Temperature Effects on the Capacitance of an Imidazolium-based Ionic Liquid on a Graphite Electrode: A Molecular Dynamics Simulation

Temperature-dependent electric double layer (EDL) and differential capacitance-potential (C-d-U) curves of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+/PF6+) were studied on a graphite electrode by molecular dynamics simulations. It was found that all C-d-U curves were asymmetric camel-shaped with higher C-d at negative polarization, attributed to the specific adsorption of BMIM+. In addition, the maxima of C-d at the negative polarization decrease monotonically with temperature due to the thicker EDL, whereas at the positive polarization they gradually increase from 450 to 550 K and decrease at 600 K. Such temperature effects at positive polarization may be understood in terms of the competition between two aspects: the weakening specific adsorption of BMIM+ allows more effective screening to the positive charge and overall increasing EDL thickness. Although the former dominates from 450 to 550 K, the latter becomes dominant at 600 K.