Semester

Fall

Date of Graduation

2012

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu

Committee Co-Chair

Darran R Cairns

Committee Member

Randall S Gemmen

Committee Member

Kirk R. Gerdes

Committee Member

Edward M. Sabolsky

Committee Member

Nick Wu.

Abstract

Solid oxide fuel cells (SOFCs) are energy conversion devices that produce electricity by electrochemically combining a fuel and an oxidant across an ionic conducting oxide electrolyte. As it is regarded as the most efficient and versatile power generation system, SOFCs have attracted more substantial interest in recent years. Oxygen reduction at the cathode is considered as the main rate limiting factor to the performance of the whole system. In this work, experimental study of oxygen transport in single phase and infiltrated cathode materials using electrical conductivity relaxation (ECR) technique are combined with physical modeling to benefit SOFCs cathode improvement.;The conductivity relaxation technique involves measurement of time variation of the electrical conductivity of a sample after a stepwise change in the ambient oxygen partial pressure. Oxygen surface exchange (k) and bulk diffusion coefficients (D) can be obtained based on the correlation between a mean conductivity and the corresponding mean non-stoichiometry. Although the ECR technique has been widely used in various applications, reliability and accuracy of fitted results have been rarely discussed. Indeed, non-unique local fitting error minimums exist when fitting a single relaxation data set. Enhanced accuracy of D and k are obtained by fitting two sets of data and plotting the error intersection.;Oxygen surface exchange and bulk diffusion coefficients of the widely used cathode material La0.6Sr0.4Co0.2Fe 0.8O3-delta (LSCF) were obtained by applying the improved fitting method. The results indicated that the oxygen surface exchange coefficient depends on the final oxygen partial pressure following the P1/2O2 law. On the other hand, the oxygen bulk diffusion coefficient was considered to be influenced by the oxygen vacancy concentration and the ordering degree.;Electrical conductivity relaxation was further developed to investigate infiltrated cathode materials in this work. Ce0.8Sm0.2O 1.9 (SDC) and La0.6Sr0.4CoO3-delta (LSC) were chosen as the infiltrated materials. The oxygen exchange coefficient at the infiltrate/cathode backbone interface was deduced from the testing results. Both of the two infiltrated materials promoted the oxygen transport rate in LSCF. Under high oxygen partial pressure, the SDC spin coated LSCF sample showed a greater improvement than the LSC spin coated sample.;In addition, a model was built up to understand SOFCs infiltrated cathode. Infiltrate/cathode backbone interface and the corresponding 3PB region distinguished infiltrated SOFCs cathode from single phase cathode. Simulation results are more plausible by including the experimentally obtained oxygen interface exchange coefficient. Over-potential effects and infiltrated material optimization were included in the discussion.

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