Electric double-layer capacitors (EDLCs) based on porous activated carbon and ionic liquid electrolytes have gained significant attention due to their fast charge/discharge rates, excellent cycle stability, and wide voltage window. These characteristics make them a promising candidate for next-generation energy storage systems. To fully understand the energy storage mechanism in EDLCs, especially how the intrinsic structure of ionic liquids affects the capacitance performance of activated carbon, it is crucial to investigate these interactions at a microscopic level. This knowledge can guide the rational selection of ionic liquids and help in designing high-performance EDLCs.
Recently, a research team from the Clean Energy Chemistry and Materials Laboratory at the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, made notable progress in studying the energy storage mechanisms of ionic liquids in EDLCs. They developed four types of nano-silica-grafted ionic liquids, enabling the selective analysis of cations and anions during the charge-discharge process. By allowing only one type of ion to enter and exit the activated carbon pores, they were able to isolate and study the contributions of individual ions, offering new insights into ionic liquid behavior in EDLCs.
The key feature of silica-grafted ionic liquids is that one type of ion—either a cation (such as BMIM+ or NBu4+) or an anion (like NTf2– or PF6–)—is covalently bonded to silica nanoparticles (7 nm in size), while the counter-ion remains free. Since most pores in the activated carbon used in this study are smaller than 4 nm, the grafted ions are blocked from entering, while the free ions can pass through. This setup allows for a simple electrochemical test to quantitatively analyze the ion movement, with cyclic voltammetry curves directly reflecting the contribution of each ion to the overall capacitance.
Using commercial activated carbon YP-50F as the electrode, the team characterized the specific capacity contributions of different ions and their respective voltage windows. Further insights were obtained using quartz crystal microbalance (EQCM) to analyze the energy storage mechanism of YP-50F in BMIM-NTf2 ionic liquid. Combined with the electrochemical properties of both BMIM+ and NTf2–, the study provided a deeper understanding of the hierarchical energy storage mechanism.
These findings were published in *Nature Communications* and supported by the National Natural Science Foundation of China and the "1-3" key projects of the Lanzhou Institute of Chemical Industry. The research opens new pathways for optimizing ionic liquid-based EDLCs and advancing the field of electrochemical energy storage.
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