Research On The Proton Conductivity Of Metal-Organic Frameworks Based On Substituted Carboxylate Ligands

As the core component of proton exchange membrane fuel cells(PEMFCs),the proton exchange membrane(PEM)directly affects the working life and energy conversion efficiency of PEMFCs.Therefore,the research and development of proton conductive materials with excellent conductivity and stable structure has been paid much attention by researchers.Recently,the proton conductivity of metal-organic framework materials(MOFs)with high crystallinity,adjustable structures,and the rich hydrogen-bonded network has become a hot spot.Preliminary studies have shown that small N-heterocyclic molecules(such as imidazole,pyrazole,triazole,etc.)and carboxylate groups can be used as proton sources and can also participate in the construction of H-bonded networks,which are very conducive to the transmission of protons.Therefore,we have extensive interest in the proton conductivity of MOFs constructed by organic ligands containing N-heterocyclic and carboxylate groups.Firstly,four complexes,{[Zn2(1,2,4,5-BTA)](4,4'-tmdp)2]·5H2O}n(1)(1,2,4,5-H4BTA=1,2,4,5-benzenetetracarboxylic acid;4,4'-tmdp=4,4'trimethylenedipyridine),{[Zn2(1,2,4,5-BTA)(4,4'-tmdp)(H2O)3]·2.25H2O}n(2),{Cd(1,2,4,5-BTA)0.5}n(3),[Sr2(3,4-DCB)4(Phen)2(H2O)4](4)(Phen=1,10phenanthroline)were successfully synthesized by solvo/hydrothermal reactions using pyromellitic dianhydride and substituted carboxylic benzene ligand,3,4dichlorobenzoic acid(3,4-HDCB).The Single-crystal test showed that 1 is a 3D structure with a great amount of cry stalline water and complicated H-bonded network.2 contains coordination and crystalline water molecules showing a 2D layered structure.In 3,the central metal Cd(?)atoms are bridged by 1,2,4,5-BTA4-ligands to form a 3D framework.4 is a 3D supramolecule bearing chlorine atoms.Secondly,to enrich the research,we also prepared other two substituted carboxylate ligands,4-hydroxyphenyl4,5-imidazole dicarboxylic acid(p-HOPhH3IDC)and N-benzoyl-glycine thiourea ligand(H3L),and synthesized the following two MOFs,{[Cd2(pHOPhHIDC)2(Phen)2]·H2O}n(5)and {[Cd2(p-HOPhHIDC)2(2,2'-bipy)2]·H2O}n(6)(2,2'-bipy=2,2'-Bipyridine),and two 3D supramolecules,[Ho2(?2-?2-H2L)2(?2H2L)2(Phen)2(NO3)2(CH3OH](7)and[Gd2(?2-?2-H2L)2(?2H2L)2(Phen)2(NO3)2(CH3OH)](8)according to the literature.Thermogravimetric analysis,powder X-ray diffraction,and scanning electron microscope were used to explore their thermal and chemical stability.The results indicate that the abovementioned complexes have good thermal and chemical stability.Water vapor adsorption and N2 adsorption tests showed that the 1-8 have a certain water vapor adsorption capacity and there are certain holes in the structures.These characteristics are very conducive to proton conduction under water vapor.Research on the proton conductivity of 1-8 demonstrated that their proton conductivity has a significant dependence on temperature and relative humidity(RH),which means that with the increase of temperature and RH,the proton conductivity gradually increases.Under 100? and 98%RH,the proton conductivities of 1-8 reach the maximum values of 1.10×10-4,1.44×10-4,1.45×10-5,2.34×10-6,1.41×10-4,1.03×10-4,8.66×10-5 and 1.58×10-4 S·cm-1,respectively.They all demonstrate good proton conductivity.Among them,the proton conductivity of 1,2,5,6,and 8 can attain 10-4 S·cm-1.It's due to that there are abundant H-bonded networks in their structures,which is beneficial to increase the migration rate of protons,thus exhibiting higher proton conductivity.Finally,based on analyzing the crystal structures,nitrogen and water vapor adsorption tests,and activation energy calculation,the proton conduction mechanism of the complexes was explored and highlighted.