Abstract
Room-temperature ionic liquids (RTILs) have received significant attention as electrolytes due to a
number of attractive properties such as their wide electrochemical windows. Since electrical double
layers (EDLs) are the cornerstone for the applications of RTILs in electrochemical systems such as
supercapacitors, it is important to develop an understanding of the structure–capacitance
relationships for these systems. Here we present a theoretical framework termed ‘‘counter-charge
layer in generalized solvents’’ (CGS) for describing the structure and capacitance of the EDLs in
neat RTILs and in RTILs mixed with different mass fractions of organic solvents. Within this
framework, an EDL is made up of a counter-charge layer exactly balancing the electrode charge,
and of polarized generalized solvents (in the form of layers of ion pairs, each of which has a zero
net charge but has a dipole moment—the ion pairs thus can be considered as a generalized solvent)
consisting of all RTILs inside the system except the counter-ions in the counter-charge layer,
together with solvent molecules if present. Several key features of the EDLs that originate from the
strong ion–ion correlation in RTILs, e.g., overscreening of electrode charge and alternating layering
of counter-ions and co-ions, are explicitly incorporated into this framework. We show that the
dielectric screening in EDLs is governed predominately by the polarization of generalized solvents
(or ion pairs) in the EDL, and the capacitance of an EDL can be related to its microstructure with
few a priori assumptions or simplifications. We use this framework to understand two interesting
phenomena observed in molecular dynamics simulations of EDLs in a neat IL of 1-butyl-3-
methylimidazolium tetrafluoroborate ([BMIM][BF4]) and in a mixture of [BMIM][BF4] and
acetonitrile (ACN): (1) the capacitance of the EDLs in the [BMIM][BF4]/ACN mixture increases
only slightly when the mass fraction of ACN in the mixture increases from zero to 50% although
the dielectric constant of bulk ACN is more than two times higher than that of neat [BMIM][BF4];
(2) the capacitance of EDLs near negative electrodes (with BMIM+ ion as the counter-ion) is smaller
than that near positive electrodes (with BF4as counter-ion) although the closest approaches of
both ions to the electrode surface are nearly identical.
