Abstract
Technological advances in various fields are frequently hampered by the insufficient capacity of energy storage devices. Electrochemical capacitors (ECs) emerge as high-power effective energy storage devices with a short charge/discharge time, long-term cycle life, and wide temperature range. The electrolyte is an important component influencing EC performance because of its physical and chemical properties. There are various types of electrolytes including organic, ionic liquids, water in salt and redox active electrolytes which offer diverse behavior to the ECs. Liquid state electrolyte as well as solid-state systems are described. This review emphasizes the significance of the electrolyte type and its influence on the EC performance. Different aspects are discussed starting from the specific ion effect at the electrode/electrolyte interface that affects the EC charge/discharge mechanism. Furthermore, emerging electrolytes and new directions in ECs are highlighted. Herein, the progress and challenges in water in salt (WIS) and deep eutectic solvent (DES) electrolytes are presented. Moreover, a discussion on the redox-active electrolyte for hybrid capacitors is carried out. ECs electrolytes are governing the crucial metrics of the device like the voltage window, the energy and power density as well as the long-term cyclability. Hence, as “a rule of thumb” the EC electrolyte should not be ignored.