Effect of gel polymer electrolyte based on poly vinyl alcohol (PVA)/poly ethylene oxide (PEO) blend and sodium salts on the performance of solid

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1 re to view linked References Effect of gel polymer electrolyte based on poly vinyl alcohol (PVA)/poly ethylene oxide (PEO) blend and sodium salts on the performance of solid state supercapacitor Hop Tran Thi Thanh, Phuoc Anh Le, Mai Dang Thi, Tuan Le Quang and Tung Ngo Trinh 1 * 1 Insititute of Chemistry, Vietnam Academy or Science and Technology (VAST), Ha Noi, Viet Nam * Author for correspondence (ngotrinhtung@gmail.com) Abstract. In this work, the effect of gel polymer electrolytes containing PVA/PEO (9/1, wt. %) blend and different contents of sodium salt mixture (sodium acetate (CH 3COONa)/sodium sulfate (Na 2SO 4) = 5/5 wt. %) on the performance of solid state supercapacitor was investigated. The active electrode of the solid state supercapacitor was made from graphene nanoplatelets and carbon black. The results indicate that the sodium salt mixtures were easily mixed in polymer blend gel to make excellent gel polymer electrolyte with large concentration of ionic liquid. At the sodium salt mixture content of 3%, the solid state supercapacitor showed the best performance of electrode specific capacitance of F/g at current density of 1 A/g, energy density of 3.25 Wh/kg and power density of W/kg. These results highly recommend the good potential of gel polymer electrolytes for developing solid-state supercapacitor in the future. Keywords. Solid state supercapacitor, gel polymer electrolytes, polymer blend, sodium salt. 1. Introduction Solid-state supercapacitors have recently attracted much research attention in modern technology because they provide many advantages than the original capacitors such as higher charge-discharge, avoiding leaking electrolyte liquid, good mechanical stability. To further improve the specific capacitance and energy density, many gel polymer electrolytes (GPE) have been developing by using new polymer or polymer blends as host polymer frame of the electrolyte to avoid the leaking of liquid. Additional, there are many different types of metal salt systems added into polymer to provide higher conductive ionic such as lithium salt (Li 2SO 4, LiCl, LiClO 4, LiBr, LiTFSI), sodium salts (Na 2SO 4, NaClO 4, NaBr, NaI, CH 3COONa) which has been studied widely [1-6]. Poly vinyl alcohol (PVA) is widely used in gel polymer electrolyte for supercapacitors. The main advantages of PVA in gel polymer electrolyte are their ability to solvate large amount of alkali-metal salts, their high ionic conductivities at temperature below the glass temperature, high mechanical strength and reduction of electrolyte leakage risk. However, gel polymer electrolyte using PVA faced a problem of low ductility as well as inherently brittle and easily precipitation due to the broken of OH bonding in the presence of organic and inorganic ion such as,, [7-12]. In order to fill up this draw back, the appearance of poly ethylene oxide (PEO) as doping mediator is considered good idea to make strong gel polymer electrolyte. Both of PVA and PEO is nontoxic, water soluble polymer, low cost which use widely in industry. The blending of PVA-PEO provides hydroxyl group and hydrogen bond and good charge storage capability [13-15]. In this paper, a solid state supercapacitors with the gel polymer electrolyte consisting of PVA/PEO blend as polymer host frame, and mixture of sodium acetate, sodium sulfate as ionic liquid were fabricated and investigated. In the best of our knowledge, this is the first report on the use of two sodium salts as ionic liquid by the fabrication of solid state supercapacitor. The two sodium salts could provide different sizes of anions, which could diffuse deeply in the graphene/carbon black electrode network. Thus, the use of sodium salt mixture will increases the ionic conductivity of the gel polymer electrolyte and also will improves the specific capacitance as well as the energy storage. In addition, the partly replace of sodium sulfate by sodium acetate is desired for environmental protection [18]. All electrochemical properties of the solid state supercapacitors were

2 investigated via three electrodes testing cell to determine their performance. 2. Experimental 2.1. Materials Poly (vinyl alcohol) (PVA, 95% hydrolyzed, average MW=95,g/mol) was from Across company. Poly (vinylidene fluoride) (PVDF, M w= 534, g/mol), sodium acetate (CH 3COONa), sodium sulfate tetrahydrate (Na 2SO 4) were from Sigma Aldrich. Poly (ethylene oxide) (PEO, M w= 1,, g/mol), 1-methyl-2-pyrrolidinone (C 5H 9NO) (NMP) and carbon black acetylene (1% compressed) were purchased from Alfa Aesar company. Graphene nanoplatelets (6-8 nm thick x 5 µm wide) were from Strem Chemicals Preparation of electrodes Graphene nanoplatelets (8 mg), carbon black (1mg) and PVDF (1 mg) were dispersed in.2 ml NMP to make the slurry. This slurry was pasted on two titanium substrate with the area of cm 2 (1 cm x 1 cm) and dried at 6 C for 12 hours in air to obtain electrode. The weight of each electrode is about 1 mg Preparation of gel polymer electrolytes Gel polymer electrolyte was prepared by mixing the solution of polymer blend and sodium salt in different weight ratio. Firstly, 1 g PVA/ PEO (9:1, wt. %) was dispersed into 1 ml deionized water at 8 C and stirring for 8h to obtain clear gel solution. Then, different amounts of CH 3COONa/Na 2SO 4 (5:5, wt. %) were added in gel polymer solution (see table 1). After that, the gel polymer electrolyte was stirred for 6h to obtain homogenous gel solution. The maximal weight ratio of polymer blend to sodium salt mixture was 7:3 in order to keep the form of gel polymer electrolyte. In the case of higher sodium salt ratios, the gel electrolytes transform to liquid form causing the leaking problem by fabrication of solid state supercapacitor Fabrication of testing cells and characterization of electrochemical properties The testing cells consist of a commercial reference electrode Ag/AgCl, a working electrode, a counter electrode and the gel polymer electrolyte. The electrochemical properties of supercapacitor were examined by cyclic voltammetry (CV) from -.5 V to.5 V, impedance spectroscopy (EIS) in range of 1 mhz to 1 khz with 5 mv amplitude, and by galvanostatic chargedischarge with different current. All of these electrochemical investigations were conducted at electrochemical workstation system ZehnerZenium (Z 2.23). The specific capacitance (C cell, F/g) of testing cell was calculated via following equations [1-13]: I t Ccell (1) m V Note that for this condition, the cell used in the calculation includes the active material on electrodes immersing in the gel polymer electrolyte. And the electrode specific capacitance (C s, F/g) was described by equation: C s 4C (2) cell The energy density (E, Wh/kg) and the power density (P, W/kg) were described by following equations: E V C cell (3) 2 36 E P 36 (4) t Where is the discharge voltage after V drop, I(A) is the discharge current, (s) is the discharging time, m is the total of active material electrode including graphene nanoplatelet, carbon black and PVDF. The Raman spectrum and XRD pattern of the sample was taken at Raman JOBIN WON, HORIBA and XRD D8 Brucker. The morphology of the sample was observed on scanning electron microscopy (SEM) Hitachi SU81. The mechanical property of polymer films was tested by micro-force testing systems (MTS, Tytron 25). The sample has the dimension of 1 x 1 x.5 mm and the testing speed was 5 mm/min. 3. Results Figure 1 shows the Raman spectrum (fig. 1a) and XRD pattern (fig. 1b) of graphene nanoplatelet/carbon black electrode. The D peak around 135 cm -1 of carbon bonding and G peak around 157 cm -1 of phonon vibration of graphite are clearly observed. In addition, the graphene 2D peak around 27 cm -1 from second phonon vibration mode is also seen [17]. On the XRD pattern, strong peak at 2Ө = 26.6⁰, corresponding to the d-spacing between single graphene layers is observed. Figure 2 shows the SEM image of graphene nanoplatelet/carbon black electrode. A network-like structure of graphene nanoplatelets is seen. On this network structure of graphene nanoplatelets, carbon black

3 particles or clusters are attached. The attachment of carbon black makes the carbon based electrode more compact in order to improve the electrical conductivity of the electrode. By the solid state supercapacitor the flexibility of the polymer host frame plays an important role. Figure 3 shows the force displacement diagrams of PEO, PVA, PVA-PEO (9/1) blend. Generally, the elasticity module (Emodule) of the sample presents the stiffness of the materials. PVA has the E-module of 2.48 MPa and it is much higher than that of PEO (1.4 MPa). By the PVA/PEO (9/1) blend the E- module is about 1.15 MPa. This result indicates that the flexibility of PVA is strongly improved by the PVA/PEO (9/1) blend. Figure 4a compares the CV curves of supercapacitor with different gel polymer electrolytes at voltage scan rate of 1 mv/s. The CV curves have good cycle that explains the capacitance is stored energy as double layer capacitance by an agglomeration of ions at interface of electrodes. The increment of CV curves was coincided with the increasing of sodium salt content in the gel polymer electrolyte. Figure 4b, 4c and 4d present the CV curves of supercapacistor with different gel polymer electrolyte at different voltage scan rates from 5 to 1 mv/s. It is clear to see that for all supercapacitors with different gel polymer electrolytes the current of the supercapacitor increase with increasing of voltage scan rate. In addition, the area under the CV curves strongly broaden with increasing of voltage scan rate. In order to investigate the electrochemical behavior at the electrode/electrolyte interface, EIS measurements were employed. The Niquist plots of supercapacitors with different gel polymer electrolytes are showed in figure 5. The supercapacitor with the content of sodium salt mixture of 1% in gel polymer electrolyte presents the highest resistance around 27.4 Ω. With increasing of sodium salt mixture content in the gel polymer electrolyte, the resistance of the supercapacitors strongly decrease to 17.3 Ω and 9.94 Ω by the sodium salt mixture content of 2% and 3%, respectively. This result indicates that the ionic conductivity and the ions transfer in gel polymer electrolyte are significantly improved at high content of sodium salt mixture. Figure 6 shows the galvanostatic charge-discharge curves of supercapacitors with different gel polymer electrolytes at current density of 1A/g. Although, the resistance of supercapacitors with different gel polymer electrolytes decreases with increasing of sodium salt content in the gel polymer electrolytes, the charge-discharge time of the supercapacitors do not behave in the same fashion (fig. 6d). The reasons for the behavior of charge-discharge time of the supercapacitor with different gel polymer electrolytes are that the diffusion of ion from gel electrolyte to the electrode is time dependable. Thus, the time of charge-discharge was not changed suddenly, but the voltage drop of charge-discharge of supercapacitors increases with increasing of sodium salt content in the gel polymer electrolyte (fig. 6d). These factors influence the performance of the supercapacitors with different gel polymer electrolytes. The performance of the supercapacitor is determined according to the equations 1 to 4. By the supercapacitor PCN1 at current density of 1 A/g, the electrode specific capacitance, energy density and power density were F/g, 3.18Wh/kg and W/kg, respectively. By the supercapacitor PCN2 at current density of 1 A/g, the electrode specific capacitance, energy density and power density were F/g, 3.211Wh/kg and W/kg, respectively. By the supercapacitor PCN3 at current density of 1 A/g, the electrode specific capacitance, energy density and power density were F/g, 3.25 Wh/kg and W/kg, respectively. The electrochemical properties of the solid state supercapacitors are slightly increased with increasing of sodium salt content in the gel polymer electrolyte. One of the reasons for these electrochemical behaviors of the solid state supercapacitors at different sodium salts contents is the quite low ion transport in the ionic liquid-based gel polymer electrolyte [19]. 4. Conclusions The effect of gel polymer electrolyte based on PVA/PEO blend and CH 3COONa/Na 2SO 4 mixture on the performance of double layer supercapacitor was investigated. The results reveal that the sodium salt mixture was easily dispersed in the PVA/PEO blend gel until 3%. The performance of supercapacitor was significantly improved by increasing of sodium salt mixture content in the gel polymer electrolyte. At the sodium salt content of 3%, the supercapacitor showed the good energy density of 3.25Wh/kg and power density of W/kg. These results suggest that the sodium salt based gel polymer electrolyte is as promising environmental gel electrolyte for double layer supercapacitor with low cost and it is easy to fabricate. Acknowledgments The authors acknowledge the financial support from the Institute of Chemistry, Vietnam Academy of Science and Technology (VHH ). References

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5 Tables Table 1. Compositions of gel polymer electrolyte Sample name PVA/PEO (9:1) blend (wt. %) CH3COONa / Na2SO4 (1:1) mixture (wt. %) PCN1 9 1 PCN2 8 2 PCN3 7 3

6 Figures Figure 1. Raman spectrum (a) and XRD pattern of graphene nanoplatelet/carbon black electrode (b)

7 Figure 2. SEM image of graphene nanoplatelet/carbon black electrode.

8 16 14 PEO PVA PVA/PEO (9/1) 12 Force (N) Displacement (mm) Figure 3. Force - displacement diagrams of PEO, PVA and PVA/PEO (9/1) blend.

9 3 2 (a) 2 (b) 1 1 Current (ma) PCN1 PCN2 PCN3 Current (ma) PCN1 5 mv 1 mv 2 mv 4 mv 6 mv 8 mv 1 mv (c) (d) 1 1 Current (ma) PCN2 5 mv 1 mv 2 mv 4 mv 6 mv 8 mv 1 mv Current (ma) PCN3 5 mv 1 mv 2 mv 4 mv 6 mv 8 mv 1 mv Figure 4. CV curves of the supercapacitors at voltage scan rate of 1 mv/s (a), CV curves of supercapacitors with different gel polymer electrolytes at different voltage scan rates from 5 to 1 mv/s (b, c, d).

10 Z'' (ohm) Z'' (ohm) Z' (ohm) 1 5 PCN1 PCN2 PCN Z' (ohm) Figure 5. Niquist plots of supercapcitor with different gel polymer electrolytes.

11 1. PCN 1 a 1. PCN 2 b.8.8 Current (A).6.4 Current (A) PCN 3 c 1. V drop d.8.8 Current (A) PCN1.2.2 PCN2 PCN Time (s) Figure 6. Galvanostatic charge-discharge curves of the supercapacitor at current density of 1 A/g: (a) PCN1, (b) PCN2, and (c) PCN3, (d) comparison charge-discharge curves of the supercapacitors with different gel polymer electrolytes at current density of 1 A/g.