Self-discharge of AC/AC electrochemical capacitors in salt aqueous electrolyte

The self-discharge (SD) of electrochemical capacitors based on activated carbon electrodes (AC/AC capacitors) in aqueous lithium sulfate was examined after applying a three-hour cell potential hold at U-i values from 1.0 to 1.6 V. The leakage current measured during the potentiostatic period as well as the amplitude of self-discharge increased with U-i; the cell potential drop was approximately doubled by 10 degrees C increase of temperature. The potential decay of both negative and positive electrodes was explored separately, by introducing a reference electrode and it was found that the negative electrode contributes essentially to the capacitor self-discharge. A diffusion-controlled mechanism was found at U-i <= 1.4 V and U-i < 1.2 V for the positive and negative electrodes, respectively. At higher U-i of 1.6 V, both electrodes display an activation-controlled mechanism due to water oxidation and subsequent carbon oxidation at the positive electrode and water or oxygen reduction at the negative electrode. (C) 2016 Elsevier Ltd. All rights reserved.