Mechanism of oxygen reduction reaction on transition metal oxide catalysts for high temperature fuel cells

The solid oxide fuel cell (SOFC) with its high energy conversion efficiency, low emissions, silent operation and its ability to utilize commercial fuels has the potential to create a large impact on the energy landscape. Although SOFCs do not require noble metal catalysts, it has the disadvantage of having to operate at elevated temperatures (>800°C) due to the poor catalytic activity of the oxygen reducing cathode. This thesis research is focused on identifying the fundamental reaction mechanisms and pathways for oxygen reduction reaction (ORR) in transition metal oxide cathodes to enable lower operating temperature and improve device performance. The approach used here involves dense and thin-film microelectrodes of Lao.sSr0.2M03-d (M = Mn and Fe) supported on thin film 8YSZ electrolyte that was fabricated via microfabrication/photolithography techniques. These microelectrodes were then probed mainly via electrochemical impedance spectroscopy (EIS) under varying temperature, oxygen partial pressure, microelectrode size/thickness, and applied polarization potential to determine the rate-limiting step(s) during ORR. For La0.8Sr0.2MnO3-d (LSM), at least four reaction or transport process have been identified from EIS; (i) ion conduction in 8YSZ, (ii) possible surface diffusion on LSM, (iii) surface exchange reaction on LSM and (iv) TPB/bulk charge transfer on LSM. The rate-limiting step and the relative contribution of ORR current from the bulk and three-phase boundary pathways were found to vary with temperature. This was quantified by correlating microelectrode geometry to the impedance. For Lao.sSr0.2FeO3-d (LSF), overall impedance was found to be at least one to two of magnitude smaller than LSM.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2008

Author dc:creator

• La O', Gerardo Jose Cordova

Advisor dc:contributor.advisor

• Yang Shao-Horn and Harry L. Tuller.

• Materials Science and Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/44683

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/44683