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 pKa 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.