Confining Amphiprotic Proton Source in Polyoxometalate-Based Metal–Organic Frameworks for Enhancing Proton Conduction

Confining amphiprotic proton sources within the framework of crystalline materials is recognized as an effective strategy for improving their proton conductivities. Two new compounds ((TPA)(AlMo6O24)·3H2O, named as CUST-841 and Zn2(HSO4)2(TPA)2(AlMo6O24)·5H2O, named as CUST-842) have been successfully synthesized under hydrothermal conditions. Thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD) results demonstrated that both compounds exhibit excellent water and thermal stabilities. Alternating current (AC) impedance spectroscopy revealed that CUST-842 achieved a maximum proton conductivity of 6.05 × 10-4 S cm-1, which is an order of magnitude higher than that of CUST-841 under conditions of 95 °C and 98% relative humidity (RH). Comparative analysis between CUST-842 and CUST-841 demonstrates a synergistic effect of Zn2+ coordination and HSO4- incorporation in the construction of continuous hydrogen-bond networks and the promotion of proton dissociation. The amphiprotic HSO4- in CUST-842 acts as "dissociation enhancers" to facilitate efficient proton dissociation, while the abundant N-sites within the nanopores and Zn2+-coordinated water molecules strengthen the hydrogen-bond network for rapid proton diffusion, as confirmed by 1H solid-state nuclear magnetic resonance (NMR) spectroscopy. This study provides insights into the synthesis of polyoxometalate (POM)-based solid proton conductors.