People who envision replacing our fossil-fuel economy with a hydrogen economy see power-producing fuel cells in every home, car, and factory. Because hydrogen can be obtained from almost any material containing hydrogensuch as ammonia, natural gas, domestically produced biofuels, or even water, albeit at a high costa society powered by fuel cells will be less dependent on foreign supplies of oil for gasoline. Such a hydrogen economy would be directly benefited by a major indigenous primary energy source, coal, because coal would likely be one of the principal sources of hydrogen through gasification technologies that produce synthesis gas, a mixture of primarily carbon monoxide and hydrogen.
A hydrogen economy will be kinder and gentler to humans because it will greatly facil-itate the reduction of emissions of lung-damaging sulfur and nitrogen oxides. If the main source of hydrogen for most cars in the future is natural gas or coal, emissions of potentially climate-altering greenhouse gases such as carbon dioxide will also be lower.
H-Power and other companies are now producing commercial fuel cells that provide electricity to buildings and industrial plants. These fuel cells, which in their housings resemble outdoor heat-pump systems or perhaps tall dumpsters, are becoming less expensive. However, they are not yet cost-competitive with other distributed generation sources such as microturbines.
President Bush has called for the development of fuel cells for use in cars as part of the national FreedomCAR Program. (The term FreedomCAR, in which CAR stands for Cooperative Automotive Research, was coined by ORNL’s Kathy Vaughan. The term suggests that this concept will “free” us from being dependent on foreign energy sources.) Right now fuel cells are too large, too heavy, and too costly to be used in cars, so research programs are being funded to develop an affordable fuel-cell car, which may be available in 10 to 20 years.
ORNL fuel-cell research includes experiments with advanced materials and designs that could lead to the manufacture of fuel cells that are small enough, light enough, and cheap enough for use in powering buildings and cars. For many years, ORNL provided only a supporting role to the Department of Energy’s program for developing fuel cells, but its activity level has picked up with increased DOE funding since 2000. ORNL has a unique combination of expertise in many different areas related to fuel-cell developmentmaterials development and characterization, reliability of structures, power electronics, catalysts and electrochemistry, thermal management, and computer modeling of systems.
HOW A FUEL CELL WORKS
In a fuel cell, water is formed by combining hydrogen (e.g., from natural gas) and oxygen from the air while an electric current is produced in an external circuit. In a PEM (polymer electrolyte membrane or proton exchange membrane) fuel cell, as pressurized hydrogen is introduced through the channels of one electrode plate (the anode), oxygen enters the channels of the other (the cathode). Hydrogen atoms are converted to protons at the anode near the electrolyte surface. The protons are transported through the electrolyte. They react with oxygen ions, which have been formed by electrochemical reduction, at the cathode to produce water.
The electrons resulting from the oxidation of hydrogen are conducted in the external circuitry. The fuel cell will generate electricity as long as hydrogen is fed to the anode. To increase the power output, fuel-cell units each consisting of an anode, electrolyte, and cathode, are assembled in an electrical series arrangement to form so-called fuel-cell stacks.
ORNL researchers are now working on several kinds of fuel cells, which are named after their electrolytes. They include PEM, solid-oxide, and alkaline fuel cells (see photographs).
PEM FUEL CELLS
PEM fuel cells use a membrane made by DuPont called Nafion, which could be considered similar to kitchen plastic wrap. This material is a proton conductor that allows protons to pass through it but blocks the flow of electrons. PEM fuel cells are well developed and are strong candidates for cars because engineers have improved their power density so that they could each be as small as a suitcase.
Ted Besmann and his colleagues in ORNL’s Metals and Ceramics (M&C) Division have developed carbon-composite bipolar plates, or interconnects, which connect the individual fuel-cell elements into a fuel-cell “stack” for the PEM fuel cell to make it lighter and more affordable. “We’ve shown that these carbon composite bipolar plates for automotive PEM fuel cells meet cost and performance goals,” says Besmann. “These plates can be made at a low cost. They don’t corrode and they show high electrical conductivity and sufficient strength. They are substantially cheaper and lighter than conventionally used graphite.”
ORNL worked with Los Alamos National Laboratory in evaluating 100-cm2 single-sided plates. The plates were found to demonstrate good performance and good corrosion resistance. The technology for making these plates has been licensed to Porvair Fuel Cell Technology, Inc.
ORNL researchers are working with industrial partners (e.g., MER and W. L. Gore, Inc.) to make thinner, lighter interconnects and electrodes that offer a good power density. They are experimenting with altering and using alternative materials (e.g., a film of titanium nitride alloy deposited on an iron-titanium base alloy), looking for structure-property relationships, analyzing engineering designs, developing prototype fuel-cell units, and evaluating their performance.
SOLID-OXIDE FUEL CELLS
ORNL researchers are becoming more involved in supporting the development of solid-oxide fuel cells (SOFCs). These fuel cells, which may operate at a very high temperature (1000°C), are best suited for stationary power generators that can provide electricity to factories and towns. They not only produce electricity electrochemically but also can operate at sufficiently high pressure that their exhaust gases can drive a power-generating turbine, making the SOFC very efficient at power production.
Rod Judkins (ORNL’s Fossil Fuel Program director), Tim Armstrong (ORNL’s Fuel Cell and Functional Materials Program manager), and Solomon Labinov (Engineering Science and Technology Division) have conceived a highly efficient power plant. Using static and dynamic computer modeling, they designed a hybrid fuel cell–gas turbine power plant that has an energy conversion efficiency of 80%. This combined- cycle system incorporates a gas turbine, one or two solid-oxide fuel cells, a gas separation device based on the carbon-fiber composite molecular sieve, and a heat exchanger using the graphite foam with very high heat transfer (developed by M&C’s James Klett and Tim Burchell).
“We are the only DOE national laboratory doing this type of development,” says Armstrong. “This is a real system that an industrial partner could integrate into a working system with components available today.”
For SOFCs, ORNL researchers have designed metal alloys for interconnects, are developing materials to improve the electrolyte, and are testing ways to improve and reduce the cost of manufacturing anodes. Using ORNL’s Infrared Processing Center, researchers are sintering electrolyte films on substrates to reduce the production cycle time and costs of assembling fuel-cell stacks. ORNL researchers are also doing design work on fuel-cell systems and stacks, as well as computer modeling and mechanical-properties testing to improve the reliability and predict the lifetime of improved SOFCs.
Armstrong and Paul Becher (also of the M&C Division), Doug Lowndes and Christopher Rouleau (both of the Solid State Division), Michael Hu of ORNL’s Nuclear Science and Technology Division, and Meilin Liu of Georgia Institute of Technology have been working on an SOFC project funded internally by the Laboratory Directed Reseach and Development Program. This project seeks to develop a fundamental basis for advances in materials for the next generation of multistage SOFCs, which must be designed to exhibit exceptionally high energy conversion efficiencies and fuel use. “These systems will require electrolyte and electrode materials made of new oxides that function at temperatures as low as 500°C for as long as 40,000 hours,” Armstrong says. “So we are developing nanocrystalline electrolytes by employing ORNL’s synthesis expertise.”
These ORNL researchers recently showed that a 15-nanometer-thick epitaxial electrolyte film of 10% yttria-stabilized zirconia (YSZ) exhibits the highest ionic conductivity ever reported. It has 140-fold greater ionic conductivity than that of a conventional zirconia electrolyte ceramic at 500°C.
“In our fuel-cell-design activities, we found that power density in SOFCs could be doubled through novel configurations without the need for radically new materials,” Armstrong says. “The SOFC fuel-cell size and weight may be reduced to one-quarter the volume and one-half the weight through changes in geometry. The design change could lower the manufacturing cost to $155 per kilowatt, which would have a dramatic effect on overall fuel-cell-system cost.”
ALKALINE FUEL CELLS
Alkaline fuel cells have been used in the U.S. space program since the 1960s. They are quite expensive in part because of the reliability needed by the space program and because they use costly platinum for their electrode catalyst. Also, potassium hydroxide, which is used as the electrolyte, has a problem: It readily reacts with carbon dioxide (CO2), gradually degrading fuel-cell performance and lifetime. But DOE’s Office of Power Technologies believes that material substitutions and existing technologies could make the alkaline fuel cell smaller and cheaper so it would be suitable for distributed-power generation. Says Judkins: “An alkaline fuel cell may cost on the order of $100 per kilowatt compared with $1500 per kilowatt for today’s fuel cell and have an energy conversion efficiency of 50 to 55%.”
To protect and preserve the fuel cell’s electrolyte, ORNL’s Judkins and a team led by Tim Burchell have developed a regenerable scrubber that removes CO2 from the air, which is the source of the oxidant (oxygen) for this fuel cell. The scrubber is regenerated by passing an electrical current through it, a process the ORNL inventors have dubbed electrical-swing adsorption. This invention is also useful for capturing CO2 from gas streams being released to the atmosphere.
ORNL’s program will focus on developing high-power-density alkaline fuel cells, and will initially focus on testing and developing more CO2-tolerant electrolytes and better bipolar plates and electrodes. Researchers will test different materials to find the best candidates to replace the expensive platinum catalyst in alkaline fuel cells to reduce their cost. “One alkaline fuel-cell electrode could be made of carbon, silver, or a metal oxide,” Armstrong says. “The other might be made of a nickel-based oxide instead of platinum.”
Combining the right materials and designs is likely to result in a better, cheaper fuel cell. That hope is fueling the efforts of ORNL researchers who offer expertise and ingenuity in these areas.
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