Ginny Sliman
Pacific Northwest National Laboratory
Cost, reliability
and performance-Pacific Northwest National Laboratory is applying research
and development to the product triangle to produce materials and components
that will make solid oxide fuel cells affordable and reliable for diverse
applications.
PNNL is a US Department of Energy Laboratory in Richland, Wash. "We
take a multidisciplinary approach to projects," said Gary McVay,
who manages PNNL's fuel cell programs. "We have chemists, material
scientists, computer scientists and a host of other experts and resources
to solve complex problems. Our capabilities are not widely replicated
in industry or academia."
The Laboratory's range of expertise and history of fuel cell research
in materials and manufacturing, modeling and simulation, fuel reformation
and thermal management have made it the primary technology contributor
to DOE's Solid State Energy Conversion Alliance (SECA) program.
"One of the reasons we are leading SECA's technology effort is that
we are developing software tools for modeling not only electrochemical
reactions in the fuel cells, but also how these reactions interact with
fuel cell design," McVay said. "SECA industry teams already
are using our simulation tools to build virtual stacks, run them through
their paces and make design changes based on what the model tells them
the fuel cell is going to do."
Higher
Power at Lower Temperatures
As part of SECA's Core Technology Program, PNNL scientists are developing
a cathode material that will allow high power density (measured in watts
per square centimeter) and stability over time.
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| PNNL researcher Eric Mast operates a system that laminates materials to fabricate solid oxide fuel cells. The lamination process enables multilayer structures, such as the anode-supported SOFC, to be formed from thin flexible tapes. |
Scientists have found
that cathodes made of lanthanum ferrite provide two to three times more
power at operating temperatures of 600°C to 800°C than those constructed
of the classic cathode material, lanthanum manganite. One of the reasons
PNNL scientists are focusing on technologies that work at lower operating
temperatures is that they provide a less harsh environment for other SOFC
components, such as interconnects and balance-of-plant around the stack.
Researchers now are trying to manipulate the cathode composition-both
physically and chemically-to advance cathode activity at lower temperatures.
"Physically, we're attempting to engineer structured interfaces to
increase the effective contact area between the cathode material and the
electrolyte," said Steve Simner, one of about 40 material scientists
contributing to fuel cell development at PNNL.
Chemically, researchers are trying to manipulate the chemistry of the
cathode and its interfaces to enhance the cathode's electronic and ionic
conductivity and improve the kinetics of the oxygen transfer processes
that occur at the cathode/electrolyte interface. Better performance translates
into cost savings. A fuel cell with higher power per square centimeter
of cell means less cell area, less material, less volume and less mass.
"SECA's main goal is getting the cost down to $400 a kilowatt for
the whole system," McVay said. "We are supporting the industrial
teams to achieve this. Nobody's going to want them if we can't make them
reliable and inexpensive. That's what SECA is all about."
New
Anode Material Outperforms Competitors
PNNL researchers working with SECA's Core Technology program also have
been tasked with developing improved anode materials. One of the limitations
of the standard nickel-based anode is that it is intolerant of sulfur,
which is present in most fuels. "Even minute quantities of sulfur
present in the fuel stream can kill anode performance," said Jeff
Stevenson, a PNNL material scientist who directs PNNL's Core Technology
materials activities. "The challenge is to reduce sulfur concentrations
to a few parts per million or less, or to develop new anode materials
which can tolerate higher amounts of sulfur in the fuel."
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| PNNL researcher Jin Yong Kim uses button cell testing to analyze the electrochemical characteristics of electrode materials for solid oxide fuel cells. Data obtained from button cell tests is used to select electrode materials for full-sized cells. |
Previously, pretreatment
of the fuel has solved the sulfur problem, but a sulfur tolerant anode
would eliminate the need for pretreatment, helping to reduce SOFC cost.
A related issue is oxidation-reduction resistance. Nickel oxidizes rapidly
and may break apart when exposed to air, which can occurduring startup
and shutdown of the fuel cell system. An anode material that does not
need protection from air would simplify the system and cut costs by eliminating
the need to protect the anode during system startup and shutdown.
PNNL scientist Olga Marina has discovered a new class of materials that
provides a good balance among the many anode performance criteria. The
composite material, made of strontium titanate and ceria, provides excellent
oxidation resistance and sulfur tolerance, has electrocatalytic activity
comparable to nickel and offers adequate electrical conductivity.
"It's a breakthrough because up until now, there had been no good
alternative to nickel," Marina said. "This material shows promising
performance quite comparable to nickel and it outperforms other known
non-metal fuel cell anodes."
Metal
Interconnects Lower Costs
Another driver for lowering SOFC stack operating temperatures is the ability
to eliminate expensive and difficult-to-work-with ceramic interconnects,
which have been the standard for traditional high temperature technology
and use less expensive and easier-to-fabricate metal interconnects. PNNL
scientists are evaluating the performance of newly developed metal alloy
interconnects under actual SOFC operating conditions.
Although interconnect testing is usually done in either a fuel or an air
environment, PNNL scientists have discovered that the oxidation/electrical
behavior of metals alloys in a dual atmosphere, which simulates the actual
SOFC interconnect exposure conditions, is quite different from their behavior
in an air-only or fuel-only environment. "What we're really sorting
out now is why these alloys behave so differently in a dual atmosphere
environment," Stevenson said.
PNNL scientists are focusing on chromium-forming ferritic stainless steels-which
offer good thermal expansion matching, an electrically conductive oxide
scale and low cost-as well as other oxidation-resistant alloys. "We're
evaluating the state-of-the-art alloys, and also making further modifications
to them both in terms of bulk composition and surface modifications,"
Stevenson said.
While reducing cost is the main reason for using metal interconnects,
Stevenson insists that reliability is a theme in all of his team's work.
"You must have materials that are reliable over the lifetime of the
stack," he said. "In order to achieve that reliability, we need
improved materials, especially in the areas of seals and interconnects."
SECA goals include a 40,000-hour lifetime for stationary fuel cell stacks
and up to a 20,000-hour lifetime for auxiliary power units in transportation
applications.
New
Seal Material Brings Greater Reliability
Reliability is a major issue in fuel cell seal development because during
long periods of operation, and especially during thermal cycling, the
fuel cell's thermal profile can vary considerably, creating a variety
of stresses.
"None of the fuel cell materials are perfectly matched in their expansion,
so if you heat the cell from room temperature to 800°C, the cell may
want to expand more than the interconnect does," Stevenson said.
"It is not uncommon for the resulting stresses to cause failure of
the seals, which can drastically reduce the performance of the stack.
This is one of the biggest challenges to getting planar stacks out of
the laboratory and into the marketplace."
Rather than using traditional glass-based materials, which provide a rigid
seal, PNNL researchers are working with mica materials to construct a
more flexible, compressive seal. Researchers are using a gasket approach,
where mica materials are put between fuel cell components and an external
load is applied, making it similar to an "O" ring or a head
gasket in a car.
The mica-based seal is a more forgiving seal in terms of thermal expansion
mismatch because it is made of parallel layers that de-bond from each
other under high temperatures. As a result, if a certain fuel cell component
expands significantly more than an adjacent component, the mica material
can mechanically de-couple the two components, preventing the buildup
of destructive stresses.
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SECA:
A Unique Alliance The
Solid State Energy Conversion Alliance (SECA) is a US Department
of Energy program devoted to promoting and accelerating the development
and commercialization of low-cost, environmentally friendly solid
oxide fuel cells for stationary, transportation and military applications.
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PNNL
Partners with Industry
Pacific Northwest
National Laboratory is working with industrial partners to develop
and test advanced cell and stack designs, component materials and
low cost fabrication processes that help accelerate the development
of low cost solid oxide fuel cell systems. The Laboratory's extensive
knowledge base in materials synthesis, testing, design optimization,
fuel processing and catalysis provide the foundation for solving
the challenges faced by SOFC system developers. PNNL's industry-related
activities include: |
For more information about PNNL's fuel cell research, contact Jeff Stevenson at jeff.stevenson@pnl.gov.