Exfoliated graphite as electrochemical double layer capacitor electrode: Electrochemical impedance study

Exfoliated graphite (EG) is one of the best alternatives for graphene in the electrochemical devices since graphene needed a cost-effective mass production to reach the commercialization. Liquid phase exfoliation has been identified as an inexpensive, ecofriendly method for high quality EG production. Sri Lankan natural graphite and commercially available graphite were used for exfoliation and the converted EG samples were used to fabricate electrochemical double layer capacitors (EDLCs). They were characterized using Electrochemical Impedance Spectroscopy (EIS). EG prepared by Sri Lankan natural graphite showed higher specific capacitance values compared with commercial graphite EG. Thereby, the value of Sri Lankan graphite over commercial graphite was emphasized.


INTRODUCTION
Electrochemical Double Layer Capacitors (EDLCs) have been widely used as an energy storage device to fulfill the energy requirement 1 . EDLCs have higher power density and longer cyclic life than rechargeable batteries 2 . Therefore, many research works have been carried out to improve the performances. Their efforts mainly focus on improving the properties of electrode materials 3 .
Nowadays, EDLCs have been developed using various types of carbonaceous materials such as activated carbon, carbon nanotubes, carbon aerogels, carbon nanofibers and graphite due to ease of accessibility, low cost, non-toxicity, high chemical stability and wide temperature range [4][5][6][7][8] . Based on these materials, many approaches have been tested to increase the Specific Surface Area (SSA) of the electrode material 9 .
Since its discovery in 2004, graphene (one atom thickness-sp 2 hybridized-two-dimensional carbon) has gained a lot of attention of global scientists 10 . Graphene has an extraordinarily high SSA up to 2630 m 2 /g 11 . Even though graphene proved to be an ideal electrode material which has remarkable electrochemical properties, its industrial applications were restricted.
Deficiency of large-scale production of graphene and high production cost are the main reasons for this drawback. Graphite is composed of individual graphene sheets and it is the most inexpensive and easily obtained carbonaceous material. Therefore, various methodologies were carried out using graphite to achieve the requirements of a highperformance electrode material.
'Exfoliation' is a process where graphite gets separated into graphene layers. The degree of exfoliation or the graphene yield completely relies on the exfoliation method. Several methods have been identified throughout the past years. A most reasonable, inexpensive and ecofriendly method is the aqueous surfactant mediated liquid phase exfoliation 12 . This method avoids using toxic and expensive solvents, thus giving an easy and practical approach. EG is low dense graphitic carbon consists of few graphene layers with a high temperature resistance 13 . In addition to that, EG provides a porous carbon network that can act as an active material in the EDLC electrode.
Therefore, this study aims to use Sri Lankan natural graphite as electrode material in EDLCs while compromising its value over commercially available graphite. Electrochemical Impedance Spectroscopy (EIS) was used to investigate the capacitive properties of the fabricated EG based EDLCs.

Materials
Sri Lankan natural graphite samples (G1 -particle size 40 µm, G2 -particle size 10-15 µm) were obtained from Bogala Graphite Lanka Ltd, Bogala, Sri Lanka. Commercial graphite propylene carbonate (PC) and acetone were purchased from Sigma Aldrich and used without further purification.

Preparationof exfoliated graphite electrodes
Each graphite sample (G1, G2, G3) were exfoliated in aq. SDBS solution (0.1 mg ml -1 ) using ultrasonic homogenizer (Athena ATP 150) for 45 minutes and filtered separately. Another set of graphite was sonicated in acetone prior to the exfoliation. The filtered exfoliated graphite samples were then dried and deposited onto fluorine-doped tin oxide (FTO) glasses as the electrodes. Acetone was used as the solvent for coating purpose. Area of the electrode was 1 cm 2 .

Preparation of gel polymer electrolyte (GPE)
Required amounts of PVDF, EC, PC and ZnTF were stirred well and heated. The hot mixture was pressed in between two glass plates to get a bubble-free thin film and was allowed to dry 14 .

RESULTS AND DISCUSSION
EIS is related with impedance data which is one of the key characterization techniques use to study electrochemical devices. It reveals the properties of the electrolyte and the electrodes of an EDLC via Nyquist plots at different frequencies. Bulk electrolyte properties are observed in the high frequency range. Mid frequency range stands for the electrodeelectrolyte interface as well as the electrode properties. Low frequency range highlights capacitive features. Figure 1 demonstrates the Nyquist plots obtained for different graphite samples.  16 . Exfoliation process decreases the number of graphene layers stack together and thus give few layered graphite. Therefore, the surface area is higher in EG than in graphite.
When the sonication was done particle size will be reduced while the mixture is allowed to be uniform in composition. Decrease of the EG particle size due to the prior sonication permits to see the difference in the semicircle in Nyquist plots and the vertical lines getting more parallel to the imaginary axis.  Additional information can be obtained by plotting the imaginary part of the complex capacitance (C // ) versus frequency ( Figure 3). The frequency, f0 where the maximum of the curve occurred is a characteristic feature for each system. This f0 can be identified as the point where the EDLC goes from purely resistive to purely capacitive when the frequency is changed from higher to lower value 18

CONCLUSION
Different EG samples were prepared via liquid phase exfoliation and were used to fabricate EDLCs in the form of EG/GPE/EG. In this study, EIS technique was performed to evaluate the electrochemical properties of the EDLCs. EIS test revealed the capacitive behavior of the EDLC through Nyquist and Bode plots. Thus the importance of using exfoliation and prior sonication as a low cost, environmental friendly method can be highlighted. Sri Lankan natural graphite showed better capacitance values than the commercially available graphite.
Improvements may enhance the performances of Sri Lankan natural graphite based EDLCs and thus be a better solution for the demand of energy storage.