Influence of surface oxides on the impedance behavior of carbon-based electrochemical capacitors

Electrochemical capacitors made of activated carbon fabrics containing different compositions of surface oxides are analyzed using ac impedance spectroscopy. The oxides are introduced and controlled via HNO₃ treatment followed by thermal treatment at different temperatures within 150-900°C. The impedance spectra showed that the overall resistance mainly came from the fiber contact resistance, which was an increasing function of oxide number, while the overall capacitance was contributed by the interface inside micropores. Oxidation enhanced the specific capacitance of the electrodes, but it was found that a thermal treatment temperature of 450°C gave the highest capacitance. Constant phase element (CPE) analysis of the capacitive behavior showed insignificant deviation from ideality, indicating the low resistance in the carbon micropores as well as the domination of the double layer mechanism in energy storage. Thus, the capacitance increase from oxidation resulted mainly from the double layer mechanism as well. The deviation from ideality was more obvious for lower temperature treated electrodes, indicating that the surface oxides that desorb as CO₂ upon heat treatment may enhance formation of aggregates to retard the charge process, while the surface oxides that desorb as CO are responsible for the promotion of the double layer capacitance.