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Ceramic fuel-cells are part of the sustainable energy system in future
10.06.2010
Master of science in engineering and VTT’s researcher Mikko Pihlatie has improved new architecture of fuel-cells. Pihlatie presents his thesis about ceramic fuel-cells development in Aalto University School of Science and Technology in 11.6.2010 at 12.00.
An operating Ni-based SOFC can be severely damaged by inadvertent oxidation of
the nickel. A central way to improve this Achilles’ heel is to design and
prepare a dimensionally stable anode half cell that does not overload the
electrolyte upon re-oxidation. Understanding the mechanisms that lead to the
redox expansion, and designing and manufacturing modified anode support
structures that improve stability is the core of the present work.
The
behaviour of Ni–YSZ cermets for SOFCs are characterised under conditions
cyclically altered between reducing and oxidising (redox cycling). The main
operating conditions that affect redox stability are shown to be temperature
and humidity; both affect the growth of Ni particles through sintering. The
temperature of re-oxidation also plays a significant role in redox stability;
a reoxidation at a high temperature (850°C or higher) leads to larger
expansions.
The behaviour of the cermet under redox
conditions is highly dependent on microstructure; as porosity of the composite
increases, redox stability is improved. A redox cycle at 600°C speeds up the
subsequent re-reduction significantly, indicating a change in microstructure
due to the re-oxidation; also the electrical conductivity of the cermets
improves on such a redox cycle. The redox strains during redox cycles below
700°C are reversible, while cumulating strain and damage is created in the
ceramic backbone at elevated temperatures.
NiO particle
growth during oxidation, combined with low temperature pseudoplasticity is
suggested to be a decisive internal factor for redox stability. Redox cycling
at high temperatures rapidly leads to irreversible nonelastic strains
(cracking, creep) in the YSZ backbone that cause mechanical degradation.
The
combination of mild operating conditions and redox-improved cells appears to
be a plausible solution to circumvent redox failures. An intentional
lowtemperature redox treatment could lead to an improvement in performance.
The durability and stability of the anode can be improved by modifications in
the microstructure and the composition of the cermets.
Thesis
in internet:
Stability of Ni–YSZ composites for solid oxide fuel
cells during reduction and re-oxidation
http://www.vtt.fi/inf/pdf/publications/2010/P740.pdf