Thin film mixed conducting cathodes for intermediate temperature solid oxide fuel cells

Bainye Francoise Angoua, Purdue University

Abstract

This work was focused on the processing and characterization of thin (200–300 nm) film La0.6Sr0.4Co0.2Fe 0.8O3-δ-Ce0.8Gd0.2O2-δ (LSCF-CGO) composite cathodes to achieve low polarization resistances at reduced temperature (300–600°C). A single precursor solution was employed to process the LSCF-CGO films by spray pyrolysis. The films were deposited on YSZ substrates to simulate the electrolyte/cathode interface in micro-scale SOFCs. A solution of nitrate salts dissolved in a mixture of ethanol and diethylene glycol was sprayed on a sintered YSZ laminate held at 235°C. The as-deposited films were amorphous and were heated to 500–900°C for crystallization to occur. Phase segregation was observed by x-ray diffraction upon annealing above 600°C. Grain growth in the LSCF-CGO films was followed at 700–900°C and occurred to lesser extent than in LSCF films due to the CGO phase. The polarization resistance of the films with 0, 41, 66 and 100 vol% CGO fraction and annealing temperatures ranging from 500 to 900°C was studied by impedance spectroscopy at 350–600°C. Several models proposed for mixed conducting cathodes were examined. A model for co-limitation of oxygen diffusion and surface exchange processes was selected based on fit quality and confidence interval of parameter estimates. The magnitude and activation energy of the polarization resistance decreased with CGO addition, which was consistent with the properties of CGO. The resistances of LSCF and 59LSCF-41CGO films annealed at 700°C were 17 and 9 ohm.cm2 at 500°C. The variation of polarization resistance with increasing annealing temperature displayed a “v” shaped curve in the 300–500°C operating temperature range. The resistance of 59LSCF-41CGO films annealed at 500, 700 and 900°C were 290, 106 and 256 ohm.cm2 respectively, at 400°C. The respective activation energies estimated over the 350–600°C operating temperature range were 1.45, 1.11 and 1.04 eV. The films annealed at 500–600°C displayed a mixed amorphous-crystalline structure with very fine crystallites which may explain their higher activation energies. For applications to micro-SOFCS, these films require operating temperatures of 600°C to achieve low polarization resistances.

Degree

Ph.D.

Advisors

Slamovich, Purdue University.

Subject Area

Energy|Materials science

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