Date of Graduation

2014

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Chemistry

Committee Chair

Harry Finklea

Committee Co-Chair

Fabien Goulay

Committee Member

Justin Legleiter

Committee Member

Alan Stolzenberg

Committee Member

Nianqiang Wu

Abstract

The solid oxide fuel cell (SOFC) is a promising energy conversion device with high efficiency, fuel flexibility and less pollution. The requirement for high operating temperatures limits its applications due to high cost and the need for compatible cell components. Therefore, it is important to design SOFCs that operate in intermediate temperatures; which have cathodes with improved microstructures to enhance the oxygen reduction reaction. However, designing novel cathode structures needs a fundamental understanding of cell processes. Impedance spectroscopy is a very powerful tool for studying SOFCs which yields useful yet complex data due to overlapping impedance related processes. Therefore, we used a deconvolution method to analyze the impedance data and extract information about individual processes through equivalent circuit fitting.;Current collectors play a vital role in SOFC characterization. Platinum (Pt), Gold (Au), Silver (Ag) and Lanthanum Strontium Manganite (LSM) are some commonly used cathode current collectors. We carried out a preliminary study in order to investigate the current collection properties of these materials and to choose the most suitable current collectors for our symmetrical cell characterizations. Based on the results (good adhesion properties and reproducible performance), it was decided continue to using Ag and LSM as the current collectors for our characterizations.;Cathode microstructure can be improved by increasing the triple phase boundary (TPB) area of a cathode. This can be done by developing composite cathode structures which consist of both electron and ion conducting phases. Traditionally, composited cathodes are formed by mixing powders of LSM and Yttria Stabilized Zirconia (YSZ) with high temperature sintering. More recently, the infiltration technique (infiltrating a pre-sintered YSZ scaffold with a LSM solution followed by low temperature sintering) has been used to prepare LSM-YSZ composite cathodes. A previous study has reported the performance comparison of LSM infiltrated YSZ nanofiber cathode with LSM-YSZ powder composite cathode. However, this work does not provide the deconvolution analysis of impedance data. In our work, the properties of LSM infiltrated YSZ nanofiber cathodes and LSM infiltrated YSZ powder cathodes were compared. LSM infiltrated YSZ nanofiber cathodes showed better performance compared to LSM infiltrated YSZ powder cathodes and deconvoluted impedance data for both types of cells showed "Gerischer" like behavior which arises due to the thickness of the cathode. Based on this knowledge, an attempt was made to prepare improved LSM infiltrated YSZ cathodes with thin YSZ scaffolds.;LSM cathodes are usually prepared using the pastes of commercial LSM powders. Despite the extensive studies carried out on LSM infiltrated YSZ cathodes, to the best of our knowledge, no attempt has been made to study the activity of the infiltrant LSM alone as a cathode. In this work, symmetrical cells with cathodes prepared by evaporating and sintering LSM infiltrant solution directly on YSZ electrolytes were characterized using Ag, Au and Pt current collectors. Impedance studies of solution-based LSM cathodes with Ag current collectors resulted in very low polarization resistances (resistance associated with the oxygen reduction reaction) and 2-3 orders of magnitude higher peak frequencies compared to powder based LSM cathodes. Properties of these solution based LSM cathodes vary with different current collectors, which suggests that the current collector participates in the oxygen reduction reaction of the solution based LSM cathodes. Nonetheless, it would be possible to further investigate and develop these "solution-based LSM cathodes with Ag current collectors" as a composite cathode for intermediate temperature SOFCs.;The significant difference of peak frequencies between the powder based and solution based LSM cathodes led to an investigation of LSM-YSZ hybrid cells (powder based LSM cathode as the working electrode and solution based LSM cathode with Ag current collector as counter electrode) in order to isolate and study the properties of powder based LSM cathode with minimal overlapping impedance processes. According to our knowledge, this direct method that we have used to study a single electrode under applied DC bias has not been reported before.;Also, using the measurements at open circuit voltage (OCV) and with applied DC bias, we have demonstrated that the properties of the powder based LSM cathode change with time and applied potential/current, at unexpectedly low overvoltages. This observation suggests that the use of Butler-Volmer equation to represent current-potential behavior in simulations may be inappropriate.

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