Discussion Topic Posted for the NANOTUBE-99 Workshop:

Here is my brief report from the IWEPNM'98 (International Winterschool on Electronic Properties of Novel Materials: Molecular Nanostructures) Workshop in Kirchberg (Austria), February 18 - March 3, 1998.

One of the workshop participants, Paul Anderson, a graduate student working in the group of Prof. Nelly Rodriguez at Northeastern University, reported his results on unusually high hydrogen uptake by a carbon-based material used in Prof. Rodriguez' group.

This carbon-based material

• has been patented by Prof. Rodriguez for its use to store hydrogen
• may contain "carbon fiber"-like structures
• has a density of 2.2 g/cm³
• has been reported to show a 20%-30% gain in weight upon loading with hydrogen at 100 atm and room temperature

The experiment, as reported, involved a 0.2 g sample. In absence of the carbon material, the container showed no net gain in weight when loaded with hydrogen.

Please note that the above is second-hand information based on Paul Anderson's interesting report at the workshop and our converstation. None of this information, which I consider very interesting, is suposed to be confidential to the best of my knowledge. I will gladly substitute it by updated first-hand information from the group of Prof. Rodriguez should I receive such a request.

I find the above findings extremely intriguing in view of the following facts:

• A 20-30% weight gain implies that every C atom should bind 2-4 H atoms. Only atomic carbon or (unstable) carbon chains could bind such a high amount of hydrogen.
• If packed compactly in the container, the 0.2 g sample should fill a volume of 91 mm³ and absorb 0.04 g hydrogen at the conservative 20% gain value. In absence of the intriguing carbon material, assuming ideal gas law (which is surely incorrect), filling the 91 mm³ volume with 0.04 g hydrogen should require the awsome pressure of 5x10³ atm, much higher than the applied pressure of 100 atm. Hence, it appears unlikely that hydrogen should be present as "free molecules".

Addendum of 8 March 1999:

The use of the ideal gas equation is obviously very naive, as pointed out in the following contribution of Christoph Nuetzenadel.

Hydrogen can be stored very efficiently within the crystal lattice of metals such as Pd, up to one H atom per metal atom, never more. Hydrogen absorbed in this way is always in its atomic (some authors call it also "metallic") form. The net energy gain associated with dissociation and subsequent solution in the bulk had been mistaken for an indication of the so-called "cold fusion" some time ago [1]. Similar binding to the carbon matrix as found in metals appears to be highly improbable. A spontaneous conversion of the hydrogen loaded carbon matrix to CH₄ or other hydrocarbons, which would account for a 4:1 hydrogen:carbon ratio, is equally improbable.

So, if the binding is neither metallic, nor covalent, and if the amount is too high to be physisorbed in it molecular form -- what is it?

[1] Z. Sun and D. Tomanek: "Cold Fusion: How Close Can Deuterium Atoms Come Inside Palladium?" Phys. Rev. Lett. 63, 59 (1989).

Addendum of 11 April 1999:

Rumors at the 1999 APS March Meeting have it that the Rodriguez group had received substantial support for "hydrogen storage in carbon materials" from the automobile manufactorer Daimler-Chrysler. This support has apparently been withdrawn in the meantime, thus raising another "caution flag".

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