【作者】 蔡克迪；

【导师】 尹鸽平；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2008， 博士

【摘要】 直接二甲醚燃料电池(Direct dimethyl ether fuel cell, DDFC)是以二甲醚(Dimethyl ether, DME)为燃料的直接型燃料电池,最近几年开始受到关注。目前的研究主要集中在DME电氧化机理上,对DDFC膜电极(Membrane electrode assembly, MEA)的研究甚少。MEA是DDFC的核心部件,是化学能直接转化为电能的场所,其性能很大程度上决定了电池的性能。因此制备高性能的MEA,优化MEA的组成与结构就显得非常重要。本论文主要结合DME燃料的特性,针对DDFC的MEA进行研究,提高了电池性能,并设计研发了一种小型被动式DDFC电堆。详细研究了MEA的组成及结构对电极性能的影响。以Pt/C为阳极催化剂的MEA在高电流密度区表现出较好的性能;在低电流密度区,PtRu/C作阳极催化剂的电极性能较优。与Pt/Vulcan XC-72相比,Pt/MWNTs对DME显示出更高的催化活性。MEA阳极催化层中随着催化剂Pt载量的增加,电极性能先增大再有所减小,阳极扩散层中PTFE的最佳含量为20 mass %。MEA阴极催化层中Nafion含量为20 mass %,扩散层中PTFE含量为30 mass %时显示出较好的电极性能。Nafion膜厚度与电池开路电压成正比,采用Nafion112的MEA性能最差,采用Nafion115膜的MEA可以获得最大的电池功率密度(46 mW cm-2)。通过对DDFC性能的研究发现,当1.5 mol L-1DME水溶液以5 mL min-1阳极进料时显示出较好的电池性能,而当DME气体饱和加湿,流速为200 mL min-1时电池性能较好。与DME气体相比,以DME溶液为燃料的DDFC可以获得更好的电池性能和长时间放电稳定性。DDFC与DMFC相比较,DME透过Nafion115膜的渗透系数比甲醇低大约一个数量级,DME在Pt/C玻璃碳电极上的初始氧化电位较甲醇负移50 mV左右。在低电流密度区,DDFC的性能要优于DMFC;在高电流密度区,DMFC则表现出较好的性能。电池在80℃工作时,DDFC的最大功率密度为56 mW cm-2,DMFC的最大功率密度为81 mW cm-2,DDFC的最大功率密度约为DMFC的69 %。针对DME在水中的溶解度随温度升高而下降的特性,研究了一种用于DDFC的MEA新结构,其阳极扩散层在同一平面上分成了亲水与憎水两个区域。电池在50℃运行时,新型MEA表现出较优的电极性能,与亲水和憎水MEA相比较,新型MEA具有较小的传质阻抗和长时间运行性能衰减率。通过改变新型MEA阳极扩散层中亲、憎水区域的面积比可调节DDFC在不同工作温度下的电极性能。低温时,阳极扩散层中亲水区面积较大的MEA性能较优;而高温时,阳极扩散层中憎水区面积较大的MEA可以获得更好的电极性能。设计、研究了一种小型被动式DDFC电堆。各单体电池性能均一性较好,电堆的开路电压在4 V左右,最大功率为300 mW。6单体电池可以按照不同的方式进行电连接,改变的只是电流与电压值,电堆的总输出功率基本不变。电堆在恒、变电流下均可稳定运行。电堆室温21℃启动,在恒电流100 mA运行过程中,温度逐渐上升,大约60 min后温度逐渐稳定在37℃。电堆在恒电流300 mA下运行时,初始电压为1.4 V,经过1100 min放电后电压降为0,电堆的燃料利用率为57 %。更多还原

【Abstract】 Direct dimethyl ether fuel cell (DDFC) is a direct type fuel cell using dimethyl ether (DME) as fuel. DDFC has been recently studied. Up to now, a lot of work was focused on electro-oxidation of DME. The membrane electrode assembly (MEA) has not been studied in detail. MEA is the key component of the DDFC, and the chemical energy is directly converted into electrical energy on the MEA. The performance of the DDFC is greatly dependent on the performance of MEA. In this paper, the MEA of the DDFC was investigated, and the performance of the MEA was increased. A small passive DDFC stack was fabricated for room temperature applications.We investigated the effects of compositions and structure of MEA on the cell performance. The MEA with Pt/C as the anode catalyst showed higher performance than that with PtRu/C at the low cell voltage regions, but at the high cell voltage regions, the PtRu/C yielded better performance than Pt/C. The Pt/MWNTs showed a higher catalytic activity for dimethyl ether electro-oxidation, compared with the Pt/Vulcan XC-72 catalyst. The performances of the MEAs was improved with the increase of Pt loading up to 3.6 mg cm-2, and then decreased with further increase of Pt loading. And 20 mass % PTFE content and 1 mg cm-2 carbon black loading were optimal compositions in the anode diffusion layer. In the cathode, MEA with 20 mass % Nafion content in the catalyst layer and 30 mass % PTFE content in the diffusion layer presented the better performance. The (Open circuit voltage, OCV) of the cell reduced with the decrease of the thickness of the membrane. The performance of the MEA with Nafion 112 membrane is the worst. The MEA with Nafion 115 membrane displayed the highest maximum power density of 46 mW cm-2 among the three MEAs with different thickness of Nafion membranes.The DDFC with 1.5 mol L-1 and 5 mL min-1 DME solution showed the best performance. The DDFC with full humidification and 5 mL min-1 DME gas presented higher performance. Compared with DME gas, the DDFC with DME solution showed higher power density and better long-term operation performance. The fuel crossover of DME in the Nafion 115 membrane was expected to be smaller than that of methanol. The DDFC showed higher performance than DMFC at the low current density regions, but at the high current density regions, the DMFC exhibited better results than DDFC. The maximum power density of the DDFC was 69 % of the DMFC.The solubility of DME in water decrease with the increase of the temperature. We presented the novel MEA for DDFC. The anode diffusion layer of the MEA consisted of hydrophilic region and hydrophobic region. The performance of novel MEA for DDFC was enhanced due to the promotion the mass transport of DME fuel at 50℃. The electrochemical impedance spectra analyses revealed that the mass transfer resistance of novel MEA was lower than that of completed hydrophilic or hydrophobic MEA. The constant current operation curves showed that the degradation rate of the novel MEA was lower than that of conventional MEAs. It indicated that the novel MEA benefited the long-term operation of DDFC. At low temperature, the novel MEA with larger hydrophilic region showed excellent performance. The performance of novel MEA with larger hydrophobic region was better at high temperature.A small passive DDFC stack consisting of 6 single cells was fabricated for room temperature applications. The performance of each single cell is uniform. The OCV of the stack is around 4 V, with a maximum power of 300 mW. The serial connection was benificial to the OCV of the stack, and the parallel connection was in favor of the discharge current. The total power density was independent to the connection pattern. The stack operated stably at either constant or varied currents. When the stack operated at a constant current of 100 mA at 21℃, the temperature gradually rose to 37℃after 60 min and remained at this temperature. The initial voltage was 1.4 V at a constant current of 300 mA. After 1100 min operation, the voltage dropped to 0 V and the fuel utilization was 57 %.更多还原