Exceptional proton conduction in two robust zirconium(IV)/hafnium(IV)-organic frameworks constructed by tetratopic carboxylate

The fabrication of metal-organic frameworks based on zirconium and hafnium with high structural rigidity using multifunctional carboxylate and the examination of their possible uses have gained significant attention in recent times. The structural features of these porous MOFs, such as their abundant Lewy acid sites and hydroxyl groups, which are appropriate for building effective proton transport channels, have also drawn much attention in the proton conduction field. Herein, two highly stable Zr(IV)/Hf(IV) MOFs, [M6(μ3-O)4(μ3-OH)4(bptc)3]·2DMF (M = Zr (Zr-bptc) or Hf (Hf-bptc)) were solvothermally prepared by employing the tetratopic ligand, 3,3′,5,5′-biphenyltetracarboxylic acid (H4bptc), in which M6(μ3-O)4(μ3-OH)4(COO)12 clusters are linked by bptc3- anions to form an ftw type framework with 12-connected M6 cluster. X-ray powder diffraction, infrared spectroscopy, scanning electron microscopy, and thermogravimetric analyses proved that the two MOFs had remarkable water, chemical, and thermal stability. The alternating current (AC) impedance determinations manifested that the two MOFs have high proton conductivities under 100 °C and 98 % relative humidity (RH) of 0.89 × 10−3 and 1.37 × 10−3 S/cm, respectively, which are among the described MOFs with high proton conductivity. Furthermore, the two MOFs demonstrated excellent electrochemical determination durability. In addition, the activation energy estimates at high or low RHs fully illustrated the significant influence of water molecules in the environment on proton transport within the framework. This investigation again confirms that Zr/Hf-based MOFs have unparalleled structural and performance advantages in energy chemistry. [Display omitted] •Two rigid Zr/Hf MOFs bearing biphenyl tetracarboxylates were fabricated.•Their prominent proton conductivities were explored.•The differences in proton conduction are compared.•Their proton-conducting mechanisms were proposed.