Carbon nanohorns for hydrogen storage

A multinational team of researchers has developed a method for storing hydrogen gas based on single-walled carbon nanotubes with an irregular, horn-like shape.

Hydrogen has great potential as a carbon neutral and renewable energy source. Or at least it will have once the difficulties of storing the gas in a safe and cost-effective manner have been overcome.

Methods of storing hydrogen include uptake by metals and physical adsorption in porous materials. The latter is cheaper than using expensive metals, and 100% of the gas can be recovered for use. Another advantage is that the uptake-release process is recyclable.

Carbon nanotubes are promising candidates for adsorbent materials in hydrogen storage, but the weak interaction between molecular hydrogen and carbon means that the temperature must be kept below –200°C. This creates obvious practical difficulties.

The researchers – based at the Rutherford Appleton Laboratory (UK), University of the Basque Country (Spain), MER Corporation (US) and CNRS (France) – propose instead to increase the binding energy by using ‘nanohorns’, the geometry of which is thought to lead to strong hydrogen-substrate interactions. Nanohorns have an average length of 2–3 nm, and adopt a horn shape 80–100 nm across at the bell end.

Neutron spectroscopy provides much detail on the mobility, energetics and geometry of the hydrogen-nanohorn complex. In their study, the researchers showed that at room temperature, most of the hydrogen remains firmly attached to the nanostructures.

Nanohorns are currently produced in small amounts by NEC in Japan, using laser ablation, and in the US by MER with an arc process. Both processes could be easily scaled up, says CNRS scientist Marie-Louise Saboungi.

Saboungi and her colleagues intend now to look at the effects of temperature on hydrogen storage, the reversibility of the process, and differences in the behaviour of single- and double-walled nanohorns. “We would like to understand the basic differences of the interactions between carbon and hydrogen in different nanostructures of carbon.”

Figure: Carbon nanohorns (© CNRS).

Further reading: Nature of the Bound States of Molecular Hydrogen in Carbon Nanohorns, Fernandez-Alonso et al., Phys. Rev. Lett. 98, 215503 (2007).

Article first published in Nanomaterials News.