【作者】 高海丽；

【导师】 廖世军；

【作者基本信息】 华南理工大学 ， 应用化学， 2011， 博士

【摘要】 燃料电池以其高效率、高能量密度、零排放、快速启动等优点,被认为是21世纪最有发展前景的高效清洁发电技术。低温燃料电池包括质子交换膜燃料电池、直接甲醇燃料电池、直接乙醇燃料电池和直接甲酸燃料电池等。电催化剂是低温燃料电池的最为重要的关键材料之一,目前常用的基于铂的贵金属催化剂存在成本高昂、以及铂资源限制等问题,已成为制约燃料电池技术的发展和商业化进程的重要因素。因此,研究和开发非铂及低铂催化剂已成为燃料电池领域的热点研究课题,其中以降低铂使用量为目标的低铂催化剂具有十分明朗的应用前景,研究和开发新型低铂催化剂,对于有效降低铂的使用量、降低燃料电池成本、促进燃料电池技术的发展具有十分重要的意义。本文以设计和制备高性能低铂催化剂为目标,设计和制备了系列以具有良好的化学稳定性和电化学稳定性的廉价金属钌为核,以铂和铂的合金为壳的核壳结构低铂催化剂,并对这些催化剂在低温燃料电池阳极和阴极中电催化性能进行了深入的研究。首先,制备了高活性的低铂核壳结构Ru@Pt/C催化剂。采用两步浸渍-还原法制备了Pt含量为5 wt.%的核壳结构催化剂Ru@Pt/C,对于甲醇的阳极氧化反应,该催化剂的单位质量铂的催化活性分别为Pt/C,PtRu/C和商业JM PtRu/C催化剂的1.9、1.5和1.4倍。尤为重要的是该催化剂对甲醇氧化中间体具有很好的去除能力,其循环伏安正向扫描的氧化峰的峰电流密度（If）与反向扫描氧化峰的峰电流密度（Ib）之比可高达2.4,为Pt/C催化剂的I_f/I_b的2.7倍。另外,Ru@Pt/C催化剂的稳定性也高于Pt/C,自制PtRu/C和商业JM PtRu/C催化剂。其次,考察了铂的覆盖度（Pt:Ru摩尔比）对Ru@Pt/C催化剂结构、甲醇氧化和氧还原（oxygen reduction reaction,ORR）性能的影响,研究了壳层原子与核之间的相互作用。利用两步还原法合成了一系列不同铂覆盖度（Pt:Ru比）的核壳结构催化剂Ru@Pt/C,催化剂对于甲醇的阳极氧化和氧还原反应活性先随Pt:Ru摩尔比的增加而增加后随Pt:Ru摩尔比的增加而降低,在Pt:Ru比例为0.42:1时,催化剂具有最好的催化性能。Pt:Ru摩尔比从0.13:1增加到0.81:1,If:Ib比值从5.8下降到0.8。当Pt:Ru比例为0.42:1时,I_f/I_b和Pt/C的相当;继续增加Pt:Ru比,I_f/I_b比值几乎不变。Ru@Pt/C（0.42:1）对甲醇氧化的单位质量Pt的催化活性是Pt/C的3倍。有甲醇存在时,Ru@Pt/C催化剂表现出良好的抗甲醇能力。第三,制备了具有合金壳层的核壳结构Ru@Pt_xPd_y/C催化剂,考察了壳层中不同Pt:Pd原子比对催化剂在甲酸氧化中催化性能的影响。Ru@Pt_xPd_y/C催化剂对甲酸的催化活性随壳层中Pt:Pd原子比而变化。和Pd/C催化剂相比,Ru@Pt₁Pd₂/C对甲酸氧化的峰电位负移了约200 mV。Ru@Pt₂Pd₁/C催化剂对甲酸的催化氧化性能是Pt₂Pd₁/C的3.5倍,表明了其贵金属利用率的提高。微型直接甲酸燃料电池单电池测试结果显示,在0.5 V处,以Ru@Pt₂Pd₁/C做阳极催化剂的单电池电流密度达到7.5 mA·cm^-2,分别是Pt₂Pd₁/C和JM Pt/C做阳极催化剂的单电池电流密度的1.3和3.5倍。这些结果表明以Ru@Pt2Pd1/C做催化剂的单电池比以Pt₂Pd₁/C和JM Pt/C做催化剂的单电池展现了更好的电池性能。这种提高可能是壳与核协同作用的结果。第四,以Ru作为核以PtPd或PtIr合金作为壳,制备了Ru@PtPd/C和Ru@PtIr/C催化剂,考察了两种催化剂在碱性介质中对乙醇氧化催化性能的影响。Ru@PtPd/C（Pt:Pd原子比1:0.2）对乙醇氧化的单位质量PtPd的催化活性分别为PtPd/C,PtRu/C,Pd/C和Pt/C的1.3,3,1.4和2倍。Ru@PtIr/C催化剂对乙醇氧化的催化活性分别是Pt/C和合金PtRu/C催化剂的1.8和3.0倍,另外其If/Ib值高达2.4,分别是Pt/C和合金PtRu/C的2.4倍和2.0倍,表明Ru@PtIr/C催化剂具有高的催化活性和高的抗乙醇氧化中间体（CO）的中毒能力。核壳结构Ru@PtIr/C和Ru@PtPd/C催化剂的稳定性也高于Pt/C。第五,制备了一系列不同Pt:Se摩尔比的Pt@Se/C氧还原催化剂。循环伏安研究结果表明,Pt@Se/C催化剂的Pt氧化峰消失;随催化剂中Se含量增加,H吸、脱附峰越来越不明显,Se的氧化峰越来越强,表明Se在Pt的表面沉积,说明Se的加入使金属Pt的表面特征发生了改变,抑制了对H的吸附。Pt@Se/C催化剂对ORR的活性随Pt:Se摩尔比的减少呈先增加后降低的趋势,Pt:Se摩尔比为0.63:0.37时,Pt@Se/C对ORR的起始电位和半波电位分别比Pt/C催化剂正移了20和41 mV。Pt@Se/C催化剂与Pt/C催化剂相比具有非常高的抗甲醇中毒性能。第六,制备了RuFeSe@Pt/C催化剂。核组分中,Ru,Fe和Se形成合金。循环伏安研究表明,在RuFeSe/C上负载Pt以后,Se的氧化峰消失,间接证明了Pt在RuFeSe表面的还原沉积,形成核壳结构。RuFeSe@Pt/C对ORR半波电位比Pt/C和Ru@Pt/C分别正移了61 mV和46 mV。当在有甲醇存在的条件下,它的催化活性要比Pt/C催化剂好。RuFeSe@Pt/C催化剂与Pt/C催化剂相比具有非常高的抗甲醇性能。RuFeSe@Pt/C催化剂催化氧还原的机理主要是以4电子反应机理进行,反应中主要生成产物为水。更多还原

【Abstract】 Low temperature fuel cells have been acknowledged as the most promising clean power technology in 21st century due to their advantages, such as high efficiency, high power density, zero or low exhaust, and quick startup at low temperature, etc. Low temperature fuel cells include hydrogen/oxygen proton exchange membrane fuel cell （PEMFC）, direct alcohol fuel cell （DAFC）, as well as direct formic acid fuel cell （DFAFC）. Electrocatalyst is one of the most important materials for low temperature fuel cells. Up to now, platinum is still the widely used active component for fuel cell electrocatalyst, the limited resource and high price of platinum is becoming into the most important factor that restricts the development and commercialization of low temperature fuel cells. Thus, research and development of low Pt and non-Pt catalysts have become the hot research topic in the fuel cell field, among which low Pt catalyst with the aim of reducing Pt loading attracted more attention due to its application prospect in near future. Consequently, research and development of novel low Pt catalyst with high rare metal utilization and low Pt content are of great importance to the cost reduction of fuel cell and development of fuel cell technology.In this thesis, a series of core-shell structured low Pt catalysts, with relative inexpensive Ru as core which is chemically and electrochemically stable and with Pt or Pt base alloy as shell, were designed and prepared, and the performances of the core-shell catalysts used for the anode and cathode of low temperature fuel cells were investigated intensively.Firstly, a highly active core-shell structured low platinum Ru@Pt/C catalyst was prepared using a two-stage impregnation-reduction method. It was found that the mass catalyst activity in terms of the Pt load was 1.9 and 1.5 times as high as that of Pt/C and alloy PtRu/C catalysts towards the anodic oxidation of methanol, respectively, and it was also much higher than that of the commercial JM PtRu/C catalyst. It is important that the ratio of forward peak current density （I_f） to backward peak current density （I_b） reached 2.4, which is 2.7 times higher than that of Pt/C catalyst, implying that the Pt-decorated Ru/C catalyst possessed high tolerance towards intermediate poisoning species. In addition, the stability of Ru@Pt/C was higher than that of Pt/C, alloy PtRu/C and JM PtRu/C catalysts.Secondly, effects of Pt coverage （Pt:Ru atomic ratio） on the catalyst structure and performance for the methanol oxidation and oxygen reduction reaction （ORR） were investigated, and the interaction between shell atoms and core was studied. A series of Ru@Pt/C catalysts with different atomic ratios of Pt to Ru were prepared. The activity of methanol oxidation and ORR on Ru@Pt/C was firstly increased with the increacement of Pt:Ru ratio and then decreased. The highest activity was reached when the ratio of Pt to Ru was 0.42:1. I_f/I_b is decreased from 5.8 to 0.8 when the Pt:Ru ratio increases from 0.13:1 to 0.81:1. When the ratio of Pt to Ru is 0.42:1, I_f/I_b is close to that of Pt/C, and further increase of the Pt:Ru ratio leads to almost no decrease in I_f/I_b. The activity of methanol oxidation on Ru@Pt/C （0.42:1） was 3 times higher than that on Pt/C. The methanol tolerance of Ru@Pt/C is higher than that of Pt/C. With the presence of methanol, Ru@Pt/C showed high methanol tolernace towards oxygen reduction reaction.Thirdly, core-shell structured Ru@Pt_xPd_y/C catalysts with Pt_xPd_y alloys as shell and nano-sized Ru as core were prepared by a successive reduction procedure. Influence of the atomic ratio of Pt to Pd in the shell on the performance for the formic acid oxidation reaction was studied. It was found that the activity of Ru@Pt_xPd_y/C catalysts is varied with the variation of the atomic ratio of Pt to Pd. The peak potential of formic acid oxidation on Ru@Pt1Pd2/C is shifted negatively for about 200 mV compared with that of Pd/C. The activity of formic acid oxidation on Ru@Pt2Pd1/C was 3.5 times higher than that on Pt₂Pd₁/C, indicating the higher utilization of noble metals. The test of micro direct formic acid fuel cell （DFAFC） showed that the current density of the MEA with Ru@Pt₂Pd₁/C as the anode catalyst is 7.5 mA·cm^-2 at 0.5 V, over 30% higher than that of the MEA prepared with Pt₂Pd₁/C and 3.5 times higher than that of the MEA prepared with JM Pt/C. The micro DFAFC test showed that the Ru@Pt₂Pd₁/C could significantly improve the catalytic activity of Pt in formic acid oxidation reaction. The improvement of the activity is ascribed to the interaction between shell and core.Fourthly, core-shell structured Ru@PtPd/C and Ru@PtIr/C catalysts were prepared by using Ru as core and PtPd or PtIr alloy as shell and the anodic oxidation of ethanol on these core-shell structured catalysts in alkaline media was studied. The activity of ethanol oxidation on Ru@PtPd/C （Pt:Pd =1:0.2） in terms of PtPd loading is 1.3, 3, 1.4, and 2.0 times as high as that on PtPd/C, PtRu/C, Pd/C, and Pt/C respectively, indicating high utilization of Pt and Pd. The activity of Ru@PtIr/C is 1.8 and 3.0 times as those of Pt/C and PtRu/C respectively. And the I_f/I_b is as high as 2.4, which is 2.4 and 2.0 times as those of Pt/C and PtRu/C catalysts respectively, revealing the high activity and high poisoning tolerance. In addition, the stability of Ru@PtPd/C and Ru@PtIr/C is higher than that of Pt/C.Fifthly, a series of Pt@Se/C catalysts with different Pt:Se atomic ratios were prepared by a two-stage procedure for ORR. The cyclic voltammograms results showed that the peak corresponding to oxidation of platinum disappeared; as the selenium content increased, the peak intensity corresponding to oxidation of selenium increased but the peak of hydrogen desorption decreased, indicating that Se was reduced on the surfaces of Pt nanoparticles. Hydrogen desorption on Pt was hindered due to the change in the surface property of Pt which was caused by addition of Se. Compared to Pt/C, the onset potential and half-wave potential for ORR on Pt@Se/C （0.63:0.37） was shifted positively for about 20 and 41 mV respectively. In the presence of methanol, Pt@Se/C has higher methanol tolerance than Pt/C catalyst.Finally, core-shell structured RuFeSe@Pt/C catalyst was prepared for ORR in direct methanol fuel cells. XRD result showed that Ru was alloyed with Se and Fe in the core. The cyclic voltammograms results showed that the peak corresponding to selenium oxidation disappeared, which may use as indirect evidence for the core-shell structure of RuFeSe@Pt. Compared to Pt/C and Ru@Pt/C, the onset potential and half-wave potential for ORR on RuFeSe@Pt/C was shifted positively for ca. 61 and 46 mV respectively. RuFeSe@Pt/C has higher methanol tolerance than Pt/C. To obtain kinetic information, a rotating disk electrode was used and the results showed that the synthesized RuFeSe@Pt/C catalyst had a 4-electron transfer mechanism for oxygen reduction.更多还原