New insights into the carbon/polymer electrolyte interface in the electric double layer capacitor

To use a polymer electrolyte successfully in the electric double-layer capacitor (EDLC), it is important to design the interface between the carbon electrode and the polymer electrolyte. In this work, interfacial properties of carbon/poly (ethylene oxide) (PEO) solid electrolyte and carbon (or activated carbon)/polymer gel electrolyte were investigated. The all-solid-state EDLC with a pair of isotropic high-density graphite (HDG) electrodes possesses a high capacitance in PEO/LiClO₄ ([EO]/[Li⁺] = 8) solid polymer electrolyte. The capacitance of HDG electrode was strongly influenced by temperature and the degree of crystallinity of PEO-LiClO₄. Furthermore, various gel electrolytes were evaluated. The PMMA (poly(methyl methacrylate)), PVdF (poly(vinylidene fluoride)) and PVdF-HFP (poly(vinylidene fluoride-hexafluoropropylene) based gel electrolytes show a good electrochemical stability on the HDG electrode and a sufficient mechanical strength. In the case of using activated carbon (AC) powder (as well as ordinary carbon powder) as the electrode material for EDLC with PVdF-HFP gel electrolyte, the electrode composed of AC (or carbon powder) and the gel electrolyte exhibits a higher specific capacitance and a lower ion diffusion resistance than does the electrode prepared with a dry polymer binder. The highest specific capacitance of 123 F g^-1 was achieved with a composite electrode containing AC powder with a specific surface area of 2500 m² g^-1.