Study of Perovskite Structure Cathode Materials and Protective Coatings on Interconnect for Solid Oxide Fuel Cells

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dc.contributor.authorShen, Fengyuen
dc.contributor.committeechairLu, Peizhenen
dc.contributor.committeememberReynolds, William T. Jr.en
dc.contributor.committeememberAning, Alexander O.en
dc.contributor.committeemembervon Spakovsky, Michael R.en
dc.contributor.departmentMaterials Science and Engineeringen
dc.date.accessioned2017-02-09T09:00:51Zen
dc.date.available2017-02-09T09:00:51Zen
dc.date.issued2017-02-08en
dc.description.abstractSolid oxide fuel cells (SOFCs) are promising devices to convert chemical energy to electrical energy due to their high efficiency, fuel flexibility, and low emissions. However, there are still some drawbacks hindering its wide application, such as high operative temperature, electrode degradation, chromium poisoning, oxidization of interconnect, and so on. Cathode plays a major role in determining the electrochemical performance of a single cell. In this dissertation, three perovskite cathode materials, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), Ba0.5Sr0.5Co0.2Fe0.8O3 (BSCF), and Sm0.5Sr0.5Co0.2Fe0.8O3 (SSCF), are comparatively studied through half-cells in the temperature range of 600-800 ºC. Sm0.2Ce0.8O1.9 (SDC) block layer on the yttria-stabilized zirconia (YSZ) electrolyte can lead to smaller polarization resistances of the three cathode materials through stopping the reaction between the cathodes and the YSZ electrolyte. SDC is also used as a catalyst to increase the oxygen reduction reaction (ORR) rate in the LSCF cathode. In addition, interconnect is protected by CoxFe1-x oxide and Co3O4/SDC/Co3O4 tri-layer coatings separately. These coatings are demonstrated to be effective in decreasing the area specific resistance (ASR) of the interconnect, inhibiting the Cr diffusion/evaporation, leading higher electrochemical performance of the SSCF-based half-cell. Only 1.54 at% of Cr is detected on the surface of the SSCF cathode with the Co0.8Fe0.2 oxide coated interconnect and no Cr is detected with the Co3O4/SDC/Co3O4 tri-layer coated interconnect. Finally, single cells with LSCF, BSCF, and SSCF as the cathodes are operated in the temperature range of 600-800 °C fueled by natural gas. BSCF has the highest power density of 39 mW cm-2 at 600 °C, 88 mW cm-2 at 650 °C, and 168 mW cm-2 at 700 °C; LSCF has the highest power density of 263 mW cm-2 at 750 °C and 456 mW cm-2 at 800 °C. Activation energies calculated from the cathode ASR are 0.44 eV, 0.38 eV, and 0.52 eV for the LSCF, BSCF, and SSCF cathodes respectively, which means the BSCF cathode is preferred. The stability test shows that the BSCF-based single cell is more stable at lower operative temperature (600 °C) while the LSCF-based single cell is more stable at higher operative temperature (800 °C).en
dc.description.abstractgeneralSolid oxide fuel cells (SOFCs) are promising devices to convert chemical energy to electrical energy due to their high efficiency, fuel flexibility, and low emissions. However, there are still some drawbacks hindering its wide application, such as high operative temperature, electrode degradation, chromium poisoning, oxidization of interconnect, and so on. A single cell is composed of an anode, electrolyte, and cathode. Interconnect can connect individual single cell to stack to increase voltage and current. In order to improve the electrochemical performance, such as resistance and power density, cathode materials and protective coatings to interconnect are studied. Three perovskite cathode materials, La<sub>0.6</sub>Sr<sub>0.4</sub>Co<sub>0.2</sub>Fe<sub>0.8</sub>O<sub>3</sub> (LSCF), Ba<sub>0.5</sub>Sr<sub>0.5</sub>Co<sub>0.2</sub>Fe<sub>0.8</sub>O<sub>3</sub> (BSCF), and Sm0.5Sr0.5Co0.2Fe0.8O3 (SSCF), are comparatively studied in 600-800 ºC to obtain the optimal cathode at different operating temperatures. BSCF has the smallest resistance at 600 ºC, LSCF at 700 ºC, and SSCF at 800 ºC. A thin Sm<sub>0.2</sub>Ce<sub>0.8</sub>O<sub>1.9</sub> (SDC) block layer on the yttria-stabilized zirconia (YSZ) electrolyte can lead to smaller resistances of the three cathode materials through stopping the reaction between the cathodes and the YSZ electrolyte. SDC is also used as a catalyst by three methods to lower the resistances of the LSCF cathode. In addition, interconnect is protected by Co<sub>x</sub>Fe<sub>1-x</sub> oxide and Co<sub>3</sub>O<sub>4</sub>/SDC/Co<sub>3</sub>O<sub>4</sub> tri-layer coatings separately. They are demonstrated to be effective in decreasing the resistance of the interconnect, inhibiting the Cr diffusion/evaporation outward to poison cathodes. Only 1.54 at% of Cr is detected on the surface of the SSCF cathode with the Co<sub>0.8</sub>Fe<sub>0.2</sub> oxide coated interconnect and no Cr with the Co<sub>3</sub>O<sub>4</sub>/SDC/Co<sub>3</sub>O<sub>4</sub> tri-layer coated interconnect. Finally, single cells with LSCF, BSCF, and SSCF as the cathodes are operated in 600-800 °C fueled by natural gas. BSCF has the highest power densities at lower operating temperatures while LSCF has the highest power densities at higher operating temperatures. Activation energies are 0.44 eV, 0.38 eV, and 0.52 eV for the LSCF, BSCF, and SSCF cathodes respectively, which means the BSCF cathode is preferred. The stability test shows that the BSCF-based single cell is more stable at 600 °C while the LSCF-based single cell is more stable at 800 °C.en
dc.description.degreePh. D.en
dc.format.mediumETDen
dc.identifier.othervt_gsexam:9476en
dc.identifier.urihttp://hdl.handle.net/10919/74973en
dc.publisherVirginia Techen
dc.rightsIn Copyrighten
dc.rights.urihttp://rightsstatements.org/vocab/InC/1.0/en
dc.subjectSolid Oxide Fuel Cellen
dc.subjectPerovskiteen
dc.subjectChromium Poisoningen
dc.subjectProtective Coatingen
dc.subjectSingle cellen
dc.titleStudy of Perovskite Structure Cathode Materials and Protective Coatings on Interconnect for Solid Oxide Fuel Cellsen
dc.typeDissertationen
thesis.degree.disciplineMaterials Science and Engineeringen
thesis.degree.grantorVirginia Polytechnic Institute and State Universityen
thesis.degree.leveldoctoralen
thesis.degree.namePh. D.en

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