Date : September
10, 2014
Source : Pacific
Northwest National Laboratory
Summary : More
efficient fuel cells might gain wider use in vehicles or as quiet,
pollution-free, neighborhood electricity generating stations. A serendipitous
finding has resulted in a semiconducting material that could enable fuel cells
to operate at temperatures two-thirds lower than current technology, scientists
report.
Researchers
have been trying to increase the efficiency of solid oxide fuel cells by
lowering the temperatures at which they run. More efficient fuel cells might
gain wider use in vehicles or as quiet, pollution-free, neighborhood
electricity generating stations. A serendipitous finding has resulted in a
semiconducting material that could enable fuel cells to operate at temperatures
two-thirds lower than current technology, scientists reported August 18 in Nature
Communications.
In
an attempt to create a metal oxide with the properties of metal, researchers at
the Department of Energy's Pacific Northwest National Laboratory created a new
form of the metal oxide. This particular strontium-chromium oxide performs as a
semiconductor, or a material whose ability to conduct electricity can be turned
on and off. It also allows oxygen to diffuse easily, a requirement for a solid
oxide fuel cell. Best yet, it allows diffusion at a temperature that can lead
to much more efficient fuel cells.
Nothing
is Something
Energy
researchers need improved materials to make fuel cells more widely used. Solid
oxide fuel cells require oxides capable of absorbing and transmitting
negatively charged oxygen atoms at low temperature. Current materials require
temperatures around 800 degrees Celsius (for reference, car engines run at
about 200 degrees Celsius and steel melts around 1500).
Researchers
at PNNL were trying to make strontium chromium oxide in a kind of crystalline
form called perovskite, which has many useful electronic properties. In this
material, the strontium, chromium and oxygen atoms stack together in a cube.
The metal atoms -- strontium and chromium -- bond completely to the oxygen
atoms around them.
However,
in the material that formed, the strontium chromium oxide packed into a
rhombus-shaped crystal -- think diamond -- and many of the oxygen atoms were
missing.
What's
more, the holes where the oxygen atoms had been, also called oxygen vacancies,
had come together to form well-defined planes within the new crystal structure.
The researchers found that these planes act as channels that allow oxygen from
outside the material to diffuse through the material at an exceptionally low
temperature for these materials, about 250 degrees Celsius.
"At
high enough concentrations, oxygen vacancies aggregate and form new mesoscale
structures with novel properties that the original material doesn't have,"
said PNNL materials scientist Scott Chambers, who led the research. "In
this case, the mesoscale crystalline structure transmits oxygen very efficiently."
Bad
Angle Bonds
The
scientists inadvertently generated the material by taking advantage of the
natural tendency of chromium atoms to avoid certain bonding environments. They
found that their attempts to make metallic SrCrO3 (strontium chromium oxide in
a ratio of 1:1:3) lead instead to the formation of semiconducting SrCrO2.8
(with a ratio of 1:1:2.8).
Because
chromium as an ion with a charge of +4 does not like to form 90o bonds with
oxygen, as it must in SrCrO3, SrCrO2.8 forms instead with a completely different
crystal structure. This material contains oxygen-deficient regions through
which oxygen can diffuse very easily. Those regions might provide a way to take
better advantage of the material's electronic properties.
"As
an additional benefit, ordered arrays of oxygen vacancies might allow us to
separate the material's electronic and thermal properties," said Chambers.
"This would help us improve the performance of thermoelectrics, in either
generating power from heat or for use in refrigeration."
The
team made ultra-pure crystalline films of the new material and used instruments
and expertise at EMSL, DOE's Environmental Molecular Sciences Laboratory, to
understand the material's properties. A DOE Office of Science User Facility,
EMSL scientists worked with Chambers to develop a new instrument called an
oxygen-assisted molecular beam epitaxy deposition system that is specifically
designed to make and study these kinds of crystalline films.
Towards
Light and Electrons
In
the future, the team plans to apply the understanding gained to other
materials, such as the deposition, characterization, and understanding of
epitaxial strontium-doped lanthanum chromite, which has potential importance in
visible light harvesting.
In
the long term, the team plans to exploit the observed phenomenon to carry out
nanofabrication of novel heterogeneous catalytic structures by depositing
submonolayer quantities of catalytically important metals on the surface of
rhombus-shaped, semiconducting oxide, and using the intersection of the defect
planes with the free surface to order the incoming metal atoms into nanowires.
This
work was supported by the Department of Energy Office of Science, EMSL and
PNNL.
Story
Source:
The
above story is based on materials provided by Pacific
Northwest National Laboratory. Note: Materials may be edited
for content and length.
Journal
Reference:
1.
K. H. L. Zhang, P. V. Sushko, R.
Colby, Y. Du, M. E. Bowden, S. A. Chambers. Reversible nano-structuring of
SrCrO3-δ through oxidation and reduction at low temperature. Nature
Communications, 2014; 5 DOI: 10.1038/ncomms5669
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