Surface-modified oxygen electrodes for reduced temperature SOFC applications

As population and its energy demand increased, fossil fuels became a requirement for sustainment of daily life. However, their extended use has brought along several unwanted consequences that are now a subject of concern, for example global warming, increase to greenhouse gasses emissions, and changes to major meteorological events. Out of these concerns, the emergence of alternative energy devices, such as fuel cells, have attracted a significant amount of attention due to their superior efficiency and reduced dependency on fossil fuels. Out of the many fuel cells, solid oxide fuel cells (SOFC) are a specific type of fuel cell that operates at high temperatures (600 – 1000°C). Their inherently high operation temperatures broaden the spectra of potential fuel sources, and their excess heat can be integrated into other processes, further increasing the overall efficiency of the system. In recent years, SOFC technology has leaned towards reducing the operation temperature (600- 800°C). However, at lower temperatures the oxygen reduction reaction (ORR) kinetics become increasingly sluggish, challenging the use of traditional cathode materials, hence, the need for highly active cathodes, like LSCF-6428 (La0.6Sr0.4Co0.2Fe0.8 O3–δ). One of the major drawbacks of LSCF is its inherent tendency to degrade due to Sr-surface segregation (SSS), phenomena driven by elastic and electrostatic forces within the material structure that results in hindered ORR activity. Infiltration of a secondary phases has been reported to suppress SSS while enhancing ORR activity for Sr-containing electrodes. This work investigated Gd0.1Ce0.9O1.95 (GDC) and HfO2 at various concentrations as surface-modifiers on porous LSCF-6428 electrodes as well as dense substrates to evaluate their effect on electrode performance and SSS prevention when compared to unmodified LSCF when all materials are subjected to typical IT-SOFC operation conditions. It was observed from experimental results of unmodified electrodes that degradation rate for electrodes (dRp%) decreased for electrodes that were stored before testing (~31% and ~4%) in contrast to electrodes that were tested closer to their fabrication date (~60%). Additionally, SEM images showed the appearance of particles along the grain boundaries that grew with increasing ageing time, consistent with SSS in literature. The absence of SSS signs on the rest of the samples was theorised to be caused by a calendar ageing effect, that has yet to be reported in literature to the authors knowledge. When analysing surface-modified electrodes and substrates, it was observed that both surface modifications produced more stable, i.e. less degradable, and better performing electrodes as proved by EIS and Rp monitoring for 50h. GDC proved to be most effective when using 0.125M and 0.250M, while for HfO2 13.16 and 26.32 mg mL-1 concentrations worked best to produce electrodes with better ORR performance, this is lower Rp values, than non-infiltrated ones. Additionally, no surface particles that could be associated with SSS were identified using XRD nor SEM on flat substrates when surface modifications were used, suggesting that even the lowest concentrations were effective in preventing Sr-segregation. Although the previously identified calendar ageing effect on bare samples could also be identified on surface-modified measurements, it was ultimately concluded that due to time constraints and limitations, more research into this subject is necessary to elucidate the causes behind these observations.