Application of Nanocomposites for Supercapacitors: Characteristics and Properties

Abstract

Supercapacitors, ultracapacitors or electrochemical capacitors (ECs), are energy storage de‐ vices that store energy as charge on the electrode surface or sub‐surface layer, rather than in the bulk material as in batteries, therefore, they can provide high power due to easier to re‐ lease energy from surface or sub‐surface layer than from the bulk. Since charging‐discharg‐ ing occurred on the surface, which does not induce drastic structural changes upon electroactive materials, supercapacitors possess excellent cycling ability. Due to those unique features, supercapacitor is regarded as one of the most promising energy storage de‐ vices. There are two types of supercapacitors: electrochemical double layer capacitors (EDLCs) and pseudocapacitors. In EDLCs, the energy is stored electrostatically at the elec‐ trode–electrolyte interface in the double layer, while in pseudocapacitors charge storage oc‐ curs via fast redox reactions on the electrode surface. There are three major types of electrode materials for supercapacitors: carbon‐based materials, metal oxides/hydroxides and conducting polymers. Carbon‐based materials such as activated carbon, mesoporous carbon, carbon nanotubes, graphene and carbon fibres are used as electrode active materials in EDLCs, while conducting polymers such as polyaniline, polypyrrole and polythiophene or metal oxides such as MnO2, V2O5, and RuO2 are used for pseudocapacitors. EDLCs de‐ pends only on the surface area of the carbon‐based materials to storage charge, therefore, often exhibit very higher power output and better cycling ability. However, EDLCs have lower energy density values than pseudocapacitors since pseudocapacitors involve redox active materials to store charge both on the surface as well as in sub‐surface layer.