| Summary: | 碩士 === 元智大學 === 化學工程與材料科學學系 === 94 === In this study, Nafion-117 membranes were modified by layer-by-layer electrostatic self assembly (LBLESA) technique and tested for their application as proton exchange membranes in direct methanol fuel cells (DMFCs). Polyelectrolyte multilayered (PEM) membranes were constructed by electrostatic absorbed polycation poly(allylamine hydrochloride) (PAH) and polyanion poly(acrylic acid) (PAA). A series of standard methods were used to characterize the membrane properties, such as ATR-FTIR analysis, contact angle experiment, XPS analysis, TEM observation, AFM observation, water uptake test, pervaporation test of aqueous methanol solution, AC impedance analysis. The properties of modified membranes were compared with those of pristine Nafion-117 with an aim to be used for the fabrication of membrane electrode assembly (MEA).
Polycation and polyanion layer showed difference in water contact angle；therefore monolayer adsorption of polyelectrolytes could be confirmed. The averaged thickness of each layer in the multilayer was about 30-45 nm by TEM observation. ATR-FTIR confirmed the presence of polyelectrolyte element group in the modified membranes. After absorbed four double layer, membranes surface have a non-homogeneous layer growth by AFM observation. XPS analysis suggests the sulfonate group through polyelectrolyte layer as a path to permit easy proton transport.
In pervaporation experiment, Six double layers were sufficient to minimize the methanol crossover ( 9.6 wt % in feed, methanol conc. of permeate 0.33 wt% ). The proton conductivity of LBL6 (σ = 2.4×10-1 S/cm) was higher than Nafion-117 membrane (σ = 3.2×10-2 S/cm) at 80℃, relatively humidity of 95 %. Base on all tested results, LBL6 membranes have potential to be used for DMFC.
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