| Recently,research on proton conductive metal-organic frameworks(MOFs)has been in the ascendant.Although the design and preparation of high-performance proton-conducting MOFs have been carried out by adopting methods such as switching central metal ions and carefully designing organic ligands.Nevertheless,there are few reports on the proton conductive hafnium-based MOFs with high stability and porous structures and related composite membranes.In this thesis,a series of three-dimensional Hf-based MOFs,Hf-UiO-66(1),Hf-UiO-66-(OH)2(2),Hf-UiO-66-NH2(3)and Hf-UiO-66-NO2(4),with excellent stability were prepared by rapid synthesis or solvothermal methods.The various tests show that they not only have good water,acid-base and thermal stabilities but also have outstanding proton conductivities.At 100℃ and 98%RH,their optimal proton conductivities are 2.51 × 10-3,4.33 × 10-3,1.10 × 10-3 and 0.89 × 10-3 S·cm-1,respectively.On the basis of comprehensively considering their porous and highly stable structural characteristics,we adopted the evaporation method to encapsulate imidazole molecules in the frameworks of the above MOFs to obtain modified products,Im@1-4.In addition,carboxylic acid groups were introduced into the frameworks of 2 and 3 through ring-opening reaction and Schiff base reaction,resulting in chemical post-modification products,SA-2 and CBD-3.Powder X-ray diffraction(PXRD),thermogravimetric analysis(TGA),scanning electron microscopy(SEM)and other characterizations proved that the post-modified products still maintained the frameworks and high stability of the original MOFs.The post-modified products Im@1-4,SA-2 and CBD-3 have higher proton conductivity of 1.15 ×10-2,1.32 ×10-2,1.32 × 10-2,0.8 ×10-2,1.23 × 10-2 and 0.71 × 10-2 S·cm-1 at 100℃ and 98%RH(relative humidity),respectively,which can be compared with previously reported MOFs with high proton conductivity and comparable to Nafion,showing the important application prospects of such MOFs and their post-modified products in the field of proton conduction.Subsequently,we obtained the composite films CS/Im@1,CS/Im@2,CS/Im@3,CS/Im@4,CS/SA-2 and CS/CBD-3 by casting the above post-modified products with chitosan(CS).The morphology and properties of the composite films were characterized by XRD,SEM,cross-section SEM,TG,and other approaches.The results denoted that the surfaces of the composite films were compact and smooth with good stability.The Electrochemical Impedance Spectroscopy(EIS)test reveals that the composite membranes with different doping amounts exhibit different proton conductivity,and the best proton conductivities are obtained when the doping amounts of MOFs are 10%,2%,2%,2%,6%and 8%,respectively.In addition,the proton conductivity of the composite membrane increased with the increase of temperature and humidity,and reached the maximum values of 1.76 × 10’2,1.72 x 10-2,1.31 × 10-2,1.51 × 10-2,2.19 × 10-2 and 1.85 × 10-2 S·cm-1 for CS/Im@1-10,CS/Im@2-2,CS/Im@3-2,CS/Im@4-2,CS/Im@SA-2-6 and CS/Im@CBD-3-8,respectively,at 100℃ and 98%RH,and most of which are better than the proton conductivity of the pristine MOFs and pure CS membranes.These composite membranes with high proton conductivity reveal great potential in proton exchange membrane fuel cells and impedance recognition fields.Finally,combined with the calculated activation energy,XRD determinations,structural analyses,nitrogen and water vapor adsorption measurements,etc.,the proton-conducting mechanisms of the above MOFs and composite membranes were speculated,which is helpful for the design and synthesis of crystalline materials with high conductivity. |
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