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.