Proton conduction in two highly stable cadmium(II) metal-organic frameworks built by substituted imidazole dicarboxylates

Recently, the fabrication of metal-organic frameworks (MOFs) with high proton conductivity (σ) still attracts the attention of many researchers. Herein, a multifunctional organic ligand, 4-hydroxyphenyl-4,5-imidazole dicarboxylic acid (p-HOPhH3IDC) was adopted to solvothermally prepare two stable one-dimensional Cd(II) MOFs, {[Cd2(p-HOPhHIDC)2(phen)2]·H2O}n (1) (phen ​= ​1,10-phenanthroline), {[Cd2(p-HOPhHIDC)2(2,2′-bipy)2]·H2O}n (2) (2, 2′-bipy ​= ​2, 2′-bipyridine) in the presence of the N-containing ligands. Consequently, thermogravimetric analysis (TG), powder X-ray diffraction (PXRD), and scanning electron microscopy (SEM) determinations were employed to explore their structural stability. Importantly, their water-mediated proton conductivity demonstrated significant humidity- and temperature-dependence, increasing with the increase of both, and could attain the best value above 10−4 ​S ​cm−1 under 100 ​°C and 98% RH. The structural analyses along with the calculation of activation energy revealed that the rich H-bonded networks inside their frameworks are responsible for the higher proton conductivity. Finally, the proton-conducting mechanisms of the two MOFs were highlighted.