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.