March 20, 2008

Buckyballs can contain superpressured hydrogen

Buckyballs (carbon 60) can be loaded with hydrogen up to a few times smaller than the pressure [up to 140GPa] when the hydrogen turns metallic (400GPa) This is a detailed computational analysis, they did not perform a physical experiment and there is as yet no clear strategy for performing the actual loading of hydrogen into the C60. The highest pressures are achieved only with significant and near unstable strain. Could multi-walled carbon nanotubes be loaded with silane up to a critical pressure to a make a superconducting wire ? What is the peak critical superconducting temperature for the silane class of material ? How can we load gases into fullerenes up to those pressures ?

Internal pressure in fullerene nanocage producing various average relative C-C bond elongations (1%, 2%, 3%, 4%, 6%, 8%, and 10%) vs radius of undeformed cage. Vertical dashed lines mark radii of some nearly spherical fullerenes.

This relates to the new class of superconductors made from supercompressing silane (silicon and hydrogen). Note: I had to amend that report on superconductors. The investigators did not get good readings in the critical range of 96-120GPa, where the readings that they did get showed an indication of a spike in critical temperature. So highly pressured silanes could be a route to room temperature superconductors but we do not know yet. Also, now if silane were superconducting under high pressure than those conditions might be achieved with buckyballs loaded with silane.

The research appears on the March 2008 cover of the American Chemical Society's journal Nano Letters. "Based on our calculations, it appears that some buckyballs are capable of holding volumes of hydrogen so dense as to be almost metallic," said lead researcher Boris Yakobson, professor of mechanical engineering and materials science at Rice. "It appears they can hold about 8 percent of their weight in hydrogen at room temperature, which is considerably better than the federal target of 6 percent."

Buckyballs, which were discovered at Rice more than 20 years ago, are part of a family of carbon molecules called fullerenes. The family includes carbon nanotubes, the typical 60-atom buckyball and larger buckyballs composed of 2,000 or more atoms.

If a feasible way to produce hydrogen-filled buckyballs is developed, Yakobson said, it might be possible to store them as a powder.

"They will likely assemble into weak molecular crystals or form a thin powder," he said. "They might find use in their whole form or be punctured under certain conditions to release pure hydrogen for fuel cells or other types of engines."

The full paper is here

Estimated hydrogen density in Hn@C60 structures vs hydrogen pressure. Horizontal dashed line marks the density of liquid hydrogen at boiling point (20 K) under ambient pressure.

Fullerene Nanocage Capacity for Hydrogen Storage by Olga V. Pupysheva, Amir A. Farajian, and Boris I. Yakobson

We model fullerene nanocages filled with hydrogen, of the general formula Hn@Ck, and study the capacity of such endohedral fullerenes to store hydrogen. It is shown using density functional theory that for large numbers of encapsulated hydrogen atoms, some of them become chemisorbed on the inner surface of the cage. A maximum of 58 hydrogen atoms inside a C60 cage is found to still remain a metastable structure, and the mechanism of its breaking is studied by ab initio molecular dynamics simulations. Hydrogen pressure inside the fullerene nanocage is estimated for the first time and is shown to reach the values only a few times smaller than the pressure of hydrogen metallization. We provide a general relation between the hydrogen pressure and resulting C−C bond elongation for fullerene nanocages of arbitrary radii. This opens a way to assess possible hydrogen content inside larger carbon nanocages, such as giant fullerenes, where significant capacity can be reached at reasonable conditions

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