Interfaces of dicationic ionic liquids and graphene: a molecular dynamics simulation study.

Molecular dynamics simulations were performed to investigate the interfacial structure and capacitance of electrical double layers (EDLs) in dicationic ionic liquids (DILs) 1-alkyl-3-dimethylimidazolium tetrafluoroborate Cn(mim)22 (n = 3, 6, 9), with respect to a baseline of a monocationic ionic liquid [C3mim][BF4], near planar carbon electrodes consisting of graphene sheets. The simulation results show that an adsorbed layer with double peaks is exclusively found for C3(mim)22, while a single peak of the other three cations is observed at the neutral electrode, due to the difference in ion-wall interaction and cation-anion association. As the electrode becomes negatively charged, the second peak of C3(mim)2 is dramatically reduced, whereas those of C6(mim)2 and C9(mim)2 become non-trivial. The capacitance-potential curve of EDLs in DILs manifests a transition from camel shape to bell shape as the cation chain length increases, which is attributed to the enlargement of ion adsorption (per unit charge) on the electrode and the decrease of attractive interaction between ions.