Design And Synthesis,Crystal Structure And Proton Conduction Property Study Of Sulfonic Acid-functionalized MOFs

In the context of environmental pollution and energy shortage,the development of clean energy has received extensive attention.Proton exchange membrane fuel cells（PEMFC）are considered to be the most promising because of their high specific energy,low pollution and low noise.Among them,the proton exchange membrane is an important component.The proton conductive membrane performance is directly related to the efficiency of PEMFC,so the study of the mechanism of proton conduction becomes very important,because it can help us understand the proton conduction process and guide the optimization of proton conduction performance for practical applications.It is not easy to understand the proton conduction process using Nafion as a model due to its hierarchical structure without definite molecular structure.Therefore,we have chosen the crystallinemetal-organic framework（MOFs）material to understand and study the mechanism of proton conduction.We used 3,3’-disulfonic acid-4,4’-diphenyl phthalate（BPDSDC）ligand to form a MOF with copper ions to form a one-dimensional channel with sulfonic acid group modification:Compound 1:H2[CuK4（BPDSDC）2]+x（solvent）.It is worth noting that during the coordination of biphenyl ligands,the steric hindrance effect led to the sulfonic acid groups on both sides of the ligand rotating to the same side,so that the sulfonic acid groups in the ligand can overcome the hindrance of the biphenyl ligand.The interaction occurs,forming a continuous hydrogen bond network with the adsorbed solvent water molecules,resulting in high proton conductivity,reaching 1.13×10^-2 S/cm at 45^oC and 97%relative humidity.Notably,the water vapor adsorption test at 25^oC and the proton conductivity test in the humidification and dehumidification process show that compound 1 has a strong water retention capacity and retains a certain proton conduction performance when the ambient humidity is greatly reduced.We synthesized a novel mixed ligand MOF by complexing 2-sulfonic acid terephthalic acid（L1）andpiperazine（L2）withcopper（Ⅱ）.Compound2:[Cu₂（μ₃-O）（HL1）（L2）（H₂O）]（DMF）（H₂O）_0.5（DMF=N,N’-dimethylformamide）,which has a one-dimensional pore with a sulfonic acid group modification along the c-axis,but does not show high proton conductivity,as the sulfonic acid groups in the pores are sparsely arranged and the sulfonic acid groups are difficult to form a continuous hydrogen bond network with water molecules.The MOFs material cannot only be functionalized,but also can accommodate a large number of guest molecules.In order to improve the proton conductivity of MOF,we have distilled the proton carrier imidazole（Im）into the pores of MOF.The imidazole can form a hydrogen bond network with the sulfonic acid group forming an acid-base pair in the channel.The test results show that the proton conductivity of the material is improved by nearly three orders of magnitude after the addition of imidazole.