Prominent Intrinsic Proton Conduction in Two Robust Zr/Hf Metal-Organic Frameworks Assembled by Bithiophene...

To date, developing crystalline proton-conductive metal-organic frameworks (MOFs) with an inherent excellent proton-conducting ability and structural stability has been a critical priority in addressing the technologies required for sustainable development and energy storage. Bearing this in mind, a multifunctional organic ligand, 3,4-dimethylthiophene[2,3-b]thiophene-2,5-dicarboxylic acid (H₂DTD), was employed to generate two exceptionally stable three-dimensional porous Zr/Hf MOFs, [Zr₆O₄(OH)₄(DTD)₆]·5DMF·H₂O (Zr-DTD) and [Hf₆O₄(OH)₄(DTD)₆]·4DMF·H₂O (Hf-DTD), using solvothermal means. The presence of Zr₆ or Hf₆ nodes, strong Zr/Hf-O bonds, the electrical influence of the methyl group, and the steric effect of the thiophene unit all contribute to their structural stability throughout a wide pH range as well as in water. Their proton conductivity was fully examined at various relative humidities (RHs) and temperatures. Creating intricate and rich H-bonded networks between the guest water molecules, coordination solvent molecules, thiophene-S, -COOH, and -OH units within the framework assisted proton transfer. As a result, both MOFs manifest the maximum proton conductivity of 0.67 × 10^-2 and 4.85 × 10^-3 S·cm^-1 under 98% RH/100 °C, making them the top-performing proton-conductive Zr/Hf-MOFs. Finally, by combining structural characteristics and activation energies, potential proton conduction pathways for the two MOFs were identified.