In the last few years the quest towards a hydrogen based economy has intensified
interest for effective and less expensive catalysts for fuel cell applications. Due to its
slow kinetics, alternative catalysts for the oxygen electroreduction reaction are actively
researched. Platinum alloys with different transition metals (for example: Ni, Co and Fe)
have shown improved activity over pure Pt. The design of a Pt-free catalysts is also
highly desirable, and different alternatives including metalloporphyrins and Pd-based
catalysts are being researched. Pd-based catalysts constitute an attractive alternative to Pt
alloys in some fuel cell applications, not only because of lower costs but also because of
the lower reactivity of Pt alloys towards methanol, which is important for improved
methanol crossover tolerance on direct methanol fuel cells.
In this work we apply density functional theory (DFT) to the study of four catalysts
for oxygen electroreduction. The electronic structure of these surfaces is characterized
together with their surface reconstruction properties and their interactions with oxygen
electroreduction intermediates both in presence and absence of water. The energetics
obtained for the intermediates is combined with entropy data from thermodynamic tables
to generate free energy profiles for two representative reaction mechanisms where the
cell potential is included as a variable. The study of the barriers in these profiles points
to the elementary steps in the reaction mechanisms that are likely to be rate-determining.
The highest barrier in the series pathway is located at the first proton and charge transfer
on all four catalytic surfaces. This is in good agreement with observed rate laws for this
reaction. The instability of hydrogen peroxide on all surfaces, especially compared with
the relatively higher stability of other intermediates, strongly points at this intermediate as the most likely point where the oxygen bond is broken during oxygen reduction. This
adds to the argument that this path might be active during oxygen electroreduction.
A better understanding behind the reaction mechanism and reactivities on these
representative surfaces will help to find systematic ways of improvement of currently
used catalysts in the oxygen electroreduction reaction.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/5826 |
| Date | 17 September 2007 |
| Creators | Lamas, Eduardo J. |
| Contributors | Balbuena, Perla B. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 1668148 bytes, electronic, application/pdf, born digital |
Page generated in 0.0019 seconds