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低温直接醇类燃料电池阳极催化剂研制

Research on the Anode Catalysts for Low-Temperature Direct Alcohol Fuel Cells

【作者】 周卫江

【导师】 辛勤; 孙公权; P.Tsiakaras;

【作者基本信息】 中国科学院研究生院(大连化学物理研究所) , 物理化学, 2003, 博士

【摘要】 阳极催化剂研制是直接甲醇燃料电池研究中最具有挑战性的任务之一,催化剂的制备方法对催化剂的性能有显著的影响。采用三种方法—浸渍—甲醛还原方法、过氧化氢氧化分解亚硫酸钠盐方法和本论文发明的多元醇或混合醇为溶剂、还原剂和保护剂的新方法,以碳载铂钌和铂为主,兼顾其他铂基催化剂的制备,考察了制备过程中主要参数对催化剂性能的影响,优化制备过程,筛选更合适的制备方法。通过比较发现,本论文发明的新方法具有操作灵活、制备过程简单易行、可制备的催化剂种类多、金属担载量高、金属平均粒径小且粒子大小可控等优点。其中,金属前体溶液浓度、载体的加入顺序、混合体系中水的含量及其加入顺序、还原过程中的pH值、金属载量等对催化剂的制备具有程度不等的影响。催化剂的平均粒径和各组分间的相互作用对甲醇的电化学氧化反应具有关键性的影响。在此基础上制备其他铂基催化剂,进而扩展了各类铂基催化剂在直接醇类燃料电池中的应用。同时本论文还考察了氧化钛、氧化钨和氧化钼等过渡金属氧化物的调变方式及处理条件对铂和铂钌催化剂的性能的影响。 在铂中添加钌或锡助剂均能促进甲醇和乙醇在铂催化剂上的电化学氧化反应,但催化效果却有着明显差别。铂钌催化剂更适合用作直接甲醇燃料电池的阳极催化剂,而采用铂锡阳极催化剂的直接乙醇燃料电池则展现出更好的放电性能,并且直接乙醇燃料电池在不同的操作温度下所需要的阳极催化剂中的铂锡原子比不同。即使仅在90℃时,采用铂锡阳极催化剂的直接醇类燃料电池无论使用甲醇还是乙醇作燃料其电池性能尤其是输出功率密度都极为接近。本论文对甲醇和乙醇反应机理以及影响因素也进行了探索和讨论。

【Abstract】 A significant enhancement of electrocatalytic activities for the 6-electron transfer electro-oxidation of methanol has been, and still is, thought as the most challenging problem for the development of direct methanol fuel cells (DMFCs). PtRu and other Pt-based catalysts are extensively investigated and employed presently as anode catalysts for methanol electro-oxidation. Their nature and structure, which are significantly influenced by catalyst preparation or/and treatment procedure, play a key role in the adsorption and electro-oxidation of methanol, and consequently the performances of DMFCs. In this dissertation, three methods were employed to prepare carbon supported Pt-based catalysts. They are the impregnation-reduction of metal precursors by formaldehyde, the oxidative decomposition of sodium sulphites of platinum or/and ruthenium by H2C>2 and the novel method originated in this dissertation, respectively. Ethylene glycol and other glycols were used as reducing agents, solvents and protective agents in the novel catalyst preparation procedure. Carbon supported Pt, PtRu and other catalysts synthesized by different methods were characterized by several technologies such as XRD, XPS, TEM and pulse titration. The parameters such as pH, temperature and so on were also investigated for catalyst preparation. The catalytic activities of these catalysts for methanol electro-oxidation and CO tolerance were compared. It was found that particle sizes, parameter lattice andinteraction between Pt and Ru play important roles in the electro-oxidation of methanol.Generally, particle sizes of Pt-based catalysts prepared by the method of impregnation-reduction of metal precursors by formaldehyde are relatively big and the interaction of Pt and Ru is also relatively weak. The catalyst types prepared by the second method are mainly restricted to Pt, Ru or PtRu. PtRu catalyst synthesized by the novel method, in comparison with its counterparts synthesized by other two methods, has smaller particle sizes and stronger interaction between Pt and Ru, and accordingly has improved electrocatalytic activity for methanol electro-oxidation. Consequently, the single direct methanol fuel cell employing the PtRu catalyst has demonstrated superior performances. The novel method, which is easy to control and operate, can be used to prepare various catalysts with nano-sized particle even in the presence of higher metal loadings. In addition, the particle size of catalysts can be controlled by the addition and sequence of water during this novel method.Titanium oxide, tungsten oxide and molybdena were used to modify Pt and PtRu catalysts in the present work. The modification mode and treatment condition have resulted in important effects on the activities of those catalysts toward methanol electro-oxidation and consequently the performances of single DMFCs with those catalysts. The addition of Tin to PtRu anode catalysts has showed no obvious improvement of DMFCs.It was also found hi this dissertation that the addition of either Ru or Sn to Pt improved its catalytic activity for electro-oxidation of methanol and ethanol, although the reactivities of methanol and ethanol on PtRu or PtSn catalysts were different from each other.As demonstrated in the previous work, PtRu was proved by methanol CV and single DMFC experiments here to be a better anode catalyst for DMFC in comparison to other binary Pt-based catalysts. On the other hand, direct ethanol fuel cells (DEFCs) demonstrated significant performances when PtSn was employed as anode catalyst The optimized atomic ratio of Pt to Sn has been found to vary with the operation temperatures of DEFCs. The direct alcohol fuel cells (DAFCs) employing PtSn as anode, whenever fueled by either methanol or ethanol, have demonstrated the similar cell performances, especially the maximum output power density, even at the relatively low temperature of 90癈. The electro-oxidation mechanisms of methanol and ethanol on PtRu and PtSn have also been primarily studied.

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