Study On Fabrication Of Superhydrophobic Micro Channels And Its Effect On Gas Subduction In DMFC Anode Flow Field

Direct methanol fuel cell (DMFC) receives concerns from institutes and automakers of the world because it offers many advantages such as high energy conversion efficiency, start-up quickly at low temperature, convenience in fuel handling and environmentally friendliness, etc. It has broad application prospects in the fields of portable devices and electric vehicles. As one of the essential components of DMFC, the flow field design has direct influences on the gas-liquid flow as well as performance of the DMFC. In this thesis, a novel DMFC anode flow field with superhydrophic micro channels for gas subduction has been designed and fabricated. The design was based on the principle of wetting characteristic and slipping effect of the superhydrophobic surface. Effect of the flow field design on gas subduction in DMFC anode flow field also has been studied.Superhydrophobic surfaces on stainless steel and aluminum alloy substrates were fabricated by using chemical etching and fluorination technique. The effects of etching time on surfaces wettability of fluorinated substrates were studied, and the result revealed that the optimal etching time of304stainless steel and6063aluminum alloy substrates are about15min and18min. The roughness of etched substrates was measured by step profiler, and the microstructures of etched substrates were observed by SEM. Both roughness and SEM images of substrates showed that the micro rough structures play important roles in surface wettability, and the etched surface which have good hierarchy and higher depth-to-width exhibited excellent superhydrophobicity after fluorination treatment.Fabrication technique of the anode flow field was also studied in this thesis. It is found that in photolithography the concentration of photoresist and the feature of substrate surface were essential to spin coating quality. Experimental results show that in order to get uniform coating surface, spinning coating with cyclohexanone diluted photoresist and surface-activated substrate should be used. Results also show that the prebake temperature and time have great impacts on the pattern quality after photoresist developed, it is recommended that the prebake temperature of95℃and prebake time of30～40min can be adopted to improve the pattern quality. Moreover, the effects of etching temperature and spraying pressure on etching rate and etching factor of the flow field substrate in a spray etching process were studied. Result indicated that both etching rate and etching factor increase along with the augment of temperature and pressure. However. the photoresist would fall off from the substrates easily if the temperature and pressure are too high. From experiments it was found that etching parts under the condition of temperature32℃and pressure0.7MPa. can not only guarantee the excellent quality (the etch factor is about1.59). but also can obtain higher etch rate (about4μm/min).Five flow field plates were fabricated by using spray etching and fluorination technique. Simulating cells were assembled with the five flow field plates. Then the relationship between the bubble shape and the width of superhydrophobic gas channel was studied. The impacts of reactant flow rate and concentration on gas subduction in DMFC anode flow field, which can be characterized by the bubbles behavior and inlet-outlet pressure drop of the flow field, were investigated by using a micro-fluid experimental setup. Results reveald that at the same condition the quantity and volume of bubbles in the flow fields with superydrophobic micro channels are all less than that in the reference flow field, the inlet-outlet pressure drop of the flow field elevates with the increase of flow rate and concentration. The pressure drops in novel designed flow fields are all lower than that of simulated cell with the reference flow field. The pressure drop in anode flow field with0.5mm wide superhydrophobic micro channels is lowest among the5flow fields.