Preparation And Investigation Of High Performance And Low-Pt Loading Catalysts For Fuel Cell Application

Low temperature fuel cells (LTFC) have attracted much attention all over the world due to their abilities to deliver clean and high power density, safety, portability, convenience in operation and commercialization viability in the future. LTFCs are predicted to be the best candidate for industrialization applications. Low temperature fuel cells include proton exchange membrane fuel cells (PEMFCs), direct alcohol fuel cells (DAFCs) and direct acid fuel cells et al. Commercialization of these fuel cells is seriously hindered by the usage of expensive and scare Pt catalysts, which led to high cost. Besides the high cost, Pt catalysts for fuel cell applications are suffered from poisoning as well. Therefore it is of vital importance for development of novel catalyst preparation technology for highly active catalyst with low platinum loading. Research in these areas has become hot topics in fuel cell community.In the thesis, a series of high performance catalysts with low Pt loading are prepared from the aspects of catalysts preparation, supports and catalyst structure for enhanced utilization of noble metal Pt. In the meanwhile, oxide addition to Pt/C catalysts is also attempted for anti-agglomeration of active catalytic components.Firstly, we synthesized highly active Pt/CNT and PtRu/CNT catalysts using carbon nanotube as supports via a high-pressure colloidal method. The particle sizes of the catalysts are around 1.2 nm with high dispersion. The performance for methanol oxidation and H2/Air single cell was investigated when these catalysts were used as anode. The experimental results showed that Pt/CNT and PtRu/CNT displayed good catalytic activity toward methanol oxidation, up to 4 times higher than that of Johnson Matthey Hispec 2000 Pt/XC-72R and 5 times better than Hispec 5000 PtRu/XC-72R catalysts respectively. In a single hydrogen/air fuel cell test, these anodic catalysts displayed better cell performance than commercial catalysts.Carbon nanotubes as supports are difficult in dispersing catalysts and can be easily entangled. In this work, carbon nanotubes were shortened from 5–15μm to ca. 200 nm using modified ball milling with ethanol as miller and a platinum catalyst supported on these shortened carbon nanotubes (SCNTs) was prepared by a high-pressure colloidal method. It was found that the catalyst supported by shortened carbon nanotubes showed much higher activity than a platinum catalyst supported by conventional carbon nanotubes (CNTs). Pt/SCNT showed much higher activity for anodic oxidation of methanol than Pt/CNT catalyst; the peak current density for Pt/SCNT catalyst was 1.4 times and 2.8 times higher than that of Pt/CNT and commercial Pt/C catalyst respectively. The peak current density for PtRu/SCNT catalyst (0.43 A·mg-1 metal) is 1.3 times higher than that of PtRu/CNT(0.32 A·mg-1metal). Based on characterization results, possible reasons for catalytic enhancement are due to the increased surface area caused by the cutting edges and the interaction between supports and the active components. Some aggregation and entangled problem caused by the long length of CNTs during the catalyst spraying onto carbon paper or membranes were solved by using SCNTs. SCNTs are expected to be good supports for catalyst with high activity and cell with high performance.A core-shell structured PdPt@Pt/C is prepared by a novel two-step colloidal approach with low Pt loading(8 wt.%)and high performance. Small amount of Pt and PdPt as core are used to prepare PdPt@Pt/C catalyst since pure Pd can hardly be dispersed uniformly. The peak current density of PdPt@Pt/C for methanol oxidation is as high as 2.93 A·mg-1 Pt and its activity is 2.85 and 3.91 times higher than that of commercial Pt/C (Tanaka, 50 wt.%) and home-made Pt/C catalyst (20 wt%) respectively. Furthermore, the ratio between forward current If and backward current Ib is 1.04, whereas the ratio is only 0.71 for Tanaka catalysts and 0.75 for home-made catalysts. Obviously, PdPt@Pt/C showed improved specific mass activity (high Pt utilization efficiency) and enhanced anti-poisoning capability caused by methanol oxidation intermediates. The catalyst also displayed high catalytic activity towards oxygen reduction reaction.PdPt@Pt/C was used as anodic catalysts in a hydrogen-air single cell to evaluate its performance. It was found that the MEA performance made by PdPt@Pt/C is 80% higher than that made by Tanaka Pt/C under identical loading and operational conditions. Possible reasons for these performance enhancements can be attributed to the high utilization of Pt on PtPd surface and the specific interaction between the outer Pt shell and the inner PtPd core. Since Pd catalysts are suffered from low activity and poor poisoning resistance in formic acid oxidation, a core-shell Pt@Pd/C using Pt core is prepared to solve these issues. It was found that the particle size for Pt@Pd/C is smaller (4 nm) than single Pd catalyst. A three-fold formic acid oxidation activity enhancement was obtained on Pt@Pd/C relative to Pd/C. Moreover, the formic acid oxidation on Pt@Pd/C followed the direct pathway, suggesting a less poisoning rate and high stability, making it a promising anodic catalyst for direct formic acid fuel cells.The effects of oxide addition to carbon for anti-aggregation of active components were investigated. A serial of SiOx-carbon supported Pt catalysts were prepared by a high pressure organic colloidal method. The effects of SiOx weight percent, heat treatment temperature and time on the aggregation of active particles in the Pt/C were studied. It was found that appropriate addition of oxide was favorable for inhibiting growth and agglomeration of active components.