Preparation And Properties Of Pore-impregnated Proton Exchange Membranes For Applications In Direct Methanol Fuel Cells

Direct methanol fuel cells(DMFCs)can convert chemical energy into electrical energy.As a type of potential future power device,they have attracted significant attention because of their light weight,high fuel utilization efficiency,high energy density,low emission,and portability.The proton exchange membrane(PEM),which acts as an electrolyte to transport protons from the anode to the cathode and prevent fuel leakage,is the key component in a fuel cell assembly.For the fabrication of a PEM,various perfluorinated,partially perfluorinated,or sulphonated wholly aromatic polymers(such as polyetheretherketone,polyethersulphone,polybenzimidazoles and polyphenylene oxide)are commonly used and have been thoroughly characterized in terms of their proton conductivity,mechanical properties,swelling and chemical resistance.Currently,perfluorinated ionomers(such as Nafion)have become the most widely used PEM materials due to their high proton conductivity as well as acceptable chemical and electrochemical stability.However,the further use of the perfluorosulphonic acid membrane is hindered due to its well-known drawbacks,including its high cost,slow anodic reaction kinetics,limited durability,high degree of water swelling,and most importantly,undesirable methanol crossover,which causes fuel waste and contamination of the platinum catalyst on the cathode side.To minimize fuel crossover,a low methanol concentration(less than 2 M)has been used,lowering the fuel efficiency and causing a potential loss at the cathode.These factors have impacted the energy density of DMFCs compared to Li-ion batteries.To develop a larger power density(to compete with Li-ion batteries)and at the same time considering the importance of water in the proton conductivity efficiency of the membrane,the concentration of methanol must be approximately 10 M in aqueous solution.Such a high concentration requires a membrane with minimum methanol crossover and high mechanical strength.This thesis is consisted of threes parts including one part of a pore-filling membrane and two parts of impregnated fibrous composite membranes.The composite membranes are all based on the same concept,involving a mechanical strong substrate and a polymer which provides proton conductivity.Firstly,a poly(ether sulphone)-based pore-filling membrane was successfully fabricated and tested against a conventional Nafion-based membrane in direct methanol fuel cells.An amino-containing polymer with a low degree of sulphonation(DS)was synthesized and used as the supporting substrate.The porous substrate was prepared by introducing the porogenic agent(tetrafluoroborate)into the membrane casting solution.The effects of the content of the porogenic agent on the pore morphologies were evaluated using field emission scanning electron microscopy.Then,an epoxy resin was introduced into the filling electrolyte for the first time to minimize the swelling and methanol crossover that resulted from the high degree of sulphonation.In essence,solidification of the amino groups in the substrate results in 3D crosslinking of epoxy resins,which greatly suppresses the swelling and methanol crossover of the composite membranes with enhanced mechanical properties and enhances the thermal and oxidation stability compared to Nafion 117.The resulting composite membrane also shows high proton conductivity that is only slightly lower than that of Nafion 117.However,the selectivity between the proton conductivity and methanol permeability is higher for the composite membranes than that of Nafion 117.The composite membrane also shows a better performance in single cell tests with 10 M methanol.Secondly,two kinds of PDMS-reinforced Nafion membranes including a Nafionimregnated PDMS fiber membrane and a PDMS impregnated Nafion fiber membrane are developed tested against a conventional Nafion membrane in direct methanol fuel cells.Both Nafion and PDMS were successfully electrospun into nanofibers membranes and impregnated with each other in different weight ratio.The morphological properties of the fibres and impregnated membranes were observed by SEM.The volumetric swelling and gravimetric water uptake of the composite membranes were investigated in comparison with Nafion membrane and it turned out the fibre-impregnated membranes all exhibited a great decrease on swelling and water uptake,which was attributed to the high hydrophobicity of PDMS.The composite membranes also exhibited better thermal and oxidative stabilities due to the TGA and Fenton’s test results.The single cell performance of the different fabricated fibreimpregnated membranes was also investigated and compared against a same-casted Nafion membrane and the results showed the fibre-impregnated membranes have highly improved performances in 10 M methanol,indicating the promising practical usage in DMFC.In the third part of the thesis,PVDF and PTFE/PVDF nanofibre membranes were successfully electrospun.The Nafion impregnated membranes with various layers of extremely thin PVDF nanofibers were fabricated and tested against Nafion membrane.The morphological structures,mechanical properties,thermal stabilities,chemical resistance,proton conductivity and single cell performances were investigated for each membrane.The introduce of PVDF fiber mats greatly enhanced the thermal and oxidation stabilities and suppressed swelling and water uptake of the membranes as compared to Nafion.The resulting composite membranes also show higher single cell performances than that of Nafion in 10 M methanol,while no significant differences occurred along with the increasing number of PVDF layers.According to the testing results above,Nafion 115 with a single layer of PTFE/PVDF nanofibre membrane on both sides(anode and cathode)were also fabricated and their single cell performances in both 5M and 10 M methanol were detailed investigated.The results showed the composite membranes had greatly improved performances compared to the commercial Nafion 115 membrane.The power density of the membranes also significantly increases with the increasing weight ratio of PTFE.Additionally,the Nafion 115 with PTFE/PVDF nanofibre membrane on anode performs better than that with PTFE/PVDF fibres on cathode side.In summary,several kinds of impregnated composite membranes were fabricated and tested against conventional Nafion membranes for applications in DMFC.The properties of the resulting membranes were evaluated in high concentration of methanol aqueous.The composite membranes were all found to be with enhanced performances,which proves the effectiveness of the involved materials and manufacture processes.