Influence of cation shape asymmetry on the interfacial features and capacitance curve of ionic liquids inside the spherical cavity of the porous electrode as an ionic liquid-based supercapacitor

Abstract

Two imidazolium-based ionic liquids with the same anions but different cations (one was considered as a dimer model while the other as a monomer one) have been compared in some interfacial phenomena such as overscreening, surface charge amplification, and crowding, besides the shape of the capacitance curve inside the spherical cavity of the porous electrode. The coarse-grained model in the framework of classical density functional theory, CDFT, has been used to investigate the different electric double layer structures of these two models through different behaviors for their mean electric potential, MEP, local volume charge density, LVCD, and normalized integrated charge density, NICD, at negative electrode potentials. Although both models illustrate charge reversal, CR, and crowding phenomena, respectively, at moderate and high electrode potentials, at low electrode potentials close to the potential of zero charge, PZC, the monomer model illustrates CR, while the dimer model shows surface charge amplification, SCA. This different behavior may be explained by the fact that anions, due to their small sizes, are positioned in the spatial region that is created by the orientations of dimer cations at the electrode surface. This leads to a negative LVCD and partial charge density in the first layer near the electrode. The normalized integrated charge density as a function of r/σ can be used as a criterion for determining the type of screening phenomenon with curvature. Our results show that regardless of the cavity size, the screening behavior of ionic liquids in different cavities is similar, although its intensity varies with size. Other consequences of cation shape asymmetry for dimer models are related to fewer stored net ion charges inside the cavity at the negative potential region and also an asymmetric shape for the capacitance curve. It has also been identified that the shape of the capacitance curve is a charge concentration effect rather than a geometric effect.