Development of cathode materials for intermediate temperature solid oxide fuel cells

Abstract

A solid oxide fuel cell (SOFC) is a promising energy device that can generate electricity by converting chemical energy of nearly all types of fuels with very high efficiency. However, its high operating temperature (> 850°C) is the main impediment to deploying this technology, because high temperature can lead to sealing issues, slow start-up/shut-down procedures, poor thermal cycling stability, poor fuel cell durability, as well as high material and operational cost. Lowering the operating temperature down to intermediate temperature (IT, 500 °C – 750 °C) is an effective and significant strategy to solve these issues, but it makes the kinetics of electrolyte and electrodes especially the cathode sluggish. Despite slow kinetics of electrolyte have been significantly alleviated by using novel electrolyte materials and thin film fabrication technology, low electroactivity of IT-SOFC cathode still remains a major challenge. Besides, the susceptibility of cathodes containing alkaline-earth elements to CO2 is another concern on long-term cathode stability, especially at low temperature. Therefore, developing a robust cathode material with high electroactivity is significant for commercialising SOFC technology, and have received growing research interest and efforts in recent years. This thesis is mainly focused on developing highly active and stable cathode materials based on SrCoO3-δ perovskite oxide for IT-SOFC. The factors affecting catalysis on oxygen reduction reaction (ORR), and the CO2-poisoning mechanisms on the SrCoO3-δ-based cathodes at intermediate temperature were investigated. In this thesis, we developed and evaluated SrCoO3-δ doped with high-valence elements such as P, Nb, and Ta as cathodes for SOFC by studying their crystal structures, compositions, microstructures and electrochemical properties as well as electroactivity in ORR at intermediate temperature. In the first part of the experimental chapters, we mainly worked on developing SrCoO3-δ-based cathode materials and studying the effects of high fixed valence dopants (P, Ta, and Nb) on SrCoO3-δ perovskite cathode for IT-SOFC. We successfully doped P and Ta into SrCoO3-δ oxide separately, and found these dopants at certain doping level can stabilise the beneficial perovskite structure at both room temperature and intermediate temperature. The study on P-doped SrCoO3-δ reveals that the stabilising effect of P is a result of the high-valence that prevents oxygen vacancy ordering and phase distortions. The electrical conductivity of SrCoO3-δ can be enhanced by small amount of P or Ta (≤ 5 mol%) due to the stabilized perovskite structure and high valence of P and Ta, but can be adversely affected for higher doping level as shown in study on SrCo1-xTaxO3-δ. Additionally, SrCoO3-δ doped with <20 mol% Ta shows superior electroactivity on ORR at IT, with a cathode polarisation resistance as low as 0.089~0.11 Ω·cm2 at 550°C for SrCo0.95Ta0.05O3-δ. However, the high fixed valence can decrease oxygen vacancy content, so high doping level (e.g. 40mol%) of Ta can seriously deteriorate cathode electroactivity at intermediate temperature. In the second part, we investigated other non-geometry factors that have an effect on cathode electroactivity. We doped 20mol% of Nb and Ta separately into SrCoO3-δ oxides, and compared their ORR-related properties. The reason we chose Nb and Ta as dopants is that these dopants have the same valence state and very similar ionic radii. These similarities allow us to explore other factors that may affect ORR activity by constraining their geometry factor. This comparative study shows that lower electronegativity of Ta than Nb can reduce the average valence of neighbouring Co, thus creating more oxygen vacancies and leading to higher electroactivity. Moreover, we developed a highly active ORR catalyst by co-doping Nb and Ta into SrCoO3-δ, showing a remarkably low polarisation resistance of ~0.16 Ω·cm2 at 500 °C. The outstanding cathode performance is likely attributed to an optimised balance of oxygen vacancy content, oxygen ionic mobility and surface electron transfer ability. The focus of the third part of experimental chapters is to address the susceptibility of SrCoO3-δ-derived cathode materials to CO2 at intermediate temperature. We incorporated Sm-doped ceria (SDC) into SrCo0.85Ta0.15O3-δ cathode by either mechanical mixing or wet impregnation, and significantly improved the CO2 tolerance of SrCo0.85Ta0.15O3-δ by over 5 times in the presence of 10% CO2 at 550 °C as compared to pure SrCo0.85Ta0.15O3-δ. The CO2 resistance improvement of SDC is a result of the low CO2 reactivity and adsorption on SDC. More importantly, this strategy prevails for other cathode materials containing alkaline-earth elements, such as benchmark IT-SOFC Ba0.5Sr0.5Co0.8Fe0.2O3-δ.