Figure 3. Schematics of zinc electrodeposition on (a) bare Zn and (b) Zn@Zn2(bim)4. Electrochemical characterization of the Zn@Zn2(bim)4anode. (c)-(d) EIS profiles of symmetric cells at temperatures from 30-60 ℃. (e) Arrhenius curves. (f) Zinc nucleation on different substrates. (g) Chronoamperometric curves for bare Zn and Zn@Zn2(bim)4. (h) Potentiodynamic polarization curves.
Then, we characterized the electrochemical performance of the in situ growing MOF zinc anodes. Firstly, Zn//Cu asymmetric cells were employed to examine the coulombic efficiency (CE) of Zn plating/stripping within situ MOF interphase. As shown in Figure 4a, Zn//Cu@Zn2(bim)4 exhibits higher steady CE (98.04%) than its counterparts 94.74% for bare Cu at 0.5 mA cm-2 and 0.5 mAh cm-2 with an obviously longer cycle life, indicating that the HER is effectively suppressed with the in situ MOF layer. The voltage profiles of the zinc depositing and striping in different cycles (Figure 4b and c) show that the Zn//Cu@Zn2(bim)4 has a larger overpotential, due to the increased mass transport barrier and nucleation potential as discussed earlier.
Furthermore, the cycling stability of Zn anodes was examined with Zn//Zn symmetric cells. The rate performance of the Zn@Zn2(bim)4 symmetric cell shows an increased overpotential compared with the cell with bare zinc, which is consistent with the above-discussed electrochemical tests. With the current density steadily increasing from 0.25 to 5 mA cm-2 and then returning to 2 mA cm-2, the cell can also steadily cycle with an overall time exceeding 500h. However, the bare zinc cell got short-circuited much earlier, even though it returns at a smaller current density of 1 mA cm-2 (Figure 4d). Then the long-term cyclability of anodes with MOF layer is verified in symmetric cells under galvanostatic cycles. At a current density of 0.5 mA cm-2 with a capacity of 0.5 mAh cm-2, the Zn@Zn2(bim)4 symmetric cell can cycle for over 1000h, while the bare zinc cell suffered from a short circuit after around 250 h (Figure 4e). It is worth noting that the regular fluctuations in the voltage profile are due to temperature changes. Under different testing conditions (1 mA cm-2 and 1 mAh cm-2), the service time of Zn@Zn2(bim)4 symmetric cells reaches 700 h as compared to a lifespan of shorter than 100 h for bare zinc (Figure 4f). It is interesting to note that during cycling, the overpotential for the Zn@Zn2(bim)4 symmetric cells undergoes an initial increase, followed by a decrease, and then gradually reaches a steady state. When observing the voltage profile in a single stripping and depositing cycle (Figure 4g), we notice that there is a relatively large overpotential for zinc stripping to take place in the 10th and 25th cycles, and the overpotential also increases towards the end of each cycle. However, as cycling continues, the profiles gradually become flatter. The possible reason for this phenomenon is that the transport of zinc ions in the MOF layer with angstrom-level pores may need to be activated. During the first several cycles of testing, the pathways for zinc ion transport may not be continuous, leading to an increased overpotential. However, after the activation stage, the overpotential returns to a relatively small level.