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.