Uncoordinated\nCarboxyl Groups as Proton Sources in\nPolyoxometalate-Based Metal–Organic Frameworks Enhance Proton\nConduction

To select appropriate organic ligands\nis an effective\nstrategy\nto enhance the proton conductivities of polyoxometalate-based metal–organic\nframeworks (POMOFs). Two new Dawson-type POMOFs, named CUST-961 and\nCUST-962, have been designed and synthesized via combining Htzbc selected\nby hard and soft acid and base theory and density functional theory\ncalculation, transition metal ions, alkali metal ions (Na<sup>+</sup> and K<sup>+</sup>), and Dawson-type polyoxometalates ([P<sub>2</sub>W<sub>18</sub>]<sup>6–</sup>) under the hydrothermal method.\nTheir stabilities under different temperatures and relative humidities\n(RHs) have been investigated through powder X-ray diffraction and\nthermogravimetric analysis. Both CUST-961 and CUST-962 exhibited excellent\naqueous and thermal stabilities. The alternating current (AC) impedance\nspectrum tests revealed that the proton conductivity of CUST-961 could\nreach 1.4 × 10<sup>–4</sup> S cm<sup>–1</sup> at\n95 °C and 98% RH, which is about 3 times that of CUST-962. The\ndifferent proton conductivities between the two compounds are due\nto the fact that CUST-961 possesses more uncoordinated carboxylic\nacid groups, as confirmed by attenuated total reflection infrared\nspectroscopy and <sup>1</sup>H solid-state nuclear magnetic resonance\nspectroscopy, which can not only act as the proton source but also\nestablish a richer hydrogen bonding network to enhance proton conduction.\nThis work provides a new strategy and insight for the design and preparation\nof polyoxometalate-based proton conductive materials.