| 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 (If) to backward peak current density (Ib) 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. If/Ib 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, If/Ib is close to that of Pt/C, and further increase of the Pt:Ru ratio leads to almost no decrease in If/Ib. 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@PtxPdy/C catalysts with PtxPdy 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@PtxPdy/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 If/Ib 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. |
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