Construction and Proton Conductivities of Multidimensional Isopolymolybdate-Based Metal–Organic Frameworks

Two novel β-Mo8-based polyoxometalate metal–organic frameworks (POMOFs), designated as CUST-821 and CUST-822, were successfully synthesized through a hydrothermal reaction involving 4-(4-(2-(thiophen-2-yl)ethyl)-4H-1,2,4-triazol-3-yl)pyridine (Tyetp), ammonium molybdate, and either CoCl2·6H2O or NiCl2·6H2O. Single-crystal X-ray diffraction analysis revealed that CUST-821 features a two-dimensional (2D) interpenetrating layered structure, whereas CUST-822 adopts a three-dimensional (3D) non-interpenetrating framework. The proton conduction properties of these materials were thoroughly investigated. CUST-822 demonstrated a relatively high proton conductivity of 4.49 × 10–5 S cm–1 at 80 °C and 98% relative humidity (RH), which is an order of magnitude greater than that of CUST-821. Additionally, alternating current (AC) impedance spectroscopy indicated that the proton conductivity of CUST-822@PVDF-1% reached 9.92 × 10–7 S cm–1 at 80 °C, approximately three times higher than that of a pure polyvinylidene fluoride (PVDF) membrane. This study provides new insights and methodologies for the design and synthesis of crystalline solid-state electrolyte materials based on polyoxometalates (POMs).