Self-Discharge in Electrochemical Capacitors: A Perspective Article

This perspective article outlines some of the key considerations and literature that have been published on self-discharge in electrochemical capacitors. While for some consumer applications self-discharge is not considered to be a significant issue (e.g. energy storage from regenerative breaking) in applications where the electrochemical capacitor is stored in the charged state for significant times (e.g. coupled with a battery in a cell phone), the impact on energy, power and recharging frequency of both the capacitor and battery can be significant. A description is provided here of the common methods of self-discharge study: half cell vs. full cell measurements and open-circuit potential decay versus float currents. A description of some of the models used to evaluate faradaic self-discharge is presented, with a synopsis of the important aspects of the many available charge redistribution models. An overview of the current self-discharge mechanisms for various ECs is provided, highlighting that for many systems there are significant factors of the self-discharge process which remain unknown. Finally, some future directions are anticipated for the field of self-discharge in electrochemical capacitors.