Electrical Double Layer of Linear Tricationic Ionic Liquids at Graphite Electrode

The microstructure and electrochemical performance of electrical double layers (EDLs) have been extensively studied for many monocationic ionic liquids (MILs) and several dicationic ionic liquids; however, they have not been reported for linear tricationic ionic liquids (LTILs). Microstructures of three LTILs [C-n(mim)(3)](Tf2N)(3) (n = 3, 6, and 10) and a MIL [C(6)mim][Tf2N] at neutral and charged graphite electrodes have been investigated using molecular dynamics simulations. We show that the LTILs and MILs have different multilayer EDL structures, with more perpendicular orientations for imidazolium rings, especially the central ring, of trications on the negative electrode surface. We also demonstrate that capacitance-voltage curves of EDLs transform from a bell shape in the MIL to a camel shape in the LTILs, which is ascribed to the accumulation of ions on electrode surfaces and the ion-ion correlation. Moreover, the LTILs can enhance the energy storage density at high electrode potentials, regardless of positive or negative voltages.