In the search for a viable on-board hydrogen storage system, a number of proposals based on the design of advanced materials such as metal hydrides and carbon-based structures have been made to overcome the limitations of the conventional solution of compressing hydrogen in tanks, the authors note.

Recent work has investigated graphene, demonstrating on theoretical grounds the possibility to achieve the desired gravimetric capacity of 8-9 wt% set by the US Department of Energy (DoE) for 2015 for both molecular and atomic hydrogen. These results were achieved by designing complex three-dimensional patterns or by decorating graphene with appropriate chemical species.

Hydrogen can either physisorb or covalently chemisorb onto the graphene surface. Physisorption is more difficult to control, while chemisorption leads to more stably bonded hydrogen. Conversely, molecular hydrogen dissociative chemisorption is an unfavorable process with a high-energy barrier, they note.

Tensile stress was shown to lower the barriers for the chemi(de)sorption of molecular hydrogen. A similar effect was observed on the convex surfaces of nanotubes and fullerenes or on graphene rippled by the previously adsorbed hydrogen. The enhanced reactivity is attributed to the local geometry deformation of the C site, whose orbitals become prone to accept a chemical bond. The fact that the energetics of the adsorption processes is dependent on the local curvature suggests ways to control the amount and pattern of chemisorbed hydrogen.

Here we show that this effect could be used as the basis of hydrogen storage systems. To this aim, we apply density functional theory to determine the adsorption capacity of graphene sheets as a function of their local curvature. Subsequently we propose different protocols for adsorption
desorption cycles based on the external control of the sheets local curvature, and we show their operation with Car-Parrinello molecular dynamics simulations.

—Tozzini and Pellegrini

They quantified the tunability of the hydrogen-graphene binding energies by changing the sheet out-of-plane deformation up to ±0.2 Å. They found that the binding energy can be varied by more than 2 eV, with the convex regions allocating the energetically favored hydrogen binding sites.

They then simulated the process of hydrogen chemisorption on corrugated graphene and release under the application of time-dependent mechanical deformations

We envision a device constituted by multiple graphene layers separated by a relatively large distance such that hydrogen can freely flow between the layers. The layers must be corrugated and the corrugation initially maintained by lateral compression. Although the adsorbate on sites with high convexity is stable also with respect to molecular hydrogen, an efficient storage mechanism should involve some catalytic strategy to overcome the molecular hydrogen chemisorption barrier or hydrogen cracking.

Subsequently, in the storage phase, atomic hydrogen spontaneously chemisorbs on convex sites. In the storage phase the loaded hydrogen is thermodynamically stable, and the system is robust and can be easily transported at room temperature and pressure. The release is obtained by inverting the curvature of the graphene layers, which reduces the H binding energy and dynamically and cooperatively eliminates the barrier toward associative desorption. If necessary, curvature inversion cycles can be repeated until all hydrogen atoms are released in molecular form. This working scheme demands control on the local curvature of graphene.

—Tozzini and Pellegrini

Working scheme of the proposed microscopic mechanism for hydrogen storage. Three phases can be distinguished: during the injection (red) the atomic hydrogen is cracked and introduced in the system. It will chemisorb on the convex regions during the storage phase (blue). The release phase (green) is activated by inversion of curvature, causing the associative desorption of H2. The horizontal lines are placed according to the effective energies/H atom (shades represent range of energy variability). Representative snapshots are reported: the graphene sheet is represented in gray and the hydrogen in orange. Credit: ACS, Tozzini and Pellegrini. Click to enlarge.

Their molecular dynamics simulations indicated that (i) chemisorption might need a fine-tuning of the concentration of the injecting hydrogen flow to optimize the process efficiency, because chemisorption is competing with recombination; and (ii) among the different possibilities to invert the curvature the passage of a transverse wave can smoothly invert the curvature without inducing stress in the graphene sheet; for a different mechanism based on a change of conformation of suitable intercalates, the external compression must be cooperatively controlled so as not to create rupture in the graphene sheets.

In addition, they concluded, optimization should involve tuning of the level of curvature to find a compromise between efficiency and dissipation.

Resources

• Valentina Tozzini and Vittorio Pellegrini (2011) Reversible Hydrogen Storage by Controlled Buckling of Graphene Layers. The Journal of Physical Chemistry C doi: /10.1021/jp208262