Hydrogen storage in carbon nanotubes is a recent topic of research and may be an important step towards making mobile hydrogen storage feasible.
The graphic to the right depicts carbon nanotubes of three different chiralities.
Significance of Mobile Hydrogen Storage
It is well-established that pure hydrogen has a much larger energy density (per unit mass) than typical hydrocarbon fuels – 2.6 times more than gasoline, in fact. For a 500 km range, a vehicle would require only 3.1 kg of H2 . The problem is the energy density per unit volume. For the same amount of energy, about four times the volume of hydrogen would be required compared with gasoline. This is fine for stationary applications, but the large space occupied is problematic for mobile applications. It follows that for hydrogen fuel cell vehicles to become prominent, the priority must be in compact and safe on-board hydrogen storage.
Existing Methods of Storage & Associated Problems
Currently, five primary technologies exist that hold promise for mobile hydrogen storage :
- 1. Compressed H2 gas is the simplest. It can be done at ambient temperature, and in- and out-flow are simple. The density, however, is low compared to other methods.
- 2. Metal hydride storage involves powdered metals that absorb hydrogen under high pressures (~1000 psia). Heat is produced upon insertion. With pressure release and applied heat, the process is reversed. A major problem is the weight of the absorbing material – a tank’s mass would be 592 kg compared to the 80 kg of a comparable compressed H2 gas tank.
- 3. Liquid H2 storage is just what it sounds like. The hydrogen storage itself has very high density, but Hydrogen boils at about -253ºC. From 25% to 45% of the stored energy is required to liquefy the H2 and maintain this low temperature (else the hydrogen will boil away), and bulky insulation is needed.
- 4. Sponge iron can be treated with steam to cause rapid oxidation, which gives off hydrogen as a byproduct. The iron itself can be considered a consumable fuel – once the entire mass of it has rusted, it is replaced with fresh iron. The steam itself, of course, requires energy to produce (the process occurs at 250ºC).
- 5. Carbon absorption is the newest field of hydrogen storage. Under pressure, hydrogen will bond with porous carbon materials such as nanotubes.
The inherency of the problem is clear: mobile hydrogen storage is currently not competitive with hydrocarbon fuels, and it must become so in order for this potential environmentally life-saving technology to be realized on a great scale. A significant standard of measuring the quality of a hydrogen storage scheme is what percent of the entire system’s weight is recoverable hydrogen:(1)
The US Department of Energy has issued goals for mobile hydrogen storage research, the most characteristic of which are 6.5 weight percent H2  and 62 kg of H2 per m3 (18.7 molecules per nm3) . Carbon nanotubes may well be a way of efficiently meeting this goal.
Carbon nanotubes (CNTs) essentially consist of sheets of graphite rolled into seamless tubes and capped at the ends. There are two forms CNTs take: single-walled and multi-walled. As the name implies, singled-walled nanotubes (SWNT) are composed of a single sheet of graphite; a diameter range of 0.4 to > 3nm is common. Multiwalled nanotubes (MWNT) are composed of several sheets, arranged concentrically in increasingly larger diameters with diameters in the range of 1.4 to 100 nm. Due to their diminutive dimensions, CNTs have unique physical and electrical properties. These include ultra high thermal conductivities (>3000 W/m-K), a Young’s modulus of ≈0.64 TPa, and the elastic ability to extend ≈5.8% of its original length before breaking. More appealing still is the disproportionately large surface area to volume that these materials possess, for this allows for a greater potential of interactions, whether they be physical or chemical in nature. Also, consider that their dimensions are relative to those of atoms and molecules. This increases the strength that these interactions have between one another, particularly from Van der Waals forces.
In order for CNTs to be a viable solution to the problem of mobile hydrogen storage, they must be both easy and cost-effective to produce. Currently CNTs are not inexpensive to produce, especially when the need for specific control over dimensions and purity of the samples is necessary.
The production of CNTs is achieved in three distinct methods:
- Arc discharge
- Laser ablation
- Chemical vapor deposition.
Arc discharge is the most basic of these and involves the use of two graphite electrodes between which a high current is passed. This produces a carbon vapor, in which CNTs form. While arc discharge is fairly inexpensive, it tends to produce samples of low purity. Also it requires special precautions due to the high amperage (100 A) and high temperatures (2000-3000°C).
Laser ablation, utilizes a laser directed at a graphite target which is then heated to roughly 1200°C. The vapor is sent down stream to condense on a cooled collector. This method has the benefit of producing high quality CNTs, but this is offset by the high initial costs and low output.
Chemical vapor deposition(CVD), the last method to be discussed here, probably has the greatest potential for use in industry of those mentioned. In CVD, a heated chamber is injected with some carbon based gas, carbon monoxide for instance. Located inside the chamber is a substrate, on top of which a matrix of metal catalysts particles is distributed, when the gas is flows through the chamber the carbon disassociates and begins forming vertical structures on the aforementioned metal particles. This takes place at a relatively low temperature of 700-800°C, as opposed to the first two methods. Also, it is far easier to implement this method in terms of large scale production, though the quality of this method leaves something to be desired.
The image above shows the growth of CNT in which the catalyst is elevated by the tube growth.
Though impurities are often a problem, they can be removed. This plays an important role in hydrogen absorption as NTs consisting of uniform carbon are shown to have higher rates of absorption . These impurities are often generated by the catalysts used in the synthesis of the CNTs, but can be removed by means such as pretreatment of the sample in an acidic bath or by heating in a vacuum at high temperature for short durations. Though any additional processes the nanotubes must undergo only accrues further expenses and thus puts the ultimate goal of affordable nanotubes further from reach.
Hydrogenation of Carbon Nanotubes
Hydrogen storage in carbon nanotubes occurs by two mechanisms: physisorption and chemisorption. The former is characterized by condensation of H2 molecules inside or between CNTs. Chemisorption, in contrast, uses a catalyst to dissociate the molecular hydrogen and allow it to bond with some of the unsaturated carbon bonds along the tube.
Early research into potential means of hydrogen storage in CNTs focused on physisorption as the primary storage mechanism. The initial studies were done on H2 adsorption of untreated carbon soot, which contained only 0.1-0.2 weight % SWNTs, it was found that this amorphous carbon was able to absorb 0.01 % H2 by weight. From these results it was extrapolated that a sample of highly pure SWNTs could reach a 5 to 10 % weight adsorptivity and thus the overall goal of 6.5 weight % put forth by the DOE.
Following up on these findings, others performed similar research into various conditions under which to improve the percent of hydrogen uptake, and while insightful these were done under unrealistic conditions; such as near cryogenic temperatures or extremely high pressures.
The most realistic take on the ability of CNTs to absorb H2, through physisorption, was performed by C. Lui et al. . Here the viability of CNTs was examined at room temperature and only modest pressures ≈10-12MPa. Through the use of hydrogen arc-discharge, three samples of carbon nanotubes were fabricated. Samples 2 and 3 underwent a pre-treatment process, which involved soaking in a solution of HCl acid. Following this, sample 3 was then heat treated in a vacuum. The results of hydrogen absorption can be seen the figure to the right.
The purpose of the acid bath was to remove all traces of the catalysts. This had little overall effect in increase H2 storage potential. The greater gain was found by the heating of sample 3 in a vacuum which evaporated any and all organic compounds that had formed of the surfaces of the CNT, thus demonstrating the need for clean and unobstructed surface interactions between hydrogen and the carbon atoms of the nanotubes. Though this showed a potential for hydrogen storage, the results were still far from ideal and made evident the inherent limitation of physisorption.
Attempts to model physisorption have lead to a study comparing analytical atomic modeling (AFEM) with a continuum model, in which the carbon tube is equated to a pressure vessel and the hydrogen stored is equated to an internal pressure. The figure to the left, from Chen et al in 2008, shows the analogy between the two methods (where (a) is AEFM, and (b) is the continuum model).
This study showed hope that CNTs could achieve the desired 18.7 molecules per nm3 as mentioned earlier in this article. The figure above, from the same study shows a correlation between nanotube radius and simulated storage capacity.
Density function modeling indicates that chemisorption holds more promise than physisorption for percent weight hydrogen. C-H bond strength prediction indicates that it is theoretically possible to release the C-H bonds at STP. The specific means by which to do so are still out of our reach technologically, though Nikitin et al  predict that an appropriate metal catalyst and a precisely calculated CNT diameter should be able to accomplish this. Under laboratory conditions, this team achieved 5.1 ± 1.2 wt% H2 storage at STP.
Through alkali doping, chemisorption in carbon nanotubes can be increased a good deal under laboratory conditions, though it must be done at higher temperatures . Lithium doping at 650 K reached a storage capacity of 20 wt% H2. Potassium doping at much lower temperatures (about room temperature) can yield 14 wt% H2, though the resulting hydrogen-rich tubes are unstable and prone to spontaneous combustion. Both processes involved 2 hours of hydrogen uptake, which is not practical for vehicle use .
A method of chemisorption was also proposed by researchers at Penn State, in which clusters of metal nanoparticles are chemically affixed to the surface of carbon nanotubes. These metal clusters, in this case platinum, act as doorways into the surface of the tubes. Some of the hydrogen is absorbed by the metal, converting them to metal hydrides, while the bulk is absorbed into the CNT where it adheres to the walls. Conveniently this increases the temperature at which the nanotubes can absorb hydrogen from near cryogenic temperatures to those temperatures more convenient for implementation. The temperature is dependent exclusively on the selection of metal used, thus there are possibilities to provide a range of functionality. Nickel and magnesium were other alternatives which were considered due to the heavy weight of platinum. Still a major concern with this method is the cost of the nanotubes themselves, which were roughly $25,000 per pound.
Hydrogenation of carbon nanotubes holds promise for the future of mobile hydrogen storage. Moreover, it holds some advantages over other existing methods of hydrogan storage, which have their own impracticalities. Though there are setbacks, research suggests that sufficient storage is theoretically possible. With the talent and unrelenting efforts of today's innovators, hydrogenated carbon nanotubes may someday make a hydrogen vehicles more viable and competitive than at present.
Written 19 November 2008