| dc.description.abstract | Alkaline fuel cell is one of the major thrust of fuel cell research in recent years, because it offers better opportunity to use non-platinum based catalyst, with potential of lowering the prices. But a suitable membrane for alkali fuel cell is still absent from the market. A main function of the membrane allows ions to pass but hinders fuel permeability. Variety of membranes systems are being explored which
include anion exchange membranes (AEM), cation exchange membranes (CEM) and polyphenylene imidazole membranes (Alkali doped PBI). In the case of anion exchange membranes, its low thermal stability and the tendency to loss alkalie functional group precludes its application in fuel cells to operate at high temperature. In this study we will focus on modification of Nafion-based cation exchange membrane, because it offers both durability and mechanical stability at high
temperature. Its main disadvantage, i.e. the low electrical conductivity, can be improved relatively easily by blending with a alkali functionalized moiety. Our design is to physically cross-linke Alkalie (K+ ) exchanged Nafion with bismaleic
imide hyper-branched polymer. This composite membrane shows good conductivity at elevated high temperatures because the hyper-branched oligomer bears base
functions augments OH- transport in the membrane; effectively blocked the fuel permeation while preserving water at elevated temperature to sustain high ion conductivity. As a result, the composite membranes simultaneously improved both the conductivity in low humidity (or at elevated temperature) and reduce permeability of ethanol. In this study, physical and electronic properties of the two series of
K-Nafion samples containing Bismaleimide (BMI) monomer and modified Bismaleimide oligomers (mBMI) with mole ratios of 28:72 (mBMI(72)) and 2:98 (mBMI(98)) are compared in detail by SEM, TEM, XRD and TGA analyses. The ionic conductivity works upward to 0.25 S/cm in DI water and 0.43 S/cm in 1M KOH at room temperature. The improvement of ion conductivity is more pronounced at low
humidity conditions where the conductivity maintains about one order higher than that of recast K-Nafion below 40% RH and shows continuous increase when temperature is
raised above 80oC. These series of cation exchange membranes contain mBMI show alcohol barrier property where ethanol permeability is substantially reduced to
2.38×10-7 cm2/s in the case of mBMI(72)/K-Nafion. Selectivity(C/P ratio) of the membrane with mBMI is also higher than the recast K-Nafion membrane, especially in
the case of 5%-mBMI(72). Under air atmosphere, these membranes are stable in basic media with KOH concentration up to 8 M. Performance of composite membrane (5%-mBMI (72) / K-Nafion) in the ethanol fuel cell at 90℃ is about 65 mW/cm2 which is higher than K-Nafion membrane (60 mW/cm2). The results shows the hyperbranch composite membranes can be effectively applied to alkaline fuel cells
with more pronounced effects at elevated temperature. | en_US |