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Anwarul Azim Akib
Ragib Shakil Chanchal Kumar Roy Al-Nakib Chowdhury
In modern society, the potential demand for energy is rising due to rapid population growth and socioeconomic advancement. In order to supply this huge amount of energy, various renewable technologies like solar, wind, tidal, biogas, and nuclear technology have already been developed. Producing clean and renewable energy is not the only challenge, rather storing the energy has also appeared a major concern. As a result, electrochemical energy storage devices like supercapacitors have become an important topic of study . As a novel energy storage technology, electrical double-layer capacitors (EDLCs) employ novel carbon as the electrode material and may supply high power in a short period while maintaining an outstanding cycle . This is due to the well-tuned hierarchical porous structure, high specific surface area, superior electrical conductivity, and good chemical and thermal stability, biomass-derived activated carbon (AC) has attracted tremendous attention for its potential applications as an electrode material or electrode modifier in supercapacitor technologies [3-5].
However, AC-based-supercapacitor electrodes are often made by spraying or coating an electrode paste or slurry of AC on a current collector such as graphite, aluminum foil, nickel foam, etc. In this work, a combination of AC, conductive agents, and binders (cohesive agent) is ultrasonically mixed to create the electrode paste or slurry. The binders bind active materials from falling off and give enough strength during electrochemical operation. Interestingly, the binders, on the other hand, invariably cover certain surface regions or pores of active molecules. As a result, the characteristics of binders and their contents in electrodes will have a direct impact on the electrochemical performance of supercapacitors. Thus, it is critical to understand how to select a good binder .
Among others, the chemical structures of carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyvinylidene fluoride (PVDF) are shown in Fig. 1(A), which are favorable for AC- based supercapacitors. The CMC possesses lower mechanical stability and it could not be utilized beyond 40 with any water or oxygen but demonstrated a reduced resistivity. Whereas PVDF is made up of a CH2-CF2 unit, it possesses conventional fluoropolymer stability, but interacting groups create a unique polarity, resulting in a better chemical and oxidative resistance, low wettability, and considerable swelling in the electrolyte.
In this study, we have investigated the interaction of a highly porous activated carbon with different polymeric binders (CMC, PVP, and PVDF) and tested its applicability as AC-based supercapacitor electrodes. The AC was synthesized from banana leaves followed by carbonization under Ar flow at 750 with an activating agent (K2CO3). The total synthesis route is shown in Fig. 1(B). The presence of different types of functional groups was confirmed by analyzing the Fourier Transform Infrared Spectroscopy (FTIR) spectrum. The morphology of the as-prepared AC was investigated by using a Field Emission Scanning Electron Microscope (FESEM), X-Ray Diffraction (XRD), and Ramon spectroscopy.
The supercapacitor performance of the AC was evaluated in a standard 3-electrode setup containing NaSO4 as electrolyte by performing cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS), and cyclic stability tests. In fabricating the supercapacitor, the modified graphite electrode was used as the working electrode, Ag/AgCl, and platinum wire was used as the reference and counter electrode, respectively. To examine the effect of the binder on the supercapacitor’s performance, three different kinds of binders were employed in fabricating the AC- modified graphite electrode of the supercapacitor. The highly porous AC with PVDF binder shows the best current response as well. The highest specific capacitance of 255 Fg-1 is found at 0.5 Ag-1 in the potential window of 1.4 V. The overall performance of AC with different binder materials is shown in Table 1. It is expected that the present work will significantly contribute to constructing an AC-based supercapacitor with the proper selection of polymeric binder materials for high efficiency.
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