Proton Conduction In Metal-Organic Frameworks Constructed By N-heterocyclic Carboxylate Ligands

As one of the essential components of proton exchange membrane fuel cells,proton conductors directly determine the life and conversion efficiency of fuel cells.Therefore,the development of proton conductors with stable structure and excellent electrical conductivity has been receiving widespread attention.Recently,there has been great interest in the use of metal-organic frameworks(MOFs)with high crystallinity,controllable structure,and affluent hydrogen bond networks and water clusters as high-performance proton conductors.Preliminary research found that N-heterocyclic groups(imidazole,triazole,etc.)and carboxylate groups can be used as proton sources and can effectively participate in the formation of H-bonding networks,which are very conducive to transport of protons.Therefore,we have a strong interest in the proton conductivity of MOFs constructed by N-heterocyclic carboxylate ligands.Firstly,two substituted imidazole dicarboxylate ligands:2-pheny 1-4,5-imidazole dicarboxylic acid(H3PhIDC)and 2-(3,4-dichlorophenyl)-4,5-imidazoledicarboxylic acid(H3DClPhIDC)were synthesized.Subsequently,four new metal-complexes,[Cd(HPhIDC)2(H2O)2]·4H2O(1),[Cd4(HPhIDC)4(CH3CN)(H2O)]n(2),[Co(4,4'-bipy)(PhHIDC)]n(3)and {(Co3(DCIPhIDC)2(H2O)6]·2H2O}n(4)were successfully prepared by solvothermal or hydrothermal methods.Single-crystal X-ray diffraction analysis shows that 1 is a supramolecular structure bearing a large number of crystallization and coordination water molecules.2 is a 3D MOF containing a great number of 1D hydrophilic channels.In 3,PhHIDC2-ligands bridge Co2+ions to form the 1D chains,which were connected by 4,4'-bipy to form a layered structure.4 is a 3D MOF containing high density of water molecules.To enrich our research,another N-heterocyclic carboxylate ligands,such as 5-(lithium formate-1H-1,2,4-triazol-1-methyl)benzene-1,3-diLithium carboxylate(Li3ctbd),2-(3,4-dimethylphenyl)imidazoledicarboxylic acid(H3DMPhIDC),2-p-(1H-1,2,4-triazolyl)phenyl-1H-4,5-imidazoledicarboxylic acid(p-TIPhH3IDC)were adopted to prepare five MOFs,1D {[Er(ctbd)2(H2O)3]·25/4H2O}n(5),1D{[Yb(ctbd)2(H2O)3]·25/4H2O}n(6),1D[Cd(HDMPhIDC)(H2O)]n(7),3D[Sr(H2DMPhIDC)2]n(8),1D[Cd(p-TIPhH2IDC)2(H2O)]n(9)according to reference methods.Moreover,TG,PXRD and SEM were used to determine their thermal and water stability.The results display that they all have execellent water and thermal stability.Water vapor adsorption and N2 adsorption measurements demonstrate that the nine complexes have relatively high water vapor adsorption capacity and certain holes in the structures.The structural characteristics mentioned above are very helpful to the research of proton conductivity.Further,their water-assisted proton conductive properties were systematically studied.The temperature and humidity-dependent proton conduction properties of the nine MOFs were demonstrated.That is,as temperature and humidity increase,proton conductivity gradually increases.At 98%RH and 100?,the highest proton conductivities of 5.38×10-4,2.22×10-4,1.35×10-4,1.36×10-3,5.09×10-3,3.09 ×10-3,1.30×10-4,0.92×10-3,and 1.24×10-4 S·cm-1 for 1-9,respectively,can be attained showing that they are promising proton conductive materials.Among them,the conductivity of MOFs 4,5,6 and 8 is around 10-3 S·cm-1,and the hydrophilic channels in the structures of 4 and 8 are conducive to improving the mobility of protons.The high conductivity of isomorphism 5 and 6 is due to the efficient proton transmission under the synergistic action of a complicated hydrogen bond network formed by external water molecules,triazole and-COOH groups.Based on the analysis of crystal structures,PXRD tests of the samples before and after electrochemical determinations,proton conductive test in a D2O atmosphere,the water vapor adsorption values and calculated Ea values,their proton conduction mechanisms were investigated and discussed.