The automobile fleet is contributing significantly to air quality degradation in large cities. The use of hydrogen as a fuel is interesting since combustion delivers a lot of energy without polluting. However, some obstacles, like the efficient storage of this gas, remain before it can be implemented in the transportation industry.

Figure 1: Hydrogen powered car built by BMW

Hydrogen storage solutions

The United States Department of energy (DOE) has fixed two targets for hydrogen storage solutions applied to automotive transportation. The first target requires a ratio of hydrogen weight / tank weight that is superior to 0,065 (6,5% weight). This target limits the weight of the tank. The second target requires a hydrogen volumetric density higher than 62 kg/m in order to limit the volume of the tank.

Four main solutions were proposed to solve this problem:

• Compression or liquefaction of hydrogen
• Metal hydrides
• Chemical tanks
• Adsorbent materials

Figure 2 shows the commercially available solutions. Each one is compared to the DOE targets. To satisfy both targets, a solution must be located in the grey region of the graph. Carbon nanostructures are not displayed on the graph since they are not readily available in the market at the moment.

Figure 2: Comparison of storage solutions available on the market

Carbon nanotubes

Solid state carbon may exist under three different crystaline forms named allotropes. Diamond and graphene, are the two well known allotropes. The third allotrope is called fullerene. It is a new class of material which is formed by a spherical or cylindrical wrapping of graphene sheets.

Nanotubes are fullerenes formed by graphene sheets wrapped as shown in Figure 3. The diameter of those tubes can be as small as 1 nm.

Figure 3: Representation of the carbon nanotube structures

The porosity of nanotubes is large. This property enables the adsorption of various gases including hydrogen.

Hydrogen storage in carbon nanostructures

Experimental results obtained in the evaluation of hydrogen storage capacity of carbon nanotubes vary significantly from one research team to another. Some groups (Dillon et al., Chambers et al.) obtained results indicating this material allows to reach DOE targets. Others (Ahn et al., Hirscher et al.), are indicating the contrary.

We chose to evaluate the storage capacity of nanotubes with Monte-Carlo numerical simulations. Our results indicate that carbon single-walled nanotubes can store 0.22% to 0.79% weight (3.95 to 7.94 kg/m) of hydrogen at room temperature and under a pressure of 10 MPa.

Figure 4: Simulation of the interaction between nanotubes (20,0) and hydrogen

Conclusion

Our results indicate that carbon nanostructures are falling short of the DOE targets. Such structure as thus more likely to be inappropriate for hydrogen storage in transport applications. However, we obtained interresting results by including the effect of impurities dispersed in nanotubes. We invite you to consult those papers to learn more about this subject:

P. Guay, Modélisation Monte-Carlo de l’adsorption de l’hydrogène dans les nanostructures de carbone, INRS-EMT, Montréal, 2003 (pdf)

P. Guay, B. L. Stansfield and A. Rochefort, On the Control of Carbon Nanostructures for Hydrogen Storage Applications, Carbon 42 (2004), 2187-2193 (abstract) (pdf)

A review of the literature is also available for consultation.

Author : Patrice Guay

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