Doctoral Dissertations

Abstract

"The electrochemical behavior of single solid oxide fuel cells based on Y-stabilized ZrO2 (YSZ) was investigated. Samples were fabricated by tape casting electrolytes followed by screen printing anode and cathode compositions. Single cells were characterized at 1000°C utilizing a Pt voltage probe positioned in the center of the electrolyte. This allowed the bulk properties (ohmic) to be separated from the interfacial phenomena (overpotentials) for the total cell during operation. AC impedance measurements were used to correlate the total cell impedance to the sum of the two-half cell impedances.

Stoichiometric (La0.79Sr0.2MnO3) and nonstoichiometric (La0.75Sr0.2MnO3 and La0.70Sr0.2MnO3) cathode compositions were synthesized with varying grain sizes. This was achieved by varying the powder calcination temperature, and the temperature the cathode was sintered onto the electrolyte. The electrochemical behavior and degradation rates were monitored for all cathodes over a 24 h period and correlated with the microstructure and composition. Cathodes prepared with fine grain sizes initially exhibited low overpotentials, but degraded within 24 h in comparison to cathodes prepared with larger grain sizes. The nonstoichiometric composition, La0.70Sr0.2MnO3, showed the best performance, η ~ 50 mV at 1000 mA/cm2. This was due to the suppression of the interfacial reaction product, La2Zr2O7. In addition, the influence of gas composition and operation temperature were also investigated in relation to the electrode overpotential.

Ni-YSZ anode compositions were initially fabricated with varying Ni volume fractions (40-55 %) using the glycine nitrate process to prepare the NiO. Low vol % Ni cermets (40 and 45 %) had the lowest overpotentials, η ~ 160 mV at 1000 mA/cm2, and smallest degradation rates. To further improve the performance, various processing strategies were employed to stabilize the microstructure; to decrease the sintering between Ni grains. The electrochemical response, in-plane (x-y) electrical conductivity, and degradation within a 24 h period was investigated and correlated with the composition and microstructure. Results indicated that the best performance was achieved using high sintering temperatures (1500°C), and low vol % Ni (40 %), or a co-fired anode/electrolyte structure. Gas composition was not investigated but was found to be rate limiting in the anode reaction due to the small percentage of fuel present (10% H2)"--Abstract, page iii.

Advisor(s)

Huebner, Wayne

Committee Member(s)

Anderson, H. U. (Harlan U.)
Moore, Robert E., 1930-2003
Switzer, Jay A., 1950-
Kosacki, Igor

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Ceramic Engineering

Publisher

University of Missouri--Rolla

Publication Date

Fall 1996

Pagination

xvii, 240 pages

Note about bibliography

Includes bibliographic references (pages 226-239).

Rights

© 1996 David Matthew Reed, All rights reserved.

Document Type

Dissertation - Restricted Access

File Type

text

Language

English

Thesis Number

T 7202

Print OCLC #

36458354

Electronic OCLC #

1089930915

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