Figure 4 . (a) Normalized density distribution of MeCN near a full-atomic graphene wall, and (b) the corresponding layer-by-layer two-dimensional density map. (c) The solvation number and (d) solvation diameter of Cl- versus the pore width of nanoslit. The grey dash lines represent the corresponding bulk values. The red and black dotted lines are drawn to guide the eye.
3.2.Microstructure of Liquid Electrolytes in Nanoslit
The electrochemical properties of supercapacitors are determined by the microstructure of electrolytes in each nanopore. The electrolytes are usually modeled with RPM model with the ion size identical with its solvation diameter.20, 32To explore the ion size effect, the density distributions of cation and anion are investigated in the same nanopore with identical ion concentration. Three typical sizes of cation and anion () are chosen, i.e ., = 0.33 nm, 0.57 nm, and 0.73 nm, which corresponds to, respectively, the bare ion size, the solvation diameter in confined MeCN with pore size = 0.92 nm, and the solvation diameter in bulk MeCN. The other system parameters are given as follows: the relative dielectric constant = 1.0, the system temperature = 298 K, the surface voltage = 1.15 V, and the bulk ion concentration = 1.50 M. The predicted ion density profiles by the CDFT are shown in Figure 5 . As expected, anions accumulate near the positively charged surfaces while cations are repelled to the center of nanoslit. In addition, we note that the ion distribution is sensitive to the solvation diameter. Specifically, when increasing the solvation diameter of ion, the contact value of anion increases, and the accumulation of both cations and anions are enhanced.