Polymer Electrolytes for Supercapacitor Applications

Abstract

The electrical energy in supercapacitors is stored at the electrode–electrolyte interfaces. Hence, besides the electrodes, the electrolytes do play a crucial role in determining the electrochemical performance of these energy storage systems. Accordingly, scientists worldwide are endlessly working on improving the features of electrolytes to bring about a renaissance in this technological sector. Narrow potential windows, low energy and power outputs, leakage issues, corrosiveness, self-discharge, poor ionic conductivity, and so on are the common problems associated with the currently employed electrolytes in this industry. In this context, polymer-based electrolytes have garnered global attention as they furnish numerous advantages, both mechanically and electrochemically, to fabricate portable and wearable electronics as well as assisting in combating complications related to low energy density, corrosion, self-discharge, and bulky designing issues, which are frequently associated with conventional liquid electrolytes. The ability of polymer electrolytes in formulating paper-thin, miniature, and light-weight devices has been one of the supreme attractive features that have widened the application domain of these renewable and sustainable energy storage systems. The classification of various polymer-based electrolytes employed for supercapacitor applications, namely: solvent-free (solid) polymer electrolytes; gel polymer electrolytes such as hydrogel and organogel; ionogel-based polymer electrolytes; proton conducting polymer electrolytes; and polyelectrolytes, are comprehensively discussed in this chapter. The performance aspects of these polymer electrolyte-based supercapacitors are demonstrated and compared so as to analyze and interpret the advantages and shortcomings of each kind of these polymer electrolytes in subsequent sections. The mode of ionic conduction which plays a major role in dictating the electrochemical signature of these energy storage cells is also discussed. The various multifunctional features displayed by these versatile polymer electrolyte-based supercapacitors, such as self-healing, stretchability, self-charging, self-memory, and electrochromism, which have diversified their implementation in various technological fields are also described. Further, current progress on polymer electrolyte-based supercapacitors integrated with various sensors for practical applications are also emphasized. Finally, the challenges, as well as possible improvements, for gaining practicability of this class of flexible supercapacitors are outlined as well.