Preparation And Properties Of PEEK-based Composite Proton Exchange Membranes Doped With ZIF-8 And TiO₂ Nanoparticles

Proton exchange membrane（PEM）is one of the core components of fuel cells,and its performance determines the operating efficiency of fuel cells.PEMs based on sulfonated polyether ether ketone（SPEEK）are favored by researchers due to low cost,simple preparation,environmentally friendly,and high proton conductivity.However,the weak acidity of SPEEK with sulfonate groups leads to phase separation between the hydrophilic phase and hydrophobic phase,narrow hydrophilic channels,and a significant decrease in proton conductivity under low humidity conditions,which affect the normal operation of fuel cells.In recent years,zeolite imidazolium ester skeleton material（ZIF-8）,which combines the characteristics of rigidity of inorganic materials and flexibility of organic materials,has attracted much attention in the field of fuel cell due to many advantages such as high porosity,large specific surface area,and excellent chemical and thermal stability.In this thesis,a novel composite PEM with self-wetting properties will be prepare by introducing ZIF-8 nanofillers with different particle sizes and Pt-TiO₂ nanoparticles into the SPEEK matrix.The following milestones were achieved as follows.1.By adjusting the molar ratio between 2-methylimidazole and zinc nitrate hexahydrate,the solvent types and the stirring time of the reaction,three ZIF-8nanofillers with different particle size were synthesized at room temperature,respectively.The surface morphology,chemical structure,particle size and size distribution and properties of ZIF-8 were investigated by SEM,TEM,nanometer,TG,XRD,FTIR and N₂ adsorption-desorption.The results showed that the particle sizes of ZIF-8-1,ZIF-8-2 and ZIF-8-3 nanoparticles are 200 nm,600 nm and 1000 nm respectively,and the nanoparticles show regular shape,good dispersion,and high specific surface area,high porosity,excellent chemical and thermal stability and good compatibility with organic materials.The preparation method of ZIF-8 is simple,short and controllable reaction time and no any additives.2.A series of novel ZIF-8/SPEEK composite PEMs were prepared by solution casting method by incorporating ZIF-8 nanofiller into SPEEK matrix,and the surface morphology,stability,mechanical properties,water absorption,swelling ratio,proton are discussed.The effects of the particle size and content of ZIF-8 nanofiller on the comprehensive performance of the composite PEM were also investigated.The results showed that the composite membrane had smooth and uniform surface,dense cross-section,and good compatibility between filler and substrate;compared with pure SPEEK membrane,the composite membrane showed some improvement in dimensional stability,oxidative stability,mechanical properties,proton conductivity,and alcohol barrier properties.For example,compared with pure SPEEK,the tensile strength of the prepared composite membrane increased by 34.9%when the particle size of ZIF-8 nanofiller was 1000 nm and the content was 2 wt.%;the dissolution mass percentage decreased by 19.55%;the swelling ratio decreased by 42%（60°C）;the proton conductivity at 70°C was 0.108 S/cm,which was 35%higher than that of the pure membrane of 0.08 S/cm.The methanol permeability was 2.03×10^-7 cm²/s,which was 38%lower than that of 3.28×10^-7 cm²/s for the pure membrane.3.Pt-TiO₂ nanoparticles with both water retention and water-generating functions was prepared by depositing Pt nanoparticles on the TiO₂ surface by redox method.Then,Pt-TiO₂/ZIF-8/SPEEK composite PEM with self-wetting property was prepared by dispersing Pt-TiO₂ nanoparticles with different contents in ZIF-8/SPEEK membrane by impregnation method and solution casting method,and the effect of the size of Pt-TiO₂doping on the comprehensive performance of PEM was explored.The results showed that the comprehensive performance of Pt-TiO₂/ZIF-8/SPEEK composite membranes doped with Pt-TiO₂ nanoparticles was significantly improved compare to that before doping.For example,the Pt-TiO₂/ZIF-8/SPEEK composite membrane exhibited higher proton conductivity（0.019 S/cm）and lower methanol permeability(1.68×10^-7cm²/s)at room temperature when doped with ZIF-8 with a particle size of 1000 nm and Pt-TiO₂ nanoparticles with a content of 2 wt.%and 1 wt.%,even at 50°C and 30%RH.The composite membrane exhibited a much higher proton conductivity（0.43 m S/cm）than the pure SPEEK membrane even after heating at the extreme environment of 50°C and 30%RH for 40 min.In this study,a new SPEEK-based composite PEM with excellent overall performance and self-wetting property was constructed by introducing ZIF-8nanoparticles and Pt-TiO₂ nanoparticles into the SPEEK matrix,and the results will provide new ideas and theoretical basis for the preparation of environmentally friendly,low-cost and excellent overall performance PEM,and accelerate the commercialization of PEMFC.