Examination of Water Electrolysis and Oxygen Reduction As Self-Discharge Mechanisms for Carbon-Based, Aqueous Electrolyte Electrochemical Capacitors

Water electrolysis and oxygen reduction as possible self-discharge mechanisms for carbon-based, aqueous H 2 SO 4 electrolyte electrochemical capacitors is examined through a comparison of the predicted and actual effects of varying the dissolved oxygen and hydrogen content on self-discharge. Water electrolysis, in the form of oxygen evolution, is not the self-discharge mechanism on the positive electrode, although, the self-discharge profile is consistent with an activation-controlled Faradaic discharge or charge redistribution mechanism. The addition of hydrogen evidences no change in self-discharge from the negative electrode, and the profile is consistent with a diffusion-controlled mechanism, suggesting water electrolysis through hydrogen evolution is not the self-discharge mechanism. Oxygen reduction causes a large increase in self-discharge on the negative electrode which necessitates purging the cell of oxygen. As such, water electrolysis is likely not the cause of self-discharge in carbon-based, aqueous electrolyte electrochemical capacitors but oxygen reduction is a cause of increased self-discharge on the negative electrode.