By Susan Dieterle
An electric car whirs along the street as the driver heads home from work. Instead of the noxious fumes emitted by gasoline-powered cars, this vehicle's exhaust consists of little more than water. As the power gets low, the driver pulls into a station where a technician removes the spent fuel container and replaces it with a fresh one.
The driver grabs her cellular phone to let her family know she's on her way. This phone battery lasts longer than the older style and doesn't contain the toxic elements used in earlier batteries. The same is true of the rechargeable batteries in the power tools and electronics at her home.
If you think a scenario such as this won't unfold during your lifetime, think again. Scientists and manufacturers throughout the world are elbows-deep in efforts to develop new types of rechargeable batteries and fuel cells that are powerful, inexpensive and environmentally safe.
Part of this effort involves researchers at Ames Laboratory: Vitalij Pecharsky, scientist; Iver Anderson, director of the Metallurgy and Ceramics Program; and Jim Foley, associate metallurgist. Along with colleagues at four other institutions, they are developing a new class of hydrogen-storage materials for rechargeable batteries and fuel cells.
They hope their work will help make cleaner, safer battery power possible -- soon.
"Successful completion of our project can really affect the everyday lives of our children and future generations who will place ever-increasing reliance on electrically powered appliances, electronic devices and even electric cars that use the clean, quiet, portable power from batteries or fuel cells," says Anderson, who is also an adjunct professor of materials science and engineering at Iowa State University (ISU).
The goal of the $8.2-million, three-year project is to develop a new class of magnesium-based alloys that can reversibly store and release hydrogen. In addition to Ames Laboratory, the other participants are Ovonic Battery Co., Crucible Research, Oak Ridge National Laboratory and the Colorado School of Mines.
Hydrogen is an appealing fuel choice because it is a clean, abundant, endless source of energy. When it burns in Earth's oxygen atmosphere, it produces water, which can then be broken back down into hydrogen and oxygen.
"Hydrogen is an almost ideal fuel," says Pecharsky, who is also an associate professor of materials science and engineering at ISU.
But using it presents a few difficulties. To be used as a liquid, hydrogen would have to be maintained at 20 degrees above absolute zero, requiring costly cryogenic equipment. It also can be stored as a compressed gas, but this creates an explosive hazard and would involve heavy, bulky storage tanks. The most attractive and safest option involves using certain types of metals and alloys that form unstable hydrides that are capable of absorbing hydrogen in a solid crystalline form and then releasing it when heated.
"By using the metal hydrides, you are not stuck with cryogenic equipment, you are not stuck with high pressure," Pecharsky says. "Instead, you have hydrogen trapped inside the metal matrix, which is very safe."
Foley is among those who believe that metal-hydride batteries will be the batteries of choice in the future. "Current and future metal-hydride batteries are considered 'green,' or environmentally friendly, because they don't contain any heavy metals, such as lead or cadmium, that can pollute groundwater," he says.
Some products, such as cellular phones and power tools, already use metal-hydride batteries. Nickel is a basic component in most of the current hydrogen-storage materials. Other common alloy components are rare earths, zirconium and titanium.
When a rechargeable nickel metal-hydride battery goes through the charging process, hydrogen ions from the electrolyzed water present in the battery are reduced and turned into hydrogen gas, which is absorbed by a cathode made of the metal-hydride material. When the process is reversed, the hydrogen oxidizes and reunites with the oxygen, forming water and producing an electrical current in the battery.
In a fuel cell for an electric car, hydrogen is drawn from the metal-hydride material and then chemically combined with oxygen from the air. This reaction takes place within the fuel cell and generates electricity to power the car.
So, with all of the advantages offered by metal-hydride batteries, why is it difficult to find them on the market right now? Mostly because they are too heavy and too expensive. Current metal-hydride alloys absorb only about 1 to 1.5 percent hydrogen by weight, meaning that storing 1 kilogram of hydrogen requires 99 kilograms of metal-hydride material. They also contain costly elements, such as zirconium, which are sometimes in short supply.
In the search for an abundant raw material that is lightweight, inexpensive and stores more hydrogen, project participants have turned to magnesium. Pure magnesium meets all of the criteria and stores about 6.5 percent hydrogen by weight. "If you think about the commercial applications of magnesium-based alloys, the potential is almost limitless -- it's huge," Pecharsky says.
Magnesium has one drawback. It must be heated to 500-600 C (about 1000 F) before it will release the stored hydrogen. But project participants believe they can solve that problem by modifying the magnesium with other elements.
Developing the new magnesium alloys will be the responsibility of Ovonic Battery, a world leader in metal-hydride batteries, with metallurgical assistance from Ames Lab. "The Ames Laboratory is the leading laboratory in the metallurgy field," says Rosa Chiang Young, vice president of advanced materials development for Ovonic Battery. "We need the expertise and all of the experience from Ames Lab to help design this material and really make the program move forward."
Oak Ridge National Laboratory will research different forms in which the alloy can be used. Crucible Research, a company with expertise in powder metallurgy, will concentrate on large-scale, commercial production of the powdered form of the alloys. The Colorado School of Mines will research ways to minimize the effect of corrosion on battery performance.
In addition to assisting Ovonic Battery with the development of the alloy, Ames Lab has another crucial task. During the next two years, Pecharsky, Anderson and Foley will research the most efficient, cost-effective way of turning the alloys into versatile metal powder.
"There's no easy way to manufacture sheets or wires or screens out of these types of materials because the alloys are intermetallic compounds, and they are brittle," Pecharsky says. "The basic way hydrogen-storage alloys are used in batteries is in the form of powders. A powder is much more flexible."
Turning the alloys into fine-grained metal powders is no easy task, though. Most commercial powder producers cast the molten alloy into ingots weighing several hundred pounds, let them solidify and then grind them into powder. But this process can take days, and because the metals within the ingot segregate during the solidification process, the resulting powder particles do not all have the same chemical composition. "It gives you inconsistent quality," Pecharsky says.
The Ames Lab team believes the best way to process the magnesium alloys will be through a technique known as high-pressure gas atomization (HPGA), which excels in producing ultrafine powder particles.
In HPGA, a stream of molten material is blasted with extremely cold argon or helium gas at up to three times the speed of sound. This converts the material into fine liquid particles that solidify quickly and are nearly spherical in shape. Because of the rapid solidification, the particles have virtually identical chemical compositions. Pecharsky says the homogeneity of the HPGA powder is a big advantage over the powder made from crushed ingots.
"With HPGA, essentially 100 percent of the yield is the target alloy," he says, noting that the resulting powder is also cleaner because it doesn't require any additional grinding.
Anderson says the HPGA-produced powders will also be more powerful and will enable the metal-hydride batteries to last longer. "A clear performance advantage we expect for the spherical atomized powder is a much longer cycle life in rechargeable batteries," he adds.
In fact, the metal-hydride batteries may well outlast the products they power. "When electric cars become more of the norm, the body of the car will need to be replaced before the batteries have reached their useful lifetime," Foley says.
Using HPGA would make the processing simpler, faster and less expensive than current methods -- a crucial step in lowering the overall price of metal-hydride batteries and fuel cells.
"Right now, a lead-acid car battery costs $40 to $50. If you wanted to replace it with a nickel metal-hydride battery, you would be looking at $300 to $400, and that would be a tough sell," Pecharsky says. "But if the cost of the battery could be reduced to around $100 per battery and it was smaller, much lighter, much safer and stored much more energy, people would start buying those batteries."
Foley adds, "The three main hurdles to getting metal-hydride batteries into the mainstream are cost, weight and performance. We think that our knowledge of materials and processing can overcome most, if not all, of those hurdles."
For more information:
Vitalij Pecharsky, (515) 294-8220, vitkp@ameslab.gov
Iver Anderson, (515) 294-4446, andersoni@ameslab.gov
Jim Foley, (515) 294-8252, foley@ameslab.gov
Current research funded by:
Department of Commerce, National Institute of Standard's Advanced Technology Program
DOE Basic Energy Sciences Office
Last revision: 12/17/99 sd
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