【作者】 朱小伟；

【导师】 廖强；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2008， 硕士

【摘要】 随着世界人口的迅速增长和人均能源消耗量的持续增大,全球性能源短缺问题日趋突出。与此同时,日益严重的环境污染问题也成为倍受人们关注的焦点。如果没有新型能源动力,世界将从目前的能源短缺很快走向能源枯竭,因此发展清洁、高效的新能源动力技术是刻不容缓的。燃料电池以其高效和清洁的特点为人们带来了曙光,其中直接甲醇燃料电池(DMFC)作为新型的便携能源,具有结构紧凑、环境友好、方便持久、能量密度高等优点,是一种具有广阔市场应用前景的高新技术,正成为电化学和能源科学里的一个研究热点。被动式自呼吸DMFC阴极不需要外部辅助设备(如泵或风扇)的帮助,就可以从氧气中获得氧化剂;阳极甲醇存储在紧贴在阳极基板的燃料罐内,通过浓度梯度可以到达阳极催化层表面。被动式自呼吸DMFC以其高的可靠性、低成本、简单的结构、高的燃料利用率以及高的能量密度等特点,成为未来移动设备首选的电源。本课题利用亚克力制作电池夹具和燃料罐,利用碳纸、Nafion 117等制备MEA,利用不锈钢材料加工流场板,组装完成有效面积为4.41 cm2的被动式自呼吸DMFC,并对该电池进行了性能研究、可视化实验和温度特性研究。探讨了催化剂种类、Nafion膜材料、扩散层材料、甲醇浓度、环境温度、环境湿度等因素对于电池性能的影响;可视化研究了放电电流、环境温度、环境湿度等参数对于阴极液滴生成规律的影响;讨论了甲醇浓度、放电电流对于电池温度特性的影响,并研究了该电池的能量效率特性。主要研究成果如下:①对电池进行了一系列的性能实验并对电池进行优化,发现在制作MEA催化层时,碳黑担载量为15wt %时电池性能较好;由于浓差极化和甲醇渗透的共同作用,随着甲醇浓度的增加,电池功率密度先上升后下降,在甲醇浓度为4 mol/L时电池性能最佳;与碳纸相比,碳布孔隙分布较均匀,喷涂扩散层和催化层时物质分布更加均匀,传质阻力较小,电池性能较好;Nafion膜越薄,甲醇渗透就越严重,当膜薄到一定程度时,甲醇渗透致使电池性能急剧下降;②周围环境相对湿度对于电池性能的影响取决于环境温度:当环境温度低于10℃时,相对湿度对于燃料电池性能影响很小;当环境温度为20℃和35℃时,可以发现,在低电流区域和中电流区域,湿度对于燃料电池的性能影响也不大;而在高电流区域,燃料电池的性能随着相对湿度的增加而变差;当环境温度为50℃时,在低电流区域电池的性能随着相对湿度的增加而变好,在高电流区域,燃料电池的性能随着湿度的增加而变差;③随着周围环境温度的升高,燃料电池的性能明显提高;④随着放电的进行,阴极侧液体分布从分散的小液滴到聚合形成大液滴,直到形成液膜最后覆盖整个呼吸孔,造成严重的阴极水淹现象;同一放电时刻,随着电流密度的增大,液体覆盖率增大;温度一定时,湿度越大越易引起阴极水淹;湿度一定时,温度越低越易引起阴极水淹;⑤注入燃料后,电池电压迅速上升,然后缓慢下降,最后又略有回升;而电池温度先迅速上升,升高速度渐缓,最后达到稳定;放电开始瞬间,电池电压突降,随着放电的进行缓慢下降,电池温度先升高后下降;电池温度随着放电电流密度和甲醇浓度的增大而变大;⑥随着温度的提高,电池能量效率显著提高;随着相对湿度、甲醇浓度和放电电流的增加,电池能量效率显著降低。更多还原

【Abstract】 With the rapid growth of world population and continued increase of per capita energy consumption, the problem of global energy shortage is getting increasingly pronounced. Meanwhile, the increasingly serious problem of environmental pollution has become the focus of attention. If there were no new power sources, the existing power sources would be exhausted. Therefore it is urgent to develop clean and efficient energy technology. It is Fuel cell that brings the dawn for people because its efficiency and cleanliness. Direct methanol fuel cell (DMFC) is one of the high technologies with broad market prospect and now becoming a research hotspot in electrochemical science and energy field due to its high energy density, long-life and no need for charging. The passive air-breathing DMFC operates without the help of external devices for pumping methanol and blowing air into the cell, and thus oxygen diffuses into the cathode from the ambient air, and methanol diffuses into the anode from a built-in feed reservoir driven by concentration gradient between the anode and the reservoir. The passive air-breathing DMFC becomes the preferred power for mobile devices because its high reliability, low cost, simple structure, high fuel efficiency and high energy density.In the present study, a passive air-breathing direct methanol fuel cell with an active area of 4.41 cm2 and a built-in methanol solution reservoir of 6.7 ml was used to investigate the cell performance, visualization study of cathodic flooding and temperature characteristics. The effects of catalyzer, Nafion membrane, gas diffusion layer material, methanol concentration, relative humidity (RH) and temperature of ambient atmosphere on cell performance were discussed. The distribution of water droplets at different discharging current densities, relative humidities and temperatures were visualized and discussed. The effects of methanol concentration and discharging current density on the cell temperature characteristics were also discussed. Some remarkable conclusions are drawn as follows:①A series of experiments were carried out to optimize the cell performance. It was found that in the production of MEA catalyst layer, a better cell performance was gained when the amount of carbon was 15 wt%. The performance becomes better with higher concentrations, which can be attributed to the higher temperature caused by the exothermic reaction between the permeated methanol and oxygen on the cathode. However, when the concentration was too high, methanol crossover would cause excessive mixed potential and result in the reduction of cell performance. A maximum power density of 12.5 mW/cm2 was obtained with 4.0 M methanol solution. As to different GDL material, carbon cloth got a better cell performance because the pores distribution of carbon cloth was more uniform, resulting in more evenly distributed catalytic layer and smaller mass transfer resistance. Methanol crossover was more serious when the thin membrane was used, which caused a sharp decline in cell performance. So a thicker membrane was recommended for passive DMFC that operate with high methanol concentrations.②It was found that the effects of RH on cell performance were dependent upon temperature. When the temperature is below 10 oC, RH shows no effect on the cell performance. At the temperature of 20 oC and 35 oC, RH also shows little influence on the cell performance at low to medium current densities, and the cell performance decreases with the increase of RH at high current densities. In the case of 50 oC, the increasing RH has a greater effect on the cell performance than those at lower temperature: the cell performance is improved at low to medium current densities, whereas it is depraved at high current densities.③It was found that the cell performance improved with the increase of ambient temperature.④Some dispersed liquid droplets appeared at the beginning of constant current discharge, and then grew big enough to coalesce. The liquid film formed and covered the whole air-breathing hole at the end of discharge. The cell voltage dropped evidently when the serious water flooding happened in the cathode. The cover ratio of liquid in the cathode increased with the increase of discharging current density, and it increased sharply at the beginning of discharge. A higher relative humidity and lower ambient temperature led to more serious water flooding in the cathode.⑤The open circuit voltage rise rapidly after the fuel injection, then declined slowly, and a slight rebound in the final, while the cell temperature increased because of methanol crossover. After discharge started the cell voltage dump firstly, and then declined slowly with the discharge. The cell temperature increased at first and then decreased continuously, and it dropped evidently after constant current discharge. The cell temperature increased with an increase in discharging current density. A higher methanol concentration led to a higher temperature due to much more permeated methanol.⑥The cell energy efficiency improved significantly with the increase of ambient temperature, and reduced with the increase of relative humidity, methanol concentration and discharge current density.更多还原