Dielectric-free electrowetting on graphene

Measuring the Capacitance of Carbon in Ionic Liquids: From Graphite to Graphene

The physical electrochemistry of the carbon/ionic liquids interface underpins the processes occurring in a vast range of applications spanning electrochemical energy storage, iontronic devices, and lubrication. Elucidating the charge storage mechanisms at the carbon/electrolyte interface will lead to a better understanding of the operational principles of such systems. Herein, we probe the charge stored at the electrochemical double layer formed between model carbon systems, ranging from single-layer graphene to graphite and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI). The effect of the number of graphene layers on the overall capacitance of the interface is investigated. We demonstrate that in pure EMIM-TFSI and at moderate potential biases, the electronic properties of graphene and graphite govern the overall capacitance of the interface, while the electrolyte contribution to the latter is less significant. In mixtures of EMIM-TFSI with solvents of varying relative permittivity, the complex interplay between electrolyte ions and solvent molecules is shown to influence the charge stored at the interface, which under certain conditions overcomes the effects of relative permittivity. This work provides additional experimental insights into the continuously advancing topic of electrochemical double-layer structure at the interface between room temperature ionic liquids and carbon materials.

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 of 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/electrolyte interface. We demonstrate that the dependence of γSL on the applied surface charge exhibits an asymmetric behavior 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 a 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, introducing new conceptual routes for the investigation of wetting phenomena under potential bias.

Hydration Energy-Dependent Ion Intercalation on Graphite and the Asymmetric Electrowetting

Ion intercalation in graphite is widely used in desalination, batteries, and graphene stripping; it has high value in the fields of industry and research. However, selective ion transport, particularly (de)hydration energy and the hydration shell effect on the intercalation of ions into the graphite interlayer spaces, is still unclear. Here, we report low-voltage ion intercalation as observed by electrowetting on highly oriented pyrolytic graphite of an aqueous drop containing various inorganic salts. The electrowetting response exhibits asymmetric behavior with no contact angle change for the negative polarity and a threshold voltage for the onset of the contact angle change for the positive polarity. To explain the asymmetric electrowetting behavior and quantitatively predict the threshold voltage, we developed a physical model based on the hydration shell energy and size of the ion that undergoes partial breaking/deformation during the co-intercalation into the spaces between graphite layers. Electrowetting experiments using ions with various hydration energies and hydration radii were performed to confirm the prediction of the model. Further, we show a strategy to make the electrowetting response of LiCl drops symmetric via tuning the hydration energy of the Li+ ions using a binary solvent of a glycerol-water mixture. This article will provide an understanding of the hydration (solvation) energy dependence intercalation mechanism in graphite for electrowetting, which underpins various processes such as ion battery applications and the graphene exfoliation process.