The supercapacitor (SC) is an important energy storage device due to its high power density, fast charge-discharge, and long-term cyclic stability. Two important parameters are energy density and cost of supercapacitors that are a constraint against its ability to replace batteries. Various research efforts have been done to enhance the energy density and to reduce the cost of SC so that it can be an alternative to batteries. The electrolyte plays a crucial role in SC as energy density; specific capacitance is linked with a voltage window of electrolyte. Polymer electrolytes (PE) emerged as the new material, also as an alternative to liquid electrolyte. The inherent flexibility, various shape geometry and light in weight features of PE motivated the research community to focus on them. The important parameters that need to be checked are ionic conductivity, voltage stability window and mechanical properties. These properties of PE were tuned by adding different guest species such as ionic liquids, plasticizers, and nanofiller. The enhancement of the aforesaid properties was achieved by various researchers as discussed in the upper section. The SC cell performance was superior in various aspects than liquid electrolytes. One challenge that still remains is to achieve the optimum combination of the electrode and electrolyte material so that high energy density can be achieved. This can be achieved by tuning the electrode active surface area, morphology via different synthesis methods. The combination of these aspects will result in the fulfillment of safe, flexible, having high energy density and power density supercapacitor. Therefore, the development of novel electrode and electrolytes material will be the focus of research. In conclusion, the important goal of the research community is to explore the supercapacitor application range, enhancement of energy density, and cost reduction.
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