Transcript SOFC - I.T. at The University of Toledo

Solid Oxide
Fuel Cell
INDEX
 History
 Technology
 Operation
 Advantages
 Limitations
 Applications
 Self-Test
HISTORY

Both solid oxide and molten carbonate fuel
cells are high temperature devices. The
technical history of both cells seems to be
rooted in similar lines of research until the late
1950s. Research into solid oxide technology
began to accelerate in late 1950’s at the
Central Technical Institute in The Hague,
Netherlands, Consolidation Coal Company, in
Pennsylvania, and General Electric, in
Schenectady, New York. A 1959 discussion of
fuel cells noted that problems with solid
electrolytes discovered by Nernst(called
Nernst Mass) included relatively high internal
electrical resistance, melting, and shortcircuiting due to semi conductivity. It seems
that many researchers began to believe that
molten carbonate fuel cells showed more

Not all gave up on
solid oxide, however.
The promise of a hightemperature cell that
would be tolerant of
carbon monoxide and
use a stable solid
electrolyte continued
to draw modest
attention. Researchers
at Westinghouse, for
example,
experimented with a
cell using zirconium
oxide and calcium
oxide in 1962.
More recently, climbing energy prices and
advances in materials technology have
reinvigorated work on SOFCs, and a
recent report noted about 40 companies
working on these fuel cells.
TECHNOLOGY

A solid oxide fuel cell (SOFC) uses a
hard ceramic electrolyte instead of a
liquid and operates at temperatures
up to 1,000 degrees C (about 1,800
degrees F). A mixture of zirconium
oxide and calcium oxide form a
crystal lattice, though other oxide
combinations have also been used as
electrolytes. The solid electrolyte is
coated on both sides with specialized
porous electrode materials.

At these high operating temperature, oxygen
ions (with a negative charge) migrate
through the crystal lattice. When a fuel gas
containing hydrogen is passed over the
anode, a flow of negatively charged oxygen
ions moves across the electrolyte to oxidize
the fuel. The oxygen is supplied, usually
from air, at the cathode. Electrons generated
at the anode travel through an external load
to the cathode, completing the circuit and
supplying electric power along the way.
Generating efficiencies can range up to
about 60 percent.
SOFC Stacking



A 40-cell SOFC stack with 16-cm diameter
cells achieves 1.40 kW at 0.428 A/cm2
with 80% fuel utilization and an average
cell voltage of 0.673 V
Four common SOFC stack configurations
have been proposed and fabricated
Flat-plate design
The simplest way to envisage a SOFC as a
single plate cell is by stacking components
on top of each other. This design offers
simple
cell
geometry
and
multiple
fabrication
options
such
as
tape
calendaring or tape casting.

Seal less tubular design
The seal less tubular design consists of
the cell components configured as thin
layers on a tubular support closed at
one end. The problems encountered
previously with gas seals are now
eliminated with this array, although the
cell is still hampered by the limited gas
flow through the tube and relatively
long current path.

Segmented cell-in-a-series
This design consists of segmented
cells connected in electrical and gas
flow series. The cells are either
arranged as a thin banded structure
on a porous support or filleted one
into the other to form a tubular selfsupporting structure. The major
problem is low gas flow, which
results from the thick support tube
and the robust seals required at the
ends of the tube

Monolithic
The monolithic design is the newest
SOFC stack concept. It consists of
many cells fabricated as a single unit.
The design has the potential to
achieve high power density because
of its compact and lightweight
structure. Nevertheless, they are
difficult to manufacture and have a
higher likelihood of cracking during
operation due to the expansion
mismatches of the materials.
OPERATION
An SOFC consists of two porous electrodes
separated by a dense oxygen-ion conducting
Electrolyte. Oxidant is reduced at the cathode
side and fuel is oxidized at the anode. The
difference in oxygen activity of the two gases
at the electrodes provides a driving force for
motion of oxide ions in the electrolyte. Oxide
ions formed by dissociation of oxygen at the
cathode under electron consumption migrate
through electrolyte to the anode where they
react with hydrogen to form water and release
electron.

Electrolyte
SOFC is based on the concept of an
oxygen-ion conducting electrolyte where
oxide ions (O2-) migrate from cathode to
anode and react with the fuel to generate
electricity. Oxide materials with fluorite
crystal structure such as yttria-stabilized
zirconia (YSZ), rare earth-doped ceria and
rare earth-doped bismuth oxide have been
widely investigated as electrolytes for fuel
cell. Zirconia doped with 8-10mole% yttria
(YSZ) is the most wide-used electrolyte
for SOFC because it conducts only oxygen
ions over a wide range of oxygen partial
pressure.

Anode (Fuel electrode)
The electrode must be stable in the
reducing environment of the fuel, should
be electronically conducting and must
have sufficient porosity to allow transport
of the products of fuel oxidation away
from electrolyte-electrode interface.
In this region the fuel oxidation reaction is:
O2- (s) + H2 (g) H2O (g) + 2e-

Cathode (Air electrode)
The cathode is the site for the
electrochemical reduction of oxidant.
Therefore, the cathode material must
be chemically and thermally stable in
the oxidizing atmosphere. In addition,
a potential cathode material should be
reasonably compatible with other cell
components.

Sealing materials
Method of sealing the ceramic components to
obtain gas-tightness is a major issue of
SOFC. Glass ceramics are used as sealant,
although migration of the silica component
can still be a problem on anode and cathode.
 Interconnecting
materials(External
Circuit)
As the name implies, the interconnecting
material connects anode of one cell with
cathode of another cell so that voltage output
could be enhanced. Doped lanthanum
chromite (LaCrO3) has been used as the
interconnecting material since the 1970s

Catalyst (Nickel, Copper)
The catalyst within the anode
promotes release of free electrons
from the cells fuel source.
The
catalyst
within
the
cathode
promotes the generation of oxygen
ions from the cells oxygen source.

Hydrogen feed
SOFC devices can internally reform some
fuels to deliver hydrogen fuel, and they
can be fabricated in a variety of shapes
and form factors.

Oxygen Feed
At the cathode, the oxygen provided by
the air or oxidant feed reacts with
electrons in-bound from the external
circuit and H+ protons coming through the
electrolyte to form water. The water is
expelled, along with any other compounds
in the oxidant feed stream out through the
cathode exhaust.

The Exhaust
The SOFC exhaust exits the
generator module at a temperature
of between 800 degrees C and 850
degrees C and in atmospheric
pressure systems is passed through
the exhaust gas heat recovery train.
This heat can be adapted to generate
process heat or hot water for a
combined heat and power application
ADVANTAGES
High Electrical Efficiency
SOFCs can achieve electrical efficiency of
up to 50% using natural gas and can also
achieve this performance with other
hydrocarbon fuels such as liquefied
petroleum gas.
 As a result of its high operating
temperatures, SOFCs can also be
combined with heat recovery technologies
such as heat exchangers to create a total
system efficiency of up to 85%.

Fuel Flexibility

SOFC is the most inherently fuel
flexible of the fuel cell types
High Reliability

SOFCs are made from commonly
available ceramic materials. SOFC
technology has no moving parts or
corrosive liquid electrolytes. They are
expected to lead to electricity
generation systems that are highly
reliable and require low maintenance
Solid Electrolyte

One of the big advantages of the SOFC
over the MCFC is that the electrolyte is a
solid. This means that no pumps are
required to circulate hot electrolyte,
moreover there is no leakage problem with
the electrolyte
Broad product range capability


SOFC technology can support distributed
generation (DG) products such as generators
and combined heat and power units in the
capacity range from small residential to large
industrial sizes, as well as automotive
applications such as auxiliary power units. In DG
applications, the low air and noise emissions of
SOFC-based systems allow for ease of siting
and permitting.
The high operating temperature also results in
high-grade exhaust heat, which can be utilised
in a wide range of cogeneration applications.
LIMITATIONS
Solid Electrolyte

Though the Solid Electrolyte cannot leak, it
can break and provide a severe problem
Manufacturing Cost

Higher stack temperatures demand
exotic materials, which add to
manufacturing costs. Heat also
presents a challenge for longevity
and reliability because of increased
material oxidation and stress.
Stationary applications

Unfortunately, the dominant SOFC
developers aim at stationary applications
Such ceramic solutions are indeed heavy,
sluggish, expensive and fragile and must
be operated at high temperatures. But
totally different SOFCs are presently
developed for mobile applications.
APPLICATIONS
RESIDENTIAL APPLICATION
SOFC micro-power plants take away the
dependence and limitations of the electric
distribution grid, in a remote standalone
package that can also provide heat for the
home. This lets the homeowner live just about
anywhere, in the mountains or deep woods, in
the desert or on an island.
COMMERCIAL APPLICATION
Solid oxide fuel cell (SOFC) power systems for
commercial and industrial applications are
designed to provide clean, highly efficient
power for on-site grid-support, grid-back-up
or grid-independent electrical generation
needs.
MILITARY APPLICATION
SOFC can be used as uninterruptible power
supplies (UPS), especially in military
application as back up power
IN COMMUNICATIONS
For high-priority carrier, provider, corporate
or government networks SOFC provides high
grade power