Development of symmetric and asymmetric supercapacitors–a step towards efficient and practical energy storage

The steady growth of human civilization has led to a steep rise in the global demand for energy. It has therefore been a daunting task for researchers not only to utilize the currently available energy with maximum efficiency but also, to figure out the possible measures to store the same. In the past decade, supercapacitors or electric double-layer capacitors (EDLCs) have developed as a potential technology in the field of sustainable energy storage. The classification of the symmetric and asymmetric supercapacitors is done mainly based on their fabrication principle and charge storage mechanism. The obtained capacitance in symmetric supercapacitors mainly originates from the double layer formation. While in asymmetric supercapacitors, both the Faradaic reactions (pseudo-capacitance) and the electric double layer participate in the total charge. In this chapter, the working principle, fundamental design aspects and charge storage mechanism of symmetric/asymmetric supercapacitors are analyzed and based on the individual electrode material capacitance and deliverable energy/power densities. Strong emphasis has been given to the role of different materials for designing the working electrodes (cathode and anode) and electrolytes in determining the performance and stability of the symmetric and asymmetric supercapacitors on a lab-scale or for practical device applications. This chapter provides the basic understanding and significance of symmetric and asymmetric supercapacitors for efficient electrochemical energy storage.