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首页> 外文学位 >Optimization of the cathode catalyst layer composition of a PEM fuel cell using a novel 2-step preparation method.
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Optimization of the cathode catalyst layer composition of a PEM fuel cell using a novel 2-step preparation method.

机译：使用新型的两步制备方法优化PEM燃料电池的阴极催化剂层组成。

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摘要

For good performance and high durability PEM fuel cells run at high water saturation levels. However, excess liquid water generated by the oxygen reduction reaction at the cathode can block pores in the catalyst layer so that reactant gases can't access the active catalyst sites. Thus, to prevent electrode flooding, the optimal catalyst layer structure has to provide channels for gas and liquid water transport, while maintaining high ionic and electronic conductivity at the same time. In detail the catalyst layer contained Nafion RTM as the ionic component to extend the three-dimensional reaction zone of the electrode, and needed TeflonRTM to provide continuous hydrophobic pathways for reactant gas transport. A simple intermixing process of the components doesn't allow optimal placement of Nafion RTM and TeflonRTM within the catalyst layer leading to coverage of active catalyst sites by TeflonRTM. By means of a two-step process the formation of the catalyst ink was separated into two parts. In the first step, a mixture of NafionRTM ionomer and catalyst particles was annealed to form ionomer coated catalyst particles. In the second step, these ionomer coated catalyst particles were mixed with nano-sized TeflonRTM particles and additional Nafion RTM ionomer, which was needed to crosslink the ionomer coated catalyst agglomerates. Since the catalyst particles have been covered by Nafion RTM ionomer before TeflonRTM was added, active sites were not blocked by TeflonRTM, which could be placed into void spaces between the catalyst clusters to form continuous hydrophobic pathways for gas transport. To determine the optimal composition for the catalyst ink in the two-step process, a matrix study with 16 different catalyst compositions was developed, and electrodes prepared from this matrix were tested in a fuel cell using the operating conditions chosen from this study. From this test two regions of catalyst composition that resulted in electrodes with good fuel cell performance were identified. The best performing fuel cells with peak powers of >0.5 W/cm2 were obtained with cathode catalyst layer with a composition of NafionRTM:TeflonRTM :C of 1.375:0.375:1 and 0.875:0.875:1, respectively. A comparison study of a two-step and one-step prepared catalyst was also done to characterize the effect of air flow rates with the different catalyst layer structures.;This catalyst composition study for the two-step process resulted in the following understandings. First, an adequate amount of NafionRTM is needed to provide ionic conduction within the catalyst layer and extend the 3-D reaction zone. Too little NafionRTM resulted in poor ionic conductivity and too much NafionRTM led to high liquid water entrapment, because of its high hydrophilicity, and poor oxygen diffusion. Second, an adequate amount of TeflonRTM was needed to provide continuous hydrophobic pathways within the catalyst layer for gas transport. The amount of TeflonRTM depended greatly on the Nafion RTM content, which determined the void volume available. While too little TeflonRTM didn't result in continuous hydrophobic pathways, too much TeflonRTM resulted in separation and isolation of reactive particles agglomerates and poor electronic conductivity. In general the two-step approach led to better performing catalyst layers which were less sensitive to liquid water flooding. This was even more evident at lower air flow rates where liquid water flooding is more severe. The better performance was attributed to the more ordered catalyst layer structure. Future work should confirm this finding in the whole composition range and have a closer look on the annealed catalyst particles and the final micro-structure of the catalyst layer.
机译：为了获得良好的性能和高耐久性，PEM燃料电池在高水饱和度下运行。但是，由阴极处的氧还原反应产生的过量液态水会堵塞催化剂层中的孔，从而使反应气体无法进入活性催化剂部位。因此，为了防止电极溢流，最佳的催化剂层结构必须提供用于气体和液体水传输的通道，同时保持高的离子和电子电导率。详细地，催化剂层包含Nafion RTM作为离子成分，以扩展电极的三维反应区，并且需要TeflonRTM提供连续的疏水性路径，以输送反应气体。组分的简单混合过程无法将Nafion RTM和TeflonRTM最佳地放置在催化剂层内，从而导致TeflonRTM覆盖活性催化剂部位。通过两步法将催化剂油墨的形成分为两部分。在第一步中，将NafionRTM离聚物和催化剂颗粒的混合物退火，以形成离聚物涂覆的催化剂颗粒。在第二步中，将这些离聚物涂覆的催化剂颗粒与纳米尺寸的TeflonRTM颗粒和另外的Nafion RTM离聚物混合，这是交联离聚物涂覆的催化剂团聚物所必需的。由于在添加TeflonRTM之前，催化剂颗粒已被Nafion RTM离聚物覆盖，因此活性位点不会被TeflonRTM阻断，可以将其放置在催化剂簇之间的空隙中，以形成连续的疏水性路径进行气体传输。为了确定两步法中催化剂墨水的最佳组成，开发了一种具有16种不同催化剂组成的基质研究，并使用选自该研究的工作条件在燃料电池中测试了由该基质制备的电极。从该测试中，鉴定出导致电极具有良好燃料电池性能的催化剂组合物的两个区域。在阴极催化剂层中，NafionRTM：TeflonRTM：C的组成分别为1.375：0.375：1和0.875：0.875：1，获得了性能最佳的峰值功率> 0.5 W / cm2的燃料电池。还对两步法和一步法制备的催化剂进行了比较研究，以表征空气流速对不同催化剂层结构的影响。这项针对两步法的催化剂组成研究得出以下理解。首先，需要足够量的NafionRTM以在催化剂层内提供离子传导并扩展3-D反应区。太少的NafionRTM会导致离子电导率变差，而太大的NafionRTM会导致高液态水截留，因为它具有很高的亲水性和较差的氧扩散性。其次，需要足够量的TeflonRTM才能在催化剂层内提供连续的疏水路径进行气体传输。 TeflonRTM的量在很大程度上取决于Nafion RTM的含量，后者决定了可用的空隙体积。尽管太少的TeflonRTM不会导致连续的疏水路径，但太多的TeflonRTM会导致反应性颗粒团聚物的分离和分离以及不良的电导率。通常，两步法可产生性能更好的催化剂层，这些催化剂层对液态水驱油的敏感性较低。这在空气流速较低的情况下更为明显，而液态水的泛滥更为严重。更好的性能归因于催化剂层结构更有序。未来的工作应在整个组成范围内确认这一发现，并仔细研究退火的催化剂颗粒和催化剂层的最终微观结构。

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