A symmetric supercapacitor based on 30% poly (methyl methacrylate) grafted natural rubber (MG30) polymer and activated carbon electrodes

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1 A symmetric supercapacitor based on 30% poly (methyl methacrylate) grafted natural rubber (MG30) polymer and activated carbon electrodes Nur Hamizah Mohd Zaki, Zaidatul Salwa Mahmud, Oskar Hasdinor Hassan, Muhd Zu Azhan Yahya, and Ab Malik Marwan Ali Citation: AIP Conference Proceedings 1875, (2017); View online: View Table of Contents: Published by the American Institute of Physics Articles you may be interested in Supercapacitors specialities - Materials review AIP Conference Proceedings 1597, 98 (2014); / Effect of conductive additives to gel electrolytes on activated carbon-based supercapacitors AIP Advances 5, (2015); / The device application of electrochemical exfoliated graphene AIP Conference Proceedings 1875, (2017); / MWCNTs-PANi nanocomposite films prepared by AC-EPD technique and its potential for enhance supercapacitor electrode AIP Conference Proceedings 1875, (2017); / Effect of polyaniline on MWCNTs supercapacitor properties prepared by electrophoretic deposition AIP Conference Proceedings 1875, (2017); / Studies on graphene zinc-oxide nanocomposites photoanodes for high-efficient dye-sensitized solar cells AIP Conference Proceedings 1877, (2017); /

2 A Symmetric Supercapacitor Based On 30% Poly (Methyl Methacrylate) Grafted Natural Rubber (MG30) Polymer and Activated Carbon Electrodes Nur Hamizah Mohd Zaki 1, Zaidatul Salwa Mahmud 2, Oskar Hasdinor Hassan 3, Muhd Zu Azhan Yahya 4 1, 5, a), Ab Malik Marwan Ali 1 Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam, Selangor, MALAYSIA 2 Faculty of Applied Sciences, Universiti Teknologi MARA, Cawangan Perak, Kampus Tapah, Tapah, Perak, MALAYSIA 3 Department of Industrial Ceramic, Faculty of Art and Design, Universiti Teknologi MARA, Shah Alam, Selangor, MALAYSIA 4 Department Of Defence Science, National Defence University Of Malaysia, Kuala Lumpur, MALAYSIA 5 Institute of Sciences, Universiti Teknologi MARA, Shah Alam, Selangor, MALAYSIA a) Corresponding author: ammali@salam.uitm.edu.my Abstract. This article focuses on polymer-based gel electrolytes because basic features good self-standing characteristics, conductivity, and excellent window stability for supercapacitor devices when compared to aqueous electrolytes. Gel polymer electrolytes (GPEs) based on 30% poly (methyl methacrylate) grafted natural rubber (MG30) doped with ammonium triflate (NH 4 CF 3 SO 3 ) and plasticized with ethylene carbonate (EC) were prepared by a solution casting method. Owing to being plasticized, the GPEs exhibit high room temperature ionic conductivity of 9.61x10-4 S.cm -1 at the composition of 26:14:60 wt% for MG30: NH 4 CF 3 SO 3 : EC. Linear sweep voltammogram study shows the highest conducting GPE exhibited electrochemical window stability of 2.7V. The GPEs has been employed to demonstrate the possibility of fabricating supercapacitor. Symmetric devices assembled using activated carbon as electrodes and GPEs (highest conducting) exhibit a specific capacitance of 32 F.g INTRODUCTION The global energy demand is expected to increase because of the depletion of fossil fuel based sources. Therefore more efficient and green alternative energy such as solar power, wind power, and biomass have been developed to fulfill the demand [1]. However, extreme power fluctuations cause to use significant amounts of energy. To overcome this issue, researchers looking for another alternative to energy sources which offer excellent performance in energy storage. Electrochemical capacitors (ECs) is at the forefront of energy storage technology since it has been used in numerous applications including electronic equipment, computer memory backup, electric/hybrid vehicle, etc. [2 4]. ECs also were known as supercapacitors, which are commercially available in the market with the use of liquid electrolytes (LEs) as electrolytes. However, the liquid electrolytes have many disadvantages like being leak-prone, corrosive, and devices using liquid electrolytes may be bulky with high self-discharge rate. The use of liquid electrolytes also acquires high cost because of the need of a separator to prevent electrode-electrolyte contact. To overcome these problems, researchers endeavor to develop supercapacitor based on solid polymer electrolyte. The advantages of using solid polymer electrolytes are due to their characteristics such as lightweight, leak free and easy International Conference on Applied Physics and Engineering (ICAPE2016) AIP Conf. Proc. 1875, ; doi: / Published by AIP Publishing /$

3 in processing but suffer from small ionic conductivity at ambient temperature due to less amorphousness of the polymer host. It is essential to improve the amorphousness and increase ionic conductivity by increasing the molecular motion. Therefore, the ace solution to overcome the problem is develop the gel polymer electrolytes (GPEs) which give advantages over leak-free characteristics and high conductivity. Many reports have been done on the polymeric gel electrolytes due to excellent features of high ionic conductivity (~10-3 S.cm -1 ) and wide electrochemical windows stability [7 10]. These features are close to the liquid electrolytes likes behavior. For application in supercapacitors, it requires the polymer electrolyte possess excellent characteristics in mechanical strength, high thermal stability, high ionic conductivity, and electrode-electrolyte compatibility. 30% PMMA-g-NR (MG30) polymer have received much attention due to excellent mechanical stability, high elasticity and their molecular structure possess a carbonyl (C=O) polar functional group which able to dissociate the dopant salt into ions [11,12]. Hence, the GPEs based on MG30 may produce better ionic conductivity and flexible film which help proper electrode-electrolyte contact in supercapacitors. In the present work, MG30 was doped with ammonium triflate at different concentration and plasticized with EC to produce GPEs. It is considered that the plasticized MG30-NH 4 CF 3 SO 3 complexes enhanced the ionic conductivity and improved the electrode/electrolyte interface. The highest ionic conductivity of GPE along with freestanding films obtained was employed in EDLC fabrication. 2.0 EXPERIMENTAL 2.1 Preparation of Gel Polymer Electrolytes Polymer electrolytes will be prepared by solution cast technique. First, the solid polymer electrolytes (SPEs) films composed of 1.0 g MG30 (Green HPSP (Malaysia) Sdn. Bhd) were dissolved in 30ml of anhydrous solvent tetrahydrofuran (THF) from J.T Beaker and continuously stirred to obtain a homogeneous solution. Then, this solution was added with different stoichiometric ratios of NH 4 CF 3 SO 3 salt (Sigma-Aldrich) and stirred continuously for about 24hours till all dopant salt entirely dissolved. The solutions were cast on Petri dishes and left at room temperature to evaporate the THF solvent. It is further heated at 50 C for 48hours in the vacuum oven to remove the residual solvent. The impedance spectroscopy studies were performed to acquire the optimized composition of NH 4 CF 3 SO 3 salt at highest conductivity. The same procedure did further preparation of GPE films by introducing ethylene carbonate (EC) (Sigma-Aldrich,98%) as above. 2.2 Preparation of Electrodes Activated carbon (AC) powder (specific surface area 1644 m 2.g -1 ), carbon black (Super P) and poly (vinylidene fluoride) (PVdF) (Sigma, Aldrich) were used as an active material, conducting material and binder, respectively. AC powder, carbon black and PVdF in the ratio of 80:10:10 (wt%) was thoroughly mixing with N-methyl pyrrolidone to prepare a slurry. Then the slurry mixture was spread on the aluminum mesh followed by heating the at 60 C for 30 minutes. Finally, the pressure 250mbar will be imposed into the electrode to ensure intimate contact will form between the electrode and aluminum mesh

4 2.3 Electrochemical Measurement The conductivity of polymer electrolytes was obtained by AC impedance technique using ALL Hi-Tester software (HIOKI LCR HI TESTER) in the frequency range of 100 Hz to 1 MHz. The data were processed using ALL Hi-Tester software. The total ionic conductivity was calculated using equation (1): σ l/(r A) b (1) Whereas R b is the bulk resistance obtained from the intercept on the real impedance axis of the impedance plot, l is the film thickness, and A is the contact area of the thin film. The FTIR technique was performed using Perkin Elmer Spectrum-One spectrometer in the frequency range between 4000 and 400 cm -1 with 2 cm -1 of resolution. The electrochemical stability window of the electrolyte was carried out by linear sweep voltammetry (LSV) using the Auto Lab Potentiostat/Galvanostat by applying an anodic voltage to a cell, which has composed of an electrolyte sandwiched between an SS electrode with scanning rate at 1.0 mv.s -1. The supercapacitors were fabricated using the highest conductivity of gel polymer electrolyte, which has sandwiched between two symmetrical AC electrodes. The electrochemical performance of supercapacitors was characterized by using cyclic voltammetry (CV) and galvanostatic charge-discharge. These performances were carried out by using Wonatech-battery cycler. The capacitance value was calculated by using equation (2): C i/mv (2) Where i is the measure of the constant current (A), v is the scan rate (V.s -1 ), and m is the weight. The charge-discharge test was measured at a constant current (1mA). By using equation (3), the discharge curve obtained and the discharge capacitance can be calculated. C (i t)/(m V) i/(m V/ t) (3) Where i is the discharge current (A) and ΔV/Δt is the gradient of the linear curve from the charge-discharge profile (V.s -1 ). The Coulombic efficiency, η of supercapacitors was calculated using equation (4): η (td/tc)x100% (4) Where t d represents the time for galvanostatic discharging and t c represents the time for galvanostatic charging. 3.0 RESULTS AND DISCUSSION 3.1 Characterization of Polymer Electrolytes Figure 1(a) shows the conductivity plot of the SPEs (MG30-NH 4 CF 3 SO 3 ) at room temperature (T=303K) reaches a maximum value of 1.96x10-4 S.cm -1 at 35 wt% NH 4 CF 3 SO 3 and decreases at higher salt loading. The increase in ionic conductivity at lower salt concentration indicates that the number of mobile ions increased proportionally with dopant salt [13,14]. It is to be noted that the present of ions will enhance the amorphousness of the polymer matrix then increase the segmental motion of the polymer chain. Therefore, a favor to the ion migration quickly takes place. As such, the results reach an agreement with FTIR studies where the increasing of free ions eventually enhanced the conductivity. The decreasing in conductivity at higher salt concentration was attributable to the formation of ions aggregation. It was due to the formation of the transient of polymer crosslink which reduces the segmental mobility [12]. The reduction of segmental mobility will lead to restriction of ionic migration resulting in decreasing in ionic conductivity

5 log (Scm -1 ) log (Scm -1 ) (a) (b) weight percentage of NH 4 CF 3 SO 3 (wt%) weight percentage of EC (wt%) Figure 1 Conductivity plot of (a) MG30-NH 4 CF 3 SO 3 (b) MG30- NH 4 CF 3 SO 3 -EC at room temperature Figure 1(b) elucidates the conductivity plot of MG30-NH 4 CF 3 SO 3 -EC at room temperature (T=303K). It can be inferred that the use of EC enables to improve the ionic conductivity [15]. The EC (small molecule than polymer host) capable of penetrating in between the polymeric chain and will create a more free volume that favor to the polymer chain flexibility. Also, the existence of plasticizer just acts as a lubricant without involving the interaction with another component thus makes the ions more mobile within the polymer system. The sample with the composition 26:14:60 (MG30: NH 4 CF 3 SO 3 : EC) is a free standing film at conductivity 9.41x10-4 S.cm -1. Figure 2 shows the conductivity plot at various temperature ( K) for the plasticized and unplasticized films. Based on equation (5), represent the Arrhenius plot of log σ versus 1000/T: Ae (Ea/RT) (5) Where A is constant that is proportional to a number of charge carrier; Ea is activation energy which indicates the energy needed to ion jump to a free hole, k is rate constant, T is absolute temperature, R is universal gas constant. The nonlinearity of the graph for unplasticized and plasticized films imply that in this system do not follow the Arrhenius behavior. The curvature observed is suggested that ionic mobility in the electrolyte system obey Vogel-Tamman-Fulcher (VTF) model where the majority of ion transport take place in the amorphous phase. This phase will reflect the high segmental motion of the polymeric chain resulting increase in ionic transport. The gain in conductivity as temperature increase is due to the presence of larger mobile ions, as a consequence of the ionic migration induced by the external energy [16]

6 log (Scm -1 ) MG30-35wt.% NH 4 CF 3 SO 3 MG30-NH 4 CF 3 SO 3-60wt.% EC /T (K -1 ) Figure 2 Ionic conductivity plot at 303K to 373K Figure 3 shows the FTIR spectra of SPEs in the region cm -1 that depicts band characteristics of triflate anion. The symmetric SO 3 stretching mode is highly sensitive, and the ion association occurs at the SO 3 end of the anion [7], [17]. The presence of free ions and ion aggregates can be observed at the band peak of 1032 cm -1 and 1051 cm -1 respectively. Further explanation regarding the ions can be understood from the area of each band. 0.4 (a) (I) (II) %T (b) (c) wavenumber (cm -1 ) Figure 3 Symmetric stretching υ s (SO 3 ) for SPEs containing (a) 30, (b) 35, (c) 40 wt% NH 4 CF 3 SO 3 in the region (I) free triflate ions, (II) ion aggregation

7 area under the band ion aggregates free ions weight percentage (wt.%) Figure 4 Plot of area under the assigned band of free ions and ion aggregates against concentration of salt for unplasticized sample MG30-NH 4 CF 3 SO 3 Figure 4 shows the area of the band of the various composition of salt concentration. At a low content of salt, some free ions gradually increase until an abrupt rise occurs at 30 wt% of salt and reaches the maximum at 35 wt% salt. This phenomenon is due to the ion dissociation in the polymer salt system that correlates to ionic conductivity. Hence the ion aggregation exhibits a minimum at 35 wt% salt. High free ions concentration signifies more ions were dissociated and migrated. One of the important properties in the PEs application is the voltage limitation of its electrochemical stability window. The breakdown voltage is necessary to identify prior the performance of supercapacitors measurement was carried out in order to prevent electrolyte damage at the interface of electrode-electrolyte during charge-discharge cycling test. Figure 5(a) and Figure 5(b) show the linear sweep voltammogram result (LSV) for the unplasticized MG30-NH 4 CF 3 SO 3 and plasticized MG30-NH 4 CF 3 SO 3 polymer electrolyte respectively. There was no visible difference in both LSV curves except the windows stability of the plasticized electrolyte is higher than that of the unplasticized electrolyte. The windows stability obtained are 2.2V and 2.7V for MG30-NH 4 CF 3 SO 3 and MG30- NH 4 CF 3 SO 3 -EC respectively. The addition of plasticizers in polymer electrolyte system increases the electrochemical stability of the electrolytes. This characteristic is influenced by the dynamic formation of a passivating layer between EC molecules at the electrode. The presence of this layer causes the diffusion potential barrier of GPE molecules to keep increasing preventing further decomposition of the electrolyte and in turn, increases the decomposition voltage [10]. 2.5 (a) 2.5 (b) Current (ma) Current (ma) Voltage (V) Voltage (V) Figure 5 Linear sweep voltammetry curves of (a) MG30-35wt% NH 4 CF 3 SO 3 (b) MG30-NH 4 CF 3 SO 3-60wt%EC

8 3.2 Characterization of Supercapacitors Cyclic voltammetry is one of the reliable tools which indicates the material in capacitive behavior. Figure 6 shows the cyclic voltammograms (CV s ) for the highest conducting GPE sample at different scan rates (1-50 mv.s -1 ). The curves show no distinct peak due to redox reaction on both positive and negative sweeps in the whole potential range. The profile of current response is ideally close to rectangular which shows capacitive behavior. The specific capacitance was calculated using equation (2) and the value achieved is 32 F.g -1 at 5 mv.s -1. At high scan rates, 50 mv.s -1 the curves (Figure 6(d)) seem to deviate from the rectangular shape due to ohmic drop/ substantial value of ESR, practically present in real capacitors [18] (d) Current (A) (c) (b) (a) Potential (V) Figure 6 Cyclic voltammogram of EDLC using gel polymer electrolyte containing MG30-NH 4 CF 3 SO 3-60wt%EC sample at different scan rate (a) 1 mv.s -1 (b) 5 mv.s -1 (c)10 mv.s -1 (d) 50 mv.s Voltage (V) Time (sec) Figure 7 Charge - discharge curve of supercapacitor using gel polymer electrolyte containing MG30-NH 4 CF 3 SO 3-60wt%EC sample at a current density of 1 ma.cm 2 More constant capacitance values will be obtained by performed galvanostatic discharge characteristics at a current load 1mA to estimate the specific capacitance. Figure 7 depicts the typically charged discharge characteristics of the supercapacitor employing highest conducting polymer electrolyte. Almost linear and symmetrical feature exhibited by charge-discharge curves confirms good capacitance behavior of the GPE/AC cell

9 The specific capacitance is calculated from the slope of the discharge to be 30 F.g -1. The Coulombic efficiency (η) is found to be in the range 90%. For electrode and liquid electrolyte interface, it is attained the higher value of coulomb efficiency. Compared to this, it is considered that the polymeric gel electrolyte behaves liquid-like ionic conduction. 4.0 CONCLUSION The freestanding film with excellent dimensional stability obtained at the highest composition of gel polymer electrolyte, MG30-NH 4 CF 3 SO 3 -EC (60 wt% EC) of 9.61x10-4 Scm -1. The capacitance of supercapacitor employed NH 4 CF 3 SO 3 salt based gel polymer electrolyte is 32 F.g -1. also, the coulombic efficiency was determined almost 90% could represent the liquid electrolytes like behavior. Thus, these present the gel polymer electrolyte has potential in energy storage for flexible and lightweight devices. ACKNOWLEDGEMENT Authors would like to thank Ministry of Higher Education (MOHE) Malaysia for funding this research under the RAGS grant (600-RMI/RAGS 5/3 (23/2014)) and Institute of Sciences, Universiti Teknologi MARA (UiTM) for the facilities provided. REFERENCES [1] Nishino A Capacitors: operating principles, current market, and technical trends. Journal of Power Sources. 60 : [2] Staaf L.G.H., Lundgren P. and Enoksson P Present and future supercapacitor carbon electrode materials for improved energy storage used in intelligent wireless sensor systems. Nano Energy. 9 : [3] Frackowiak E Carbon materials for supercapacitor application. Phys. Chem. 9: [4] Pumera M Graphene-based nanomaterials and their electrochemistry. Chem. Soc. Rev. 39(11): [5] Gray F.M., MacCallum J.R. and Vincent C.A Poly(ethylene oxide) - LiCF3SO3 - polystyrene electrolyte systems. Solid State Ionics (PART 1): [6] Armand M Polymer solid electrolytes - an overview. Solid State Ionics.9 10(PART 2): [7] Asmara S.N., Kufian M.Z., Majid S.R. and Arof A.K Preparation and characterization of magnesium ion gel polymer electrolytes for application in electrical double layer capacitors. Electrochim. Acta. 57(1): [8] Kamisan A.S., Kudin T.I.T., Ali A.M.M. and Yahya M.Z.A Electrical and physical studies on 49% methyl-grafted natural rubber-based composite polymer gel electrolytes. Electrochim. Acta. 57 : [9] Qian X., Gu N., Cheng Z., Yang X., Wang E. and Dong S Plasticizer effect on the ionic conductivity of PEO-based polymer electrolyte. Materials Chemistry and Physics. 74: [10] Ali A.M.M., Subban R.H.Y., Bahron H., Yahya M.Z.A. and Kamisan A.S Investigation on modified natural rubber gel polymer electrolytes for lithium polymer battery. J. Power Sources. 244: [11] Kumutha K., Alias Y. and Said R FTIR and Thermal Studies of Modified Natural Rubber Based Polymer Electrolytes. Ionics. 11: [12] Ali A.M.M., Subban R.H.Y., Bahron H. and Winie T Grafted natural rubber-based polymer electrolytes : ATR-FTIR and conductivity studies. Ionics

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