【作者】 叶丁丁；

【导师】 廖强； 李俊；

【作者基本信息】 重庆大学 ， 工程热物理， 2009， 博士

【摘要】 传统的能量转化是通过热机过程来实现的,热机过程受卡诺循环的限制,不但转化效率低、导致严重的能源浪费,而且产生大量的有害物质以及噪声,造成严重的环境污染问题。而燃料电池发电技术与热机发电过程不同,它是一种直接将储存于燃料或氧化剂中的化学能直接转化为电能的能量转换装置,发电过程步骤少、没有燃烧过程及转动部件,不受卡诺循环的限制,是一种清洁高效的发电技术。近年来,随着移动电话、个人数字助手、笔记本电脑等便携式电子产品的迅猛发展,而且产品新的耗电型功能也不断增强,目前现有的充电电池技术已无法满足日益增长的高能耗需求。空气自呼吸式直接甲醇燃料电池(DMFC)因具有系统结构简单、能量密度高、环境友好、更换燃料方便、可在常温下工作等优点,成为便携式设备最有前景的可替代电源,是电化学和能源科学领域的研究热点。与主动式DMFC相比,空气自呼吸式DMFC反应物传递速率较慢,导致电化学反应速率较低,电池结构上的差异不但使得性能远不及主动式的DMFC,也造成了电池内部的热质传输特性有所不同。因此,对空气自呼吸式DMFC内两相流动及传输特性进行研究具有非常重要的意义。本文从工程热物理学科角度出发,采用自制的膜电极组件对空气自呼吸式DMFC性能及传输特性进行实验研究,包括电池的环境适应性实验,阴极侧水滴积聚特性及其对放电性能影响的可视化实验,电池在搁置、恒电流放电及放电结束后电压稳定整个测试过程中电池的温度特性实验等,进一步深入研究了阳极微孔层的浸润性对电池性能及放电时间的影响;针对阳极侧主动进料的空气自呼吸式DMFC,考虑了扩散层/催化层界面处液相饱和度的不连续性及催化层内的质量传递,建立了一个全面描述电池内两相流动及传质的数学模型,分析甲醇及水渗透的机理。主要研究成果如下:1)对组装的电池采取加入甲醇溶液搁置24 h活化的方法,电池性能提高到活化前的4倍;提出了一种更为简便且高重现性的微孔层制备方法—抽滤法,制得的微孔层更为平整,可获得较高的性能和较长的恒电流放电持续时间。2)研究了催化剂类型、扩散层材料、甲醇溶液浓度、集流板结构等因素对电池性能及传质的影响,实验结果表明:采用高担量的催化剂或碳布制作的膜电极可以强化反应物的传质且易于获得较高的电池性能;适当提高甲醇浓度、采用平行集流板可获得较高的电池性能。3)对环境温度及相对湿度对电池性能的影响进行了研究,实验结果表明:在相同湿度下,空气自呼吸式DMFC电池性能随着环境温度的升高而增加;在相同温度下,环境相对湿度对电池性能的影响与环境温度的大小有关,温度较低时,湿度的影响不大,随着温度的升高,其影响也越来越大。4)研究了恒电流放电过程中阴极侧的水滴积聚特性,实验结果表明:液滴总在某些特定的位置出现,被水完全覆盖的呼吸孔中液体还会不断增多;在低电流密度下,电池恒流放电时间主要受阴极水积聚的影响;在高电流密度下,甲醇的消耗为控制电池放电时间的主要因素;适当提高甲醇浓度、降低放电电流密度及环境湿度均可延长放电持续时间。5)对放电前后及放电期间整个测试过程中电池的温度特性进行了实验研究,结果表明:加入燃料后,电池与环境之间的温差迅速上升,升高的速度渐缓,最后达到稳定,放电期间,温差先升高后降低,且温差随着放电电流密度的增大而增大,但在大放电电流密度下,因放电时间较短未出现下降的趋势,放电结束后,温差骤降;此外,在整个测试过程中温差随甲醇浓度的增大而增大。6)对采用不同浸润性的阳极微孔层时的电池性能及放电持续时间进行比较,发现:在快速扫描模式下,阳极采用亲水的微孔层制备的电池性能优于憎水的微孔层,但在慢速扫描模式下,中高电流密度时,由于阴极侧水滴的积聚,阻碍了氧气的传递,阳极采用憎水的微孔层可获得较好的性能;在恒电流放电初期,阳极采用亲水的微孔层制备的电池性能较好,但随着放电的进行,因采用憎水的阳极微孔层不易发生阴极水淹,电池性能更佳且放电持续时间较长。7)建立了空气自呼吸式DMFC二维两相传质模型,将计算结果与实验数据进行了比较,二者基本吻合。结果表明:在相同电流密度下,甲醇渗透通量随甲醇浓度的增加而增加;在浓度为4M和2M时甲醇渗透通量随电流密度的增加而增加,而在1M时随着电流密度的增加先增加后减少。低电流密度下,扩散在甲醇渗透中起主导作用,而高电流密度下电渗是甲醇渗透最主要的传输方式;电渗和压差在低电流密度时对于水的渗透都起了重要的作用,而在高电流密度时电渗作用占了主导地位。阴极侧总的水通量随着电流密度的增加明显增加,且在低电流密度时,主要来源于渗透的甲醇发生氧化反应生成的水,而在高电流密度下,从阳极渗透到阴极的水占了绝大部分。更多还原

【Abstract】 The traditional energy conversion process is usually carried out by the heat engines, which is limited by the Carnot cycle leading to low efficiency and serious energy waste. Meanwhile, severe environmental pollution problem arises from the generated deleterious species and noise during the process. However, unlike the power generation process of heat engines, fuel cells are energy conversion devices in which the chemical energy is directly converted into electricity without the intermediate of thermal energy. They are regarded as clean power generation technique with high efficiency due to less energy conversion steps, no combustion process and rotated components, and without the limitation of Carnot cycle. In recent years, with the rapid development of electric devices such as mobile phones, personal digital assistants and laptop computers, which demand much more power due to new functions, the present battery technology is unlikely to keep pace with these growing power demands. The air-breathing direct methanol fuel cell (DMFC) which are considered as an attractive alternative to the conventional power sources for portable devices because of its simple system, high energy density, environmental friendly emissions, fast refueling and low operating temperature, are becoming a research hotspot in the field of electrochemistry and energy science.Compared with the active DMFC, the electrochemical reaction rate of the air-breathing DMFC is lower resulted from the slow mass transport of reactants. The different structure from active ones causes not only the poorer performance but also the various heat and mass transport characteristics. Thus, it is necessary to investigate the two-phase flow and transport characteristics inside the air-breathing DMFC.The cell performance and mass transport characteristics of an air-breathing DMFC were investigated with a home-made membrane electrode assembly (MEA). The effect of ambient conditions on the cell performance was discussed experimentally. The water droplets accumulation in the cathode and its effect on discharging performance were studied with the aid of visualization technique. The operating temperature characteristics in the whole test including before, during, and after the discharging process were recorded in the present study. The effect of wettability of anode microporous layer (MPL) on the performance and operation duration was also tested. A mathematical model was developed to comprehensively describe the two-phase flow and mass transport and analyze the mechanism of methanol and water crossover inside an air-breathing DMFC with active fuel supply. The mass transport in the catalyst layer and the discontinuity in liquid saturation at the interface between the diffusion layer and the catalyst layer are particularly considered. The main results are summarized below:1) The performance of air-breathing DMFC was improved to four times after being activated by adding methanol solution into reservoir and then resting for 24 h. A simpler filtration method with high reproducibility to fabricate MPL was proposed, through which more flat MPL, higher cell performance and longer operation duration could be obtained.2) The effects of catalyst, diffusion layer material, methanol concentration and current collector structure on the cell performance and mass transfer characteristics were discussed. The experimental results show that the mass transfer of reactants and the cell performance are enhanced by using catalyst with high noble metal loading or carbon cloth in the MEA. The better performance can also be obtained by increasing methanol concentration properly or testing with parallel current collectors.3) The influences of ambient temperature and relative humidity on the cell performance were investigated. The results reveal that at the same relative humidity, the performance of air-breathing DMFC increases with the increasing of ambient temperature. At the same temperature, the effect of relative humidity on the cell performance is related to the value of the ambient temperature. There is no evident effect of relative humidity observed at a low temperature. However, with the increasing of temperature, the relative humidity plays a more significant role in the cell performance.4) The water droplets accumulation in the cathode was visualized with a digital camera during the constant current discharge test. It is observed that water droplets always emerge at some preferential locations and the amount of water still increases in the air-breathing holes fully covered by liquid droplets. The discharging time at a low current density is dominated by the water droplets accumulation at the cathode, while the consumption of methanol is the dominator for that at a high current density. The longer discharging time can be obtained by improving methanol concentration properly, reducing the discharging current density and the relative humidity.5) The experiments on the operating temperature characteristics during the whole test process were conducted. It is found that after injecting fuel into the cell, the temperature difference between the cell and ambient increases rapidly in a few minutes, but after a mild increase it tends to be a constant value. During the constant current discharge, the temperature difference rises at first and then goes down continuously, and it increases with the increasing of discharging current density. However, at a high discharging current density, the temperature difference goes up at all times due to short discharging time. Once the constant current discharge terminated, the temperature difference drops significantly. In addition, a higher methanol concentration leads to a higher the temperature difference due to a larger methanol crossover rate.6) Comparative studies on cell performance and operation duration of air-breathing DMFCs with different anode GDLs were performed experimentally. The results demonstrate that when the performance evaluation was conducted in the fast scan mode (a holding time of 45 s at each current density), the DMFC with a hydrophilic MPL (DMFC-L) in the anode shows superior performance to that with a hydrophobic MPL (DMFC-B). While the DMFC-B shows better performance at medium and high current densities in the slow scan mode (a holding time of 150 s), which results principally from blockage of the oxygen supply due to the water droplets accumulated during the cell performance evaluation. Although the DMFC-L can yield a better performance at the beginning of constant current discharge, the DMFC-B exhibits a higher performance and longer operation duration in the following discharge process due to a lower rate of water accumulation with the hydrophobic MPL in the anode.7) A two-dimensional two-phase mass transport model was developed to predict methanol and water crossover in an air-breathing DMFC with active fuel supply. The modeling results agree well with the experimental data of a home-assembled cell. The numerical results indicate that for a given current density, the methanol crossover flux increases with increasing methanol concentration; for a given methanol concentration, it increases with increasing current density for the methanol concentrations of 2 M and 4 M, while it increases slightly at low current densities and decreases at high current densities for 1 M. Diffusion predominates the methanol crossover at low current densities, while electro-osmosis is the dominator at high current densities. Water transport through the membrane depends on electro-osmosis and hydraulic pressure difference across the membrane at low current densities; however, electro-osmosis plays a critical role in the water crossover at high current densities. The total water flux at the cathode is originated primarily from the water generated by the oxidation reaction of the permeated methanol at low current densities, while the water crossover flux is the main source of the total water flux at high current densities.更多还原