Modeling, Design, Construction, and Operation of Power Generators with Solid Oxide Fuel Cells by Jakub Kupecki

Author:Jakub Kupecki
Language: eng
Format: epub, pdf
Publisher: Springer International Publishing, Cham

(4.26)

Mixed oxides with several lattice structures have not proved to be favorable options to date, namely fluorite (zirconia- and ceria-based), rutile, tungsten bronzes, and pyrochlores [77, 78]. Doped zirconium oxide (with Ti and La) and doped cerium oxide (with La, Ca and Mn) are not interesting due to their low electrical conductivity (0.1–1 S/cm). Rutile and tungsten bronzes show low ionic conductivity, which leads to large polarization resistance even in H2. A few pyrochlores (Gd2Ti2O7 and Gd2Mo2O7) couple both ionic and electronic conductivity, therefore showing MIEC properties, but tests in the presence of hydrocarbons are still needed and redox stability is questionable. Instead, some promise is shown by perovskite oxides, both single (ABO3) and double (A2BBO6). Several extensive reviews are available, which are dedicated to the application of this class of materials as anodes for SOFCs [73, 77, 78, 137]. Perovskites allow for very versatile introduction of vacancies and defect chemistry, and significant distortions can be adapted in the lattice by introducing aliovalent ions in the A site and in the B site. Oxygen ion vacancies are generated by partial substitution of the tetravalent B site ion with an ion with lower valence; interstitials are formed by doping the B site with a higher valence ion. Vacancies on the divalent A site can be formed by substituting cations with lower valence. Typically, alkali, alkaline earths, and rare earths are introduced on the A site, whereas multivalence transition elements can be introduced on the B site. Double perovskites are obtained by using two different elements on the B site, both in excess of the percolation limit (>30%): Depending on the cation combination and on the operating conditions, ordered structures can form, with alternate layers of BO3 and B′O3 corner-sharing octahedra. Among the perovskite anodes, titanates, chromites, and vanadates are mostly investigated [138].

SrTiO3 strontium titanate is a prototypical perovskite, which is widely studied due to its high compositional flexibility. It is stable in very low pO2 and becomes an MIEC when appropriately doped, on the Sr site with higher valence cations, and on the Ti site with lower valence cations. In order to confer electronic conductivity, La and Y are commonly used to dope the Sr site, leading to La-doped SrTiO3 (LST) and Y-doped SrTiO3 (YST). Nb, Sc, Mn, and Ga are dopants on the Ti site. A major drawback of these materials is the need for high-temperature calcination in a reducing atmosphere to achieve enough electronic conductivity via production of the Ti3+ ion (e.g., 1400 °C, 7% H2/Ar for YSZ), which makes it difficult to co-fire onto the electrolyte. LST has a very low tendency to form carbon, and for this reason, it has been applied as a support, scaffold, and barrier layer. Its electrochemical performance is still low, but improvement is possible by substituting Mn and Ga on the B site, individually or in co-doping (leading to LSTMG anodes).

Lanthanum chromites (LaCrO3) doped with Ca or Sr are used as interconnects in SOFC owing to their stability in both reducing and oxidizing conditions.

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