Classical Density Functional Theory Insights for Supercapacitors

Abstract

The most urgent issue for supercapacitor is to improve their energy density so that they can better compete with batteries. To design materials and interfaces for supercapacitor with higher energy density requires a deeper understanding of the factors and contributions affecting the total capacitance. In our recent works, the classical density functional theory (CDFT) was developed and applied to study the electrode/electrolyte interface behaviors, to understand capacitive energy storage. For porous electrode materials, we studied the pore size effect, curvature effect, and the surface modification of porous materials on the capaci- tance. Thought CDFT, we have found that the curvature effects on convex and concave EDLs are drastically different and that materials with extensive convex surfaces will lead to maximized capacitance; CDFT also predicts oscillatory variation of capacitance with pore size, but the oscillatory behavior is magnified as the curvature increases; an increase in the ionophobicity of the nanopores leads to a higher capacity for energy storage, and a pore-like impurity can enter the pore, makes the pore ionophobic and storage more energy. We also find the mixture effect, which makes more counterions pack on and more co-ions leave from the electrode surface, leads to an increase of the counterion density within the EDL and thus a larger capacitance.