Swift Proton Release Enhancing Proton Conductivity in Anderson-Polyoxometalates-Based Metal–Organic Frameworks

Swift proton release in solid-state proton conductors can enhance the proton conductivity. Two new polyoxometalates-based metal–organic frameworks (POMOFs), [Cu₂(HBip)₂(H₂O)₆(TeMo₆O₂₄)] (CUST-860) and [Co₃(Bip)₂(H₂O)₁₀(TeMo₆O₂₄)] (CUST-861), have been fabricated via combining Anderson-type POMs (TeMo₆), 3,5-bis(imidazole-1-yl)pyridines (Bips), and transition metal ions under hydrothermal method. The related tests such as powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) indicated that both compounds exhibit excellent thermal stabilities and water stabilities. Alternating current (AC) impedance tests showed that the highest proton conductivity of CUST-861 was 2.96 × 10^–2 S cm^–1 under 90 °C and 98% relative humidity (RH), which is 3 orders of magnitude higher than CUST-860. Furthermore, theoretical calculation results indicated that the uncoordinated pyridine nitrogen sites of ligands in CUST-861 have a lower pK_a value than the uncoordinated imidazole nitrogen sites of ligands in CUST-860, which is more conductive to proton transfer. This study provides insights into the synthesis of solid proton conductors based on POMs.