Electrical and Electronic Devices, Circuits, and Materials. Группа авторов

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       Polymer Electrolytes: Development and Supercapacitor Application

       Anil Arya1*, Anurag Gaur2 and A. L. Sharma1

       1Department of Physics, Central University of Punjab, Bathinda, Punjab, India

       2Departmet of Physics, National Institute of Technology, Kurukshetra, Haryana, India

       Abstract

      Due to the increasing demand for energy globally and the reduction of the traditional energy sources, the development of an efficient and sustainable energy source has grabbed the attention of researchers. So, supercapacitor (SC) is a crucial energy storage device that has gained attention in the energy sector. One important part of any SC cell is the electrolyte. The electrolyte plays an important role in the ion migration in the device between electrodes. Hence, polymer electrolytes are fascinating candidates and fulfil the need due to better mechanical properties and ion dynamics. Consequently, the present chapter will start with a brief introduction to supercapacitors, followed by characteristics of electrolyte, types and modification strategies for the electrolyte. Further, the important preparation techniques and advanced characterization techniques are briefed. Finally, some important developments made using the polymer electrolytes for SC cell are presented (publications and patents).

      Keywords: Polymer electrolytes, energy storage devices, supercapacitor, hybrid capacitor, energy density etc.

3.1 Introduction

The global energy crisis and depletion of traditional sources of energy (coal, fuel), has captured the attention of the scientific community and is the biggest global challenge that needs to be resolved in the 21st century. It is a top priority to switch from non-renewable to renewable sources of energy (hydro, wind, solar) which are superior in terms of efficiency, sustainability, and long service life. Energy from various sources needs to be stored somewhere and used on-demand. Two important emerging technological candidates are battery and supercapacitor. The task for the scientific community is towards up-gradation of electrochemical energy storage devices (supercapacitors, batteries) and making them efficient to reduce dependency on traditional sources of energy. From traditional capacitors to the supercapacitors available nowadays, the battery is a crucial energy storage device. Among them, Supercapacitors (SC) emerged as a promising alternative to a traditional capacitor having a low capacity and the battery having low power density [1, 2]. The low energy density in SC is due to charge storage limited to the surface, so this is an important parameter that is the focus of research. The energy density of SC is related to the specific capacitance of the electrode materials and the voltage window of the cell. The voltage window of the cell is linked to the electrochemical voltage stability window of the electrolyte used. The strategy has been built to develop novel electrodes with high surface area, high porosity and high effective interaction area for charge storage. Along with this, different charge storage mechanisms have been developed to enhance the overall cell capacitance by tailoring the cell configuration (symmetric/asymmetric/hybrid) and electrode material (carbon/metal oxide/sulfide) [3–5]. In conclusion, research is focused on the development of novel electrode and electrolyte materials as well as tailoring of the existing materials to enhance the electrochemical performance of the SC cell.

      The present chapter describes the important development in the field of polymer electrolytes of application in the supercapacitor. The characteristics of polymer electrolytes will be discussed; then the selection criteria for the polymer, salt and additives will be the focus of discussion, followed by the classification of polymer electrolytes.

       3.1.1 The Basic Principle and Types of Supercapacitors

A supercapacitor is an electrochemical device and is used to store energy and lies between the traditional capacitors and battery. The key advantages of SC are high power density, fast-charge discharge, high-performance stability, and long cyclic stability/life (∼10⁶ cycles). Along with these features, only one drawback needs to be explored, the low energy density of SC. A lot of research efforts have been demonstrated to increase the energy density of SC by tuning the electrode material, electrolyte, and device structure. Figure 3.1a shows the history of the supercapacitor worldwide [6]. The supercapacitor performance is influenced by the electrode material, electrolyte, and separator. These are further linked to the performance parameter of the SC cell examined by different characterization techniques. Figure 3.1b shows the schematic diagram which highlights the relation between different performance metrics, the major affecting factors, and the corresponding test methods. For clarity and good visibility to readers, several color schemes are employed. Three core parameters are highlighted in yellow; the power and energy densities in dark blue; time constant and cycling stability in light orange; all the important affecting factors in light purple; and the corresponding test methods in white [7].

The supercapacitor is different from the traditional capacitor or electrostatic capacitors as shown in Figure 3.2a. Depending on the charge storage mechanism, electrode material, electrolyte, and cell design are classified into three types. SC store energy and charge storage phenomena is an important criterion that decides SC performance. On the basis of the charge storage mechanism, SC is of three types [8]:

      Electric double-layer capacitors (EDLCs), where the capacitance is produced by the electrostatic charge separation (no charge transport between electrode and electrolyte) at the interface between the electrode and the electrolyte (Figure 3.2b). To maximize the charge storage capacity, the electrode materials are usually made from highly porous carbon materials for achieving

SC has a high power density and low energy density. So, various strategies have been adopted by researchers to improve the energy density of the SC cell. Novel cell design (symmetric, asymmetric, hybrid), cell voltage (E ∝ V²) and synthesizing new electrode nanostructures and electrolyte material opens new doors of opportunity to researchers. Figure 3.3 depicts the overview of the different strategies used to improve the energy density of the SC cell [13].