Effects of Surface Transition and Adsorption on Ionic Liquid Capacitors

Room temperature Ionic liquids (RTILs) are a type of synthesized electrolytes possessing superb electrochemical stability and low vapor pressure compared with conventional aqueous-based electrolytes, which offers significantly enlarged electrochemical window and ease of maintenance for capacitors. Experiments measuring capacitance in concentrated RTILs often found hysteresis indicating that an underlying phase transition might exist. Current theories explaining the hysteresis in terms of phase transition either assume RTIL mixtures with neutral solvents or omit ion-ion correlations in RTILs. In this study, a variant of an existing RTIL model is established for solvent-free RTIL capacitors incorporating both ion-ion correlations and nonelectrostatic interactions. We first use the model to explore the spontaneous charge separation in the capacitors and find that this transition is a common feature for realistic choices of the model parameters for most RTILs. Next, we investigate the effects of preferential ion adsorption on this charge separation transition. The results show that preferential ion adsorption can be a useful design parameter for optimizing the energy storage of the capacitors.