Study On The Preparation And Properties Of Low-Platinum Electrocatalysts For Polymer Fuel Cells

Considerable attention has been paid to alternative energy sources in an attempt to relieve the pollution and energy crisis. One of the promising fields of clean and sustainable power is polymer fuel cell technology, based on direct conversation of fuel into electricity. Despite considerable advances in recent years, many technical barriers still need to overcome in the development of electrocatalysts for widespread commercialization.Platinum is the most effective catalyst to facilitate both methanol oxidation and oxygen reduction in polymer fuel cells. However, platinum is very expensive, resource limited and irreversibly inactivated by CO-like poisoning species. Therefore, it is essential that the utilization of platinum should be kept as low as possible without sacrificing the catalytic performance.The thesis mainly focus on high utilization of platinum in the catalysts. To match this requirements, a series of low-Pt catalysts for ORR and methanol oxidation are designed and prepared by several methods. And the catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX), etc. Additionally, the catalysts were tested in a micro fuel cell system by measuring the performance of MEA/s, which were made from our lab-made catalysts. It was found that these catalysts showed quite good activity for ORR and methanol oxidation in polymer fuel cells. The thesis is consists of three parts and the research details are as follows:Part I: Carbon supported PdCo-core with a thin Pt-shell electrocatalyst for methanol oxidationWe report a core-shell structured PdCo@Pt/C catalyst with a monolayer Pt shell encapsulated PdCo alloy. It is prepared via a two-step procedure, which is consist of an organic colloid method step and a surface replacement reaction step. The electrochemical surface areas (ECSA) are 4.5 and 4 times that of Pt/C and PtRu/C catalysts, respectively. Based on the Pt mass, the cyclic voltammetry (CV) and chronoamperometry results demonstrate that the electrocatalytic activity and stability of the PdCo@Pt/C catalyst for methanol oxidation are much higher than that of the Pt/C and PtRu/C catalysts. This method makes the more efficient use of Pt in electrocatalysts and can be concluded that PdCo@Pt/C represents one of the best alternative candidates for the polymers fuel cells anode catalysts.Partâ…¡: An effective carbon supported Pt-PdFe catalyst for methanol oxidationA low-cost and high performance Pt-PdFe/C catalyst for methanol oxidation is prepared by deposited Pt onto the carbon supported PdFe nanoparticles using replacement reaction. Electrochemical results show that the Pt-PdFe/C catalyst has much larger Pt active surface area (285.33 m²Â·g^-1Pt) than that of the PtRu/C and Pt/C catalyst. Furthermore, the mass specific anodic peak current is 1.01 A mg^-1Pt for methanol oxidation on the Pt-PdFe/C electrode, an increase by a factor of 3.5 times and 12.6 times as compared to PtRu/C and Pt/C, respectively. The studies sheds light on the advanced way to reducing Pt loading while has excellent electrocatalysts activities for polymer fuel cells.Partâ…¢: Pt overgrowth on carbon supported PdFe seeds in the preparation of core-shell electrocatalysts for the oxygen reduction reactionA novel core-shell structured Pd₃Fe@Pt/C electrocatalyst, which is based on Pt deposited onto carbon-supported Pd₃Fe nanoparticles, is prepared for the oxygen reduction reaction (ORR) in polymer fuel cells (PEMFCs). The carbon-supported Pd₃Fe nanoparticles act as seeds to guide the growth of Pt. The higher surface area, mass specific activity and the half-wave potential of the synthesized catalyst, suggests that the utilization of Pt in the Pd₃Fe@Pt/C catalyst is higher than that in Pt/C. Furthermore, The Koutecky-Levìch plots indicate that the ORR, which catalyzed by the Pd₃Fe@Pt/C, could go through a four electron pathway similar to that of the ORR on the Pt. The high activity of the Pd₃Fe@Pt/C catalyst for the ORR suggests that the synthesized catalyst could be an economically viable candidate to replace pure Pt catalysts as an ultralow-Pt cathode catalyst in polymer fuel cells.