Density Functional Theory for Confined Electrolytes: Phase Behavior, Interfacial Phenomena and Application to Capacitive Energy Storage

Electric Double Layer Capacitor (EDLC) is a very promising candidate for the next generation energy storage device. The breakthroughs in developing novel electrode and/or electrolyte material in experiments have greatly improve the energy density and kinetics in EDLC, which usually limits the application of EDLC. Meanwhile, theory, modeling and simulation can effectively complement experimental efforts and can provide insight into mechanisms, predict trend, identify new material and guide experiments. Among different methodologies, Classical Density Functional Theory is a versatile and efficient tool to study interfacial phenomena and phase behavior in confined fluid, which is critical important for the fundamental understanding of EDLCs. In this thesis, CDFT will be applied to study several important topics in capacitive energy storage, including electrolyte composition, charge storage mechanisms and wettability of non-aqueous electrolyte to porous material.We first explored how the electrolyte composition will influence the capacitive energy storage in the EDLC. We applied coarse grained model for the electrolyte consisting of ionic liquid and a small amount of chemical, which can be regarded as either additives or impurities. The effect of dipole moment, particle size, and binding energy on the capacitive curve was discussed. Then, we examined the idea of improving the energy storage by creating ionophobic environment in the pore. Different charge storage mechanisms were found, and the theoretical predictions showed that creating an ionophobic environment at low voltage was beneficial for the energy storage. This can be achieved by adding additives that have lower transfer energy into the electrolyte. Last, we discussed the importance of pore wettability to the energy storage in porous material. We showed the capillary evaporation of electrolyte may occur in the subnanometer pores and demonstrated that the accessibility of micropores depends not only on the ionic diameters but also on their wetting behavior intrinsically related the vapor-liquid or liquid-liquid phase separation of the bulk ionic systems. We hope the fundamental insights gained in this thesis work will help people to understand the complex physical process in EDLC and to better design systems to realize their full potentials