Preparation And Investigation Of High Platinum Loading And High Performance Catalysts For Low Temperature Fuel Cells Application

Fuel cells are power generation equipments which directly convert the chemical energyto electrical energy by electrochemical reactions, and are considered to be one of the mostpromising power generation technologys in the21st century. Low temperature fuel cells,including proton exchange membrane fuel cells （PEMFCs）, direct alcohol fuel cells （DAFCs）and direct formic acid fuel cells （DFAFCs）, have attracted much attention due to theiradvantages such as high power density, quick startup, high energy conversion efficiency andcommercialization viability in the future. Electrocatalyst, one of the key materials in lowtemperature fuel cells, plays a crucial role for the performance of fuel cells, the researchesrelated to the electrocatalyst has been becoming one of the hottest topics in fuel cells area.Currently, Pt/C catalyst is the most wide used catalyst in low temperature fuel cells.Recent years, it is found that the high platinum loading Pt/C catalyst is beneficial to thepreparation of high performance membrane-electrode-assembly （MEA） with thin catalystlayer and better activities. However, it is difficult to prepare high platinum loading Pt/Ccatalyst with smaller nanoparticle size, because of the easy self-agglomeration of Ptnanoparticles, especially at high loadings. On the other hand, although a lot of excellentresults have been reported for the preparation of Pt/C in milligrams scale, volume preparation（in terms of grams per pot） of high loading Pt catalysts with particles smaller than3nm is stillchallenge. In addition, in order to improve the performance of catalysts and reduce costs, Pt-based catalysts have been extensively investigated. It is of great importance to developPt-based catalysts with high activity and high tolerance to poison species.Based on the above fact and consideration, in this thesis, a series of catalysts used forlow temperature fuel cells, were prepared with a simple and effective microwave-assistedorganic colloid approach.Firstly, Pt/C catalyst with high loading （40wt.%） was prepared by microwave-assistedorganic colloid method. To optimize the preparation parameters, the effects of reactionconditions were investigated extensively. The optimized microwave power and microwavetime are400W and100s respectively for a50ml ethylene glycol reaction system, the Pt/Ccatalyst prepared at optimum condition and with pretreated carbon black as support exhibited the best catalytic activity for methanol oxidation, and highest electrochemical surface area（ESA）. It is found that the oxidation pretreatment degree of carbon should be moderatebecause insufficient or excessive oxidation groups on carbon surface had adverse effect onstructure and performance of the catalyst. Compared to the conventional preparation methods,e.g. impregnation-NaBH4reduction method and reflux heating method, microwave-assistedorganic colloid method shows following advantages:（1） quick and uniform heating;（2） insitu reduction, which would make the preparation of high dispersive and homogeneous Ptnanoparticles easy. Moreover, the influence of Pt loading to activity of catalyst had beeninvestigated, the results showed that Pt/C （40wt.%） has the highest ESA while Pt/C （20wt.%）had the highest mass specific activity.Secondly, high platinum loading （40wt.%） and high performance Pt/C catalyst has beenvolume prepared in gram scale by the microwave-assisted organic colloid approach. Theactive components were dispersed on the XC-72carbon support with an uniform sizedistribution and an average particle size of2.3nm, which is smaller than that of commercial40wt.%Pt/C catalyst （Johnson Matthey）. The catalyst showed better performance than theJohnson Matthey Pt/C for both methanol oxidation and oxygen reduction reactions. In ahydrogen-air single fuel cell, the current density of the MEA prepared with the home-madePt/C catalyst reached630mA·cm^-2（at0.7V）, which is comparable with that of thestate-of-the-art Johnson Matthey Pt/C.Thirdly, to further improve the activity and anti-CO poisoning ability of catalyst, PtRu/Ccatalyst was prepared by microwave-assisted organic colloid method. The optimumpreparation conditions are as follows: reacting at400W microwave power for200s, the bestsolvent and reductant composition is4:2:1for ethylene glycol, glycerol and water. The effectof different pH adjustive reagents on the catalyst had been studied, it was found that urea isthe best pH adjustive reagent, we speculated that in-situ OH-generated by urea hydrolyzationmay protect precursor to homogeneously depositing on the carbon support, simultaneouslykeep pH stable during the course of reaction. Consequently, the particles of PtRu/C catalystprepared by urea adjustment were highly dispersed on the carbon support, and the particlesize was about2.2nm, along with ESA of102m²·g¹Pt, and current density of0.56A·mg^-1Ptfor methanol oxidation. It was found that heat treating the PtRu/C catalyst in hydrogen atmosphere could result to the increase of particle size and the decrease of catalytic activitytoward methanol oxidation, we also found that heat treating in hydrogen atmosphere wouldresult to the reduction of RuO₂·xH₂O, the content decreased from19%to12.3%, it may beanother reason of activity decline. However, we found that the stability of catalyst wasenhanced with the treatment.A series of PtxMoy/C catalysts with different atomic ratios of Pt to Mo were prepared bymicrowave-assisted organic colloid method. The electrochemical tests revealed an enhancedperformance of the PtxMoy/C catalysts for the CO oxidation, in comparison with Pt/C catalyst.It was very important to control the atomic ratios of Pt to Mo. Pt2Mo1/C exhibited the bestcatalytic activity to CO oxidation.Pd₃Pt/C catalyst for oxygen reduction reaction was prepared by the microwave-assistedorganic colloid process. It was found that the addition of Pd increased the particle size. In0.5M H₂SO₄solution, the Pd₃Pt/C catalyst showed an activity similar to that of the Pt/C catalystfor the oxygen redcution. However, in0.5M CH₃OH+0.5M H₂SO₄solution, differentperformances had been observed for two catalysts, for Pt/C catalyst, the onset cathodicpotential shifted cathodically by220mV, while there was no obvious increase in onsetpotential for Pd₃Pt/C catalyst could be observed, indicating the higher methanol tolerance ofPd₃Pt/C catalyst, which may make it a better cathode catalyst for oxygen reduction in DMFC.