Wouldn’t it be great if we could fuel our cars with hydrogen? One gram of molecular hydrogen contains three times the energy of a gram of gasoline, and the only by-product of running a car on a hydrogen fuel cell is water.
Those are the best reasons for fueling cars with hydrogen. The bad news—part of it, anyway—is that molecular hydrogen is a gas at room temperature, and there aren’t a whole lot of hydrogen molecules per volume under normal pressure. That means your Honda Accord would need a fifteen-gallon hydrogen tank pressurized at one thousand times the atmosphere for you to be able to drive typical distances. Such a contraption would be an explosion waiting to happen. And lowering tank pressure simply means that you’d need a ridiculously large tank.
Yue Wu is part of a national team trying to find a way to store hydrogen in a smaller tank. His team has created a way to use nuclear magnetic resonance (NMR) to determine how well and how much hydrogen molecules adsorb onto carbon-based nanomaterials.
The idea, in theory, is simple. Scientists have created carbon-based nanomaterials such as carbon nanotubes (see Endeavors, Spring 2000, “The World’s Tiniest Tubes”), lightweight tubes made from graphene sheets with walls a single atom thick. “One gram of some carbon-based nanomaterials could contain the surface area of up to nine basketball courts,” Wu says.
If hydrogen atoms could bind with carbon atoms and cover the entire surface of such nanomaterials, then you could fill a tank with nanomaterial, pump in hydrogen, and presto—there’s your hydrogen tank that feeds the fuel cell.
“Unfortunately, nature is not too kind with this approach,” Wu says. “Carbon and molecular hydrogen don’t like each other enough at room temperature. For this to work, the nanomaterial has to be stored at low temperatures, such as minus one hundred ninety-six degrees Celsius, which is somewhat undesirable.”
At room temperature, hydrogen either binds too strongly—as it does in some metallic alloys, causing problems when it is released it from the material—or it binds too weakly, as it does in carbon-based materials, which means that storing hydrogen would only work at very low temperature.
“So we’re trying to create a surface that works in between, with three or four times the hydrogen binding energy of the current carbon-based nanomaterials,” Wu says.
When Wu says “we,” he means the U.S. Department of Energy’s (DOE) Center of Excellence for Carbon-Based Hydrogen Storage, which, along with his team, includes the National Renewable Resources Laboratory in Golden, Colorado, and eleven other research teams from U.S. companies and universities.
Other scientists create the nanomaterial, such as nanohorns (picture an ice cream cone sized down to one billionth of a meter), and Wu’s team tests the material with a unique NMR system that his team created for the sole purpose of measuring how gases interact with nanomaterials. Wu and research partner Alfred Kleinhammes designed the NMR system so they could study hydrogen storage at high pressure. The DOE took notice and granted them $780,000 to be part of the center.
Nuclear magnetic resonance occurs when the nuclei of a substance are placed in a static magnetic field and then exposed to radio waves with a specific frequency. NMR allows scientists to study nuclei that have intrinsic spin, such as those in hydrogen.
In medicine, NMR is used as an imaging tool known as MRI—magnetic resonance imaging. For instance, when a patient breathes in oxygen, the MRI shows where the gas goes to image the details of brain activity. Studying nanomaterial is similar; NMR shows where the hydrogen goes.
Wu’s NMR system includes a long tube—a probe, they call it—that contains a pencil-thin sapphire cylinder and a coil that resembles a bent paper clip. The coil, which surrounds the sapphire cylinder, emits the radio waves and picks up the responding radio wave from the nanomaterial inside the sapphire cylinder. The entire probe is housed in a cryostat—a temperature-controlled container—which slips into the top of the superconducting magnet (a large, metal, can-like container).
Wu’s team attached stainless steel pipes to the sapphire cylinder to feed hydrogen into the nanomaterial at up to one hundred times the atmospheric pressure. The team added valves and pumps to control the amount of hydrogen, as well as the amount of liquid nitrogen, which cools the cryostat so that scientists can conduct experiments at various temperatures.
Then when everything is ready to go, Wu’s team bombards the substance with radio waves.
“It’s like a microwave oven in which water in food absorbs energy and makes food hot,” Wu says. “The idea is similar; we shine the sample with this radio wave—a pulse of energy—and then see how much energy is absorbed.”
The NMR allows Wu to watch how hydrogen reacts inside a substance. “We can tell how much hydrogen the nano-material absorbs and how much pressure you need to cover the surface of a material,” Wu says. “And we can also tell the binding energy—we can say, ‘this material has that many binding sites for hydrogen per gram.’ And we can tell how strongly the hydrogen is bound.
“We are looking for nanomaterials that could absorb a lot of hydrogen at room temperature, and we want to tell people why it works or why it doesn’t work.”
The NMR method has unique advantages over the traditional technique of measuring the weight-gain of a substance when exposed to hydrogen. For years, scientists who used that traditional technique went back and forth on whether carbon nanotubes worked for hydrogen storage. Part of the problem, Wu says, was the lack of specific information about weight measurements.
“Our test is very specific,” Wu says. “We really can tell how the molecule feels; we can tell the details of the situation. If the hydrogen goes inside the nanomaterial, we can see that. And we can change the pressure to see how this changes the amount of adsorption at different places inside the nanomaterial. At the molecular level, substances have their own unique signatures, and NMR provides detailed information on those signatures.”
The team can also study why hydrogen binds well or does not. Full experiments on one substance take about a week, Wu says.
In the past two years, there’s been no eureka moment, but Wu’s team keeps learning more about carbon-based nanomaterials, especially those laced with boron, another lightweight element that, in theory, should allow hydrogen to bind better. Wu’s team found that boron does do this, but the number of binding sites is not quite high enough for transportation purposes. So DOE center scientists are trying to find different ways to add more boron to carbon-based nanomaterials without losing vital surface area in the process. This is where Wu’s NMR method is essential; it can reveal previously undetectable details when hydrogen interacts with altered nanomaterial.
Last year, Mike Chung, a DOE center scientist at Penn State, created boron-doped graphitic carbon, and Wu’s team showed that such material has significantly better binding energy than bare carbon nanotubes. “But we still need to improve the material by about fifty percent,” Wu’s research partner Kleinhammes says.
Wu says, “We’ve seen so far that adding boron has led to improved binding energy, which is favorable. But we don’t know exactly the local structure of the binding sites and the structure surrounding the boron atoms. The number of such binding sites with favorable binding energy is still too small. If we want to design this material, we need a better understanding of the structure, and we’re working on that now.”
Another material called nanoporous carbon expands when heated, leaving channels on its surface, which means more surface area for hydrogen binding. It, too, is laced with boron. And Wu’s team is also running experiments on carbon nanohorns decorated with metal particles.
“So that gives us three types of samples,” Wu says of his current projects. He hopes his team will find out if hydrogen-binding has more to do with adding boron to carbon, or if the porosity of a substance matters more.
If the materials show promise, Wu’s team will try to determine why, and consult with team members who will then work to improve the material within the narrow parameters for hydrogen storage at room temperature. Many materials have already been ruled out because they’re too heavy, too expensive, or react with hydrogen so slowly that filling a gas tank would take hours. Few consumers would stand for that.
The DOE has given Wu and other team scientists strict guidelines that would keep society humming along the way it is, only with hydrogen fuel cells instead of combustion engines.
Whether that’s a pipe dream or not, Wu can’t say. “I don’t think anybody knows for sure if this will work,” he says. “That’s why we do the research. But we’ll definitely know what won’t work.”
Yue Wu, a professor of physics and astronomy in the College of Arts and Sciences, receives funding from the United States Department of Energy. His team includes research professor Alfred Kleinhammes, also of the Department of Physics and Astronomy, and graduate student B.J. Anderson.