The size dependence of the integral capacitance (CI) has been investigated before by both
experimental and theoretical means.[31, 45, 46] The integral capacitance is defined as the ratio of
the charge to the electrical potential at the electrode surface, that is, CI Q/
s. By modelling the microscopic environment of porous carbon electrodes as slit pores, CDFT predicts anoscillatory dependence of CI on the pore size ( H ), resembling that for the potential of mean force between two charged surfaces surrounded by an RTIL.[31] For a pure ionic liquid in a slit
87
EDLs from the two charged walls. The peak capacitance appears when the EDLs near the two
surfaces have the most constructive interference. If the RTIL contains an impurity, how is the
I
C H curve different from that of a pure ionic liquid?
Figure 4.2 The integral capacitance (C ) versus the pore size ( H ). Here, the surface electrical potential I
is s = 1.5 V; the bulk mole fraction of the impurity, x0 0b/
0bbb
, is 10-4. The linescorrespond to different surface energy w.
Figure 4.2 shows CI as a function of H when the surface potential
s is fixed at 1.5V. Here the impurity is modelled as inert particles except that they can bind to the surface, i.e. ij =0 for the interaction between the impurity segments with all ionic species. Throughout this work,
the parameters and the bulk concentration for the ionic liquid are fixed, approximately
corresponding to those for Emim-TFSI at 298 K and 1 bar ( = 0.5 nm,
= 2.32 nm−3). In the range of pore size about 1–4 times the ion diameter, the integral capacitance becomes remarkablydifferent when the binding energy increases from 0 to 40 k TB . If there is no surface binding energy, that is,
w = 0, the impurity has negligible influence on the capacitance due to its extremely low bulk concentration. In this case, the CI H curve is almost identical to that of a pure ionic liquid: the capacitance displays multiple peaks with a decaying envelope and the peaks88
appear at the positions corresponding to the layer-by-layer ionic distributions. When the surface
binding energy increases to 20k TB , the integral capacitance is significantly reduced, in particular for large pores, while the oscillation in the CI H curve is weakened. A maximum reduction of ∼38% (at H = 1.1 nm) can be observed in the CI H curve. Further increasing the surface
energy to 40k TB , which is about the same order of magnitude for the energy of a covalent bond, the capacitance is reduced to an even lower value, and no oscillation can be observed in the
I
C H curve. The reduction in capacitance varies from ∼48% to ∼72%, depending on the pore size. Compared to the case in which
w = 0, strong adsorption of impurity leads to a drastic decrease of the integral capacitance and the disappearance of its oscillatory dependence on thepore size.
Why does the impurity generate such a drastic reduction of the integral capacitance and
make the oscillatory profile vanishing? To address this question, we first analyze the composition
of the contact layer to explain this harmful effect of an impurity. For convenience, here the
contact layer is defined within a region of 0.30 nm from the surface, which corresponds to the ion
radius (0.25 nm) plus the range of the surface attraction (0.05 nm). Ions or impurity molecules in
this region can be considered as in direct contact with the electrode. The composition of the
contact layer is described by the fraction of each segment,is, inside the contact layer:
0 s s i i s s s (4.6)
where 0s , s and s are the surface number density (nm−2) of the impurity, cations (counterion in this case) and anions (co-ions) in the contact layer, respectively.
89
Figure 4.3 The composition of the contact layer versus the pore size at s =1.5 V. The impurity bulk mole
fraction is fixed at with x =100 -4. From top to bottom, the surface energy acting on the impurity is w = 0, 20 and 40k T , respectively. B
Figure 4.3 shows the composition of the contact layer for the three ionic systems
discussed above. One can see that the adsorption of impurity on the electrode can be greatly
enhanced and the degree of enhancement depends on the surface energy and the pore size. For the
case with no surface binding energy for the impurity molecules, the interfacial region is mainly
dominated by counterions due to the strong electrostatic attraction. With the surface binding
energy, the impurity is also able to be enriched in the contact layer. Due to the competition of the
surface energy and the electrostatic potential, the contact layer, or the interfacial region, is
90
Essentially, the oscillation of the capacitance is the result of the interference of alternating cations
and anions layers. When the contact layer is occupied by the neutral impurity, the effective
surface charge density within the first ionic layer becomes significantly smaller, making the
layering structure less distinctive. Therefore, the oscillation of the capacitance with the pore size
vanishes. For the same magnitude of the surface binding energy, the depletion of the counterions
is stronger in larger pores than in smaller pores. In contrast to the electrostatic potential, the
surface binding energy is short-ranged. In a small pore, the confinement effect is responsible for
the strong overlap of the electrostatic potential. A large binding energy is required for the
impurity molecules to overcome the energy barrier and accumulate at the electrode surface. Even
when the surface binding energy is as large as 40k TB , the counterions still contribute up to ∼30% of the contact layer. In large pores, the overlap of the electrostatic potential from the surface becomes less distinctive so the impurities are much easily enriched at the interface. The
smaller effective surface charge density as well as the excluded volume effects contribute to the
reduction of the capacitance for large pores.
To better understand the effect of adsorbed impurity, we extend our analysis of the ion
distributions to the entire pore. For the pore size comparable to the ion diameter, the ions can only
form a monolayer. The composition of the contact layer can be deduced from the ionic
distributions. Therefore, here we focus on a larger pore, such as 2.0 nm to delineate the EDL
structure in detail: the distributions of ions and impurity molecules near the interface and their
mutual effects on each other. Figure 4.4(a–c) shows the ionic distributions across the pore.
Similar to prior studies, cations and anions form alternating layers near the electrode surface.
However, the oscillation of the ionic density profiles varies in response to the change in the
surface binding energy. For the hybrid RTIL containing an impurity but without the surface
91
composed of counterions. As the surface energy changes from0 to 40k TB , the contact layer evolves into two parts: counterions and the impurities, while both the counterions and co-ions
tend to accumulate primarily at the central plane of the slit. The depletion of counterions from the
electrode surface can be attributed to the competition between the surface energy and the
electrostatic potential.
Figure 4.4 Density distributions of the counterions, co-ions and impurity molecules across a 2.0 nm pore at
s
= 1.5 V. The impurity bulk mole fraction is x =100 -4. From top to bottom, the surface energy acting on
the impurity is w = 0, 20 and 40k T , respectively. B
Figure 4.5 shows the effect of surface energy on the integral capacitance and the contact
layer composition in various pore sizes. One can see that both the capacitance and the interfacial
92
the impurity molecules need to overcome an energy barrier, around 10 k TB for the given electrical potential, to enter the slit pore. Increasing the surface energy from 10 to 40k TB , the capacitance decreases monotonically with the surface energy. This implies that the impurity
molecules begin to interfere the EDL structure and change the ionic density profiles in the pore.
Meanwhile, the impurity molecules eventually accumulate inside the porous electrode until
saturation. For the larger pores of H =1.2 and 2.0 nm, increasing the surface energy to 10 k TB or more, both the capacitance and the contact layer composition show little changes. It means that,
in these cases, the surface energy plays a more important role than the pore size effects.
Figure 4.5 The integral capacitance (a) and the mole fraction of the impurity in the contact layer (b) versus
the surface energy wat s=1.5 V. The impurity bulk mole fraction is fixed at x =100 -4. The different lines
correspond to the pore size H = 0.55, 1.2 and 2.0 nm, respectively
A natural question one may ask is whether the capacitance is sensitive to the impurity
93
investigate the impurity bulk density effect in a 1.2 nm slit pore, which equals to the pore size
when the second peak in the capacitance curve appears (see Figure 4.2). In Figure 4.6, one can
see that with zero or a very small surface energy, adsorption of the impurity is almost negligible
and the impurity is difficult to accumulate on the electrode surface; when the surface energy is as
high as 40k TB , the slit pore is almost saturated with the impurity even at an extreme low impurity bulk concentration (x0=10−6), suggesting that the capacitance is relatively insensitive to the impurity bulk concentration. Both the integral capacitance and the contact layer composition
show low sensitivity to the bulk density of the impurity for some magnitudes of the surface
energy. As different surface energy reflects the different chemical nature of the impurity, Figure
4.6 indicates the importance of electrolyte purification for different types of impurities.
Figure 4.6 The integral capacitance (a) and the mole fraction of impurity in the contact layer (b) versus the