High proton conduction in a series of three-dimensional lanthanide(III)-organic frameworks constructed by 2,5-dihydroxyterephthalic acid

In designing and preparing new proton-conductive materials, using cheap and easily available raw materials to efficiently prepare metal-organic frameworks (MOFs) with high stability and excellent proton conductivity is still a huge challenge. Herein, six lanthanide(III)-MOFs, {[Ln₂(DHBDC)₃(DMF)₄](DMF)₂}_n [(Ln ^III = Pr ^III (1), Nd ^III (2), Sm ^III (3), Eu ^III (4), Gd ^III (5), Tb ^III (6))] with high stability were solvothermally synthesized utilizing 2,5-dihydroxy-1,4-benzenedicarboxylic acid (H₄-DHBDC) as a bridging ligand. These isostructural MOFs all possess a three-dimensional framework and a dense H-bond network formed by the carbonyl groups in the framework, the non-coordinated hydroxyl groups, and the coordinated and free DMF molecules, which ensure efficient proton conduction. Their good water and thermal stability were verified using various characterization techniques (powder X-ray diffraction, thermogravimetric analysis, and infrared). Then, their proton conductivity was investigated in detail concerning temperature and relative humidity (RH). At 100 °C and 97 % RH, their optimum proton conductivity can reach up to 0.96 × 10^-2, 0.67 × 10^-2, 0.85 × 10^-2, 1.03 × 10^-2, 0.53 × 10^-2, and 0.93 × 10^-2 S/cm for 1-6, respectively. Finally, their proton-transport processes were thoroughly examined through detailed structural analyses, adsorption-property determinations, and activation energy values. Notably, these MOF materials have the advantages of easy preparation and relatively low cost, which paves the way for their practical applications.