Investigation of Electrical Double Layers of Ionic Liquids in Graphene-Capped Liquid Cells by Multimodal Synchrotron Infrared Nanospectroscopy for Advanced Energy Storage Devices

Electrical double layer capacitors (EDLCs), often categorized as a subset of supercapacitors, are prominent energy 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 current 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 graphene-capped liquid cells, which allow for synchrotron infrared nanospectroscopy (SINS) investigation of IL EDLs, in combination with density functional theory (DFT) analysis and other atomic force microscopy (AFM) characterizations, including high-resolution structural imaging and Kelvin probe force microscopy (KPFM). This approach effectively correlates the chemical and vibrational bond information of IL EDLs with their nanoscale ion ordering and displacement behaviors, leading to a comprehensive understanding of IL EDLs that was previously inaccessible. The insights into IL EDL behaviors is expected to reveal the molecular-level dynamics corresponding to charge storage, thus providing valuable guidance for the design and creation of next-generation EDLCs based on novel IL-containing electrolytes.