| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 98 === Proton exchange membrane fuel cell (PEMFC), due to its advantageous characteristics of zero pollution (the product is water), simplicity of operation, high proton conductivity and chemical stability has high potential for renewable energy. However, at high temperature, the degradation in the proton conductivity of membrane could be caused by an inappropriate hydration level. The transportation of proton from the anode to the cathode needs water in the membrane of the PEMFC. In reality, the membrane near the anode could be dried out and lose too much water making de-watering. On the other hand, water molecules could accumulate inside the flow channels of the membrane near cathode, resulting in flooding in the cathode, and deterioration of the cell performance.
The γ-alumina was synthesized by a sol-gel procedure. In this research, different amounts of γ-aluminas (0 %，5%，10%，20%) were added to Nafion solution to form a composite material and to raise its water content. Due to its lewis acid sites on the surface of γ-alumina, it can adsorb the base of water and keep the membrane in an appropriate humidity in the PEM. The composite Nafion membrane was recast and heat treated. After pre-treatment, the composite membranes were characterized by tensile test, field emission scanning electron microscopy, X-ray diffraction and water-uptake measurement. In addition, the electrical properties of the composite membranes were evaluated by an electrochemical analyzer and fuel cell testing system.
The experimental result indicates that the water absorption content increases with γ-alumina addition; however, the tensile strength and thermal stability of the composite membranes decrease with an increasing of γ-alumina. It was found that the defect in the membranes increases with the addition of γ-alumina and the interface between membrane and the catalyst layer is easily broken at high strains. At room temperature, the electrical conductivity of the composite membrane increases with the quantity of γ-alumina.
In the fuel cell test, the γ-alumina in the composite membrane can increase the current density of the fuel cell, however, too much γ-alumina in membrane will deteriorate the performance of fuel cell; the addition of γ-alumina in the composite membrane with the lewis acid site on its surface assists absorption of water molecules and maintains the humidity in the membrane. When the fuel cell temperature reaches 70℃, water in the composite membranes form steam and make carrying capability of protons decreases, As a result the PEMFC performance of the composite membrane decreases. When increasing the anode humidity temperature, the water in fuel cell increases slowly, and the performance also improved slightly, because the water content in the composite membrane is almost saturated.
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