Study of catalyst layer for polymer electrolyte fuel cell

Fan Xu, Purdue University

Abstract

There are three parts in this work centered on the catalyst layer of Polymer Electrolyte Fuel Cell (PEFC) in this thesis. The first part is for making best MEA structure. One of the major aims of this investigation is to understand the micro-structural organization of ionomer particles and Pt/C aggregates dispersed in a catalyst ink. The dispersion of Nafion® ionomer particles and Pt/C catalyst aggregates in liquid media was studied using ultra small angle x-ray scattering (USAXS) and cryogenic TEM technologies. A systematic approach was taken to study the dispersion of each component (i.e. ionomer particles and Pt/C aggregates) first, then the combination, last the catalyst ink. A multiple curve fitting was used to extract the particle size, size distribution and geometry from scattering data. The results suggests that the particle size, size distribution and geometry of each system are not uniform, rather, vary significantly. The results also indicate that interaction among components (i.e. ionomer particles and Pt/C aggregates) exists. The cryogenic TEM, by which the size and geometry of particles in a liquid can be directly observed, was used to validate the scattering results, which shows the excellent agreement. Based on this study, a methodology of analyzing dispersion of Pt/C particles, Nafion® particles in a catalyst ink has been developed and can serve as a powerful tool for making a desired catalyst ink which is a critical step for making rational designed MEA. The carbon corrosion process is the second part of the thesis. The carbon corrosion process of low-surface-area Pt/XC72 and high-surface-area Pt/BP2000 was investigated using an developed accelerated durability testing (ADT) method under simulated fuel cell conditions (a Rotating Disk Electrode (RDE) approach). Compared with the complex MEA system, this innovated approach using RDE can simply focus on carbon corrosion process and avoid the use of MEA in which many degradation/corrosion processes simultaneously occur. It was observed that different carbon corrosion processes resulted in different performance (electrochemical active surface area, mass activity and double layer capacity) decay of catalysts. The corrosion process was studied using TEM. It was found that in the case of Pt/XC72, major corrosion occurred at the center of the Pt/XC72 particle, with some minor corrosion on the surface of the carbon particle removing some amorphous carbon black filaments, while in the case of Pt/BP2000, the corrosion started on the surface. The understanding of carbon corrosion process provides the guidance for making high corrosion resistance catalysts to increase the durability performance of PEFC. Based on the second work, XC72 carbon blacks were etched using steam under different time and used as a new high corrosion resistance catalysts support for the oxygen reduction reaction. TEM results show that the center part of the XC72 particle was more easily etched away. XRD results show that the 002 and 10 peaks of the XC72 based samples are initially sharp, but then broaden during the corrosion process. TEM results of Pt particles show that the steam etching can improve dispersion uniformity of Pt nanoparticles on the surface of carbon support and reduce the Pt particles size. Electrochemical characterization results show that the mass activity of etched carbon black for 1 hour was 1.3 and 34 times greater than that of the carbon blacks etched for 3h and that of carbon blacks non-ecthed. ECSA of the carbon blacks was also significantly increased after etching. MEA test showed after 45 hours testing, the performance MEA with steam etching 1 hour XC72 based catalyst decreases much less than the MEA with commercial catalyst. Clearly, steam etching is a simple and efficient method to increase the performance and durability of the fuel cells catalysts.

Degree

M.S.M.E.

Advisors

Xie, Purdue University.

Subject Area

Mechanical engineering|Materials science

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