Preparation And Investigations Of High Performance And Self-humidifying Catalysts For Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells （PEMFC） have attracted much attention due totheir various advantages, such as high specific power density, quick startup at low temperatureand zero or low emission. It is well known that the membrane in a PEMFC system requireswater to maintain its proton conductivity during operation. To wet the Nafion ionomer in themembrane electrode assembly （MEA）, significant energy is consumed in humidifying theanode and cathode gases. This auxiliary humidification equipment increases systemcomplexity and adds additional cost to fuel cells. In regard to these reasons, the developmentof self-humidifying MEA with self-humidifying catalysts has always been one of the hottopics in fuel cells. Its implementation can not only reduce the system cost, but also greatlysimplify PEMFC structure, consequently easier cell management and improved overallsystem performance.This thesis is targeted for the reduced cell cost and improved cell performance with thedevelopment of self-humidification catalysts.Firstly, through in situ hydrolysis of tetraethyl orthosilicate （TEOS）, amorphous silica isintroduced into the carbon support and SiO_x/C composite support is therefore formed uponbaking. A series of Pt/SiO_x/C catalysts （supported on SiO_x/C） was prepared using a modifiedorganic colloidal method. The effects of modification conditions on the catalyst performanceand structure had been investigated. Optimization has been made to identify the bestmodification parameters. It is found that the optimized baking temperature for SiO_x/C supportis3500C. In the nitrogen atmosphere, this baked support can improve the dispersion of activecomponents and reduce particle size of the catalysts. It has been found that, as the dopingSiO_xcontent reached6wt.%, the resulted catalysts displayed the highest electrochemicallyactive surface area （ECSA） and highest peak current density for methanol oxidation. Thewater uptake test shows that the Pt/SiO_x/C catalyst displays desirable self-humidificationability.Secondly, with the dual doping of RuO₂and SiO_x, multifunctional Pt/RuO₂-SiO_x/Ccatalysts with different RuO₂content have been prepared. It is found that, with the addition of3wt.%RuO₂, Pt/RuO₂-SiO_x/C （PRSC3） catalyst shows the highest ECSA, the best methanoloxidation and oxygen reduction activity. Compared with the similarly prepared Pt/SiO_x/C, thecatalyst PRSC3shows at least30%higher activity at room temperature. The particle size ofPRSC3catalyst was about2.4nm. PRSC3also showed better dispersion than that of PSC.Notably, the addition of ruthenium oxide significantly improves the performance of MEAs under non-humidified conditions. At0.6V, the current density of MEAs prepared withPt/RuO₂-SiO_x/C （using optimized3wt.%RuO₂） as the anode catalyst is0.71A·cm^-2, whichenhanced by20%compared with MEAs prepared with Pt/SiO_x/C as the anode catalyst (0.59A·cm^-2). It is suggested that the promotional effect of RuO₂may result from the highdispersion of platinum.Thirdly, a series of Pt/MO_y-SiO_x/C （M=Ir、 Ru、 Co） catalysts were prepared usingMO_y-SiO_x/C（M=Ir、Ru、Co） as catalyst supports, ethylene glycol as solvent cum reducingagent, and sodium citrate as ligand. It was found that the addition of group partⅧ metaloxides was favorable for the reduction of Pt particle size and particle dispersion, consequentlyimproved electrocatalytic activity. Under the identical conditions, the oxygen reductionactivity of the Pt/SiO_x/C series catalyst decreased in the order PCSC>PRSC>PISC>PSC.Fourthly, Pt/WO₃-SiO_x/C catalysts have been prepared by supporting platinumnanoparticles on WO₃modified carbon black using an organic colloidal method. It was foundthat the addition of WO₃significantly improves the methanol oxidation activity of thePt/SiO_x/C catalysts. When the mass ratio between phospho-wolframic acid and carbonsupport is1:1, the Pt/WO₃-SiO_x/C catalyst shows the highest activity toward methanoloxidation,2.1times higher than that of Pt/SiO_x/C. The influence of baking temperature of thesupport on the stability of the catalysts has been investigated is found that the stabilityincreased with the increase of heating temperature. However, the catalytic performancedecreased with the increase of support heating temperature. Therefore the support heatingtemperature was set at5000C to balance between stability and catalytic activity.