Oxygenated Functional Groups Localize Charge and Increase Capacitance in Aromatic Ionic Liquids

High Resolution Image Download MS PowerPoint Slide Ionic liquids are promising supercapacitor electrolytes due to high thermal and electrochemical stability. However, interfaces formed by ionic liquids with electrode surfaces are dominated by strong ion–ion correlations and ion crowding, which decrease interfacial capacitance. Here, we report how modulating ion correlations influences interfacial capacitance with the goal of advancing the molecular design of ionic liquid-based supercapacitors. We focus on studying how the addition of polar substituents impacts electric double layer capacitance compared to ionic liquids functionalized with conventional alkyl groups. Specifically, we evaluate how cation size, charge distribution, and polarity impact capacitance using electrochemical impedance spectroscopy. We find that a n -butyl-substituted pyrrolidinium ionic liquid exhibits higher capacitance than its n -butyl-functionalized imidazolium counterpart, suggesting localized charge density plays a larger role in dictating capacitance than ion size. Further, we reveal that replacing alkyl groups with polar oxygenated functional groups doubles capacitance in imidazolium ionic liquids while minimally impacting pyrrolidinium ionic liquids. Connecting interfacial capacitance to chemical environment, we investigate proton shifts resulting from oxygen functionalization in imidazolium and pyrrolidinium ionic liquids using nuclear magnetic resonance spectroscopy. While adding oxygen functionalities introduces electron-withdrawing effects that draw electron density from all cation charge centers, we find that oxygenation breaks the symmetry of the electronic environment in aromatic imidazolium rings, localizing charge. Thus, we reveal that charge localization is key to determining interfacial capacitance in ionic liquids, providing pathways towards tuning ionic liquid interfaces for use in energy storage.