Ion Selectivity in the Nanoporous Electrodes of Electrical Double Layer Capacitors with Room-Temperature Ionic Liquids

Ion selectivity in nanoporous materials plays an important role in the performance of electrical double layer capacitors (EDLCs) containing room-temperature ionic liquids (RTIL) but remains poorly understood due to difficulties in both materials characterization and monitoring of ion partition at small scales. In this work, we study the partitioning of RTIL mixtures in the nanopores of carbon derived carbide (CDC), an amorphous porous material commonly used in EDLCs. Based on a course-grained model for RTILs, the classical density functional theory (CDFT) allow us to identify key parameters controlling ion selectivity and to optimize EDLC performance by tuning the electrolyte composition. The theoretical predictions on the effect electrolyte composition on capacitance find good agreement with voltammetry measurements for EDLCs consisting of CDC and a ternary ion mixture containing 1-ethyl-3-methylimidazolium (EMIM) cation, bis(trifluoromethane)sulfonimide (TFSI) and tetrafluoroborate (BF₄) anions.