 cycles at 2.67 A g1. The capacitance falls ~12% in the initial 200 cycles followed by a stabilized performance. The initial fade may be attributed to activation of the device. However, after activation the device shows stable performance which is an evidence for electrochemical stability of HEG e PMMA/[BMIM][TFSI] electrodeeelectrolyte system. Ragone plot relates the energy density and power density of an energy storage device. The energy density (E) and power density (P) of supercapacitor have been calculated using (2) and (3), respectively.
1 E ¼ Ccell V 2 2
P¼
E Dt
(2)
(3)
Fig. 11. Ragone plot and Inset: power dissipation profile of HEG e PMMA/[BMIM][TFSI] supercapacitor.
energy storage capacity (Emax) and the maximum available power (Pmax) of the supercapacitor. The Ragone equation is represented as.
Emax EL ¼ 2
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! P 1þ 1 L Pmax
(4)
where, EL, PL, Emax and Pmax are available energy, power to the external load, maximum energy storage capacity and maximum available power respectively [44]. The fitting parameters reveal that the values of Emax and Pmax are 25.7 Wh kg1 and 35.2 kW kg1, respectively, which is lightly larger than the recent report [28]. The Emax and Pmax can be related by the time constant.
to ¼
Emax 2Pmax
(5)
where, Ccell is the specific capacitance of the device, V is the cell potential and Dt is the discharge time for the corresponding potential [43]. The Ragone plot of HEGePMMA/[BMIM][TFSI] supercapacitor (Fig. 11) shows the energy density and power density of 26.1 Wh kg1 and 5 kW kg1, respectively, at the specific current of 2.67 A g1 corresponding to the Cs of 83 F g1, in terms of mass of total electrode material. The Ragone plot drops faster at high power due to the fast voltage decay during discharge. Ragone equation has been fitted to the experimental data in order to extract maximum
where, time constant (to) is the time necessary to discharge the capacitor to 36.8% of its initial voltage in a short found to be 101 ms for the HEGePMMA/[BMIM][TFSI] supercapacitor [44]. The inset in Fig. 11 shows the internal power dissipation (PESR¼I2RESR) in the device with respect to discharge current. Nyquist plot of supercapacitor, at 500 mV sinusoidal measuring potential, is given in Fig. 12. The linear fit to the low frequency region reveals that the equivalent series resistance (RESR) is 16 U at
Fig. 10. Cyclic stability of HEG e PMMA/[BMIM][TFSI] supercapacitor at specific current of 2.67 A g1.
Fig. 12. Nyquist plot and Inset: Change in RESR with strain cycles.
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room temperature, which is much lesser than the previous report on stretchable supercapacitor (~350 U) [28]. This effectively helps to reduce the internal power dissipation. Here RESR is composed of ionic and electronic contributions. The ionic contribution comes from the mobility of electrolyte ions due to its mass and interaction with host polymer. In addition, the electrodeeelectrolyte interface resistance also contributes as there is a possibility for the incompatibility between electrode and bulky electrolyte ions. The electronic contribution comes from the ohmic resistance in the current collector and electrode. Hence, the low RESR can be attributed to the compatibility at the electrodeeelectrolyte interface as shown in FESEM image of the interface. The strained device shows higher RESR than the strain-free one (inset in Fig. 12). A 30% strained device shows 69 U, which can be attributed to the ohmic resistance, added by the electric discontinuities in graphene layers due to the electrolyte elongation. The inset in Fig. 12 shows the change in RESR of retained as a function of number of strain cycles. The measurements were taken after 5 min of every strain cycle. It is clear that the retained device shows a slight increase in RESR in the initial 4 cycles followed by the nearly unchanged values. 4. Summary This study described the fabrication of a stretchable supercapacitor using highly stretchable ionic liquid incorporated poly(methyl methacrylate) (PMMA/[BMIM][TFSI]) electrolyte where hydrogen exfoliated graphene has been used as electrode. The structural, molecular vibrational, optical and electrochemical properties of the electrolyte have been determined. A stretchable supercapacitor comprised of PMMA/[BMIM][TFSI] electrolyte has been fabricated with HEG electrode. The specific capacitance, energy density and power density of the device was 83 F g1, 26.1 Wh kg1 and 5 kW kg1, respectively, at the specific current of 2.67 A g1, in terms of mass of total electrode material. At least 90% of initial capacitance has been retained even after repeated strain cycles. The equivalent series resistance (RESR) of 16 U leads to 20e35% internal power dissipation. We observed a slight raise in RESR due to strain cycles. The detailed analysis suggests that PMMA/ [BMIM][TFSI] electrolyte can play both roles as electrolyte and stretchable substrate for electrode in transparent, stretchable electric devices. Acknowledgements Authors acknowledge SAIF-IITM for FTIR analysis. One of the authors (Tamilarasan) acknowledges Indian Institute of Technology Madras (IITM) for the financial supports (Senior research fellowship). References [1] K. Yu Jin, C. Haegeun, H. Chi-Hwan, K. Woong, All-solid-state flexible supercapacitors based on papers coated with carbon nanotubes and ionic-liquidbased gel electrolytes, Nanotechnology 23 (2012) 065401. [2] C.L. Pint, N.W. Nicholas, S. Xu, Z. Sun, J.M. Tour, H.K. Schmidt, R.G. Gordon, R.H. Hauge, Three dimensional solid-state supercapacitors from aligned single-walled carbon nanotube array templates, Carbon 49 (2011) 4890e4897. [3] L. Su, Z. Xiao, Z. Lu, All solid-state electrochromic device with PMMA gel electrolyte, Mater. Chem. Phys. 52 (1998) 180e183. [4] K.H. Wen Lu, Craig Turchi, John Pellegrino, Incorporating ionic liquid electrolytes into polymer gels for solid-state ultracapacitors, J. Electrochem. Soc. 155 (2008) A361eA367. [5] H. Yang, M. Huang, J. Wu, Z. Lan, S. Hao, J. Lin, The polymer gel electrolyte based on poly(methyl methacrylate) and its application in quasi-solid-state dye-sensitized solar cells, Mater. Chem. Phys. 110 (2008) 38e42. [6] S. Rajendran, O. Mahendran, R. Kannan, Lithium ion conduction in plasticized PMMAePVdF polymer blend electrolytes, Mater. Chem. Phys. 74 (2002) 52e57.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Stretchable supercapacitors based on highly stretchable ionic liquid incorporated polymer electrolyte P. Tamilarasan, S. Ramaprabhu* Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology Centre (NFMTC), Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A stretchable supercapacitor has been fabricated using stretchable electrolyte. Here ionic liquid incorporated polymer plays dual role as electrolyte and stretchable support. The developed device shows low equivalent series resistance. The device has specific capacitance of 83 F g1, at the specific current of 2.67 A g1. The energy density and power density of 25.7 Wh kg1 and 35.2 kW kg1, respectively.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2013 Received in revised form 26 June 2014 Accepted 9 July 2014 Available online 1 August 2014
Mechanical stability of electrolyte in all-solid-state supercapacitor attains immense attention as it addresses safety aspects. In this study, we have demonstrated, the fabrication of stretchable supercapacitor based on stretchable electrolyte and hydrogen exfoliated graphene electrode. We synthesized ionic liquid incorporated stretchable Poly(methyl methacrylate) electrolyte which plays dual role as electrolyte and stretchable support for electrode material. The molecular vibration studies show composite nature of the electrolyte. At least four-fold stretchability has been observed along with good ionic conductivity (0.78 mS cm1 at 28 C) for this polymer electrolyte. This stretchable supercapacitor shows a low equivalent series resistance (16 U) due to the compatibility at electrodeeelectrolyte interface. The performance of the device has been determined under strain as well. © 2014 Elsevier B.V. All rights reserved.
Keywords: Polymers Nanostructures Chemical synthesis Evaporation Mechanical properties Electrochemical techniques
1. Introduction All-solid-state supercapacitors have great advantage as it offers mechanical robustness. Several attempts are experimented towards a high performance, mechanically robust, electrochemically stable supercapacitors [1e3]. In this regards, solid state electrolytes,
* Corresponding author. E-mail address: [email protected] (S. Ramaprabhu). http://dx.doi.org/10.1016/j.matchemphys.2014.07.010 0254-0584/© 2014 Elsevier B.V. All rights reserved.
especially gel polymer electrolytes, attain more interest as they offer excellent mechanical stability and ionic conductivity [4e10]. Here, mobile phase is imbibed in host polymer, where polymer matrix gives mechanical stability and the mobile phase gives ionic conductivity to the electrolyte. In general, gel polymer electrolytes contain aqueous or organic solutions of suitable salts. Here, aqueous solutions limit the potential window of the electrolyte to the maximum of 1.23 V, whereas the flammability of organic solvents make the device non-robust and results in safety issues. Ionic liquids (ILs) attain more importance as electrolytes for energy
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storage applications due to their wide operating potential, high ionic conductivity, wide liquid range along with high thermal and electrochemical stability [11e13]. In addition, ILs are generally known as a unique mixture of 100% salt and 100% solvent, which accounts electrolyte depletion problem. Hence, conventional salt based electrolytes have been replaced by ionic liquids for superior performance. The scientific community have employed ionic liquid as mobile phase in gel polymer electrolyte. Recently, Ye et al., have reviewed the progress and development in various aspects of ionic liquid based polymer electrolytes [14]. Since, ionic liquid ions are made up of bulk organic moieties, the interaction with other organic molecules, like solvents, polymers, gases etc., is unique. In ionic liquid based gel polymer electrolyte, the ionic conductivity, electrochemical stability and mechanical properties, depend on the interaction between the host polymer and the mobile phase [15,16]. Several host polymers have been studies including, polyvinylidene fluoride (PVDF), polyvinylidene fluorideehexafluoropropylene (PVDFeHFP), polyacrylonitrile (PAN), polyethylene oxide (PEO) and poly (methyl methacrylate) (PMMA) [17]. Among them, PMMA and its co-polymers are well known for their mechanical strength with good electrochemical properties [5e8,18]. Duluard et al., made a detail investigation on PMMA based gel polymer electrolytes with ionic liquid as mobile phase [19]. In the recent past, graphene grabbed the interest of material science community, due to its unique physical, electrical and electrochemical properties, which promote graphene as a better choice for energy storage applications, especially in supercapacitors [20e22]. The electrodeeelectrolyte interface plays a major role in all-solid-state supercapacitors as it is one of the factor decides the equivalent series resistance (RESR) of the device. Hence, graphene based supercapacitors with mechanically robust, highly conductive electrolytes, which can make unique interface with electrode, are anticipated to have potential advantages. Graphene based all-solidstate supercapacitors with ionic liquid incorporated PVDFeHFP [23,24], PEO [24], PAN [25,26] electrolyte have been reported. But, much studies have not been done on ionic liquid incorporated PMMA as electrolyte for graphene based supercapacitor though it has good mechanical, thermal and electrochemical properties [19]. In this study, we have prepared [BMIM][TFSI] incorporated PMMA electrolyte (PMMA/[BMIM][TFSI]) and studied its properties. PMMA/[BMIM][TFSI] (1:2) electrolyte exhibits high ionic conductivity (0.78 mS cm1 at 28 C) and has good transparency in visible range (>98%). The electrolyte is a highly stretchable film with at least four-fold stretchability. From the inspiration drawn from the recent report on buckled single walled carbon nanotube based stretchable supercapacitors, [27e29] we fabricated stretchable supercapacitor device with highly stretchable polymer electrolyte (PMMA/[BMIM][TFSI]) and hydrogen exfoliated graphene (HEG) electrode and demonstrated. To the best of our knowledge, this is the first study on supercapacitor based on PMMA/[BMIM] [TFSI] electrolyte and graphene electrode. 2. Experimental section 2.1. Material synthesis PMMA/[BMIM][TFSI] was synthesized as follows: 300 mg of PMMA (M.W ¼ 550,000 g mol1) was dissolved in 10 ml of acetone by stirring at 1000 rpm for 30 min, which resulted in a transparent gel. Acetone has been chosen as solvent medium, since PMMA has good solubility and the solution has better miscibility with [BMIM] [TFSI] ionic liquid. Appropriate amount of [BMIM][TFSI] was added in to the gel under stirring. Further the solution was stirred for 2 h at 1000 rpm. A transparent gel was obtained which was casted on a bare or graphene coated glass surface at ambient conditions for
49
24 h. Then the film was removed from the surface and dried under vacuum (400 mbar) and room temperature before use for further analysis. A solid state blending route is also employed to obtain PMMA/[BMIM][TFSI] blend, where PMMA powder was mechanically mixed with [BMIM][TFSI] using mortar and pestle. But the obtained blend was inhomogeneous at low IL to polymer ratios as IL interacts with polymer rapidly. Graphene was prepared from graphite by a two step topedown approach, reported elsewhere [30]. Briefly, graphite was oxidized using water-free mixture of conc. H2SO4, NaNO3 and KMnO4 (Hummers' method [31]) to result graphite oxide (GO). Graphene was obtained by thermal exfoliation of GO at 200 C under hydrogen atmosphere in a tubular furnace and labelled as HEG. The as-exfoliated HEG has the surface area as high as 443 m2 g1, with 1.64 103 S m1 of electrical conductivity with 14.6 C/O ratio [32]. 2.2. Fabrication of supercapacitor The stretchable supercapacitor has been fabricated as follows: HEG was dispersed in the solution of 5 wt % PMMA/[BMIM][TFSI] blend as a binder in acetone medium under ultrasonic irradiation. We chose acetone medium because PMMA/[BMIM][TFSI] blend is either insoluble or phase separated in most of the solvents, other than acetone. But, graphene with large amount of residual functional groups cannot be dispersed in acetone. Hence, PMMA/ [BMIM][TFSI] was first dissolved in acetone followed by the addition of required amount of graphene. We found a good dispersibility of graphene in this solution, which may be attributed to the deposition of PMMA/[BMIM][TFSI] blend on graphene surface. The dispersion was coated (~1 mg cm2) on a smooth glass surface. The PMMA/[BMIM][TFSI] electrolyte gel was poured on graphene film (50 mg cm2) and allowed to evaporate acetone for 24 h at ambient conditions. Then the electrodeeelectrolyte assembly (EEA) was removed from the surface. A thin layer of [BMIM][TFSI] was coated on electrolyte sides of two EEAs and the assemblies were joined as shown in Scheme 1. The device was dried for 1 h under vacuum (400 mbar) at room temperature in order to remove residual solvent molecules. Here, graphene film itself plays a role as current collector. In addition, electrolyte acts as support for electrode material, unlike the previous reports [27e29]. Stainless steel foils were used as electrical connections. This device was denoted as HEG e PMMA/[BMIM][TFSI] supercapacitor. 2.3. Characterization techniques The structural analysis of synthesized materials was done by X'Pert Pro PANalytical powder X-ray diffractometer. The morphological analysis was carried out by electron microscopy (FEI Quanta and Technai G-20). WITec alpha 300 Confocal Raman spectrometer, equipped with a Nd:YAG laser as the excitation source, has been employed to acquire Raman spectra while Fourier transform infrared spectra were acquired by PerkineElmer FT-IR spectrometer. The transmittance studies were carried our using JASCO corp., V-570 spectrophotometer. Electrochemical behaviour was studied using CH instrument (CHI608C). 3. Results and discussions 3.1. Morphological analysis The cross-sectional view of FESEM image of HEG e PMMA/ [BMIM][TFSI] supercapacitor (Fig. 1) reveals the graphene electrode thickness of ~150e200 mm. The cross-sectional view of roughly tore PMMA/[BMIM][TFSI] shows a smooth morphology. It is a consequence of microscopic elastic deformation of the film which gives
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Scheme 1. Fabrication of stretchable HEG e PMMA/[BMIM][TFSI] supercapacitor.
extraordinary mechanical stability. The incorporation of ionic liquid reduces the glass transition temperature of PMMA which makes it stretchable [19]. This can be confirmed by X-ray diffraction pattern and thermal analysis. The white spots embedded in the film may be nano-sized solid like short range ordering of ionic liquid. It is reported that ionic liquid ions makes short range ordering at the solid surfaces [33e35]. The morphological analysis clearly displays the unique, well connected interface between polymer electrolyte and graphene electrode produced by this method which can effectively reduce the RESR. The HRTEM image of HEG shows a highly wrinkled nature of graphene layers due to the rapid removal of functional
groups from the lattice. In energy storage applications the lattice defects (hexagons and pentagons) in graphene lattice plays a role as anchoring sites for charge storage which enhances the binding energy with ions. In addition the wrinkles present in graphene layers may assist the electrode to follow the expansion of electrolyte under strain and retain the electrodeeelectrolyte interface. 3.2. Powder X-ray diffraction analysis The X-ray diffractogram of PMMA (Fig. 2(a)) shows a peak at ~16 , which is a strong proof for the tactic nature of PMMA. It is
Fig. 1. FESEM image of HEG e PMMA/[BMIM][TFSI] supercapacitor.
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51
original shape within 2 min. Ionic liquids are identified as a good plasticizer for a wide range of polymers. Unlike other plasticizers, ions of IL interact with the polar side chain of PMMA which increases the mechanical stability and high stretchability. In addition, the digital image illustrates the transparency of the electrolyte also. The optical transmission spectrum (Fig. 3) of the dry PMMA/ [BMIM][TFSI] shows at least 95% transmittance in the complete visible region which is in good agreement with literature [19]. These properties promote PMMA/[BMIM][TFSI] as a unique electrolyte for stretchable electrochemical devices. 3.4. Molecular vibration spectra of PMMA/[BMIM][TFSI] electrolyte
Fig. 2. X-Ray diffraction pattern of (a) PMMA and (b) PMMA/[BMIM][TFSI].
clear from the XRD pattern of PMMA/[BMIM][TFSI] (Fig. 2(b)) that the tacticity of PMMA is reduced significantly due to the incorporation of ionic liquid. Here, it is anticipated that the ionic liquid ions are accommodated in the inter-chain space and disturb the orientation of building blocks. This reduces the glass transition temperature of tactic PMMA matrix and makes it stretchable. We observed a new peak at ~13 , corresponding to the inter-layer distance of ~0.7 nm, along with PMMA peaks. This may arise due to the short range ordering of IL in the polymer matrix which is seen in FESEM image as well [33e35]. 3.3. UVeVisible spectrum of PMMA/[BMIM][TFSI] electrolyte The electrolyte is a transparent, stretchable film with excellent mechanical retainability. The inset in Fig. 3 clearly demonstrates at least four-fold stretchability of the PMMA/[BMIM][TFSI]. We anticipated, as a conclusion of XRD analysis, ionic liquid ions may accommodate in the inter-chain space and reduce the interaction, which results in highly stretchable film. The given external force may elastically deform the polymer array to elongate it. As soon as the force is released, the film restores its shape with less than 150% of its initial length which is also displayed. The film retains its
The interaction between host polymer and mobile phase has been analysed by molecular vibration spectroscopy (Raman and FTIR spectroscopy). Table 1 shows the detailed assignments for the Raman and IR features of PMMA/[BMIM][TFSI]. The Raman spectrum of PMMA (in Fig. 4(a)) shows peaks corresponding to stretching and bending vibrations of CeH at 3000, 2950 cm1 and 1454 cm1, respectively, which has the corresponding features in IR spectrum also. In addition, the presence of stretching vibrations of CH2 (2950, 883 cm1), C]O (1727 cm1), CeO (1243 cm1), CeCOO (1243 cm1), CeOeCe (1131 cm1 in IR), CeC (1065 cm1), CeCeO (603 cm1) and bending vibration of CeH (1454 cm1) along with rocking modes of OeCH3(979 cm1), a-CH3(964 cm1 in IR) confirms the structure of PMMA [36,37]. The long polymeric chain results in the skeletal mode of v(CeC) (1065 cm1) vibrations. A combination band involving OeCH3 vibrations has been assigned to the feature present at 2845 cm1. The incorporation of ionic liquid into the polymer matrix introduces the bands, corresponding to the constituents. The in-plane (ip) stretching vibrations of imidazolium ring appear at 900 to 1600 cm1 region along with a peak at 3172 cm1 while the bending vibrations occur at low wavenumber region (600e800 cm1). The out-plane (op) asymmetric bending vibration of imidazolium ring is positioned at 654 cm1. The NeCH2 and NeCH3 stretching modes (600e1600 cm1 region) confirms that the side chains are attached with the nitrogen atom of the imidazolium ring. The butyl chain produces the d(CeCeCeCeH) and v(CeCeCeC) vibrations. The sulfonylimide (1392 cm1), symmetric (1142, 603 cm1) and asymmetric (1339, 1243 cm1) vibrations of SO2, stretching (1243 cm1) and bending (746 cm1) vibrations of CF3 confirms the structure of trifluoromethylsulfonyl imide anion. The appearance of symmetric stretching mode of SO3, from free triflates (1026 cm1) reveals the non-aggregation of electrolyte ions [38]. Fig. 4(b) clearly shows that the v(CeC) mode (1065 cm1) is strengthened due to the incorporation of ionic liquid. The v(CeO) feature is superimposed with vs(SO2) and produces a strong signal centred at 1177 cm1 [39,40]. Table 1 and Fig. 4 clearly show that all the existing peaks are assigned to either PMMA or [BMIM][TFSI] and no new vibrational signal is produced. This detailed analysis suggests that there is no chemical bonding between host polymer and ionic liquid and ions simply stay at inter-chain or inter-side chain space. This indeed reduces the interaction between polymer chains and makes it stretchable. In addition, the presence of foreign moieties (ionic liquid) disturbs the tacticity of PMMA which is confirmed by XRD pattern. 3.5. Electrochemical analysis
Fig. 3. UVeVisible spectrum of PMMA/[BMIM][TFSI]. Inset: digital image of demonstration of transparency and stretchability.
The electrochemical analysis has been carried out using a more practical two electrode supercapacitor setup at ambient conditions. The ionic conductivity of PMMA/[BMIM][TFSI] is determined by AC impedance analysis where stainless steel foil has been used as
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Table 1 Spectral assignments for molecular vibration spectra of PMMA and PMMA/[BMIM] [TFSI]. Wavenumber (cm1)
PMMA [36,37]
[BMIM][TFSI] [39,40]
vs(ring HCeCH), v(CeH of NeCHeN), vs(ring ip) va(CH3eNeCH) va(CeH) vs(CeH) &va(CH2) vs(terminal CH3HCH) e
Raman
IR
3172
3159
e
3113 3000 2950 2879 2845
3120 3003 2960 2880 e
1727 1568 e 1454
1726 1571 1486 1453
e va(CeH) vs(CeH) &va(CH2) e Combination band involving OeCH3 v(C]O)from (CeCOO) e d(CH2) da(CeH)
1421
e
e
1392
1385
e
1339
1348
e
e 1243 1142 e 1065 1026
1330 1276, 1228 1177 1131 1052 e
v(CeO) va(CeOeCe) v(CeC)skeletal mode e
979 e 883 & 817 e 746
988 964 844 787 739
OeCH3 rock aeCH3 rock v(CH2) e e
e 603
654 613
e v(CeCOO), vs(CeCeO)
v(CeO), v(CeCOO)
e va(ring ip), v(CH3(N)), v(CH2(N)) d(CH2) ds(CCH HCC), ds(CH3(N)HCH), terminal ds(CH3 HCH) va(ring ip), v(CeC), v((N)CH2), v(CH3(N)CN) va(ring ip), v(CeC), v((N)CH2), v(CH3(N)CN), sulfonamide from anion vs(ring ip), v(CH2(N)), va(SO2), v(CeC), v(CeN of NeCH3) v(CeCeCeC), va(ring ip) va(SO2), vs(CF3) vs(SO2) e v(CeC) vs(CeF), vs(ring ip), v(CH3(N)), v(CH2(N)) vs (SO3) from free triflates anion va(ring ip), v(CeC), e v(CH2) d(CeCeCeCeH) 2 conformers of [TFSI], ds(ring HCCH), ds(CF3) v(CH3(N)), v(CH2(N)), da(ring op) ds(ring ip), ds(SO2), v(CH2(N)), v(CH3(N)CN)
current collectors. Un-doped PMMA has the ionic conductivity in the order of 1016 S cm1. It is anticipated that, ionic liquid ions are simply stored in the inter-chain space by a weak hydrogen bonding between the hydrogen from host polymer and highly electro
Fig. 4. (a) Raman spectrum and (b) FTIR spectrum of PMMA/[BMIM][TFSI].
negative nitrogen in IL cation. Storage of IL in polymer matrix has a threshold, known as percolation threshold. Hence, the ionic conductivity should depends on the IL concentration, not surprisingly. Fig. 5 shows significant improvement in ionic conductivity of the polymer due to the incorporation of IL. However, the improvement in ionic conductivity and stretchability are not much impressive at low ionic liquid concentration. We observed a considerable raise in ionic conductivity above 1:1 polymer to IL ratio. This may be attributed to the polymer-IL interaction. Briefly, IL ions are immobilised in the inter-chain space of polymer matrix by weak hydrogen bonding between the hydrogen from host polymer and highly electro negative nitrogen in IL cation. The amount of ionic liquid has a threshold limit known as percolation threshold. Above this limit, the excess IL ions make long-range connectivity in the GPE system and move freely under the applied potential. In polymer electrolytes, the segmental motion or alignment of polar moieties of host polymer also contributes in ionic conductivity along with the mobile phase. PMMA is a polar material at above the glass transition temperature (Tg) [41]. Puyu et al. have reported that [BMIM][PF6] ionic liquid can play a role as plasticizer for PMMA to attain a low Tg and high thermal stability along with good compatibility [42]. The low Tg facilitates improvement in segmental motion or alignment of polar moieties and thus high ionic conductivity. We have studied different compositions up to 1:2 ratios of PMMA to IL. PMMA/[BMIM][TFSI] (1:2) has the ionic conductivity of 0.78 mS cm1 at 28 C, which is comparable to that of PMMA/ LiClO4/EC/PC electrolyte, along with remarkable stretchability [38]. The polymer electrolytes, up to 1:1 composition, are flexible, but hard to stretch. Further addition of IL, gradually decreases the hardness. The compositions above 1:2.5 result in a transparent, sticky, mechanically unstable gel. Since IL plays a role as plasticizer, higher composition reduces the polymer chain interaction and thus the mechanical stability. The inset in Fig. 5 shows ionic conductivity of PMMA/[BMIM][TFSI] (1:2) as a function of temperature which suggests the temperature assisted improvement in ionic transport above 310 K. At low temperatures, ionic liquid produces solid-like long range ordered particles in the polymer matrix which is confirmed by FESEM image. As temperature increases, immobilized ions get high kinetic energy and tend to move freely. Further increase in temperature increases the kinetic energy ions and shows a linear behaviour. The performance of the device has been reported in terms of mass of electrode materials. The device was activated to stabilize
Fig. 5. Ionic conductivity as a function of composition. Inset: Variation of ionic conductivity with temperature.
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Fig. 6. Cyclic voltammograms of HEG e PMMA/[BMIM][TFSI] supercapacitor. Fig. 8. Discharge behaviour of HEG e PMMA/[BMIM][TFSI] supercapacitor at various specific currents.
the performance by scanning 100 cycles at 100 mV s1 scan rate. Specific capacitance (Cs) was calculated using the equation-
Cs ¼ 2
I m dV dt
(1)
where, I is the current, dV/dt is the potential scan rate and m is the mass of electrode material at each electrode. A factor of two is multiplied due to the series capacitance formed in two electrode system [43]. The cyclic voltammograms (CV) of HEG e PMMA/[BMIM][TFSI] stretchable supercapacitor is shown in Fig. 6 at various scan rates, which clearly shows that the device is capable of operating at least up to 3 V, which is less than that of neat [BMIM][TFSI] (3.5 V). The CV deviates from the rectangular shape above 3 V and shows rapid increase in current. We expect irreversible electrochemical reaction (like a pseudocapacitor) of PMMA at this potential. The CV shows no oxidation or reduction peak, which ensures the complete removal of solvent molecules. The rectangular shape of the CV ensures the better supercapacitive behaviour. However, at high scan rates the CV deviates much from the ideal response. Here, specific capacitance has been calculated from Equation (1). The device is highly flexible and stretchable with at least twofold stretchability. The performance of the device under strain
Fig. 7. Discharge behaviour of HEG e PMMA/[BMIM][TFSI] supercapacitor under strain. Inset: change in specific capacitance with strain cycles.
and the effect of strain cycles have been determined by chargeedischarge cycles. Fig. 7 shows that the device losses 40% of its specific capacitance under strain (30%). This may be attributed to the discontinuities introduced in graphene network by the elongation of polymer which affects the electrical conductivity of electrode. However the device retains at least 90% of its initial capacitance when the strain is released (inset in Fig. 7). It has to be noted that the subsequent strain cycles did not result much change in capacitance. The chargeedischarge behaviour of the device is shown in Fig. 8 at various specific current. Specific capacitance (Cs) of the device has been measured employing Equation (1). Here, I is the specific current and dV/dt is the slop of the straight line fit of discharge curves from Vmax to Vmax/2 [43]. The intercept of linear fit with potential axis gives the potential drop (Vdrop) due to Equivalent series resistance (RESR ¼ Vdrop/I). The performance of the supercapacitor with chargeedischarge cycles is reported at different current with at least 96% coulombic efficiency. The device shows specific capacitance of 83 F g1 at 2.67 A g1 which is almost 60% higher than the previous reports [27e29]. The increase in scan rate or current affects capacitance (Fig. 9). The specific capacitance of the device was ~220 F g1 at 5 mV s1 scan rate. However, a slight raise in scan rate results a large fade in capacitance (Fig. 9(b)). This may be attributed to the following reasons, first, the finite mobility of the massive electrolyte ions and second, the ionic liquid ions play a role as plasticizer by
Fig. 9. (a) Specific capacitance as a function of specific current and (b) scan rate.
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accommodating the inter-chain space of PMMA matrix, which effectively traps the ions from moving under applied potential. At very low scan rates, the ions find sufficient time to get out of the trap. At high scan rates, the polarity of the device may reverted before the ion move significant distance in the polymer matrix which drives the ion in opposite direction. This suggests that this electrolyte can perform two-fold better in slow chargingedischarging applications. Capacitance falls with increase in current due to the rapid motion of massive ions which results in poor accommodation of ions inside the pores of electrode (Fig. 9(a)). At high currents, the ions get insufficient time to diffuse into the pores of electrode which results in a simple polarization followed by the accumulation of charges at the electrodeeelectrolyte interface. An electrochemical device should have long life time. The cyclic stability of the device with repeated chargeedischarge cycles has been determined with a newly fabricated cell. Fig. 10 shows at least 88% of its initial performance even after 1000 cycles at 2.67 A g1. The capacitance falls ~12% in the initial 200 cycles followed by a stabilized performance. The initial fade may be attributed to activation of the device. However, after activation the device shows stable performance which is an evidence for electrochemical stability of HEG e PMMA/[BMIM][TFSI] electrodeeelectrolyte system. Ragone plot relates the energy density and power density of an energy storage device. The energy density (E) and power density (P) of supercapacitor have been calculated using (2) and (3), respectively.
1 E ¼ Ccell V 2 2
P¼
E Dt
(2)
(3)
Fig. 11. Ragone plot and Inset: power dissipation profile of HEG e PMMA/[BMIM][TFSI] supercapacitor.
energy storage capacity (Emax) and the maximum available power (Pmax) of the supercapacitor. The Ragone equation is represented as.
Emax EL ¼ 2
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! P 1þ 1 L Pmax
(4)
where, EL, PL, Emax and Pmax are available energy, power to the external load, maximum energy storage capacity and maximum available power respectively [44]. The fitting parameters reveal that the values of Emax and Pmax are 25.7 Wh kg1 and 35.2 kW kg1, respectively, which is lightly larger than the recent report [28]. The Emax and Pmax can be related by the time constant.
to ¼
Emax 2Pmax
(5)
where, Ccell is the specific capacitance of the device, V is the cell potential and Dt is the discharge time for the corresponding potential [43]. The Ragone plot of HEGePMMA/[BMIM][TFSI] supercapacitor (Fig. 11) shows the energy density and power density of 26.1 Wh kg1 and 5 kW kg1, respectively, at the specific current of 2.67 A g1 corresponding to the Cs of 83 F g1, in terms of mass of total electrode material. The Ragone plot drops faster at high power due to the fast voltage decay during discharge. Ragone equation has been fitted to the experimental data in order to extract maximum
where, time constant (to) is the time necessary to discharge the capacitor to 36.8% of its initial voltage in a short found to be 101 ms for the HEGePMMA/[BMIM][TFSI] supercapacitor [44]. The inset in Fig. 11 shows the internal power dissipation (PESR¼I2RESR) in the device with respect to discharge current. Nyquist plot of supercapacitor, at 500 mV sinusoidal measuring potential, is given in Fig. 12. The linear fit to the low frequency region reveals that the equivalent series resistance (RESR) is 16 U at
Fig. 10. Cyclic stability of HEG e PMMA/[BMIM][TFSI] supercapacitor at specific current of 2.67 A g1.
Fig. 12. Nyquist plot and Inset: Change in RESR with strain cycles.
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room temperature, which is much lesser than the previous report on stretchable supercapacitor (~350 U) [28]. This effectively helps to reduce the internal power dissipation. Here RESR is composed of ionic and electronic contributions. The ionic contribution comes from the mobility of electrolyte ions due to its mass and interaction with host polymer. In addition, the electrodeeelectrolyte interface resistance also contributes as there is a possibility for the incompatibility between electrode and bulky electrolyte ions. The electronic contribution comes from the ohmic resistance in the current collector and electrode. Hence, the low RESR can be attributed to the compatibility at the electrodeeelectrolyte interface as shown in FESEM image of the interface. The strained device shows higher RESR than the strain-free one (inset in Fig. 12). A 30% strained device shows 69 U, which can be attributed to the ohmic resistance, added by the electric discontinuities in graphene layers due to the electrolyte elongation. The inset in Fig. 12 shows the change in RESR of retained as a function of number of strain cycles. The measurements were taken after 5 min of every strain cycle. It is clear that the retained device shows a slight increase in RESR in the initial 4 cycles followed by the nearly unchanged values. 4. Summary This study described the fabrication of a stretchable supercapacitor using highly stretchable ionic liquid incorporated poly(methyl methacrylate) (PMMA/[BMIM][TFSI]) electrolyte where hydrogen exfoliated graphene has been used as electrode. The structural, molecular vibrational, optical and electrochemical properties of the electrolyte have been determined. A stretchable supercapacitor comprised of PMMA/[BMIM][TFSI] electrolyte has been fabricated with HEG electrode. The specific capacitance, energy density and power density of the device was 83 F g1, 26.1 Wh kg1 and 5 kW kg1, respectively, at the specific current of 2.67 A g1, in terms of mass of total electrode material. At least 90% of initial capacitance has been retained even after repeated strain cycles. The equivalent series resistance (RESR) of 16 U leads to 20e35% internal power dissipation. We observed a slight raise in RESR due to strain cycles. The detailed analysis suggests that PMMA/ [BMIM][TFSI] electrolyte can play both roles as electrolyte and stretchable substrate for electrode in transparent, stretchable electric devices. Acknowledgements Authors acknowledge SAIF-IITM for FTIR analysis. One of the authors (Tamilarasan) acknowledges Indian Institute of Technology Madras (IITM) for the financial supports (Senior research fellowship). References [1] K. Yu Jin, C. Haegeun, H. Chi-Hwan, K. Woong, All-solid-state flexible supercapacitors based on papers coated with carbon nanotubes and ionic-liquidbased gel electrolytes, Nanotechnology 23 (2012) 065401. [2] C.L. Pint, N.W. Nicholas, S. Xu, Z. Sun, J.M. Tour, H.K. Schmidt, R.G. Gordon, R.H. Hauge, Three dimensional solid-state supercapacitors from aligned single-walled carbon nanotube array templates, Carbon 49 (2011) 4890e4897. [3] L. Su, Z. Xiao, Z. Lu, All solid-state electrochromic device with PMMA gel electrolyte, Mater. Chem. Phys. 52 (1998) 180e183. [4] K.H. Wen Lu, Craig Turchi, John Pellegrino, Incorporating ionic liquid electrolytes into polymer gels for solid-state ultracapacitors, J. Electrochem. Soc. 155 (2008) A361eA367. [5] H. Yang, M. Huang, J. Wu, Z. Lan, S. Hao, J. Lin, The polymer gel electrolyte based on poly(methyl methacrylate) and its application in quasi-solid-state dye-sensitized solar cells, Mater. Chem. Phys. 110 (2008) 38e42. [6] S. Rajendran, O. Mahendran, R. Kannan, Lithium ion conduction in plasticized PMMAePVdF polymer blend electrolytes, Mater. Chem. Phys. 74 (2002) 52e57.
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