Electrode Models for Ionic Liquid-Based Capacitors

We investigate the influence of different electrode models on properties of a BMIM PF6 ionic liquid-based capacitor system using molecular dynamics simulations. We compared atomic graphene electrodes against smooth walls of equal average interaction energy. Furthermore, two different methods for simulating a constant potential drop between the electrodes were employed and compared to constant charge electrode models. In the evaluation of the voltage dependence of the ion configuration in the interfacial electrode regions, the influence of the electrode structure was determined by analyzing the in-plane radial distribution of the adsorbed ions, the interlayer ion exchange process, and the differential capacitance. We see similar results for the constant charge and constant potential approach for capacitance and structure of the adsorbed ions. We demonstrate that, due to the underlying ionic models, anions and cations adapt to the structural pattern of the graphene electrodes in different ways. This affects the charging process and leads to distinct asymmetric features in the differential capacitance.