A SYSTEMATIC STUDY OF SELF-DISCHARGE MECHANISMS IN CARBON-BASED, AQUEOUS ELECTROLYTE ELECTROCHEMICAL CAPACITORS

Abstract

This work focused on the study of self-discharge mechanisms of carbon electrochemical capacitor electrodes in 1.0 M H2SO4 electrolyte. Electrochemical capacitors have an increasingly important role in the future of energy storage for specific applications due to their high cycle lives, high power capabilities and the ability to use environmentally friendly materials. Remediation of the occurrence of self-discharge – the loss of charge over time when left in open-circuit configuration – must take place before electrochemical capacitors can be used more widely as this diminished potential results in a reduction of stored energy. By examining the now poorly understood causes and mechanisms of self-discharge, beneficial modifications to the electrochemical capacitors systems can be made, improving device performance. Three-electrode electrochemical set-ups were used to separate self-discharge mechanisms on the negative and positive electrodes. Various electrode and electrolyte reactions were investigated in relation to self-discharge, including Fe-contamination reaction, electrolyte decomposition, oxygen-reduction, carbon oxidation, and carbon surface group development. All experiments were conducted on porous carbon electrodes. It was determined that Fe-contamination increased self-discharge on both carbon electrodes at concentrations >10-3 M, and that previously developed planar kinetic models applied to these porous systems. Electrolyte decomposition did not result in increased self-discharge on either electrode. Electrolyte oxygen content must be minimized as oxygen is believed to undergo reduction to hydrogen peroxide on the negative-electrode, resulting in an increase in self-discharge. The carbon electrodes used in this work must be cycled prior to energy storage as the capacitance varies greatly with continued cycling, and the lack of cycling results in increased self-discharge. Additionally, interest in the carbon electrode’s surface functionalities resulted in the standardization of the Boehm titration.