Progress on carbon for electrochemical capacitors

Abstract

Electrochemical capacitors bridge the energy gap between conventional dielectric capacitors and batteries. The energy storage mechanism relies on purely physical electrical double-layer charging (EDL) and the faradaic process involving fast electrochemical redox reactions. These processes are strongly influenced by the surface area, porosity, electrical conductivity of the electrode materials, and the operating potential window of the electrolyte used. Carbonaceous materials play enormous roles in delivering outstanding electrochemical performance in electrochemical supercapacitors (ESCs) due to attractive material features suitable for high charge storage and release. However, due to the purely EDL-based charge storage mechanism in only carbon-based ESCs, the achievable energy density is low and hardly meets the high energy density demanding applications. Therefore, various carbon structures such as activated carbon, carbon nanotubes, graphene, and so on are designed and integrated with other hetero atoms or combined with transition metal oxides and polymer components to induce the pseudo-capacitive contributions via the electrochemical faradaic reaction. Thus, promoting the electrochemical performance of ESC based on the hybrid/composite material attributed to synergistic capacitances from EDLC and pseudocapacitance. Therefore, this review overviews the general perspective of the ESCs based on nanocarbons with various forms trending the progressive research contributions in developing high-performance ESCs.