Uncoordinated Carboxyl Groups as Proton Sources in Polyoxometalate-Based Metal–Organic Frameworks Enhance Proton Conduction
Xin Zheng, Yuming Cui, Ning-Hao Wang, Xue-Song Wu, Zhong‐Min Su, Xingqi Han
Abstract
To select appropriate organic ligands is an effective strategy to enhance the proton conductivities of polyoxometalate-based metal–organic frameworks (POMOFs). Two new Dawson-type POMOFs, named CUST-961 and CUST-962, have been designed and synthesized via combining Htzbc selected by hard and soft acid and base theory and density functional theory calculation, transition metal ions, alkali metal ions (Na + and K + ), and Dawson-type polyoxometalates ([P 2 W 18 ] 6– ) under the hydrothermal method. Their stabilities under different temperatures and relative humidities (RHs) have been investigated through powder X-ray diffraction and thermogravimetric analysis. Both CUST-961 and CUST-962 exhibited excellent aqueous and thermal stabilities. The alternating current (AC) impedance spectrum tests revealed that the proton conductivity of CUST-961 could reach 1.4 × 10 –4 S cm –1 at 95 °C and 98% RH, which is about 3 times that of CUST-962. The different proton conductivities between the two compounds are due to the fact that CUST-961 possesses more uncoordinated carboxylic acid groups, as confirmed by attenuated total reflection infrared spectroscopy and 1 H solid-state nuclear magnetic resonance spectroscopy, which can not only act as the proton source but also establish a richer hydrogen bonding network to enhance proton conduction. This work provides a new strategy and insight for the design and preparation of polyoxometalate-based proton conductive materials.