The formation of performance enhancing pseudo-composites in the highly active La1-xCaxFe0.8Ni0.2O 3 system for IT-SOFC application

Abstract

The La1-xCaxFe0.8Ni0.2O 3-δ (0 ≤ x ≤ 0.9) system is investigated for potential application as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). A broad range of experimental techniques have been utilized in order to elucidate the characteristics of the entire compositional range. Low A-site Ca content compositions (x ≤ 0.4) feature a single perovskite solid solution. Compositions with 40% Ca content (x = 0.4) exhibit the highest electrical and ionic conductivities of these single phase materials (250 and 1.9 × 10-3 S cm-1 at 800 °C, respectively), a level competitive with state-of-the-art (La,Sr)(Fe,Co)O3. Between 40 and 50% Ca content (0.4 > x > 0.5) a solubility limit is reached and a secondary, brownmillerite-type phase appears for all higher Ca content compositions (0.5 ≤ x ≤ 0.9). While typically seen as detrimental to electrochemical performance in cathode materials, this phase brings with it ionic conductivity at operational temperatures. This gives rise to the effective formation of pseudo-composite materials which feature significantly enhanced performance characteristics, while also providing the closest match in thermal expansion behavior to typical electrolyte materials. This all comes with the advantage of being produced through a simple, single-step, low-cost production route without the issues associated with typical composite materials. The highest performing pseudo-composite material (x = 0.5) exhibits electronic conductivity of 300-350 S cm-1 in the 600-800 °C temperature range while the best polarisation resistance (Rp) values of approximately 0.2 Ω cm2 are found in the 0.5 ≤ x ≤ 0.7 range. A wide compositional range of materials are proposed and applied as cathodes featuring highly competitive performance. Contrary to expectations, the formation of perovskite-brownmillerite pseudo-composites with high Ca substitution results in significant improvements to electrochemical performance and excellent thermomechanical compatibility with electrolytes. Furthermore, new insights are gained into the material surface properties controlling oxygen reduction processes. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.