Preparation And Properties Of Composite Proton Exchange Membranes Based On Functionalized Metal-Organic Frameworks

Currently,the biggest society problems facing society are energy shortage and environmental pollution,and the urgent need for renewable and clean energy is driving the development of fuel cells.Proton exchange membrane fuel cells（PEMFCs）are one of the most promising fuel cells.Proton exchange membranes（PEMs）determine the lifetime and overall performance of PEMFCs.In the study of PEMs materials,it was found that aromatic polymers have good thermal stability and mechanical properties and can be applied to PEMs materials after sulfonation modification.However,the high proton conductivity of aromatic polymers mainly depends on the sulfonation degree,and excessive sulfonation can weaken the dimensional stability of the membranes and limit further applications.Therefore,modification of sulfonated aromatic polymers are needed to obtain a balance of high proton conductivity and good mechanical properties.Organic-inorganic hybrid modification is one of the most commonly used modification methods,and metal-organic frameworks（MOFs）have attracted a lot of attention from researchers in the further exploration of organic-inorganic hybrid materials.MOFs are new organic-inorganic hybrid materials with intramolecular pores formed by self-assembly of metal ions or metal oxygen clusters and bridging organic ligands through coordination bonds.MOFs materials have excellent features such as high specific surface area,adjustable pore size,easy functionalization,and can accommodate a variety of guest molecules.MOFs can be better applied in the PEMs materials by certain functionalization design.By introducing functionalized MOFs into sulfonated aromatic polymers can combine the advantages of both to obtain PEMs materials with excellent performance and low cost.In this thesis,an organic-inorganic modification approach was used to prepare sulfonated poly（aryl ether ketone sulfone）as a polymer matrix from the perspective of molecular design,and a series of functionalized MOFs were designed to obtain MOFs materials favorable for proton transport,and the effects of different functionalized MOFs on each property of the polymer matrix were investigated.In this paper,we first prepared side chain type PEMs based on functionalized MOFs and a sulfonated poly（aryl ether ketone sulfone）organic matrix containing long and short sulfonic acid side chains.A series of composite membranes were prepared by introducing ZIF-67/GO into this side-chain type polymer matrix.Among them,ZIF-67 was chemically grafted onto the GO lamellae through the coordination between Co²⁺and the carboxyl group of GO,and grew uniformly on the GO surface.ZIF-67/GO was not only important for the construction of proton transport channels,but also contributed to the improvement of the dimensional stability of the composite membranes.Among them,MSP-1%ZIF-67/GO had the highest proton transport ability and mechanical properties with a proton conductivity of 0.133 S cm^-1（80 ^oC,100%RH）,Young’s modulus of 877.57 MPa,and tensile strength of 41.88 MPa.Secondly,the imidazole functionalized MOF-801（IM8）was introduced into the sulfonated poly（aryl ether ketone sulfone）containing long and short sulfonic acid side chains.Using MOF-801 as a template,imidazole was ligated with Zr⁴⁺to introduce imidazole groups into MOF-801 as additional proton carriers.IM8 and the sulfonated poly（aryl ether ketone sulfone）containing inhomogeneous long and short sulfonic acid side chains synergistically improved the proton conductivity.Among them,MSP/0.5%IM8 exhibited the highest proton conductivity and mechanical properties with a proton conductivity of 0.199 S cm^-1（80 ^oC,100%RH）,Young’s modulus of 616.51 MPa,and tensile strength of 42.33 MPa.In the above experiments,it was found that although the side-chain type polymer matrix could maintain better mechanical properties of the composite membrane by constructing a distinct hydrophobic phase separation structure,its mechanical properties were still not satisfactory.Therefore,the main-chain type sulfonated poly（aryl ether ketone sulfone）was selected as organic matrix.And two MOFs（amino-sulfonic acid bifunctionalized Ui O-66（USN）and sulfonic acid-imidazole bifunctionalized Ui O-66（IUSN））were prepared by ligand exchange and Schiff base reaction,two series of superprotonic Ui O-66/sulfonated poly（aryl ether ketone sulfone）composite membranes were designed.Among them,the composite membranes with the addition of sulfonic acid-imidazole bifunctionalized Ui O-66 exhibited relatively high proton conductivity and mechanical properties.Among them,the CSP/IUSN-3%exhibited the highest proton conductivity of 0.234 S cm^-1（80 ^oC,100%RH）,the mechanical properties have also been significantly improved with a Young’s modulus of 1535.66 MPa and a tensile strength of48.670 MPa,also had a high peak power density(243.770 m W cm^-2)and an excellent open circuit voltage（0.9500 V）.The results showed that this series of membranes had excellent comprehensive performance,and demonstrated great potential for application in PEMFCs.