Two stable proton–conductive Cerium (III)–organic frameworks with high–density carboxylic groups

Metal–organic frameworks (MOFs) with high proton–conductive behavior have attracted great attention. Here, by using the organic phosphonic acid ligand H₅L (4–{[bis(phosphonomethyl)amino]methyl}benzoic acid) and auxiliary ligand oxalic acid (H₂C₂O₄), two Ce–MOFs with different structures, namely, Ce[(H₃L)(C₂O₄)_0.5(H₂O)]·0.5H₂O (1) and Ce[(H₄L)(H₃L)(H₂O)]·2H₂O (2), have been hydrothermally synthesized. Due to the existence of hydrogen–bond interactions among uncoordinated carboxyl groups, free water molecules, and protonated N atoms in the framework structures as well as their good stabilities, which provide possibilities in proton conduction. The alternating–current (AC) impedance measurements indicate that the proton conductivities of both 1 and 2 increase with the increasing of the humidity and temperature. Expectedly, the maximum proton conductivity of 1 (1.22 × 10⁻³ S cm⁻¹) is nearly 2 times higher than that of 2 (6.51 × 10⁻⁴ S cm⁻¹). The proton conduction mechanism was clarified by analyzing the structures of 1 and 2, the activation energy (E_a), water vapor and nitrogen adsorption tests. Meanwhile, the structural–activity relationship between structure and proton conductivity has also been analyzed and discussed. Proton conductive composite membrane of 1 and 2 have also been successfully prepared and investigated. This work provides more options for the design and construction of high proton–conductive materials.