Self-discharge in Manganese Oxide Electrochemical Capacitor Electrodes in Aqueous Electrolytes with Comparisons to Faradaic and Charge Redistribution Models

The self-discharge profiles of manganese oxide films were examined to identify the self-discharge processes for these pseudocapacitive films. The profile shape was compared to four self-discharge models: an activation-controlled Faradaic reaction only; resistance-limited charge redistribution only; diffusion-limited charge redistribution only; or an activation-controlled Faradaic reaction coupled with charge redistribution. Hardware circuitry models (e.g. a transmission line) were used to model the activation-controlled reaction and resistance-limited charge redistribution. Ruthenium oxide was used as an experimental model for diffusion-limited charge redistribution, while highly porous carbon undergoing surface oxidation modelled the activation-controlled Faradaic reaction coupled with charge redistribution. Comparison of the manganese oxide self-discharge to these models showed an, as yet unidentified, activation-controlled Faradaic reaction on the manganese oxide surface, with a Tafel slope of 67 ± 5 mV. Additionally, the presence of charge redistribution was indicated in these films for the first time. During self-discharge, the activation-controlled reaction discharges the manganese oxide surface, setting up a potential differential between the surface and the bulk - where the surface has a lower potential. The result of this potential differential is that during charge redistribution the bulk manganese oxide provides charge to the surface, effectively slowing self-discharge.