Imidazolium-Based Ionic Liquids in Graphene-Capped Electrochemical Cells Probed by Synchrotron Infrared Nanospectroscopy: Insights into Electric Double Layers for Advanced Energy Storage

Electric double layer capacitors (EDLCs) are prominent energy storage devices owing to their advantages such as rapid charge/discharge processes, extremely long cycle life, and environmental and safety benefits. However, the generally low energy density of EDLCs hinders their broad application. Ionic liquids (ILs) are a novel class of electrolytes typically composed of asymmetric organic cations and weakly coordinated anions. Their wide electrochemical windows enable operation voltages much higher than conventional EDLCs and thus significantly improve the energy density. Moreover, ILs are robust materials suited for long-cycle EDLC operation. The implementation of ILs as electrolytes in EDLCs for enhanced performance is contingent upon understanding the behaviors of IL EDLs, a knowledge gap that exists thus far. Here, we incorporate ILs in custom-made graphene electrochemical cells and perform synchrotron infrared nanospectroscopy (SINS) to investigate IL EDLs interfaced with graphene electrodes, complemented by density functional theory (DFT) analysis and electrochemical evaluation. This approach correlates the chemical characterization and vibrational response of IL EDLs with their nanoscale ion ordering and displacement under electrochemical conditions, enabling in-depth observations of IL EDLs that were previously unattainable. Insights into IL EDL behaviors highlight the dependence of capacitive performance on the molecular-level ion dynamics decided by IL chemical structures, thereby guiding the design of next-generation EDLCs employing novel IL-based electrolytes.