Development of Solid Oxide Fuel Cell cathodes for operation at 600 °C

Peter Ried

Research output: ThesisDoctoral Thesis

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Abstract

Cathodes for Solid Oxide Fuel cells were developed for a reduced operating temperature of 600 °C. The major reason is to reduce material costs and degradation of the Solid Oxide Fuel cell. The cathode mainly contributes to the losses of the Solid Oxide Fuel Cell and therefore the aim of this study was to produce cathodes with a polarization resistance 0.5 Ohm cm2. Four different cathode materials, namely La0.6Sr0.4Co0.2Fe0.8O3-d, Ba0.5Sr0.5Co0.8Fe0.2O3-d, La0.6Sr0.4Fe0.8O3-d and (La0.8Sr0.2)0.95Fe0.8O3-d, were compared regarding the relevant properties for fuel cell operation at 600 °C. After material synthesis the powder properties were evaluated regarding phase purity, particle size distribution, Specific Surface Area and sintering properties. The Coefficient of Thermal Expansion of dense bulk samples was obtained from dilatometer curves. The investigations resulted in phase pure sub micron powders with specific surface area of 3 m2/g of all four compositions and phase pure nano powders of La0.6Sr0.4Co0.2Fe0.8O3-d, (30 m2/g) additionally. The coefficient of thermal expansion of all materials except (La0.8Sr0.2)0.95Fe0.8O3-d ( 12 ppm/K) is far larger (> 16 ppm/K) than for common electrolyte materials ( 10 ppm/K) at 600 °C. Knowing the sintering behaviour, dense bulk pellets were sintered from powders of all four compositions, their electrical conductivity was evaluated in the van der Pauw geometry and the chemical diffusion and surface exchange coefficients were evaluated by conductivity relaxation. Conductivities between 48 and 411 S cm-1 were measured at 600 °C, the chemical diffusion coefficients > 10-5 cm s-2 (except La0.6Sr0.4Co0.2Fe0.8O3-d) and surface exchange coefficients compared at 700 °C were > 10-3 cm s-1. Despite large CTE and lower diffusion coefficient La0.6Sr0.4Co0.2Fe0.8O3-d was selected for the production of cathodes, because of the favourable that nano powders could be produced and the high electrical conductivity. Microstructures and the obtained polarization resistances of different cathodes, including sub micron and nano powders as well as composite mixtures were compared. Most cathodes resulted in rather dense microstructures with <20 % porosity and the lowest obtained polarization resistance was 0.76 Ohm cm at 600 °C. Literature predicts comparable values modelled from the kinetic parameter, the microstructure and the conductivity of LSCF at 600 °C and 800 °C.
Translated title of the contributionEntwicklung von Kathoden für Festelektrolytbrennstoffzellen bei 600 °C Betriebstemperatur
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Sitte, Werner, Assessor A (internal)
  • Hofer, Ferdinand, Assessor B (external), External person
Publication statusPublished - 2009

Bibliographical note

embargoed until null

Keywords

  • cathode Solid Oxide Fuel Cell synthesis nano powder composite cathode oxygen exchange kinetics diffusion surface exchange electrical conductivity polarisation resistance conductivity relaxation impedance spectroscopy dilatometry

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