Qualitative electrochemical impedance spectroscopy study of ion transport into sub-nanometer carbon pores in electrochemical double layer capacitor electrodes

Abstract

Ion adsorption onto high surface area microporous Carbide Derived Carbons (CDCs) with pore sizes in the sub-nanometer range was studied by means of the Electrochemical Impedance Spectroscopy (EIS) technique in two electrolytes, Tetraethylammonium Tetrafluoroborate (NEt4BF4) in Acetonitrile (AN) and in Propylene Carbonate (PC). Polarization at two bias voltages (0.5 V/Ref and -1 V/Ref) for EIS measurements enabled comparing the capacitive behaviors resulting from anions and cations adsorption, respectively, it was confirmed that the effective size of NEt4+ is bigger than the one of BF4-. Higher transport limitation was then observed for cations and was exalted in PC-based electrolyte. Although slow ion transport kinetics, it was found that the low frequency vertical line observed on the Nyquist plots was preserved meaning that carbon electrodes were fully charged. This study confirmed the importance of choosing an electrode carbon pore size adapted to the effective ion size. Finally, the best performances would be got in 1.5 M NEt4BF4 AN-based electrolyte with a 0.76 nm pore size negative electrode and a 0.68 nm pore size positive electrode.