Semester

Fall

Date of Graduation

2019

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu

Committee Member

Harry Finklea

Committee Member

Edward Sabolsky

Committee Member

David Mebane

Committee Member

Konstantinos Sierros

Committee Member

Wenyuan Li

Abstract

Replacing the electronically conductive (LaSr)MnO3±δ (LSM) cathode in the LSM/yttrium- stabilized zirconia (YSZ) system with the mixed ion-electron conductive (MIEC) (LaSr)(CoFe)O3-δ (LSCF) will promote cathode performance in SOFCs significantly. However, it might be hindered by the reaction between YSZ and LSCF, producing some insulating phases, which lowers the cell performance. To address this issue, a dense barrier layer of doped ceria is always adopted between these two components to eliminate the reaction as well as to boost the cell performance. In this study, a scalable and cost-effective method, electrophoretic deposition (EPD), is used to realize the deposition of gadolinium doped ceria (GDC) on non-conductive YSZ substrate. The fundamental characteristics of EPD of GDC on YSZ and the mechanism are also systematically investigated.

Highly compact GDC green layers are obtained by the EPD process in an ethanol-based suspension. GDC thin layers in a thickness range of 5-8 µm have been successfully densified at temperatures as low as 1300 oC and the adhesion between GDC and YSZ is excellent. Compared to a GDC barrier layer made by a conventional spin-coating method, the ohmic resistance of GDC made by EPD is lower. The deposition rate of GDC on PPy coated YSZ is slower than that on graphite at the same voltage. However, at constant current, the mass of GDC deposited per coulomb of charge is larger on the PPy-coated YSZ cathode. An H+ ion accumulation zone is formed near the PPy coated YSZ after applying the voltage. The thickness of this H+ ion accumulation zone increases at the beginning and then decreases. Finally, the ion accumulation zone is replaced by an ion depletion zone; the main reason contributing to the increasing resistance is the formation of an ion depletion zone. The absorbed H+ ions desorb from particles after deposition and then move through the porous deposit to the cathode to be reduced. The reduction of free H+ ions and absorbed H+ ions corresponds to the unavoidable side reaction and the deposition of GDC particles, respectively. Deposition efficiency, f, the percentage of electric charge associated with the reduction of H+ ions in the total charge, is introduced to reflect the competitive relationship between deposition and the side reactions. f decreases with the increase of current density.

A dense GDC layer is successfully obtained by AC-EPD at 500 Hz after sintering at a relatively low temperature, e.g., 1250 oC, which is anticipated to eliminate the reaction between LSCF and YSZ. An optimum frequency of 500 Hz leads to the maximum deposition rate by balancing the suppression of bubble evolution and the acceleration of the particle migration. The deposit yield in a given time grows with the increase of voltage ratio and forward width percentage. In AC-EPD with negligible faradic current, the deposition rate of GDC particles is determined by both the transport process and the desorption process, wherein the latter process is irreversible. The deposit yield is monotonically controlled by the deposition time, suggesting the possibility of fabrication of GDC layer with tunable thickness.

Dense and homogeneous GDC films have also been successfully prepared by AC-EPD in an aqueous suspension. The parameters influencing the quality of deposition films are thoroughly investigated and the optimal deposition conditions include the frequency of 1 kHz, forward width percentage of 50 % and a voltage ratio of 10/4. This work provides a facile approach to fabricating smooth GDC barrier layers with tunable thickness in an environmentally benign, rapid, versatile and low-cost manner, indicating its promise for SOFC and other applications.

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