| Summary: | Atmospheric plasma spraying is attractive for manufacturing solid oxide fuel cells (SOFCs) because it allows functional layers to be built rapidly with controlled microstructures. The technique allows SOFCs that operate at low temperatures (600 to 750°C) to be fabricated by spraying directly onto robust and inexpensive metallic supports. Processes were developed to manufacture metal-supported SOFC cathodes by axial-injection plasma spraying. Cathodes consisted of LSCF (La0.6Sr0.4Co0.2Fe0.8O3-δ) or SSC (Sm0.5Sr0.5CoO3) as the primary material. Initially, the plasma spray process parameters were varied, and x-ray diffraction analyses were performed on the cathode coatings to detect material decomposition and the formation of undesired phases. These results determined the envelope of plasma spray parameters in which coatings of LSCF and SSC can be manufactured, and the range of conditions in which composite cathode coatings could potentially be manufactured.
Subsequently, composite cathodes were fabricated by mixing up to 40 wt. % of the ionic conducting SDC (Ce0.8Sm0.2O1.9) material into the feedstock. The deposition efficiencies of these cathodes were calculated based on the mass of the sprayed cathode. Particle surface temperatures were measured in-flight to enhance understanding of the relationship between spray parameters, microstructure, and deposition efficiency. Electrochemical impedance spectroscopy was performed in symmetrical cells: at 750°C, LSCF-SDC cathodes had polarization resistances as low as 0.101 Ωcm², and SSC cathodes had polarization resistances as low as 0.0056 Ωcm².
Finer mixing of the ceramic phases was achieved by using a nano-structured feedstock that contained both LSCF and SDC phases agglomerated together in larger particles. Fuel cells containing a yttria-stabilized zirconia (YSZ) electrolyte and a nickel-YSZ anode were fabricated, and the effect of the cathode microstructure on cell impedance was studied using the analysis of differential impedance spectra.
The degradation of composite LSCF-SDC cathodes on porous 430 stainless steel supports was also investigated. To reduce degradation, La2O3 and Y2O3 reactive element oxide coatings were deposited on the internal pore surfaces of the metal supports. As a result, polarization resistance degradation rates as low as 0.00256 Ω·cm2 /1000 h were observed over 100 hours on coated substrates, compared to 0.1 Ω·cm2 /1000 h on uncoated substrates.
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