Quantum behaviour of molecular hydrogen in potassium-graphite intercalates

Inelastic neutron spectra of KC24(H2)x measured at 1.5 K on the IRIS s

Inelastic neutron spectra of KC24(H2)x measured at 1.5 K on the IRIS spectrometer. Left inset: calculated electron density difference (red = gain, blue = loss) showing strong charge interaction between H2 and K. Right inset: vicinal hydrogen sites (white circles) in the KC24 unit cell (white lines).
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Experimenting with hydrogen storage media.

Understanding how molecular hydrogen (H2) binds to materials is crucial to designing a new generation of hydrogen storage media for ‘clean’ fuel cell vehicles. By combining neutron scattering studies with numerical calculations, new insight into H2 binding sites in the layered potassium-graphite intercalate KC24 has been gained. The inelastic neutron spectra show features consistent with a single adsorption site, unlike in the similar compound CsC24. Further, H2 is strongly pinned along a single axis and the H2-substrate interaction is characterized by rotational barriers ~100 times greater than in pure graphite. The calculations suggest that hydrogen is sited close to potassium ions, but they fail to account for the underlying symmetry of the experimental H2-substrate interaction. This discrepancy disappears once the H2 position is averaged over three positions close to separate ions in each adsorption site, naturally leading to the saturation coverage of ~2H2 per metal atom in this material. Our results imply that H2 storage in metal-doped carbon substrates can be severely affected by quantum mechanical effects.

A Lovell, F Fernandez-Alonso, SM Bennington, SF Parker (ISIS), K Refson (CSE), NT Skipper (UCL)

Contact:  Dr A Lovell, arthur.lovell@stfc.ac.uk

Research date: December 2008

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