Alternative Catalysts And Synthesis Methods For Fuel Cell Reformers

Author : Irene J. Hsu
Publisher :
ISBN 13 : 9781267214935
Total Pages : pages
Book Rating : 4.2/5 (149 download)

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Download or read book Novel Catalyst Synthesis Methods for Fuel Cell Applications written by Irene J. Hsu and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Many resources have been devoted to developing novel catalytic systems in efforts to improve the activity of electrochemical reactions while more efficiently utilizing the precious metal catalyst. However, in order for these catalyst systems to be relevant industrially they must be in the form of particles. Conventional synthesis methods are too simplistic for these systems, which require a higher level of control in their synthesis. Atomic layer deposition (ALD) was used to deposit Pt layers on tungsten monocarbide (WC) particles in an effort to produce monolayer Pt-WC catalysts. DFT calculations predicted that one monolayer of Pt on a WC substrate should perform as well as a Pt catalyst for the ORR. Pt ALD was first demonstrated on WC thin films and the growth and nucleation characteristics were studied. A nucleation period was observed where the growth per cycle was quite slow; after which the growth rate increased considerably. When few ALD cycles were used, Pt particles were well dispersed on the WC surface, but eventually with increased ALD cycles, merged to form a continuous layer. Pt monolayer growth is difficult on WC because there are few available adsorption sites for the Pt precursor, and discrete Pt particles are created instead. With increasing ALD cycles, existing Pt particles are more likely to grow rather than nucleate elsewhere on the WC surface. Therefore, in order to make thinner Pt layers the nucleation density must be increased at low ALD cycles. Pt ALD was then applied to WC powders to compare growth characteristics with the thin films. TEM imaging and Cu stripping voltammetry found that Pt ALD growth on the WC powder substrate was similar to that on WC thin films. However, the presence of excess free carbon was found to affect Pt ALD on WC powders by blocking adsorption sites on WC and also providing its own sites for Pt particle nucleation. Despite this, the ALD Pt-WC powders were found to be electrochemically equivalent to the ALD Pt-WC thin films through cyclic voltammogram analysis. The Pt-WC powders were also analyzed for ORR using a rotating disk electrode apparatus to obtain quantitative activity information. The mass specific and specific activities for the 30 and 50 ALD cycle samples were found to be comparable to a 10 wt% Pt/C catalyst with more than half the Pt loading. However, the oxygen reduction mechanism on the Pt-WC catalysts was found to be less efficient than on the Pt/C catalyst. Higher overpotentials were observed, and the reduction was found to take the 2-electron peroxide reduction pathway, the less desirable pathway for fuel cell applications. To explain some of the observations in electrochemical activity found with the ALD Pt-WC samples, WC disks with one monolayer of Pt were produced to make idealized surfaces. This system was found to have a lower-than-expected ORR activity, which was attributed to the lack of stability of the WC substrate in oxygen-saturated acid media. Regardless, the Pt-WC core shell system could be potentially useful for other reaction systems, such as hydrogen evolution reaction. Additionally, shape-controlled Pt particles were produced using pulse electrodeposition. By adjusting the applied potential, Pt particles that are spherical, cubic, or dendritic can be made. These were also shown to be useful for the methanol oxidation, a structure sensitive reaction. The dendritic particles were confirmed to be more active than cubic particles, which is in agreement with the literature.