Chain length matters: Structural transition and capacitance of room temperature ionic liquids in nanoporous electrodes

Room-temperature ionic liquids (RTILs) have shown great potential in promoting the widespread application of energy storage devices. The relationship between the electrical double layer (EDL) structure and capacitive performance of different RTILs is vital to the rational design of novel RTIL electrolytes. Herein, we have investigated the microscopic structures of three types of RTILs containing cations with varying tail lengths in nanopores, their charging behaviors, and their EDL capacitances using classical density functional theory (CDFT). The results show that the interfacial structure changes from a monolayer to a bilayer as the neutral chain length of the cations increases, which is in agreement with previous experimental results (Smith et al., 2013). Moreover, the RTILs containing cations with longer chains can significantly enhance the capacitance in nearly neutral pores, while they dampen capacitance under negative electrical potential and show little disadvantage when positive potential is applied.