Effects of charge redistribution on self-discharge of electrochemical capacitors

The effect of charge redistribution on the self-discharge profile of porous carbon (Spectracarb 2225) electrodes is examined. A model pore based on the de Levie transmission line circuit is used to show that self-discharge due purely to charge redistribution results in the same self-discharge profile as that expected for an activation-controlled self-discharge mechanism (the potential falls linearly with log t), thus the linear log time profile is not characteristic of an activation-controlled mechanism. The addition of a hold step reduces the amount of charge redistribution in porous carbon electrodes, although the hold time required to minimize the charge redistribution is much longer than expected, with electrodes which have undergone a 50 h hold time still evidencing charge redistribution effects. The time required for the charge redistribution through the porous electrode is also much greater than predicted, likely requiring tens Of hours. This highlights the importance of the charge redistribution in self-discharge of systems using porous electrodes, such as electrochemical capacitors. (C) 2009 Elsevier Ltd. All rights reserved