Molecular Dynamics Simulation Study of the Capacitive Performance of a Binary Mixture of Ionic Liquids near an Onion-like Carbon Electrode Song Li, † Guang Feng, † Pasquale F. Fulvio, ‡ Patrick, C. Hillesheim, ‡ Chen Liao, ‡ Sheng Dai, ‡,§ and Peter T. Cummings* ,†,∥ † Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States ‡ Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States ∥ Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States * S Supporting Information ABSTRACT: An equimolar mixture of 1-methyl-1-propylpyrrolidinium bis- (trifluoromethylsulfonyl)imide ([C 3 mpy][Tf 2 N]), 1-methyl-1-butylpiperidinium bis- (trifluoromethylsulfonyl)imide ([C 4 mpip][Tf 2 N]) was investigated by classic molecular dynamics (MD) simulation. Differential scanning calorimetry (DSC) measurements verified that the binary mixture exhibited lower glass transition temperature than either of the pure room-temperature ionic liquids (RTILs). Moreover, the binary mixture gave rise to higher conductivity than the neat RTILs at lower temperature range. In order to study its capacitive performance in supercapacitors, simulations were performed of the mixture, and the neat RTILs used as electrolytes near an onion-like carbon (OLC) electrode at varying temperatures. The differential capacitance exhibited independence of the electrical potential applied for three electrolytes, which is in agreement with previous work on OLC electrodes in a different RTILs. Positive temperature dependence of the differential capacitance was observed, and it was dominated by the electrical double layer (EDL) thickness, which is for the first time substantiated in MD simulation. SECTION: Energy Conversion and Storage; Energy and Charge Transport R oom-temperature ionic liquids (RTILs) are promising electrolytes in energy storage devices. The application of RTILs in electrical double layer capacitors (EDLCs, also named supercapacitors) that store electric energy in the form of electrical double layer (EDL), has attracted increasing research interest due to its longer cycle life, higher power density and faster charging/discharging rates than organic or aqueous electrolytes. 1−4 The current utility of RTILs is restricted by the limited operating temperature range, which is mostly within 293−353 K. In order for supercapacitors to be used under severe cold weather conditions (specifically, temperatures as low as −50 °C, corresponding to the lower limit for automotive applications), ionic liquids with lower melting points are essential while retaining the capacitance. In this direction, binary mixtures of RTILs exhibited reduced melting temper- ature than either of the neat RTILs and widened liquidus range, 5 thus favoring the low-temperature application of RTILs. These mixed RTILs with decreased melting points are also referred to as eutectic ionic liquids, since the concentrations chosen correspond to eutectic points on the phase diagram. To achieve better electrochemical performance, electrolytes consisting of mixed RTILs have been used in lithium batteries 6 and dye-sensitized solar cells. 7 A recent study found that an equimolar mixture of piperidinium- and pyrrolidinium-based RTILs with the same anion bis(fluorosulfonyl)imide (FSI) as electrolytes near an exohedral onion-like carbon (OLC) electrode exhibited a broadened operation temperature range and increased conductivity at low temperature. 8 Because of the implications of such eutectic mixtures for energy storage and conversion devices, understanding the molecular behavior near electrode surfaces with well-defined surface curvatures is of great importance for further progresses in this field. Hence, it is scientifically interesting to understand the influence of temperature on the capacitive behavior of binary mixtures near the OLC electrode surface. Such electrodes have a simpler surface geometry than most porous carbons normally used as electrodes in supercapacitors. The nonporous OLC electrodes can be described as nearly spherical particles having a concentric fullerene shell structure and exhibiting a narrow particle size distribution. Thus, the simpler exohedral model describing the surfaces of OLCs offers a suitable reference for investigating the positive temperature issues associated with the eutectic mixtures of RTILs on carbon electrodes issue using Received: July 12, 2012 Accepted: August 20, 2012 Published: August 20, 2012 Letter pubs.acs.org/JPCL © 2012 American Chemical Society 2465 dx.doi.org/10.1021/jz3009387 | J. Phys. Chem. Lett. 2012, 3, 2465−2469