Characterization of mixed-conducting barium cerate-based perovskites for potential fuel cell applications / R. Mukundan.

Format:

Book

Manuscript

Microformat

Thesis/Dissertation

Author/Creator:

Mukundan, R.

Contributor:

Davies, Peter K., advisor.

Worrell, Wayne L., 1937-2012, advisor.

University of Pennsylvania.

Language:

English

Subjects (All):

Penn dissertations--Materials science and engineering.

Materials science and engineering--Penn dissertations.

Local Subjects:

Penn dissertations--Materials science and engineering.

Materials science and engineering--Penn dissertations.

Physical Description:

xvii, 155 pages : illustrations ; 29 cm

Production:

1997.

Summary:

Chemical modifications of barium cerium gadolinium oxide through the substitution of Bi, Tb, Pr, Nb and Ta were attempted in an effort to increase the p-type or n-type conductivity, and to develop new mixed-conducting electrodes that are chemically compatible with the $\rm Ba(Ce\sb{1-x}Gd\sb{x})O\sb{3-x/2}$ electrolyte. The structure, oxygen non-stoichiometry, electronic and ionic-conductivity of several compositions in the doped-barium cerate systems were studied by X-ray diffraction, TGA, DC and AC conductivity, and EMF measurements. The cathodic overpotential of the mixed (electronic/ionic) conducting compositions in this system, on a $\rm Ba(Ce\sb{0.8}Gd\sb{0.2})O\sb{2.9}$ electrolyte, were also studied using Current Interruption and AC impedance techniques.

The substitution of Bi into $\rm Ba(Ce\sb{0.9}Gd\sb{0.1})O\sb{2.95}$ lead to a significant increase in the electronic conductivity, and a total conductivity of about 0.94 S/cm was obtained for $\rm Ba(Bi\sb{0.5}Ce\sb{0.4}Gd\sb{0.1})O\sb3$ at 800$\sp\circ$C in air. However, the concentration of oxygen-ion vacancies and hence the ionic conductivity decreased due to the oxidation of Bi to the 5$\sp{+}$ state. Compositions in the $\rm Ba(Bi\sb{0.5}Ce\sb{x}Gd\sb{0.5-x})O\sb3$ system also exhibited significant oxygen non-stoichiometry depending upon the ordering of the B-site cations and the relative concentrations of Ce and Gd. However, the absence of any detectable EMF in the non-stoichiometric compositions implied that the oxygen vacancies are strongly associated with the Bi$\sp{3+}$ cations. Although highly conductive, chemically stable compositions were prepared in the $\rm Ba(Bi\sb{x}Ce\sb{y}Gd\sb{1-(x+y)})O\sb{3-d}$ system, their ionic conductivities were low.

The mixed-conduction properties of $\rm Ba(Ce\sb{1-x}Gd\sb{x})O\sb{3-d}$ were enhanced under cathode conditions (600-800$\sp\circ$C in air) by the substitution of Ce by Tb and Pr. While the substitution of Tb resulted in a decrease in the total conductivity, Pr induced a significant increase in the total conductivity at high Pr levels ($\ge$40 mole%) due to an enhancement of the electronic conductivity. The $\rm Ba(Pr\sb{0.8}Gd\sb{0.2})O\sb{2.9}$ sample was found to have the best mixed-conducting properties of all the perovskites evaluated, $\rm\sigma\sb{T}=0.75$ S/cm in air at 800$\sp\circ$C, $\rm t\sb{H+}=0.15$ in a wet argon//dry argon gradient, and $\rm t\sb{0.2-}\approx 0.05$ in a dry air//dry argon gradient. The cathodic overpotentials of the mixed-conducting Pr-doped barium cerates were low, and decreased with increasing ionic and electronic conductivity of the electrode. The lowest overpotential was obtained for the $\rm Ba(Pr\sb{0.8}Gd\sb{0.2})O\sb{2.9}$, cathode, and at low current densities was comparable to that of an optimized porous Pt-electrode.

While the substitution of Nb and Ta for Ce lead to an enhancement in the electronic conductivity under reducing conditions associated with the increased reduction of Ce$\sp{4+}$ to Ce$\sp{3+}$, the ionic-conductivity of these perovskites was low. There was no evidence for any protonic conductivity in the 15 mole% Nb and Ta substituted barium cerates. Moreover the anodic overpotential and the anode resistance of these perovskites on a $\rm Ba(Ce\sb{0.8}Gd\sb{0.2})O\sb{2.9}$ electrolyte were both high.

Notes:

Supervisors: Peter K. Davies; Wayne L. Worrell.

Thesis (Ph.D. in Materials Science and Engineering) -- University of Pennsylvania, 1997.

Includes bibliographical references.

Local Notes:

University Microfilms order no.: 97-27265.

OCLC:

187470445