Pore-Size-Dependent Capacitance and Charging Dynamics of Nanoporous Carbons in Aqueous Electrolytes

Abstract

In this study, we carry out molecular simulations on carbon pores of different diameters in an aqueous potassium chloride solution. We investigate the delicate correlation of the capacitance as a function of pore size and electrode potential, the in-pore ion and water distributions, as well as the charging dynamics of these systems. Our results see the capacitance enhancement of the subnanometer pores; however, the capacitance tends to become stable and pore-size independent when the pore gets larger. In addition, the charging mechanisms of pores with different sizes are found to shift from the ion-exchange mode to the permselective counterion adsorption as the size of the pore shrinks. The latter can be attributed to the desolvation of ions in the subnanometer pores, which limits the ion adsorption kinetics and causes low in-pore ionic conductivity, thus exacerbating the rate performance. These conclusions are supported by our electrochemical measurements on the assembled cells composed of four distinct activated carbon samples with different average pore sizes. This work gives molecular insights into the nanoporous carbons in aqueous electrolytes, shedding light on the future design of the aqueous-based electrical double-layer capacitors with balanced rate performance and energy density.