Relation between double layer structure, capacitance and surface tension in electrowetting of graphene and aqueous electrolytes

Deciphering the mechanisms of charge storage on carbon-based materials is pivotal for the development of next generation electrochemical energy storage systems. Graphene, the building block for graphitic electrodes, is an ideal model for probing such processes on a fundamental level. Herein, we investigate the thermodynamics of the graphene/aqueous electrolyte interface by utilizing a multiscale quantum mechanics – classical molecular dynamics (QM/MD) approach to provide insights into the effect of alkali metal ion (Li+) concentration on the interfacial tension (γSL) of the charged graphene sheet/electrolyte interface. We demonstrate that the dependence of γSL on the applied surface charge exhibits an asymmetric behaviour relative to the neutral surface. At the positively charged graphene sheet, the electrowetting response is amplified by electrolyte concentration, resulting in a strongly hydrophilic surface. On the contrary, at negative potential bias, γSL shows a weaker response to the charging of the electrode. Changes in γSL greatly affect the total areal capacitance predicted by the Young-Lippmann equation but have negligible impact on the simulated total areal capacitance indicating that the EDL structure is not directly correlated with the wettability of the surface and different interfacial mechanisms drive the two phenomena. The proposed model is validated experimentally by studying the electrowetting response of highly oriented pyrolytic graphite over a wide range of electrolyte concentrations. Our work presents the first combined theoretical and experimental study on electrowetting using carbon surfaces, probing the charge storage mechanism at the electrochemical interface and its structural characteristics. The approach developed herein introduces new conceptual routes for the investigation of wetting phenomena on carbon surfaces under potential bias.