Ionic liquid-metal interface: The origins of capacitance peaks

The complex nature of electrode charge screening is well-known for ionic liquids (ILs). Due to strong ionion correlations, these electrolytes form a distinctive layered structure at interfaces. Variations in electrode potential cause structural changes that are reflected in a peculiar shape of differential capacitance-potential dependence with characteristic peaks. Although the differential capacitance for various ILs in conjunction with metal electrodes accessed via molecular dynamics (MD) simulations has been reported in the literature, retrieving a capacitance-potential curve, C(U), from the MD trajectories is not a trivial task. In this work, we present the results of the MD simulations of the IL 1-butyl-3-methylimidazolium hexafluorophosphate at a single-crystalline Au (100) surface. The discussion focuses on the simulation data treatment for C(U) curve fitting. It is shown that the resulting C strongly depends on the fitting method used. Four capacitance peaks and three structural reorganization types were identified in the studied system. With the help of a semi-quantitative approach in the framework of the original bilayer model of electric double layer (EDL), it is argued that the ions' reorientation is in the origin of the capacitance peaks. Also, it is shown that under the conditions of this study, the multilayer structure, characteristic of EDL in ILs on the whole, is far from the lattice saturation regime. The multilayer structure possesses a steric packing effect that impedes structural changes, decreasing the capacitance. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The complexity of electrode charge screening in ionic liquids is well-known, resulting in distinctive structural changes and capacitance peaks at interfaces due to strong ion correlations. Molecular dynamics simulations have been used to study differential capacitance for various ILs with metal electrodes, highlighting the challenge of obtaining a capacitance-potential curve. This study focused on MD simulations of a specific IL at a gold surface, revealing the dependence of capacitance on the fitting method and identifying capacitance peaks and structural reorganization types.