Chemical and electrochemical ageing of carbon materials used in supercapacitor electrodes

Electrochemical double layer capacitors-supercapacitors-are usually based on highly porous activated carbon electrodes. The chemical and electrochemical inertness of carbon assures a very long lifetime. However, on the timescale of months the capacitors slowly lose some capacitance, and the equivalent series resistance rises. This "ageing" is faster when temperature and operation voltage are increased. To elucidate ageing processes in the electrodes, we analyzed aged samples of activated carbon, and also of carbon black, which is used as conductive additive. The experimental methods were infrared, Raman, and photoelectron spectroscopy, and nitrogen porosimetry. The electrodes are not completely inert, and their ageing is due to chemical and electrochemical reactions at the carbon. Moreover, the ageing procedures are much stronger at the positive pole. We found that oxygen, hydrogen, nitrogen, and fluorine, which originate from trace water and from the electrolyte, can covalently attach to the electrodes and form various chemical groups (e.g. OH, COOH, CONH, and F). (C) 2008 Elsevier Ltd. All rights reserved.