Nanometrically Confined Water Molecules in 2D Polyoxometalate-Based Metal–Organic Frameworks Enhancing Proton Conductivity

Confining
water molecules within sub1 nm channels is recognized
as an effective strategy for improving proton conductivity. Utilizing
a hydrothermal synthesis approach, two new two-dimensional (2D) polyoxometalate-based
metal–organic frameworks (POMOFs) were successfully synthesized,
named [Ni(Bip)2(H2O)2(γ-Mo8O26)]·3H2O (CUST-862) and [Co2(Bip)2(H2O)4(γ-Mo8O26)]·2H2O (CUST-863). Thermogravimetric
analysis (TGA) and powder X-ray diffraction (PXRD) results demonstrated
that both compounds exhibit excellent water stability and thermal
stability. Alternating current (AC) impedance spectroscopy revealed
that CUST-862 achieved a maximum proton conductivity of 1.97 ×
10–3 S cm–1, which is an order
of magnitude higher than that of CUST-863 under conditions of 90 °C
and 98% relative humidity (RH). The space confined effect of sub-1-nm
channels acting on free molecules in CUST-862 effectively enhances
the proton conductivity. The results obtained from attenuated total
reflection infrared (ATR-IR) and water vapor adsorption–desorption
tests offered a comprehensive explanation for the confinement effect.
This research gives a novel approach for enhancing proton conductivities
in POMs-based materials.