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.