Hydrogen Storage in Carbon Nanotubes
by Jian Cui and Kellen Schefter
On the desk of Anton Nikitin, a graduate student in Stanford's Applied Physics department, sit several stacks of Petri dishes containing small slabs of silicon, each roughly the size of a postage stamp. A thin film-like substance spread on the surface contains carbon nanotubes, extremely versatile molecules that may revolutionize the way hydrogen energy is stored.
Why Hydrogen?
Energy use is the primary cause behind today's most pressing environmental concerns such as air pollution and global warming. Fossil fuels - a major source of energy today - are limited and will become increasingly difficult to obtain as populations rise. Clearly, new ways to power the world are needed.
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| XPS analysis allowed researchers to conclude that hydrogen is binding to the carbon nanotubes. Before hydrogenation, the binding profile reflects purely C-C bonds. After hydrogenation, the binding profile reflects C-H bonds. |
Meeting the FreedomCAR's Initiatives
Ensuring that the automobile fits into a hydrogen-driven future is the task of the FreedomCAR Initiative, a partnership between the Department of Energy (DOE) and American automakers. FreedomCAR has established a set of standards that future fuel cell vehicles must meet in order to remain a practical alternative to today's vehicles.
Meeting these standards won't be easy. "The main problem," says Nikitin, "is how to store hydrogen on-board." FreedomCAR guidelines stipulate that, by 2010, hydrogen storage systems achieve an energy density of six weight percent. That is, the weight of a fuel storage system must be six percent hydrogen. Even with today's fuel cell vehicle prototypes, which, as Nikitin puts it, are "very far away from optimized," this energy density requirement translates into a range of 300 miles - very close to today's gasoline powered vehicles.
Storing Hydrogen in Carbon Nanotubes
Carbon nanotubes provide a promising new approach to storing the hydrogen needed to power fuel cells on a large scale. In addition to being an extremely light and abundant element, carbon is available in a variety of structures that facilitate unique interactions with other elements. For instance, carbon structures can 'adsorb' other elements by holding them in place on the surface of the structure. "If you want to adsorb a lot of something into a material, you need to maximize the surface area," explains Nikitin.
A tube-like structure, the walls of which are comprised of a single layer of carbon atoms, does just that: all the atoms are on the surface, ready to attach to other atoms such as hydrogen. Single-walled carbon nanotubes - tubes with a diameter on the order of nanometers - contain an estimated 1500 square meters per gram of carbon.
Two primary methods have been identified for trapping hydrogen on the surface of these nanotubes: physisorption and chemisorpt ion. Research into physisorption, a weak interaction between hydrogen and carbon, has not been able to store hydrogen at the set standard of six weight percent in a reproducible fashion. Recent theoretical studies, especially work done in Professor Kyeongjae Cho's lab in Stanford's Mechanical Engineering department, predict that chemisorption - forming chemical bonds between hydrogen and the carbon atoms of the nanotubes - may be able to store up to 7.5 weight percent of hydrogen.
To confirm this possibility, Anton Nikitin and other researchers at the Stanford Synchrotron Radiation Laboratory (SSML) within the Stanford Linear Accelerator Center bombarded carbon nanotubes with atomic hydrogen, then determined whether hydrogen was indeed attached to the nanotubes.
"We can't really see hydrogen," says Nikitin, "but if it does form C-H [carbon-hydrogen] bonds, it also should change the electronic structure of the carbon." Thus, to test their results, the researchers looked at the carbon's electronic structure. Using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS), the team saw that the electronic structure of the carbon atoms changed in a way that reflected the formation of carbon-hydrogen bonds.
"This is the first attempt to try to figure out if we can form carbon nanotube-hydrogen complexes," explains Nikitin. While this is a notable feat, the experiment would have been considered a failure if very little hydrogen bonded to the nanotubes. XPS analysis revealed that approximately sixty-five percent of the carbon atoms in the nanotubes had been hydrogenated, a hydrogen capacity of 5.1 ± 1.2 percent by weight.
The team had achieved the six weight percent storage requirement within experimental error, but before they could celebrate, they had to see if they could remove the hydrogen atoms.
Removing Hydrogen and Cycling the Process
In order to use the hydrogen in a fuel cell, it must first be released from the surface of the carbon nanotubes by exposing them to enough heat. The team was excited to find that at 600 ¼C, XAS and XPS showed that the electronic states of the carbon atoms were virtually identical to carbon atoms before hydrogenation.
The next step was to see if the carbon nanotubes could be reused for hydrogen storage. The same nanotubes were hydrogenated again and sure enough, XAS and XPS revealed the presence of carbon-hydrogen bonds. The hydrogenation/dehydrogenation process was successfully repeated twice more.
From Laboratory to Reality
While the team's results are promising, nanotubes as hydrogen storage media are far from realization. These results were obtained from proof-of-principle experiments that are far from being technologically feasible for mass production. For instance, SSML is one of only a few institutions in the world that can introduce hydrogen to the carbon nanotubes in the form of an atomic beam.
The research team is already investigating a more practical technique. "We can cover nanotubes with very, very small clusters of catalytically active metals like nickel, which can dissociate H2 molecules," Nikitin explains. With this metal catalyst, carbon nanotubes need only be exposed to molecular hydrogen gas to form the carbon-hydrogen bonds. The temperature of 600 ¼C needed to dehydrogenate the nanotubes is too energy demanding to be practical. However, the research team believes carbon nanotubes could release their stored hydrogen at temperatures as low as 50 to 100 ¼C, since theory predicts that the carbon-hydrogen bonds are weaker in carbon nanotubes with larger diameters.
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| Energy from sunlight is harnessed for a hydrogenase enzyme to produce H2 (the target product) and O2 (a byproduct) from H20. |
The Future of Hydrogen
In the future, Nikitin imagines fuel cell vehicles supplied with hydrogen from replaceable cartridges. Each cartridge would contain an array of carbon nanotube material preloaded with hydrogen at a central facility. "If this complex is pretty stable, you don't need to have special [fueling] places, you could go to any store and buy a cartridge." At this point, the implementation is just a guessing game. "It's simply an idea," Nikitin says.
With each new idea, we may be closer to a future where hydrogen is safely and easily stored so that it may power our cars, our buildings and our electronic devices. There is a lot of work to be done, but it all starts with the theory and the basic research. This is why Nikitin has accumulated Petri dishes on his desk. The road to a hydrogen economy, like any path blazed by advancing science, is littered with such artifacts.
XAS and XPS Analysis
X-ray absorption spectroscopy (XAS) involves exciting electrons into a higher energy state. By changing the wavelength and measuring the absorption of X-rays, information is obtained on the unoccupied molecular orbitals of chemical species. These empty density states are very sensitive to their chemical surroundings, namely nearby bonds, so absorption of X-rays provides information on the nature of chemical bonds. In the hydrogenation of carbon nanotubes, researchers found a decrease in the signal for the pi* resonance and an increase in the signal for C-H* resonance, meaning that the double bonds of the sp2 hybridized carbons in the nanotubes had transformed into single bonds with hydrogen.
X-ray photoelectron spectroscopy (XPS), excites electrons from carbon atoms into the vacuum. Using the energy of the beam and the kinetic energy of the released electron, one of Einstein's equations can be used to calculate the binding energy of the electron. The research team determined that changes in these binding energies were caused by changes in the hybridization of the carbon atoms from sp2 to sp3, indicating that carbon was no longer bound only to carbon, but to hydrogen as well.
Producing Carbon Nanotubes
Carbon nanotubes are grown under extreme conditions using carbonated gases around metal catalysts. However, as Anton Nikitin explains, "There is no good way to generate a lot of [carbon] nanotubes with controlled quality." At this point, higher quality carbon nanotubes free of defects can only be produced in extremely low quantities. For the hydrogenation experiments performed at SSML, the nanotubes were specially grown by Professor Hongjie Dai's group in Stanford's Chemistry department to meet size and quality specifications. Once they are produced, the carbon nanotubes are very delicate and prone to defects.
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| On the left is an illustration of the structure of a single-walled carbon nanotube. Carbon atoms, represented by green spheres, are bound together by sp2 bonds. After hydrogenation, the single-walled carbon nanotube structure changes to that shown on the right. The carbon atoms are no longer only bound to themselves, but to the hydrogen atoms(white spheres) as well. After heating to 600C, the hydrogen atoms completely detach from the carbon nanotubes and the structure reverts to its original form. |
Problems with Conventional Hydrogen Storage Methods
Traditionally, hydrogen has been stored as a compressed gas or as a liquid. The low density of hydrogen gas means it fills a lot of space. This problem has been mitigated with compression of up to 10,000 psi or higher, but with these pressures "it's like driving around with a bomb inside," as Nikitin puts it. Liquid hydrogen takes up less storage volume than hydrogen gas, but some of it is inevitably lost to the atmosphere since liquid hydrogen is so difficult to contain. Neither method is very energy efficient because a great deal of energy is required to compress hydrogen gas or cool it to its liquid state.
A third method, absorbing hydrogen into a medium, is more promising. Metal hydrides - compounds of a metallic element and hydrogen - have already shown a hydrogen storage capacity of up to eight weight percent. "There is one drawback," cautions Nikitin. "To get the hydrogen out, you need to heat it up to 800 ¼C." Generating this heat means less energy is available for the system that needs the hydrogen, such as a car. The need for a more practical solution has prompted researchers to "look for carbon-based materials."
To Learn More:
The SSML science highlight on the research:
http://www-SSMl.slac.stanford.edu/research/highlights_archive/swcn.html
Department of Energy website on the road to a hydrogen economy:
http://www.eere.energy.gov/hydrogenandfuelcells/presidents_initiative.html