Charging Rate Dependence of Ion Migration and Stagnation in Ionic-Liquid-Filled Carbon Nanopores

Over the past decade, interest in leveraging subnanometer pores for improved capacitance in electrochemical double layer capacitors (EDLCs) has readily grown. Correspondingly, many theoretical studies have endeavored to understand the mechanisms that dictate the capacitance enhancement once ions are confined within nanopores, typically within quasi-equilibrium conditions However, a kinetic-based understanding of the capacitance may be important, especially since the dynamics of ion transport can exhibit dramatic differences under confinement compared to the bulk liquid phase; ion transport is driven by the competition between the electrostatic electrode-ion and ion-ion interactions, which can be comparable as the internal surface area to volume ratio increases. In this work, we study the relationship between the dynamics of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM/BF4) ionic liquid and the capacitance within two idealized cylindrical subnanometer pores with diameters of 0.81 and 1.22 nm using classical molecular dynamics simulations. By adjusting the voltage scan rate, we find that the capacitance is highly sensitive to the formation of an electroneutral ionic liquid region; with rapid charging, consolidated anion-cation contact pairs, which remain trapped within the pore, restrict the local accumulation of charge carriers and, thereby, the capacitance. These findings highlight potential kinetic limitations that can mitigate the benefits from electrodes with subnanometer pores.