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 H 5 L (4–{[bis(phosphonomethyl)amino]methyl}benzoic acid) and auxiliary ligand oxalic acid (H 2 C 2 O 4 ), two Ce–MOFs with different structures, namely, Ce[(H 3 L)(C 2 O 4 ) 0.5 (H 2 O)]·0.5H 2 O ( 1 ) and Ce[(H 4 L)(H 3 L)(H 2 O)]·2H 2 O ( 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. 1 and 2 exhibit a positive correlation with temperature and relative humidity, reaching their maximum at 95 % RH and 368 K (1.22 × 10 −3 S cm −1 for 1 and 6.51 × 10 −4 S cm −1 for 2 ). Introducing oxalic acid to adjust the structure to promote the establishment of the proton conduction channels, further leading to the high proton conductivity. Our work provides a guidance for the construction of high proton conductivity MOFs. • Both Ce–MOFs exhibit high proton conductivity and the conductivity can be improved by adjusting the structure. • The mechanism and the structural–activity relationship are investigated. • The density of hydrophilic carboxyl groups between layers plays an important role in adjusting proton conductivity.