Plasma-tuneable oxygen functionalization of vertical graphenes enhance electrochemical capacitor performance

Vertical Graphene Nanosheets (VGN) is one of the most promising energy storage materials in particular for electrochemical capacitor electrode applications. Yet, the intrinsic hydrophobic nature of VGN impedes electrode-electrolyte interaction and necessitates VGN surfaces to be hydrophilic to enhance the charge storage performance. This work not only demonstrates the improved effectiveness of surface modification through oxygen plasma exposure to transform the inherent hydrophobic VGN surfaces into super-hydrophilic ones, but also fills the existing knowledge gap about the specific oxygenated functionalities that cause this transition. Here we use an innovative combination of ex-situ or in-situ exposures to reveal and quantify the specific oxygenated functionalities on VGN surfaces. In particular, a preferential increase in hydroxyl and carbonyl type functional groups is demonstrated at higher plasma powers. In this way, the super-hydrophilic VGN electrodes reveal a ten times increase in the areal capacitance over the inherent hydrophobic ones. Moreover, a significant difference in capacitance amongst the samples treated in ex-situ and in-situ manner under the same plasma power is also demonstrated. The observed variation in the capacitance is related to the type and relative presence of the oxygenated functional groups. This simple and scalable approach elucidates the critical role of surface chemistry and the possibility of decorating VGN structures with preferential functional groups to achieve superior specific capacitance.