Identification and isolation of carbon oxidation and charge redistribution as self-discharge mechanisms in reduced graphene oxide electrochemical capacitor electrodes

摘要

While carbons are common electrode materials for electrochemical capacitors (ECs) owing to their abundance, affordability, and environmental compatibility, graphene is particularly desirable due to its high electronic conductivity and high surface area for double-layer charging. However, very little is known about graphene's self-discharge (SD) - the spontaneous potential loss that occurs when a device rests idle. Knowledge of SD mechanisms is key because this process limits EC applications and reliability. Herein, we show carbon oxidation and charge redistribution - charge movement to eliminate potential gradients within a material - are key SD mechanisms for reduced graphene oxide (rGO) in acidic-aqueous electrolytes. Differentiating between these phenomena proves challenging; both processes present similar SD profiles. To address this, a novel experimental protocol is developed which resets CR to hold this process constant; when applied, this protocol separates CR contributions from carbon oxidation. We thus can identify charge redistribution as the primary SD mechanism, with a smaller but important contribution (17% of original SD) from carbon oxidation. From this work, it is clear that rGO SD can be significantly reduced by oxidizing (achieved here by CV cycling) and by more fully charging the rGO material (achieved through repeated charge/SD cycles). (C) 2018 Elsevier Ltd. All rights reserved.