Scientia: Research at the University of Tennessee
Have Hydrogen, Will Travel
By LAURA BUENNING
In response to the national emphasis on hydrogen energy research, Myvizhi Esai Selvan, David Keffer, and Brian Edwards take a nanoscale look at the molecules inside a fuel cell. Meanwhile, Sandeep Agnihotri investigates the storage capacity of carbon nanotubes.
Touted for their promise as a power source for tomorrow’s automobiles, hydrogen fuel cells still have a way to go to live up to their “clean green” potential.
At the heart of the question lies uncertainty about how the molecular structure of the fuel cell and its energy-producing processes interact to help or hinder the cell’s efficiency, says Myvizhi Esai Selvan, a chemical engineering graduate student with the UT Computational Materials Research Group (CMRG) and recipient of the 2007–08 Institute for a Secure and Sustainable Environment stipend.
If only we could journey to the center of a fuel cell to see exactly what happens there, rather like the miniaturized surgical crew who enter the brain of a wounded Russian defector in their microscopic submarine in the film Fantastic Voyage.
Such a voyage is still a fantasy, but CMRG-created computerized simulations of molecules in action may offer the next best thing. The molecular-dynamics model permits a nanoscale view of the structures that form at the borders where protons enter and leave a polyelectrolyte membrane.
We know generally how fuel cells work. Pure hydrogen enters the anode side of the cell and, aided by a platinum catalyst, splits into electrons and protons. On the other side, a platinum-coated cathode attracts oxygen from air to its surface, creating a kind of electrical pressure called a “potential.”
Separating the anode and cathode, a thin semipermeable membrane—termed a polyelectrolyte or proton exchange membrane (PEM)—allows the protons to pass but rebuffs the electrons and the hydrogen and oxygen atoms. The electrons, attracted to the oxygen on the other side of the membrane, flow into an external circuit around the membrane and back inside the cell, supplying power for electrical motors or other equipment along the way. To complete the circuit, the protons pass through the membrane and pair off to bond with the oxygen atoms and electrons on the cathode side of the cell to form water.
“People have analyzed what’s going on with the electrodes [anode and cathode] and the electrolyte [the membrane], but we don’t have a clear picture of what happens in the interface [or common boundary] between the platinum catalyst on the electrode and the membrane,” Esai Selvan says.
“We want to know how the protons leave the catalyst and enter the proton-exchange membrane,” she says, “and how water influences the process.”
The question has stumped scientists for more than 40 years, says CMRG group leader David Keffer. Keffer and Brian Edwards, UT chemical and biomolecular engineering associate professors, direct the computational materials group.
“We’re trying to determine whether wetting helps or hinders or makes little difference to the electrodes, catalyst, and proton movement through the cell,” Esai Selvan says. This information will improve the durability and cost of the membrane as well as the placement of the electrode-catalyst membrane within the cell.
Keffer, Edwards, and the CMRG can combine what they learn about the chemical engineering aspects of fuel cells with the findings of their colleagues in the Chemistry Department. Chemistry’s Distinguished Professor Jimmy Mays and Professor Mark Dadmun are working to improve polymers suitable for polyelectrolye exchange membranes. (See “Stretching the Limits.”)
Safe Transport
Fuel-cell design represents one side of the equation; hydrogen storage and transport, the other.
Lightweight, chemically active, and highly flammable, hydrogen is as hard to contain as the best escape artists. Only high pressures can condense it into useful quantities, and cryogenic temperatures are required for it to go into a liquid state. So unless we come up with a way to safely store hydrogen in reasonably lightweight containers, its use as an alternative fuel in automobiles will be limited.
“Storage materials that physically—as opposed to chemically—capture hydrogen have potential,” says Sandeep Agnihotri, assistant professor in UT’s Department of Civil and Environmental Engineering. Under the right conditions, pores inside and outside a material become containers for hydrogen gas or other molecules.
“We use a material’s gas adsorption [the capacity of a material to attract and hold molecules on its surface] to calculate its porosity. The more pores, or holes, a material has, the more surface area where the molecules can cling,” Agnihotri says. And adsorption of hydrogen is reversible; simply remove the conditions holding the hydrogen in place and it becomes available as fuel.
Enter the Carbon Nanotube
Made exclusively from carbon atoms, nanotubes are 1-atom-thick sheets of graphite rolled into seamless cylinders 1/10,000 to 1/100,000 of the thickness of an average human hair yet 100 times stronger than steel. Nanotubes have garnered attention as a possible storage medium for hydrogen because they rarely react with other elements and have both inside and outside surfaces.
“But,” Agnihotri says, “if you track studies on hydrogen storage in nanotubes over the last 10 years you find inconsistent results, and the question is why?”
To find out, he needed a model that told him theoretically how much hydrogen the tubes should be able to carry—a task complicated by the fact that tubes come in many sizes.
“A bundle of nanotubes will typically have diameters anywhere from 7 angstroms (0.7 nanometer) to 20 angstroms,” he says, pulling pens, markers, and pencils from his desk drawer and holding them between his thumb and forefinger to demonstrate.
With the help of Raman spectroscopy to tell him the actual diameters and percentages of each in a particular bundle of nanotubes, Agnihotri calculated the inside and outside adsorption potential for each diameter. By adding them together at the percentages found in the sample, he was able to arrive at a total adsorption figure for the entire bundle.
“But my calculated adsorption was much higher than experimental data from similar samples,” he says.
As he thought about it, Agnihotri began to question his assumption that 100 percent of the carbon tubes would be open to allow the gas inside. Playing with the idea, he found in one case that his adsorption calculations matched experimental data if only 45 percent of the tubes were open and 60 percent in another. In one sample nothing was open.
“Knowing what fraction of the tubes are open will help clarify experimental data,” he says. “Think about the implications for making and processing nanotubes, let alone for accurately predicting hydrogen storage capacity.”
- - -
For more information, contact Myvizhi Esai Selvan, mesaisel@utk.edu; David Keffer, 865-974-5322, e-mail dkeffer@utk.edu; or Sandeep Agnihotri, 865-974-7728, e-mail sagnihot@utk.edu.
PLUg-in Vehicles Get Charged Up at Home
According to 2003 statistics compiled for the U.S. Department of Transportation, two out of three Americans drive 15 miles or less to work each day; three out of four spend as much as to 30 minutes on that one-way commute. Eighty-one percent commute in a personal vehicle; 86 percent of those drive alone.
“Something like the plug-in hybrid [vehicle] just makes so much sense,” says Robert E. Uhrig, speaking by phone from his home in Florida. The retired University of Tennessee–Oak Ridge National Laboratory Distinguished Scientist relishes the time he now has to pursue side interests related to his long career improving the efficiency and safety of nuclear and fossil-fueled power plants.
Uhrig’s fascination with hybrids began in the early 1970s when he followed the progress of a colleague and several students as they built two vehicles. The first was a car built on a Datsun chassis and powered by eight lead-acid automobile batteries and a 14-horsepower motor-generator. The second was a 24-passenger bus with two 1,600-pound lead-acid batteries storing power from two 15-kilowatt generators driven by a 60-horsepower diesel engine.
“With a plug-in hybrid you run the battery down to a low level before charging it while you’re driving, so you can take the electricity from your home service,” Uhrig says. For distances longer than 50 to 80 miles, the hybrid switches over to a lightweight fuel-efficient internal-combustion engine.
By Uhrig’s calculations, hybrids operating in plug-in mode could replace 281 million gallons (6.7?million barrels) of fuel per day with electricity, when compared with standard lightweight vehicles averaging 20 miles per gallon. That’s nearly 75 percent of the estimated 9 million barrels of oil Uhrig calculated is used daily by 225 million automobiles and lightweight vehicles on American highways. But to do so would impose a sizeable demand on power-generating facilities, dictating that we build new nonpolluting power plants, as well as new transmission and distribution lines and substations.
Still, supplying adequate green electricity is not the most daunting obstacle.
“For the technology to work we need a lightweight, low-maintenance, economical battery with high energy density—meaning it stores and releases substantial power for its size and weight,” says Uhrig, who points out that none of today’s common batteries meet these criteria.
“With better batteries, hybrids can have all the amenities: power steering, power brakes, air conditioning, and automatic transmission,” Uhrig says.
Opinions vary, but expense tops the list of concerns. True, the lead-acid batteries in most American automobiles are inexpensive and trustworthy, but large batteries also add significant weight. In contrast, the nickel–metal hydride batteries found in many commercial hybrids more than double the energy-density level of lead-acid batteries. But nickel is expensive and in high demand.
Lightweight lithium-ion batteries, similar to those found in laptops, have even higher energy-density levels but are also expensive and limited by unresolved stability and safety issues in larger batteries.
Nevertheless, Uhrig says, escalating atmospheric carbon dioxide and soaring fuel consumption have intensified the search for better battery technology, which would make the plug-in hybrid an attractive alternative mode of travel.
— L. B.
Teambuilder
“Astounding as it seems, the number of vehicle miles traveled by individuals in the United States continues to grow by 3 to 4 percent each year,” Wayne Davis says. “That’s faster than population growth,” which increased 1.3 percent annually from 1990 to 2000 and grew by less than 1 percent in 2007, according to the CIA World Factbook.
“What we have to do is change people’s minds,” says Davis, the associate dean of research and technology for the College of Engineering.
“We could cut our personal fuel use in half if people would just double up in cars to go to work,“ Davis says. “But ideas about how to conserve energy collide with our desire to be independent, to never have to wait on others, to get to our destinations more quickly.
“Still, more people are thinking ‘energy’ these days, and with the changes to the environment being publicized and accepted, and with the recognition of our dependence on foreign oil, people are more willing to choose hybrid and alternative-fueled vehicles, to use energy-efficient lighting, or build energy-efficient homes.”
Well known for his air-quality research, Davis first earned degrees in physics but changed to engineering because, he says, “Environment issues ‘rang my bell’ in cases where solutions to real-world problems might be found.”
Today Davis’s administrative role resembles that of a catalyst, sparking alliances between UT faculty members conducting basic energy research and those studying practical engineering applications. Hydrogen-energy research caught his attention recently. Davis also leads a new hydrogen-fueled vehicle demonstration project funded this summer by the state of Tennessee.
“UT has so many powerhouse researchers working on energy issues,“ he says. “It’s exciting to be able to help create a stronger energy-related campus.”
For more information, contact Wayne Davis, 865-974-5321, or e-mail wtdavis@utk.edu.
— L. B.