Molecular Dynamic Study of [EMIM]+[PF6]- Ionic Liquid near a Monolayer Graphene Surface

One of the important applications of ionic liquids is energy storage as an electric double-layer (EDL) supercapacitor. Several theoretical studies have been reported on the mechanism of EDLsupercapacitors [1, 2, 3]; however, understanding molecular structure of ILs near an electrode surface is missing. In this study, detailed molecular dynamics (MD) simulation results of the structure of [EMIM]+[PF6]- IL near a neutral monolayer graphene electrode have beeninvestigated. Method and Simulation Details All MD simulations were carried out by DL_POLY_2.17 package. An NVT ensemble at 873 K using Nose-Hoover thermostat was accomplished. To be ensured that system reaches equilibrium a three-step 1.5 ns cooling procedure, was applied on the target system. Finally, 2 ns productionrun was performed at 373 K and atmospheric pressure, with 1 fs time step, and 12 Å cutoff distance. Results and Discussion The results of MD simulation have shown accumulation of IL ions near the surface as Fig. 1demonstrates. This aggregation has a solid like behavior over 19 Å distance away from electrode showing a layer-by-layer structure as Vatamanu et al. [2] mentioned. Number density profile of[EMIM]+ cation, [PF6]- anion, and IL shows layering structure accurately. [EMIM]+[PF6]- angle distribution of the first layer of imidazolium ring in Fig. 2 illustrates the cations with lower distance and more distance to the surface, were parallel and perpendicular,respectively Imidazolium charge distribution of ions was considered as a candidate of potential zero charge(PZC), to understand the charge behavior of IL near the surface, (Fig. 3). The sharp peaks of total charge distribution tend to negativevalues because of the higher charge density of anions than the cations The layering behavior of [EMIm]+[PF6]- and total charge distribution of ions near the surface were shown the promising application of IL as supercapacitor electrolyte. It was resulted from the low charge distribution of the first layer due to its low potential zero charge

کلیدواژه ها:

Molecular Dynamics ، Double Layer ، Supercapacitor