| dc.description.abstract | In the fuel cell system, the most urgent issue of proton exchange membrane, which separates anode and cathode are: (1) the rapid loss of water at high temperature, which leads to a rapid decline of conductivity; (2) The cross-over of methanol fuel, which deteriorated the fuel cell performance; and (3) the mechanical and electrochemical durability that membrane disintegrated after prolonged usage, thus limited fuel cell lifetime. The membrane is designed based on the idea that Nafion channel morphology modified or tunable by incorporating a base-modified SiO2 nano-particle in-beded in the hydrophilic domains. To prevent the organic-inorganic phase separation in the composite material, the novel membrane, Nafion/ pyrrolidone silane (Py-silane) modified SiO2 composite (f-SiO2) were prepared by in-situ sol-gel method where the silica particle first nucleated in Nafion solution and growth in dimension during the Nafion membrane formation.
SEM and Si-mapping confirmed the silica nano-particle is homogeneously distributed in the composite membrane. TEM results indicated the membrane 15-1p4T displayed the best morphology where the hydrophilic and hydrophobic domains maintains the most suitable size and most uniform distribution. Crystallinity provided by XRD shows the addition of alkaline-modified SiO2 has raised the crystallinity on Nafion main chain structure (PVdF), which is especially true for the 15 phr sample. The composite membrane bearing 15 phr silica content with base silane : TEOS ratio = 1:4 (15-1p4T), yielded the lowest water uptake, and smallest swelling ratio. Although the conductivity is reduced about 17%, its methanol permeability is reduced about 50%, leading to a better selectivity (C/P ratio) over recast-Nafion of 60%to 70%. In, addition, the activation energy derived from variable temperature water diffusion constant is about half of that in recast-Nafion, suggesting a novel water transport path which is more suitable for proton conduction. As moisture is preserved by the inorganic moiety, proton conductivity is not lost at low humidity (~1×10-3S/cm under 20% RH). The DMFC performance reached 88.2mW/cm2 under 70oC which is better than N117 under the same operating condition.
All results indicated this material has successfully tailored the Nafion membrane micro-structure. This proton exchange membrane promises low swelling ratio, lower proton conduction activation energy, highly dispersed inorganic composite, and higher C/P ratio; suitable for direct methanol fuel cell to operate at elevated temperature.
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