Construct Ordered Proton Channels in Polyoxometalate-Based Metal–Organic Frameworks for Enhancing Proton Conduction

Constructing ordered proton channels is regarded as an effective strategy for improving the proton conduction. Using the hydrothermal method, two new polyoxometalate-based metal–organic frameworks (POMOFs), named as CUST-973 and CUST-974, were successfully synthesized. The test results of thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD) show that both compounds exhibit excellent thermal and water stabilities. The alternating current (AC) impedance measurements indicate that at 90 °C and 98% relative humidity (RH), the proton conductivity of CUST-974 is 4.72 × 10^–3 S cm^–1, which is 2 orders of magnitude higher than that of CUST-973. The framework of CUST-974 exhibits precisely aligned channels that enable efficient proton conduction via water molecules acting as proton carriers. This well-defined proton transport pathway accounts for the superior conductivity relative to CUST-973. Complementary water vapor adsorption–desorption experiments confirm that enhanced water uptake in CUST-974 exhibits a higher water uptake capacity compared to CUST-973. This study provides new research ideas for improving the proton conductivity of POMOFs and their application as crystalline solid-state conductor materials.