Temporal-stability of plasma functionalized vertical graphene electrodes for charge storage

Vertical graphene is an emerging material for supercapacitor applications. Yet, its inherent hydrophobic nature restricts the interaction with electrolyte ions, which brings in high demand for developing hydrophilic vertical graphene surfaces. Herein, super-wetting vertical graphene nanosheets are achieved by an in-situ post-deposition oxygen plasma treatment. The plasma-treated vertical graphene nanosheets electrodes found to exhibit a ten-fold enhancement in specific capacitance. However, the super-wetting nature of the plasma-treated vertical graphene nanosheets transformed back to hydrophobic upon exposure to ambient conditions, yet the rate of transformation is governed by the plasma parameters in-turn the type of oxygen functional group attached. Noteworthy, the analogous effect is persisted in supercapacitor performance too. Furthermore, a correlation between the temporal-stability of wetting nature and supercapacitor performance is established; by measuring the charge storage capacity of vertical graphene surfaces with different water contact angle. A preferential shift in dominant oxygen functionalities from carboxyl type to hydroxyl and carbonyl type with an increase in plasma energy is evident. A symmetric coin-type electrochemical capacitor device using super-wetting vertical graphene nanosheets electrodes is fabricated and demonstrated its performance.