| Summary: | 碩士 === 長庚大學 === 化工與材料工程學系 === 101 === The objectives of this study are to investigate the suitable gas diffusion layers and to optimize polybenzimidazole (PBI) electrolyte membranes in direct methanol fuel cells (DMFCs). The commercial gas diffusion layer (GDL) often consists of a hydrophobic micro porous layer (MPL) on a porous carbon substrate. In the current study, we used gas diffusion layers with and without MPL for the electrodes and measured the resulting DMFC performance. Higher cell voltage and peak power density were obtained for the proton-exchange DMFC when the GDL without MPL on the anode and the GDL with MPL on the cathode were used. This arrangement allows the fuel to spread uniformly into the GDL and to react with the catalyst on the anode. The hydrophilic layer on the anode GDL also helps in effective removal of the produced CO2 out of the anode. The hydrophobic anode GDL prevents water flooding at the cathode. The electrochemical impedance spectroscopy confirmed the lower cell resistance in the proton-exchange DMFC using the MPL-containing cathode GDL and the pristine GDL for anode. Polybenzimidazole (PBI) films were prepared using solution casting method. The PBI membranes were immersed in 2 M phosphoric acid or 6 M potassium hydroxide solutions to prepare electrolytes for proton-exchange and hydroxide-conducting DMFCs, respectively. The effects of the methanol concentration , anode flow rate, temperature, and the use of hydrophobic GDL on DMFC performance were examined. When fed with 2M methanol fuel and humidified oxygen gas , the DMFC containing KOH-doped PBI outperformed that with the H3PO4-doped PBI (117.9 mWcm-2 vs. 46.5mWcm-2), with both cells equipped with the MPL-containing cathode GDL, the pristine GDL for anode, and operated at 90ºC. While the hydrophilic GDLs without MPL were employed on both electrodes, the KOH-doped PBI showed a peak power density of 143.7 mW cm2 due to the facilitation of water uptake to the cathode and of the conversion of oxygen to hydroxide ions. As the anode fuel flow rate was increased from 5 to 15 mL min1, the peak power density further reached 158.9 mW cm2. The KOH-doped PBI membrane has the potential in the high temperature alkaline DMFC application.
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