High protonic conduction in two metal–organic frameworks contained high-density carboxylic groups
To achieve crystalline solid MOFs showing high proton conductivity still faces great challenges. The organic functional ligands delicately designed will be very conducive to the realization of this goal. Herein, by adopting a designed ligand, p-N-imidazol-1-yl)-phenyl-1H-imidazole-4,5-dicarboxylic acid (p-IPhH3IDC) that contains two imidazole and two carboxylate units, two hydrolytically stable 2D MOFs, [Zn(p-IPhHIDC)]n (1) and [Co(p-IPhHIDC)]n (2) with high-density uncoordinated -COOH groups were successfully acquired. The water-assisted proton conductive characteristics of the two MOFs were explored. Expectedly, MOFs 1 and 2 exhibit super-high protonic conductivities up to 1.9 × 10-3 and 1.07 × 10-3 S/cm at 100 °C and 98% RH, respectively, which are among of these previous MOFs having high proton conductivities. Moreover, their proton-conducting performances maintained at last 8 days. We further discussed the conducting mechanism from the crystal structural analyses, N2 and H2O vapors adsorption determinations and the calculated Ea values, and pointed out that the uncoordinated -COOH groups in the frameworks were responsible for the high proton conductivity. Our study provides a useful strategy for constructing MOFs with high proton conductivity by assembling multiple functional units into an organic ligand.