Synthesis, Modification And Electrical Conductivity Of Metal-organic Framework-based Proton-conducting Materials

Fuel cell is considered as a new generation of energy technology to alleviate energy crisis and environmental problems with its advantages of high energy conversion efficiency and less pollutant emission.Electrolyte materials with high proton conductivity and excellent stability is the key to limit the development of fuel cell technology.Over the last few decades,Metal organic framework(MOF)as a new type of porous material has been extensively studied by scientists due to its advantages of high porosity,large specific surface area and controllable structure.It has been used in catalysis,fluorescence,adsorption and separation,drug delivery and proton conduction fields.With the development of synthesis and modification methods,MOF has made some progress in the field of proton conduction.In this paper,the MOF was modified by mixed ligand strategy and post-synthesis modification strategy to obtain proton conductive materials and proton exchange membranes with better proton conductivity.1.Developing novel proton-conducting materials is crucial to improve the performance of proton exchange membrane fuel cells for energy conversion especially at low humidity.Metal-organic frameworks(MOF)have expressed an apparent potential in proton conduction applications due to their highly ordered and tunable structure.Previous studies mainly focus on the MOF modified with homogeneous functional groups to tune the proton conductivities.In this study,an acid-base pair synergistic strategy,namely introducing the acidic and basic groups into mix-linker MOF simultaneously to obtain MOF-directed ordered acid-base coexistence materials,was firstly proposed to improve the proton conduction performance at 75%RH.Herein,10 kinds of ML-UiO-66-(A)(B)-X were synthesized and the effects of content and ratio of acid-base pairs in MOF matrix on proton-conducting properties were studied.It was found that ML-UiO-66-(NH₂)(SO₃H)-5:5 possessed the highest proton conductivity in the ML-UiO-66 materials,with the value of 2.8×10^-2S·cm^-1at 353 K.In addition,at303K,75%RH,the proton conductivity of ML-UiO-66-(NH2)(SO3H)-5:5 reached 7.4×10^-3S·cm^-1,which is 3 orders of magnitude higher than pristine UiO-66(?980times),2 orders of magnitude higher than the base-functionalized UiO-66-NH₂(?529times)and about 218%higher than the acid-functionalized UiO-66-SO₃H at 97%RH.The results show that at relatively lower humidity 75%,ML-UiO-66-(NH₂)(SO₃H)-5:5 has the better proton conductivity than other UiO-66materials at nearly 100%RH.Meanwhile,ML-UiO-66-(NH₂)(SO₃H)-5:5 shows high water and thermal stability under high humidity and high temperature,which ensures its application in complex environment.Performance and mechanism studies show that the strategy of introducing acid-base pair into MOF matrix may open an avenue to design novel proton-conducting materials with excellent properties.2.Most MOFs lack the ability to form films,doping MOF into polymer is an effective strategy to improve the proton conductivity of composite films under the condition of hydration.A series of composite membranes CS/ML-UIO-66-(NH₂)(SO₃H)-5:5-X(X=0,5,10,15)with different doping levels were prepared by solution casting method with the synthesized ML-UIO-66-(NH₂)(SO₃H)-5:5 mixed with chitosan matrix,where X represents the percentage of ML-UIO-66-(NH₂)(SO₃H)-5:5 in the total weight of composite membranes.The morphology and thermal stability of the composite mambrane were characterized by XRD,SEM and TGA.AC impedance test shows that the proton conductivity of CS/ML-UiO-66-(NH₂)(SO₃H)-5:5-X composite membrane is higher than that of pure chitosan membrane.CS/ML-UiO-66-(NH₂)(SO₃H)-5:5-15 composite membrane has a proton conductivity of 8.8×10^-2S·cm^-1at 353 K and 100%RH,the?value is increased by 260%compared to pure chitosan film.These data provide a reference for exploring high-performance MOF matrix proton exchange membranes.3.In this chapter,MIP-202 and Zn Glu,two kinds of MOF with L-aspartic acid and L-glutamic acid as organic ligands were synthesized,which have high biological safety and conform to the concept of sustainable development.Then,the MIP-202-SO₃H and Zn Glu-SO₃H were obtained by introducing hydrophilic and highly acidic sulfonic groups into the MOF structure through post-synthesis modification strategy.The crystal structure and morphology of the two MOFs were characterized by XRD,FT-IR,TGA and TEM.The results showed that the original crystal structure of the two MOFs was not damaged after the modification.AC impedance test showed that the proton conductivity of the modified MIP-202-SO₃H and Zn Glu-SO₃H reached 2.3×10^-2S·cm^-1and 2.1×10^-2S·cm^-1at 353 K and 100%RH,which were both higher than that of the unmodified materials.In this chapter,a simple post-synthesis modification strategy is adopted to improve the proton conductivity of the two amino acid based MOF,which is also applicable to other kinds of MOF materials,and is conducive to the design and development of MOF matrix conductive materials with better performance.