Novel Method For Preparing Nanocatalysts Used In Low Temperature Fuel Cells

Fuel cell is a device that can turn chemical energy into electrical energy through electrochemical reaction without combustion and is of great importance in solving the scarcity of energy sources and environmental pollution, so it is regarded as one of the most important power resources in the 21 century. Low temperature fuel cells such as PEMFC DMFC can work at low temperature with the advantages of quick start and high energy efficiency. They can be potentially used for electric car field power plant and potable equipment. The most difficult problem in the commercialization of low temperature fuel cells is the high cost. Effective solutions to this problem include developing catalyst preparation technique, preparing highly active catalyst, increasing the utilization of catalyst and decreasing the amount of catalyst used.This thesis investigates a new method to prepare carbon supported precious metal catalyst with intermittent microwave heating (IMH) process. This new method has the advantages of rapid, convenient, economic and easy comparing with other methods. Based on this method Pt/C (VulcanXC-72) and Pt/multi-walled carbon nanotubes (MCNTs) catalysts with different loadings were prepared. The catalysts were characterized by XRD, TEM, XPS, EDX, Cyclic voltammograms(CV) method, chrono-amperometric method, chronopotentiometric method and their application in PEMFC and DMFC were studied at the different conditions.TEM and XRD results showed that in the preparation of Pt/C the intermittent microwave heating procedure of Pt deposition had an obvious influence on the size and distribution of Pt particle. The average particle size of Pt in IMH-90 and IMH-100s (prepared by the procedure of 10s heating and 90s or100s intermission) were 3.2nm and 3.0nm respectively and Pt particles were uniformly dispersed on carbon. SEM and UV results showed that the as-prepared catalyst had an uniform diameter of 10 μm. Compared with E-TEK catalyst our catalyst powder size is smaller and is easier to be dispersed in solvents. Catalysts prepared by IMH-90s and IMH-100s had bigger electrochemical active area (EAA) and higher methanol oxidation activity. The 60%Pt/C catalyst with the Pt diameter of 4nm that was prepared by adjusting microwave heating procedure had more uniformly dispersed diameter and higher electrochemical activity compared with E-TEK catalyst.XRD, TEM and XPS were used to characterize the mechanism and influencing factors of the preparation of Pt/MCNTs catalysts with IMH method. Pt existed on the surface of MCNTs in the state of Pt( II) and Pt(IV) oxide, before the reduction and most of transferred to after reduction. The diameter and dispersion of Pt were influenced by the pretreatment of MCNTs, microwave heating procedure and the structure of MCNTs. Methanol oxidation on these catalysts were studied with CV and current-potential polarization. The results showed that catalysts supported on MCNTs pretreated with H2SO4+HNO3 and prepared with the heating procedure IMH-lOOs have highest electrochemical activity and best methanol oxidation performance.The dispersion of the catalysts in different solvents was studied with Zeta potential method. The results showed that the dispersion of the agglomerate Pt/C catalysts depended on the property of the solvent. In this work the catalyst was most uniformly dispersed when isopropanol was used as solvent with the smallest size. Membrane electrode assembly(MEA) was fabricated with isopropanol as disperser and 1,2-propanediol as stabilizing reagent. The performance of H2/O2 single cell reached 0.58V, 1A cm"2 using O2 as cathode. 40W PEMFC stack was fabricated with the performance of 43W at 35°C and 74W at 50"C at atmosphere pressure.DMFC was studied with the current-potential polarization curve. It was found that its performance was largely influenced by the catalyst loading of the anode. Optimized loading was PtRu(l:l):3.4 mg cm"2, DMFC could reach the power density of 115.8 mW cm"2 at 90°C, 2 mol dm'3 methanol,O2 pressure 0.2MPa and 95.8 mW cm"2 at atmosphere pressure, The performance was largely influenced by methanol concentration, 02/air pressure, 02/air flux, and temperature. The DMFC with the self-breathing cathode can discharge steadily.