Abstract
Electric double-layer capacitors (EDLCs) have the advantages of much longer life and unnecessary maintenance. However, for EDLCs used for a long period, a decrease of electrostatic capacity and an increase in internal resistance are recognized. In this study, in order to clarify the factors responsible for the deterioration of EDLCs used for a long period, chemical analysis was conducted on each component such as an activated carbon electrode and an electrolytes solution. The obtained data were compared with those from a new EDLC. The results showed that long-term use reduced electrostatic capacity about 5% and made internal resistance increase by about 1.7 times. The quantity of oxygen due to surface functional groups of the activated carbon electrode was confirmed using electron spectroscopy for chemical analysis (ESCA). As compared with results obtained from a new EDLC, long-term use made the ratio of O-C=O deriving from carboxyl group (-COOH) to C-C increase 1.2 times. The ratio of C-O deriving from hydroxyl group (-OH) to C-C also increased 1.4 times. In addition, it was found that BET surface area decreased about 40% and that pores whose diameter was in the range between 0.8 to 20 nm disappeared.
