Ionic liquids and Graphene: The ultimate combination for High-Performance supercapacitors

Abstract

Ionic liquids (ILs), due to their tunable propensities and properties, are discussed as a new class of solvents for scientific and industrial applications. Developing novel processes and applying ionic liquids further accelerate the growth of our fundamental and engineering understanding.In this work, the structure of four ILs, 1-ethyl-3-methylimidazolium tetrafluoroborate formate (Emim+BF4−)/chloride (Emim+Cl−)/Nitrate (Emim+NO3−), and thiocyanate (Emim+SCN−) is reported. Then via employing classical molecular dynamics and well-tempered metadynamics simulations their dynamics are investigated. The dynamics of ILs are represented by studying mean squared displacements. Additionally, the increased size and weight of anions interacting with the Emim+ cation cause the dynamics to become slow. Therefore, due to the higher molecular weight, the BF4− anion has lower diffusion than any other. Indeed, radial distribution functions between cations and anions reveal the structural arrangement in ILs. On the other hand, it is unexpectedly found that a certain amount of ILs are distributed near the electrode surface. Such phenomena occur because of the relatively strong ionic interactions between cations and anions; also, the π–π stacking between cations and graphene caused their interactions to become stronger. Furthermore, the atoms in molecules (AIM) analyses identified several interatomic interactions between the ILs and graphene. The free energy values for the ILs at their global minima are about ∼ -271.79, −267.93, −251.37 kJ mol−1 and ∼ -241.98 kJ mol−1 in systems Emim+/Cl−, Emim//NO3−, Emim+/SCN−, and Emim+/BF4−, respectively. Our findings represent a crucial step towards interpreting the complex interactions and nanostructures at the electrolyte–electrode interface, which could be beneficial for designing high-performance supercapacitors in energy storage using carbon electrodes and IL electrolytes in experiments.