Decoding Ionic Liquid Architecture at the Electrode-Electrolyte Interface in Supercapacitors.

The increasing demand for efficient energy storage underscores the need to enhance the capacity of electric double-layer capacitors (EDLCs), with electrolyte modification being a promising approach to improve their performance. Ionic liquids offer wide electrochemical stability windows, low volatility, thermal stability, and tunable molecular architectures, making them promising electrolytes for EDLCs. To use the molecular tunability of ionic liquids more effectively, it is important to understand how ion architecture shapes interfacial organization of ions near the electrode and controls charge storage. To understand the integrated geometry and electrostatic organization of ions at the molecular level, here we employ atomistic molecular dynamics simulations to compare imidazolium, pyridinium, pyrrolidinium, and piperidinium-based electrolytes at graphene electrodes to understand how five and six membered aromatic planar and saturated non-planar frameworks organize at the electrode-electrolyte interface and affect the device performance. A common tetrafluoroborate anion, electrode geometry, interelectrode separation, and a fixed surface charge density provide a consistent framework for comparison. Molecular orientation, two-dimensional Voronoi tessellation, ion-density distributions, and charge-screening analyses connect adsorption configurations, lateral packing, and countercharge organization at the interface with capacitance behavior of the individual architectural class. Our simulation results show that Imidazolium exhibits the highest capacitance, accompanied by predominantly surface-parallel rings, while pyridinium exhibits the lowest capacitance and is accompanied by mixed orientations. Our simulation results help establish the structure-orientation-capacitance relationship at the molecular scale, which is essential for designing a future ionic liquid-based electrolyte for high-performance EDLCs for applications such as power buffering in renewable energy systems, peak-power delivery in portable electronics, and industrial energy recovery.