Electric double-layer capacitors (EDLCs) based on porous activated carbon and ionic liquid electrolytes are promising energy storage devices due to their fast charge/discharge rates, excellent cycle stability, and wide voltage window. Understanding the energy storage mechanisms of EDLCs in ionic liquids is crucial, especially for revealing how the structural properties of ionic liquid cations and anions influence the capacitance performance of activated carbon. This knowledge can guide the rational design of high-performance EDLCs.
Recently, a research team from the Institute of Clean Energy Chemistry and Materials at the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, made significant progress in this area. They developed four types of nano-silica-grafted ionic liquids, enabling separate analysis of cations and anions by leveraging the fact that only one type of ion can freely enter and exit the activated carbon pores during charging and discharging. This approach offers a new strategy to study the behavior of individual ions in EDLCs.
The structure of these silica-grafted ionic liquids is such that one ion—either a cation like BMIM+ or NBu4+, or an anion like NTf2– or PF6–—is free to move, while the counter-ion remains as a balanced charge. For example, trifluoromethanesulfonimide anion (NTf2–) and methylimidazolium cation (MIM+) were covalently attached to 7 nm silica nanoparticles. Since most activated carbons used in this study have pore sizes below 4 nm, the silica-bound ions are blocked, allowing only the free ions (BMIM+, NBu4+, NTf2–, or PF6–) to access the pores. This setup enables simple electrochemical tests to quantitatively analyze ion insertion, with cyclic voltammetry directly reflecting the contribution of each ion to the total capacitance.
Using commercial activated carbon YP-50F as the electrode, the researchers were able to characterize the capacity contributions of each ion and their respective voltage windows. By combining quartz crystal microbalance (EQCM) with electrochemical measurements, they further explored the energy storage mechanism of YP-50F in the ionic liquid BMIM-NTf2. Their findings provided a deeper understanding of how BMIM+ and NTf2– contribute to the overall performance of the device.
These results were published in *Nature Communications*, and the research was supported by the National Natural Science Foundation of China and the Lanzhou Institute of Chemical Industry's "13th Five-Year Plan" initiative. This work paves the way for more efficient and tailored ionic liquid-based EDLCs in future energy storage applications.
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