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Engineers and environmentalists have long dreamed of being able to
obtain the benefits of clean electric power without pollution-producing
engines or heavy batteries. Solar panels and wind farms are familiar
images of alternative energy technologies. While they are effective
sources of electrical energy, there are problems with the stability of
their energy source as, for example, on a cloudy or windless day. Their
applications are somewhat limited due to lack of portability; a windmill
is not much help to the power plant of a diesel truck, a solar panel
cannot provide power at night, etc.
In 1962 a revolution in energy research occurred. Scientists at
Westinghouse Electric Corporation (now Siemens Westinghouse)
demonstrated for the first time the feasibility of extracting
electricity from a device they called a "solid electrolyte fuel cell"
[Weissbart]. Since then there has been an
intense research and development effort to develop the alternative
energy technology known as fuel cells. Now, as energy issues are at the
forefront of current events, fuel cell technology is ripening and on the
verge of being ready for large scale commercial implementation.
What is a Fuel Cell?
A fuel cell is an electrochemical device that converts the chemical
energy in fuels (such as hydrogen, methane, butane or even gasoline and
diesel) into electrical energy by exploiting the natural tendency of
oxygen and hydrogen to react. By controlling the means by which such a
reaction occurs and directing the reaction through a device, it is
possible to harvest the energy given off by the reaction. Fuel cells are
simple devices, containing no moving parts and only four functional
component elements: cathode,
electrolyte, anode and interconnect.
Why Fuel Cells?
Much development has focused on solid
oxide fuel cells (SOFC), both because they are able to convert a
wide variety of fuels and because they do so with such high efficiency
(40-60% unassisted, up to 70% in pressurized hybrid system) compared to engines and
modern thermal power plants (30-40% efficient) [Singhal]. SOFC technology dominates competing
fuel cell technologies because of the ability of SOFCs to use currently
available fossil fuels, thus reducing operating costs. Other fuel cell
technologies (e.g. molten carbonate, polymer electrolyte, phosphoric
acid and alkali) require hydrogen as their fuel. Widespread use of such
fuel cells would require a network of hydrogen suppliers, similar to our
familiar gas stations.
High efficiency and fuel adaptability are not the only advantages of
solid oxide fuel cells. SOFCs are attractive as energy sources because
they are clean, reliable, and almost entirely nonpolluting. Because
there are no moving parts and the cells are therefore vibration-free,
the noise pollution associated with power generation is also
eliminated.
How Does a Fuel Cell Work?
Figure 1 shows schematically how a solid oxide fuel cell works. The
cell is constructed with two porous electrodes which sandwich an
electrolyte. Air flows along the cathode (which is therefore also called
the "air electrode"). When an oxygen molecule contacts the
cathode/electrolyte interface, it catalytically acquires four electrons
from the cathode and splits into two oxygen ions. The oxygen ions
diffuse into the electrolyte material and migrate to the other side of
the cell where they encounter the anode (also called the "fuel
electrode"). The oxygen ions encounter the fuel at the anode/electrolyte
interface and react catalytically, giving off water, carbon dioxide,
heat, and -- most importantly -- electrons. The electrons transport
through the anode to the external circuit and back to the cathode,
providing a source of useful electrical energy in an external
circuit.
Two possible design configurations for SOFCs have emerged: a planar
design (Figure 2) and a tubular design (Figure 3). In the planar design,
the components are assembled in flat stacks, with air and fuel flowing
through channels built into the cathode and anode. In the tubular
design, components are assembled in the form of a hollow tube, with the
cell constructed in layers around a tubular cathode; air flows through
the inside of the tube and fuel flows around the exterior.
Materials Selection and Processing
Although the operating concept of SOFCs is rather simple, the
selection of materials for the individual components presents enormous
challenges. Each material must have the electrical properties required
to perform its function in the cell. There must be enough chemical and
structural stability to endure fabrication and operation at high
temperatures. The fuel cell needs to run at high temperatures in order
to achieve sufficiently high current densities and power output;
operation at up to 1000 C is possible using the most common electrolyte
material, yttria-stabilized zirconia (YSZ). Reactivity and
interdiffusion between the components must be as low as possible. The
thermal expansion coefficients of the components must be as close to one
another as possible in order to minimize thermal stresses which could
lead to cracking and mechanical failure. The air side of the cell must
operate in an oxidizing atmosphere and
the fuel side must operate in a reducing
atmosphere. The temperature and atmosphere requirements drive the
materials selection for all the other components.
In order for SOFCs to reach their commercial potential, the materials
and processing must also be cost-effective. The first successful SOFC
used platinum as both the cathode and anode, but fortunately less
expensive alternatives are available today [Weissbart].
Cathode
The cathode must meet all the above requirements and be porous in order
to allow oxygen molecules to reach the electrode/electrolyte interface.
In some designs (e.g. tubular) the cathode contributes over 90% of the
cell's weight and therefore provides structural support for the cell
[Park].
Today the most commonly used cathode material is lanthanum manganite
(LaMnO3), a p-type
perovskite. Typically, it is doped with
rare earth elements (eg. Sr, Ce, Pr) to enhance its conductivity. Most
often it is doped with strontium and referred to as LSM
(La1-xSrxMnO3). The conductivity of
these perovskites is all electronic (no ionic conductivity), a desirable
feature since the electrons from the open circuit flow back through the
cell via the cathode to reduce the oxygen molecules, forcing the oxygen
ions through the electrolyte. In addition to being compatible with YSZ
electrolytes, it has the further advantage of having adequate
functionality at intermediate fuel cell temperatures (about 700 C),
allowing it to be used with alternative electrolyte compositions. Any
reduction in operating temperature reduces operating costs and expands
the materials selection, creating an opportunity for additional cost
savings.
Fabrication of LSM depends on cell design. For example, at Siemens
Westinghouse a tubular cell design is being developed [Singhal]. The cell is constructed by
extruding a cathode tube and building the rest of the cell around it. At
NexTech Materials, where several planar cell designs are being
investigated, the cathode is designed as the bottom supporting layer,
and fabricated with tape casting
techniques using nanoscale particles
[NexTech]. In both cases, the challenge is
to sinter the cathode adequately, often
by co-sintering with the other components, while maintaining sufficient
interconnected porosity.
Electrolyte
Once the molecular oxygen has been converted to oxygen ions it must
migrate through the electrolyte to the fuel side of the cell. In order
for such migration to occur, the electrolyte must possess a high ionic
conductivity and no electrical conductivity. It must be fully dense to
prevent short circuiting of reacting gases through it and it should also
be as thin as possible to minimize resistive losses in the cell. As with
the other materials, it must be chemically, thermally, and structurally
stable across a wide temperature range.
There are several candidate materials: YSZ, doped cerium oxide, and
doped bismuth oxide. Of these, the first two are the most promising.
Bismuth oxide-based materials have a high oxygen ion conductivity and
lower operating temperature (less than 800 C), but do not offer enough
crystalline stability at high temperature to be broadly useful
[Liou].
YSZ has emerged as the most suitable electrolyte material. Yttria
serves the dual purpose of stabilizing zirconia into the cubic structure
at high temperatures and also providing oxygen vacancies at the rate of
one vacancy per mole of dopant. A typical dopant level is 10 mol% yttria
[Singhal].
A thin, dense film of electrolyte (approximately 40 microns thick)
needs to be applied to the cathode substrate. A reliable way to apply
the electrolyte is known as electrochemical vapor deposition which
offers high purity and a high level of process control. Electrochemical
vapor deposition solves the problem of depositing a dense film onto a
porous substrate by passing oxygen through the inside of the cathode
tube while chlorides of zirconium and yttrium are passed along the
outside. They react at the tube surface to form YSZ and, because the
reaction comes to the surface from both sides, the porosity is closed
off. Once the porosity is closed off, the electrolyte deposition
continues, but now the oxygen diffuses through the growing YSZ layer to
react with the chlorides, thereby ensuring a highly dense electrolyte
layer. The process, while effective, is expensive and capital-intensive
[Singhal].
Alternative electrolyte deposition methods that show promise are
spray coating and dip coating followed by sintering. Colloidal
suspensions of YSZ are applied in thin layers of at least 20 microns,
using nanosize (5-10 nm) particles in order to meet the critical
requirement of low porosity. Through careful engineering of the particle
size distribution and dispersions, these deposition methods are likely
to replace electrochemical deposition [Liou].
Cerium oxide has also been considered as a possible electrolyte. Its
advantage is that it has high ionic conductivity in air but can operate
effectively at much lower temperatures (under 700 C); this temperature
range significantly broadens the choice of materials for the other
components, which can be made of much less expensive and more readily
available materials. The problem is that this electrolyte is susceptible
to reduction on the anode (fuel) side. At low operating temperatures
(500-700 C) grain boundary resistance
is a significant impediment to ionic conductivity. Efforts are underway
to develop compositions which address these problems [Ralph].
Anode
The anode (the fuel electrode) must meet most of the same
requirements as the cathode for electrical conductivity, thermal
expansion compatibility and porosity, and must function in a reducing
atmosphere. The reducing conditions combined with electrical
conductivity requirements make metals attractive candidate
materials.
Most development has focused on nickel owing to its abundance and
affordability. However, its thermal expansion (13.3 x 10-6/C
compared with 10 x 10-6/C for YSZ) is too high to pair it in
pure form with YSZ; moreover, it tends to sinter and close off its
porosity at operation temperatures. These problems have been solved by
making the anode out of a Ni-YSZ composite. The YSZ provides structural
support for separated Ni particles, preventing them from sintering
together while matching the thermal expansions. Adhesion of the anode to
the electrolyte is also improved [Singhal].
Anodes are applied to the fuel cell through powder technology
processes. Either a slurry of Ni is applied over the cell and then YSZ
is deposited by electrochemical vapor deposition, or a Ni-YSZ slurry is
applied and sintered. More recently NiO-YSZ slurries have been used, the
NiO being reduced to particulate Ni in the firing process.
In order to maintain porosity, pore formers such as starch, carbon,
or thermosetting resins are added. These burn out during firing and
leave pores behind.
There are problems with this approach. First, the process tends to
form tortuous porosity pathways that reduce the transport efficiency of
reacting gasses through the anode. Second, there is an increased
likelihood of cracking on firing because of the thinness of the interior
solid structure left behind. Third, there are environmental issues
associated with the burning of the pore formers.
For these reasons, recent research is investigating the possibility
of a freeze-drying approach to forming porous structures without the use
of fillers. The slurry is applied through a simple dipping process and
then freeze-dried; the resulting ice is then sublimed out of the unfired
structure. The resulting pore structure -- neatly aligned because of the
way water crystallizes -- allows efficient flow of gases to and from
the electrolyte/anode interface. The fineness of the pore structure is
easily controlled by adjusting the solids content (and therefore water
content) of the slurry [Moon].
Other choices of material are under investigation as well. Although
Ni-YSZ is currently the anode material of choice and the freeze-drying
process solves most of the associated problems, nickel still has a
disadvantage: it catalyzes the formation of graphite from hydrocarbons.
The deposition of graphite residues on the interior surfaces of the
anode reduces its usefulness by destroying one of the main advantages of
SOFCs, namely their ability to use unreformed fuel sources.
Cu-cerium oxide anodes are being studied as a possible alternative.
Copper is an excellent electrical conductor but a poor catalyst of
hydrocarbons; cerium oxide is used as the matrix in part because of its
high activity of hydrocarbon oxidation. A composite of the two thus has
the advantage of being compatible with cerium oxide electrolyte fuel
cells. Initial results using a wide range of hydrocarbon fuels are
promising [Park].
Interconnect
Just as an internal combustion engine relies on several cylinders to
provide enough power to be useful, so too must fuel cells be used in
combination in order to generate enough voltage and current. This means
that the cells need to be connected together and a mechanism for
collection of electrical current needs to be provided, hence the need
for interconnects. The interconnect functions as the electrical contact
to the cathode while protecting it from the reducing atmosphere of the
anode.
The high operating temperature of the cells combined with the severe
environments means that interconnects must meet the most stringent
requirements of all the cell components: 100% electrical conductivity,
no porosity (to avoid mixing of fuel and oxygen), thermal expansion
compatibility, and inertness with respect to the other fuel cell
components. It will be exposed simultaneously to the reducing
environment of the anode and the oxidizing atmosphere of the
cathode.
For a YSZ SOFC operating at about 1000 C, the material of choice is
LaCrO3 doped with a rare earth element (Ca, Mg, Sr, etc.) to improve its
conductivity. Ca-doped yttrium chromite is also being considered because
it has better thermal expansion compatibility, especially in reducing
atmospheres [Chou]. Interconnects are
applied to the anode by plasma spraying
and then the entire cell is
co-fired.
Any reduction in component costs (either raw materials or processing)
directly translates into improved energy affordability. The strong
economic incentive to use traditional metals for the interconnect is
driving the development of intermediate and low temperature SOFCs. At
operating temperatures in the 900-1000 C range, interconnects made of
such nickel base alloys as Inconel 600 are possible [Matsuzaki]. At or below 800 C, ferritic
steels can be used. At even lower temperatures (below 700 C), it becomes
possible to use stainless steels, which are comparatively inexpensive
and readily available [Ralph].
Applications and Markets
The United States government is taking a proactive role in expediting
the technology through the Solid State Energy Conversion Alliance
(SECA), which is coordinated by the Department of Energy and Pacific
Northwest National Laboratory. The technical goal is to develop mass
producible, modular SOFC units capable of 3-10 kW at a price of $400/kW.
SECAs approach is to develop industrial collaborations and to extend
financial support of technical research [SECA].
There seems, therefore, to be little doubt that SOFC technology will
be implemented. Analysts expect that the overall market for fuel cell
technology could reach $95 billion by the year 2010 [ceramic]. The market share that will belong
to SOFCs is unclear but will surely be significant, as SOFCs are
targeted for use in three energy applications: stationary energy
sources, transportation, and military applications.
Stationary installations would be the primary or auxiliary power
sources for such facilities as homes, office buildings, industrial
sites, ports, and military installations. They are well suited for
mini-power-grid applications at places like universities and military
bases. According to the SECA, worldwide demand for electricity is
expected to double in the next 20 years. SOFC technology is ideal for
such an expansion, since much of the anticipated demand is expected to
come from growing economies with minimal infrastructure. SOFCs can be
positioned on-site, even in remote areas; on-site location makes it
possible to match power generation to the electrical demands of the
site.
Stationary SOFC power generation is no longer just a hope for the
future. Siemens Westinghouse has tested several prototype tubular
systems, with excellent results. A plant in the Netherlands has been
operational for two years and an earlier prototype installation has been
operating for 8 years. The fuel cells have been through over 100 thermal
cycles and the voltage degradation during the test time has been minimal
less than 0.1%/thousand hours. Siemens Westinghouse expects to have its
first fully operational tubular fuel cell plant in place by October 2003
[Siemens]. Meanwhile, in Australia,
Ceramic Fuel Cells, Ltd. has been operating prototype planar fuel cell
plants since 2001 and expects to be ready with market-entry products in
2003 [CFCL].
In the transportation sector, SOFCs are likely to find applications
in
both trucks and automobiles. In diesel trucks, they will probably be
used as auxiliary power units to run electrical systems like air
conditioning and on-board electronics. Such units would preclude the
need to leave diesel trucks running at rest stops, thereby leading to a
savings in diesel fuel expenditures and a significant reduction in both
diesel exhaust and truck noise. Meanwhile, automobile manufacturers have
invested at least $4.5 billion in fuel cell research (not all SOFC)
[ceramic]. There are an estimated 600
million vehicles worldwide, 75% of which
are personal automobiles, and the number is expected to grow by 30% in
the next 10 years [SECA]. With more
stringent environmental restrictions
in the United States and European Union, automobile manufacturers are
under growing time pressure to bring non-polluting cars to the
marketplace. SOFCs are attractive prospects because of their ability to
use readily available, inexpensive fuels.
Finally, SOFCs are of high interest to the military because they can
be
established on-site in remote locations, are quiet, and non-polluting.
Moreover, the use of fuel cells could significantly reduce deployment
costs: 70% by weight of the material that the military moves is nothing
but fuel [SECA].
Conclusion
Forty years have passed since the first successful demonstration of a
solid oxide fuel cell. Through ingenuity, materials science, extensive
research, and commitment to developing alternative energy sources, that
seed of an idea has germinated and is about to bloom into a viable,
robust energy alternative. Materials development will certainly continue
to make SOFCs increasingly affordable, efficient, and reliable.
© Copyright 2003, All Rights Reserved, C
SA
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