The principle of energy storage in

The principle of energy storage in a supercapacitor can be either (i) electrostatic charge accumulation at the electrode/electrolyte interface (electrical double layer capacitance, EDLC), as schematically shown in Fig. 1, or (ii) charge transfer, via reversible (Faradaic) redox reaction(s), to redox materials (e.g. conductive polymers, metal oxide nanoparticles) on the surface of electrode (pseudo-capacitance). In practical supercapacitors, the two storage mechanisms often work simultaneously [16]. Different charge transfer processes involved in the EDLC and pseudo-capacitance 4, 5, 16. In EDLC, the energy is stored through ion adsorption (a purely electrostatic process) at the electrode-electrolyte interface with no charge transfer across the electrodes, suggesting a non-faradic process. By contrast, pseudo-capacitance arises from reversible redox reaction(s) between the electrolyte and active species on the surface of electrodes. Although pseudo-capacitance higher than EDLC capacitance can be achieved, supercapacitors based on pseudo-capacitance often suffer from the poor electrical conductivity of the electroactive species, and hence demonstrate low power density and cycling stability. Therefore, the combination of both EDLC and pseudo-capacitance presents an effective means to improve the overall capacitance of a supercapacitor.