growing number of power generators and users, fuel cells are the
key to the nations energy future. Clean, quiet, efficient,
and compact, fuel cells generate electricity through chemistry instead
of combustion. (See the box near the end of this article.) As they
become more widely used, fuel cells promise to help reduce global
warming, air pollution, and U.S. dependence on foreign oil. No wonder,
then, that interest in the advanced development and commercialization
of fuel cells is at an all-time high.
of this interest is focused on four types of fuel cellssolid
oxide, proton exchange membrane, molten carbonate, and alkaline.
A major impediment to commercialization is the manufacturing cost.
In the case of solid-oxide fuel cells (SOFCs), high manufacturing,
or fabrication, costs translate into capital costs that run upwards
of $5,000 per kilowatt. In comparison, energy produced by conventional
power plants has a capital cost of about $500 per kilowatt.
Primarily because of these
high costs, the Department of Energy formed the Solid State Energy
Conversion Alliance (SECA) in 1999 to accelerate the development
and commercialization of SOFCs. The alliance is helping researchers
to discover ways to both lower fabrication costs and increase power
density, that is, the power generated per area of fuel cell. SECAs
goal is a modular, 3- to 10-kilowatt SOFC design that can be mass-produced
and used individually or in stacks to provide power for a host of
applications. In addition, the California Energy Commission (CEC)
is strongly supporting the development and demonstration of fuel
cells in the state.
As a committed developer
of fuel cell technology, the Applied Energy Technologies Program
in Livermores Energy and Environment Directorate is helping
SECA to reach its goal. Researchers in the programs Energy
Conversion and Storage Technologies Group have extensive experience
in developing several types of fuel cells, including the zincair
fuel cell, the unitized regenerative fuel cell, the direct carbon
conversion fuel cell, and the SOFC.
Why Solid-Oxide Fuel Cells?
SOFCs are particularly
attractive because they have the highest efficiencies of any conventional
fuel cell design and the potential to use many fuelsincluding
gasoline and dieselwithout expensive external reformers that
create more volatile chemicals. SOFCs can operate at high temperatures,
producing high-grade waste heat, or exhaust, which can be recovered
and used for other applications, such as space heating and cooling,
supplying homes with hot water, and even generating extra electricity
by spinning a gas turbine linked to the unit. For the military,
SOFCs offer the possibility of delivering quiet, clean, and uninterruptible
energy to armed forces stationed in remote locations. SOFCs can
also serve as auxiliary power units in motor vehicles, and leading
automotive companies are already working with industrial partners
to exploit their potential.
Before SOFCs can be fully
commercialized, however, several technological breakthroughs are
needed. A team of Livermore researchers led by materials scientist
Quoc Pham is working to address the key technological challenges.
Under Laboratory Directed Research and Development (LDRD) funding
since 1998, the team has pursued the development of low-cost, high-power-density
SOFCs that operate at temperatures below 800°C. The teams
focus is developing low-cost thin-film processing techniques and
optimizing materials and design to increase power density.
|This cross section of yttria-stabilized
zirconia electrolyte, imaged at different resolutions, is an
example of a thin film that was deposited on a porous anode
using colloidal spray deposition.
Lower Operating Temperatures, Higher Power Density
The current Livermore design is flat, or planar,
as described in the box below. Some SOFCs were originally fashioned
in a tubular design, but they proved to be too expensive and had
little potential for increasing power density. Livermore researchers
focused on a planar design because of its potential for both higher
performance and lower costs. Because a single cell has a voltage
of 1 volt or less, several cells must be connected in series using
electrical interconnections to achieve higher voltages. The complete
unit is called a fuel cell stack.
SOFCs traditionally operate
at extremely high temperatures (around 1,000°C). As a result,
the stacks interconnections are made of ceramic materials
that are expensive and difficult to manufacture. Thus, one way to
cut the fabrication cost is to reduce the operating temperature
by at least 200°C, so that inexpensive alloys can be used as
The initial challenge for
Pham and his team was to lower the SOFCs operating temperature
without compromising the power density. The researchers first made
the electrolyte layer thinner, thereby lowering the amount of resistive
energy lost during operation and increasing the efficiency. The
team developed a low-cost, thin-film deposition technique called
colloidal spray composition, which has since been patented. This
simple technique produces high-quality thin films ranging from one
to several hundred micrometers thick.
The team then turned its
attention to optimizing the fuel cell components. They created a
multilayer fuel cell structure that features different materials
to enable the use of high-performance electrodes. The structure
minimizes the stress generated by the difference in thermal expansion
characteristics at the interface between the electrodes and the
electrolyte materials, thereby achieving a significant decrease
in electrical loss. When the researchers combined the multilayer
design with their thin-film deposition technique, they improved
the power density of a single cell to 1.4 watts per square centimeter,
one of the highest values reported for power density at 800°C.
The team then set out to
demonstrate this same high power density in a stack. A three-cell
stack prototype generated 61 watts, exceeding the LDRD project goal
of 50 watts. The power density of the stack was 1.05 watts per square
centimeter (at 800°C using hydrogen fuel), a value at least
50 percent higher than any stack power density previously reported.
The latest Livermore SOFCs
operate at 700°C, a dramatic improvement. The team has received
funding from the CEC to lower the operating temperature even further
and to make other improvements.
||Livermores first prototype
fuel cell stack, consisting of three solid-oxide fuel cells,
set a record for stack power density.
The Problem of Fuel
In a separate LDRD project,
Pham and his colleagues focused on streamlining the SOFCs
fuel supply. Hydrogen is the preferred fuel for SOFCs, but hydrogens
high production cost and complex storage issues have made hydrocarbons
such as natural gas the preferred fuels. However, methane, the main
constituent of natural gas, has low reactivity, so it must be converted
to more reactive products, such as carbon monoxide and hydrogen
eliminate this conversion step, which is both expensive and complex,
the Livermore team explored the possibility of directly oxidizing
methane at the anode. Carbon deposition has been a major barrier
to the direct oxidation of methane at SOFC anodes, but the team
discovered that highly porous anodes permit direct oxidation.
the team has addressed many SOFC issues, three major materials science
challenges are preventing the commercialization of planar SOFCs.
The planar fuel cell is a difficult design because of sealing
problems, says Pham. The biggest challenge is separating the
air from the fuel, which requires that the edges of the ceramic
plate be sealed.
The second challenge concerns
the type of interconnection used in the fuel cell stack. Livermores
success in lowering the operating temperature made possible the
switch from ceramic to metal interconnections. At lower temperatures
(below 800°C), metallic interconnects are less subject to oxidation,
which leads to a loss of conductivity.
Problems with the mechanical
integrity of the fuel cell stack constitute the third challenge.
When brought up to operating temperature and then back to room temperature,
the fuel cell stack components experience dramatic thermal and mechanical
stresses. Researchers must try to minimize these stresses by both
attempting to match the thermal expansions of stack components as
much as possible and developing engineering designs that can accommodate
the inevitable level of mismatched thermal expansion.
CEC funding, the Livermore team has already developed potential,
patentable solutions to the problems of interconnection and mechanical
integrity. In total, Phams team has seven patents pending
related to fuel cell technology.
Fuel Cell Basics
Fuel cells are electrochemical energy conversion devices
that generate electricity and heat by converting the
chemical energy of fuels. Like batteries, fuel cells
can be connected together in series to produce higher
voltages. Fuel cells and batteries differ. A battery
is an energy storage device that stores its fuel internally
and that can supply only a fixed amount of energy. Reactants
used in fuel cells are supplied externally, and a fuel
cell has no fixed capacityit will generate electricity
as long as it is supplied with fuel and air.
Fuel cells can
accept almost any kind of fuel, including natural gas,
coal gas, gaseous fuels from biomass (plant materials
and animal waste), and liquid fuels (gasoline, diesel),
although some fuels may require preprocessing and purification.
Once connected to a fuel supply, a fuel cell will produce
electricity until its fuel supply is removed or exhausted.
fuel cells (SOFCs) are made from solid-state materials,
namely ceramic oxides. SOFCs consist of three components:
a cathode, an anode, and an electrolyte sandwiched between
the two. Oxygen from air is reduced at the cathode and
is converted into negatively charged oxygen ions. These
ions travel through the electrolyte to the anode, where
they react with fuel that has been delivered to the
anode. The fuel is oxidized by the oxygen ions and releases
electrons to an external circuit, thereby producing
electricity. The electrons then travel to the cathode,
where they reduce
oxygen from air, thus continuing the electricity-generating
cycle. Individual cells can be stacked together in series
to generate larger quantities of electricity. Within
this unit, called a fuel cell stack, the cells are separated
by bipolar separator planes, or interconnects.
three components of a solid-oxide fuel cell form a modular
unit that can be connected to other cells in a fuel
for the Future
the remaining materials science problems are resolved, the team
plans to construct and demonstrate a 100-watt, high-power-density
SOFC, followed by construction of 500- and 1,000-watt prototypes.
After that, we can say weve solved the materials science
issues, and our task evolves into an engineering project,
team has secured several sources of funding, including support from
DOEs Fossil Energy Program. The Livermore technology is being
licensed to Solid Oxide Systems, LLC (SOX), a private start-up company
that is matching the CEC funding with a goal of demonstrating a
10-kilowatt system. By partnering with SOX, Pham and his colleagues
hope to achieve the long-sought goal of commercializing solid-oxide
fuel cell technology and fulfilling the promise of clean, highly
efficient electric power at an affordable cost.
The work described in this highlight was done by Quoc Pham (principal
investigator), Brandon Chung, Jeff Haslam, Dave Lenz, and Ervin
Key Words: colloidal
spray deposition, energy conversion, fuel cell, fuel cell stack,
high power density, multilayer fuel cell, power generation, solid
oxide, Solid State Energy Conversion Alliance (SECA), thin-film
For further information contact Quoc Pham (925) 423-3394 (firstname.lastname@example.org).