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.