Figure 4. Zn//Cu half cell and Zn///Zn symmetric cell performance. (a) CE of Zn//Cu and Zn//Cu@Zn2(bim)4Voltage profile of zinc deposition and stripping of (b) Zn//Cu and (c) Zn//Cu@Zn2(bim)4 half cells. (d) Rate performance, (e)-(f) cycling performance of the symmetric cell. (g) Selected voltage profile of the Zn@Zn2(bim)4//Zn@Zn2(bim)4at different cycles.
The effect of angstrom-level channels on the desolvation of Zn2+ ions is studied with Raman spectroscopy, as shown in Figure 5. Typically, Zn ions are coordinated with water molecules and exist in aqueous solutions as solvent-separated ion pairs (SSIP) [Zn2+(H2O)6·SO42-], which is reflected from Raman peak at around 931 cm-1, and contact ion pairs (CIP) [Zn2+(H2O)5·OSO32-] that appears 935.5 cm-1 [37]. As the concentration of ZnSO4 increases, more Zn2+ ions forms CIPs, as seen in Figure 5a, and the peak of ν-SO42- shifts to a higher frequency. The Raman peak of ν-SO42- shifts even to a higher frequency in MOF channels, indicating the coordination of Zn2+ with even fewer water molecule. Thus, the spectral results indicate that the MOF channels with a diameter of 2.1 Å only allow for the transport of Zn2+ in a mono-hydrated coordination type and function as ion sieve interphase with the size screening effect, as schematically shown in Figure 5b.