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.