Effect of accelerated ageing on the performance of high voltage carbon/carbon electrochemical capacitors in salt aqueous electrolyte

An accelerated ageing protocol has been developed and validated to determine the factors influencing the life-time of high voltage carbon/carbon electrochemical capacitors (ECs) in aqueous lithium sulfate electrolyte, and to offer solutions reducing ageing of the cells. The study allowed the internal overpressure, increase of equivalent series resistance and corrosion of the positive current collector to be distinguished as the main failures which can appear during operation of these ECs at high voltage. To assure stability during cycling, the capacitor should not operate above 1.5 V at room temperature. Adding sodium molybdate (Na2MoO4) to the electrolyte reduces the corrosion of the positive collector and lowers the equivalent series resistance which remains constant during floating at 1.5 V; additionally, the initial capacitance is enhanced due to a pseudo-faradaic contribution of the molybdate additive. The internal resistance is further lowered when the pellet electrodes are glued to the current collectors with a carbon conductive adhesive.