Figure 1 . (a) Schematic diagram of microporous TiC-CDC
electrode with specific PSD obtained from ref. 12 . (b)
Illustration of ion desolvation caused by geometrical confinement of
nanoslit. (c)
A
representative nanoslit in the positively charged electrode.
Figure 1 (b) displays the schematic graph of ion desolvation in
confined liquid, in which the anion is encircled by solvent molecules
due to the static electronic attraction.
Here,
acetonitrile (MeCN) solvent is considered in order to perform a faithful
comparison with experiments. MeCN is a typical linear polar fluid and it
can be modeled as a linear molecule containing three
parts42 : the methyl group
(blue
sphere), the carbon group (grey sphere), and the nitrogen group (yellow
sphere).
The
solvent structure of MeCN surrounding the solute ion is described by the
local density distribution of MeCN
molecules,
, which depends
on
both the spatial position, , and the Euler angle, , of MeCN molecule in
space.
The spatial orientation of a MeCN molecule can be alternatively
characterized by the included angle between the linear MeCN molecule and
the vector connecting the solute ion to the carbon group of MeCN,
designated as
.
In
this work, the cosine value of , i.e .,, is employed to describe
the spatial orientation of MeCN,
which
can be converted from the Euler angle through a rotation matrix
operation.43In nanopores, the interfacial interaction and the geometrical
confinement effect may destroy the solvation structure of ion, leading
to the rearrangement of local density distribution of MeCN, and this
feature becomes more pronounced as the pore size decreases down to
several molecule sizes, resulting in a significant decrease of the MeCN
number within the first solvation shell. When further decreasing the
pore size, the solvation diameter of ion will decrease until the
occurrence of a bare ion.
Since
not all solvent molecules within the first solvation shell contribute to
the solvation diameter of
ions,44-45the spatial orientation distribution of MeCN can be additionally
introduced to distinguish the contributing molecules.
This
idea has been applied to evaluate the ion hydration size in confined
liquid water by using the MDFT, in which
of the contributing water molecules in the first solvation shell was
determined as [-1.0, -0.44] for cation and [0.48, 1.0] for
anion.36 In this work, we
adopt
the same concept to calculate the solvation diameter of ions in confined
MeCN:
, (1)