Preparation And Characterization Of Novel Proton Exchange Membranes For Direct Methanol Fuel Cells

Proton exchange membrane is one of the key materials in direct methanol fuel cell (DMFC). Perfluorosulfonate ionomer membranes( e.g. commercially available Nafion^? membranes, DuPont) have successfully been used in hydrogen-oxygen type proton exchange membrane fuel cell(PEMFC.).So Nafion^? membranes were used as PEM during the early years of the DMFC research. However, Nafion^? membranes have high methanol crossover for the water-dependence of proton conductivity, which has reduced cell efficiency of DMFC. These have motivated great efforts in developing new type proton exchange membranes (PEMs) based on different concept. The main aim of this work is to prepare novel PEMs with no or less methanol crossover. This thesis presents the preparation and characterization of novel PEMs for DMFC. The innovative achievements related to this aim are:(1) The methodology of membrane characterization was set up to evaluate the physical and chemical properties of the PEMs and characterize the membranes surface and cross-section morphologies. Gas chromatography was used to determinate methanol permeability through PEM according to solution diffusion mechanism. Proton conductivity measurements in normal direction were performed by the two-electrode AC impedance spectroscopy. Stability of phosphotungstic acid(PTA) in composite membrane was characterized by bledout experiment. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR/FT-IR) was measured to get information about the chemical structure of membranes. Characterization, to evaluate and improve the membranes obtained, is an important part of this work.(2) Nanometer oxide/Nafion^? composite membranes were prepared by solution casting. To realize good quality composite membranes, aliphatic alcohol and water in Nafion^? PFSA Polymer solution was firstly replaced by DMF under vacuum evaporation. XRD and ATR/FT-IR analysis showed that SiO₂,TiO₂,Al₂O₃ and ZnO were incorporated into Nafion^? polymer matrix. Compared with Nafion 112 membrane and blank Nafion^? membrane, methanol permeability of composite membrane decreased by a large extent, though proton conductivity of composite membrane decreased. SiO₂/Nafion^? and TiO₂/Nafion^? membranes have excellent methanol barrier performance, which permeability were about-10^-8cm²/s at 25℃. The order of the proton conductivity and methanol permeability are SiO₂/Nafion^?>TiO₂/Nafion^?>Al₂O₃/Nafion^?>ZnO/Nafion^? and TiO₂/Nafion^?>SiO₂/ Nation^?>Al₂O₃/Nafion^?>ZnO/Nafion ^?, respectively. It can be noticed that TiO₂/Nafion^? and SiO₂/Nafion^? have higher selectivity than Nafion^?, Al₂O₃/Nafion^? and ZnO/Nafion^? membranes.(3) Novel PTA/SiO₂/SPEEK composite membranes were firstly prepared, which was one of innovations in this thesis. ATR/FT-IR spectrographic measurements indicated that the Keggin structure characteristics of PW₁₂O₄₀^3- anion was present in PTA/SiO₂/SPEEK composite membranes. XRD analysis of the composite membranes confirmed that PTA and SiO₂ were amorphous. It was found that the composite membranes had good proton conductivity(σ=0.01S/cm at 80℃) and low methanol permeability(P≈-10^-7cm²/s).Due to enhancement of the PTA stability in composite membranes by doped SiO₂,PTA/SiO₂/SPEEK composite membranes have a potential to be considered for direct methanol fuel cells (DMFC) applications.(4) Sulfonated poly (ether sulfone) (SPES) was synthesized with chlorosulfonic acid as sulfonating agent and methylene chloride as solvent at room temperature. The presence of sulfonic acid groups in SPES was identified by FT-IR Spectra. SPES Membranes and PES/SPES Blend Membranes were cast from solutions using N,N-dimethyllformamide(DMF). Both proton conductivity and methanol permeability of SPES membranes increased with the degree of sulfonation(DS), when DS=31.2%,σ=2.3×10^-3 S/cm,P≈-10^-7cm²/s. It was found that with the increasing of the PES content in PES/SPES blend membranes, methanol permeability of blend membranes decreased(P≈-10^-9cm²/s when PES content is 50 %).Proton conductivity of blend membranes decreased due to the content of sulfonic acid groups decreased resulted from addition of PES. It was concluded that both SPES and PES/SPES blend membranes had lower methanol permeability and high proton conductivity.(5) PES/SPEEK blend membranes with various PES content were prepared by casting from solutions. The glass transition temperatures of blend membranes indicated the molecular miscibility between PES and SPEEK. It was found that methanol permeability of blend membranes decreased with the increase of PES content due to the formation of H bond between O=S=O group in PES and -SO₃H group in SPEEK. Though the proton conductivity of PES/SPEEK blend membranes decreased to some extent, their performance for barrier methanol(P≈10^-8-10^-9cm²/s when PES content is below 30%) had remarkably enhanced, which showed the feasibility for direct methanol fuel cell.(6) SPEEK-Nafion^? multilayered composite membranes were also prepared by spraying two Nafion^? coated layers on both sides of basic SPEEK membranes to prevent SPEEK membranes from oxidating and degradating in fuel cells. Compared to the blank SPEEK membrane,the properties of Nafion^? | SPEEK |Nafion^? and Nafion^?- TiO₂ | SPEEK | Nafion^?- TiO₂ multilayered composite membranes were improved. Nafion^?-TiO₂ | PES/SPEEK | Nafion^?-TiO₂ multilayered composite membranes were firstly prepared, which was another important innovation in this thesis. The Fenton experiments indicated that the longevity of multilayered composite membranes was enhanced for Nafion^? coated layers protect the SPEEK membrane from degradation due to Nafion^? layer prevented·OH radical from attacking Ar-O-Ar band, which suggested that these novel membranes were excellent candidate membrane materials for DMFCs applications.There are 85 pictures,10 tables and 168 references in this thesis.