Synthesis and Characterization of Nafion Composite Membrane for Reduced Methanol Crossover in DMFC

Abstract

The polymer electrolyte membrane (PEM) is a key component in a direct methanol fuel cell (DMFC). The primary characteristics desired of a PEM for DMFC are low methanol crossover (MCO), high proton conductivity, high mechanical, thermal and chemical stability, sufficient water uptake and moderate swelling, easy fabrication to form the membrane electrode assembly (MEA), competitive low-cost, and sufficient long-term durability. The state of the art membrane, Nafionऍ by DuPont possesses almost all of the desirable properties except the low MCO and low cost. Low MCO through nafion in DMFC is a challenging issue. Subsequently, a lot of research has been directed to the modification of nafion for DMFC application for reducing the MCO and simultaneously there has also been a search for alternative membrane with all the desirable properties of nafion including low MCO. As a contribution to the research on nafion modification for DMFC application, this thesis aims to synthesize nafion composite membranes with inorganic additives. Six different inorganic additives namely, titanium dioxide (TiO2), neodymium oxide (Nd2O3), magnesium silicate hydroxide or talc H2Mg3(SiO3)4, erbium triflate (ErTfO), neodymium triflate (NdTfO), and molecular sieve (MS), have been used for nafion modification by the casting method. Each additive selected had a characteristic property, which was anticipated to modify nafion by lowering its MCO while maintaining its other desirable properties especially proton conductivity, thermal, chemical and mechanical stability. TiO2, Nd2O3, and talc are hygroscopic and crystalline and consist of groups on the surface. Both ErTfO and NdTfO are rare earth lanthanide triflates, resembles nafion in structure, have high coordination number, are hygroscopic, thermally stable, resistant to redox environment, and is anticipated to lower MCO without compromising proton conductivity and the other desirable properties of nafion. MS is hygroscopic and crystalline in nature with pore size of 3 ध, which is anticipated to act as barrier to methanol having molecular size of 4.4 ध. All the six additives were studied for morphological analysis using particle size analyzer, X-ray diffraction (XRD), BET surface area analyzer, and scanning electron microscope (SEM). The volume median diameters of the particles were determined using particle size analyzer. The developed composite nafion membranes were characterized under ex-situ conditions by scanning electron microscopy (SEM), for thickness, X-ray diffraction (XRD) analysis, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectrometer analysis, ion exchange capacity (IEC), water uptake, methanol uptake, swelling, tensile strength, oxidative stability, proton conductivity, and methanol crossover. SEM images of the membranes indicated the formation of dense nafion composite membranes with no pore formation. XRD analysis of the nafion composite membranes indicated that the TiO2/nafion and talc/nafion membranes are more crystalline than pure cast nafion membrane. TGA thermograms of the nafion composite membranes indicated that all the composite membranes were stable upto a temperature of 310 ओC. FTIR analysis of the composite membranes indicated shifting of peaks for S-O stretching and water stretching frequency to higher wavenumbers and broader peak for water bending vibration ...