Probing Imidazolium-Based Ionic Liquid Double Layers on Graphene Electrodes via Synchrotron Infrared Nanospectroscopy for Energy Storage Applications

Electrical double layer capacitors (EDLCs), often categorized as a subset of supercapacitors, 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. A major strategy to improve the energy density of EDLCs involves employing electrolytes capable of high operation voltages. Ionic liquids (ILs) are a novel class of electrolytes typically composed of asymmetric organic cations and weakly coordinated anions. They appear to be ideal candidates to replace the diluted aqueous and organic electrolytes in traditional EDLCs for their wide electrochemical windows, which enable operation voltages much higher than conventional EDLCs and thus significantly improve the energy density. Moreover, the unique properties of ILs, including high thermal stability, non-flammability, high charge density, as well as their distinct EDL structure with oscillating ion concentration, not only make them safe and environmentally-friendly options for EDLCs, but also present great possibilities for superior performance to traditional electrolytes. The implementation of ILs as electrolytes in EDLCs for enhanced performance and functionalities is contingent upon understanding the structure of IL EDLs, a knowledge gap that exists thus far. Here, we incorporate ILs in custom-made liquid cells and perform synchrotron infrared nanospectroscopy (SINS) to investigate IL EDLs interfaced with graphene electrodes, complemented by density functional theory (DFT) analysis and electrochemical testing. This approach effectively correlates the chemical and vibrational bond information of IL EDLs with their nanoscale ion ordering and displacement behaviors under electrochemical conditions, leading to a comprehensive understanding of IL EDLs that was previously inaccessible. The insights into IL EDL behaviors are expected to reveal the molecular-level dynamics corresponding to charge storage, thus providing guidance for the design and creation of next-generation EDLCs based on novel IL-based electrolytes.