Figure 2. (a) Schematic of the sol-gel coating and GVD method toin situ grow the MOF layer. (b)-(l) Morphologies and structure
characterization of Zn@Zn2(bim)4. (b)
SEM image, (c)-(d) energy dispersive spectroscopy (EDS) mapping of the
cross-section view of Zn@Zn2(bim)4. (e)
SEM image, (f) and (g) EDS mapping of the top view of
Zn@Zn2(bim)4. (h) TEM images nanosheets.
(i) Digital photo. Contact angle of water droplet on (j) bare zinc and
(k)Zn@Zn2(bim)4. (l) XRD patterns of
bare Zn foil and Zn@Zn2(bim)4.
Typically, the zinc deposition involves the following steps: i) mass
transport of the solvated zinc ion in the bulk electrolyte; ii)
desolvation in the electric double layer; iii) charge transfer; and iv)
electro-crystallization (nucleation and
growth)[32], as depicted in Figure 3a and b. The
overpotential corresponding to each process can be denoted as
ηmt, ηrxn, ηct, and
ηec, respectively. Therefore, we investigated the
electrochemical properties of
Zn@Zn2(bim)4 anode with various
techniques from the perspective of the above-mentioned processes.
Firstly, we investigate the impedance of the symmetric cell under
different temperatures using EIS. It can be seen from the inset in
Figures 3c and d that the ohmic resistance (the first interception of
the EIS arc with the x -axis) of
Zn@Zn2(bim)4//Zn@Zn2(bim)4cell is slightly larger than that of bare zinc cell. The insulating MOF
layer with tiny pores form a barrier for zinc ion to transport, which
leads to increased ohmic resistance (ηmt in
Zn@Zn2(bim)4 cells is higher than that
in bare zinc cell). During the desolvation process, a large amount of
water in the
[Zn(H2O)6]2+ was
physically rejected before the ions could pass through the MOF layer,
which can boost the desolvation process at the zinc front surface, as
shown in Figure 3b. With the charge transfer resistance at different
temperatures determined from EIS tests (Figures 3c and d), the
activation energy (Ea) of desolvation during Zn2+plating can be calculated according to the Arrhenius
equation[33]:
\(\ln\left(T/R_{\text{ct}}\right)=\frac{E_{a}}{\text{RT}}+lnA\)(1)
Where Rct , T , R and Arepresent charge transfer resistance, absolute temperature, standard gas
constant, and preexponential constant, respectively. According to the
calculation, Ea of Zn2+ on
Zn@Zn2(bim)4 is 20.6 kJ
mol-1, which is smaller than that on bare Zn surface
(29.9 kJ mol-1), indicating an enhanced desolvation
process (Figure 3e). After that, the nucleation of
Zn2+ ion on bare Cu and
Cu@Zn2(bim)4 is characterized through
the constant current deposition process in Zn//Cu half cells and the CV
test in a three-electrode setup. As shown in Figure 3f, when applying a
constant current, the voltage shifts from the equilibrium to
EA, corresponding to the formation of crystal nuclei.
With the the deposition process going on, the voltage reaches a plateau
(EB) corresponding to a steady growth process.
Previously, the difference (Δη) between EA and
EB is regarded as the nucleation potential. With such
theory, the nucleation overpotential on
Cu@Zn2(bim)4 is increased compared to
bare Cu. However, as discussed in Ref. [32],
EA instead of Δη should be recognized as the nucleation
overpotential since Δη represents the gap of the driving force between
nucleation and growth. With this, the nucleation overpotential on
Cu@Zn2(bim)4is still larger than that on bare zinc, which can further be confirmed
from the CV results (Figure S2, supporting information). According to
the relationship between the critical Zn nucleus and nucleation
overpotential (NOP)[34], the increased NOP
indicates a more fine-grained Zn nucleus, favoring the crystallographic
orientation of Zn deposits.
Furthermore, we investigated the diffusion mode using chronoamperometry
(CA) [35], as shown in Figure 3g. In the bare zinc
setup, the current increases gradually, representing a 2D-diffusion mode
and a steady increase in the electrochemical active surface area,
corresponding to the formation of the porous and nonuniform zinc
dendrites. In contrast, the current gradually reaches a plateau on
Zn@Zn2(bim)4, representing the
homogenous and dense deposition of zinc. The corrosion-resistant
capability of the in situ grown MOF layer was investigated by linear
polarization experiments in 2M ZnSO4 solution (Figure
3h). The corrosion potential of the
Zn@Zn2(bim)4 is more positive than that
of bare Zn, indicating the inhibited tendency of corrosion caused by
hydrogen evolution [36]. In addition, a lower
corrosion current on Zn@Zn2(bim)4represents a lower corrosion rate [34]. To sum up,
the dense MOF interphase acts as an ion sieve with ordered fine pores,
which increases mass transport overpotential but boosts the desolvation
process and prevents water-induced corrosion. With an increased
nucleation overpotential, more homogenous and dense zinc deposition can
be achieved, suppressing the formation of zinc dendrites.