Solving Hydrogen storage Limit to Power Green Cars

Hydrogen fuel, because its only byproduct is steam, should be the ultimate in green alternatives to fossil fuels, but it hasn't delivered on its promise yet because of one enormous stumbling block, storage.

Now a team of chemical engineers at the University of Massachusetts Amherst has developed a computational model that shows that carbon nanotubes may offer a solution. Results are presented in the October 2009 online issue of the journal, Applied Physics Letters.

"If this works as we expect, it's perhaps no longer science fiction to hope for a briefcase-sized hydrogen battery to run a bus or car," says UMass Amherst chemical engineering professor Dimitrios Maroudas. The UMass Amherst computational model strongly lends itself to verification in laboratory experiments, say Maroudas and colleagues, and it provides ample testable hypotheses for future experimental research.

Specifically, Maroudas shows that proper arrangement of carbon nanotubes can overcome hydrogen transport limitations in nanotube bundles. It should also prevent ineffective and nonuniform hydrogenation, which is caused by nanotube swelling due to chemisorption of hydrogen atoms on the nanotube walls.

If one were to think of carbon nanotube bundles as something like a toothbrush, one strategy that Maroudas and colleagues recommend for holding hydrogen atoms most efficiently is that the brush arrangement should not be too dense.

If it is, when the tubules swell they'll block efficient passage and diffusion of the hydrogen, Maroudas explains. In addition to an optimal bundle density, further improvement can be achieved by optimizing the individual nanotube configurations to limit their swelling upon hydrogenation.

Following this approach should result in one hydrogen atom being able to chemisorb onto form a chemical bond with each carbon atom of the nanotubes, leading to 100 percent (atomically) storage capacity, he adds. This chemisorbed hydrogen, bound to the surface, can then be easily released by applying heat.

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