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Solid Oxide Fuel Cells
Ready to Market?
by Sandrine Colson-Inam, Ph.D.
Solid oxide fuel cells (SOFC) represent the second most developed fuel cell
technology after proton exchange membrane (PEM).
While PEM fuel cells are now available on the
market for stationary power as a portable consumer
electronics option, SOFC are reaching pre-commercialization
with hundreds of residential stationary power
units (about 1kW) being tested in Europe and larger
units (250kW or above) being evaluated by various
utility companies worldwide. SOFC are emerging
as an option for hybrid electric vehicles (HEV)
in auxiliary power units (APU).
The following and other issues have been reviewed
in a recently published report from BCC Inc.,
RGB-282 – Solid Oxide Fuel Cells: What are
the market incentives making SOFC attractive (cost
and performance) for a wide range of applications?
What new technological advances made it possible
for SOFC to reach pre-commercialization? How do
SOFC stand compared to other competing technologies?
What other factors will influence the success
of SOFC: legislation, funding, fuel infrastructure?
SOFC Technology
Schoenbein and Grove (1834-1854) were the first
to complete experiments on water electrolysis
using hydrogen and oxygen gases. Around 1937,
Bauer and Preis developed the first solid oxide
fuel cells using zirconia as electrolyte. Fuel
cells were further developed through the various
U.S. space programs. During the ‘60s and
‘70s, Westinghouse continued to develop SOFC
in the U.S. until a satisfactory design appeared
in the ‘80s to yield to a commercially viable
stack design in the ‘90s. (Westinghouse uses
a tubular SOFC design.) The first SOFC-powered
electrical plant was demonstrated in 1997 by Westinghouse,
now Siemens Westinghouse Power Corp. (SWPC), in
Europe with electrical conversion efficiency of
more than 45%, the ability to generate steam in
parallel, and an operating life of more than 20,000
hours. Combined heat and power generation (CHP)
using a micro-turbine with SOFC, for example,
can result in energy conversion efficiencies of
about 75-85%.
Solid oxide fuel cells refer to fuel cells where
the electrolyte (ion-conducting) medium separating
the electrodes is solid, typically a ceramic material
such as yttria-stabilized zirconia (YSZ). The
anode is usually a Ni-cermet (ceramic-metal composite
material) and the cathode, lanthanum-strontium
manganese oxide (LSM). In SOFC, hydrogen (or raw
fuel) is converted to water at the anode while
oxygen is converted in oxygen ions that migrate
through the electrolyte ceramic to the anode.
As the main current carriers are oxygen ions that
diffuse (related to conductivity) slowly at room
temperature, operating temperatures ranging from
800°C to 1000°C are typical, requiring
exotic and costly materials such as lanthanum
chromite to be used for current collectors (interconnects).
Cell designs for SOFC are either a planar geometry
where individual cells (anode/electrolyte/cathode
layers) can be stacked together to form a “stack”
or a tubular design where the various layers (anode/electrolyte/cathode)
are deposited in the form of cylindrical tubes
that are then assembled together to form stacks.
The planar designs are easier to manufacture and
have the potential to provide higher power densities,
thus lower cost. Diverse variations exist between
these designs and each manufacturer gives its
twist. Stacks then become part of the SOFC system,
which includes: the balance of plant or BOP (all
the monitoring and controlling fuel-related components),
fuel and oxidant (oxygen/air) reservoirs, the
fuel cell stack, and the electronics that interfaces
to the powered device.
Fuel cells present the advantages of high energy
conversion (40-60%), low greenhouse gases emissions,
and an alternative power option for distributed
power generation (DG). Solid oxide fuel cells
present the following added benefits: fuel flexibility
(pure hydrogen is not needed and direct fuels
such as natural gas, fossil fuels, diesel can
be used without reforming), simplified BOP, higher
power potential, low or maintenance-free, high
electrolyte stability over a wide range of temperatures,
flexible cell design, long stack life as all components
are solid, and no expensive catalysts such as
platinum required.
The main technological drawbacks of SOFC are high
operating temperature range and long warm-up time.
High temperature requires glass-ceramic seals
and exotic interconnect materials (lanthanum chromite
or high-temperature stainless steel), increasing
the cost. In addition, to prevent component failure
(thermal shock), slow startup time (up to several
hours) is required before the SOFC system can
operate at its optimal performance. Recent developments
have been related to bringing the operating temperatures
of SOFC down through new materials. Temperature
as low as 500°C have been reported (some even
claim as low as 200-400°C). However, this
presents fuel management issues such as the formation
of carbon-based compounds at the anode that requires
cleaning by water flashing for higher efficiency.
Emerging materials for SOFC operating at lower
temperatures include:
· Cathodes: doped-cobaltite (LSC), doped
ferrites (LSFC) and various combinations of several
materials.
· Anodes: doped-ceria
· Electrolytes: gadolinium-doped ceria
(CGO) or doped lanthanum gallate (LSG)
· Interconnects: high temperature stainless
steel alloys
· Sealants: metallic, mica, or ceramic
fiber sheets.
Chemical compatibility between the various materials
composing the basic SOFC cell (anode/electrolyte/cathode)
is essential to prevent cross-migration of species
(similar to shorting in batteries) and good resistance
to mechanical stress during the heating/cooling
cycles. Recent developments include the deposition
of intermediate material layers at the electrode/electrolyte
interfaces to reinforce the basic unit cell strength,
thus, the overall stack strength.
Additional cost reduction options include new
high-power design (the flat high-power density
elliptical design by SWPC, for example), reducing
the cost of the system-component part of the SOFC
system, and improved and cheaper manufacturing
processes (tape-casting versus electrodepositing
techniques, for example). Further development
is under way worldwide to reduce these costs further.
Tubular SOFC stacks presently cost about $1000-$1500/kWh,
and planar designs about $600-$800/kWh.
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