Graphene spacers

As demonstrated in the previous chapter graphene surfaces can hardly increase the hydrogen density for practical applications. Therefore, the hypothetical model system of H2 ``sandwiched'' between two graphene layers was suggested. This model demonstrates a better ability of the slit-pore cavities to trap hydrogen molecules. This ability depends strongly on the interlayer distances between the graphene layers.

For distances below c = 5 Å the H2$-$graphene interaction potential has been found to be repulsive, and no H2 can penetrate in between the graphene layers. For slit pore sizes slightly larger than c ${\approx}$ 6 Å, the maximum estimated storage capacity approaches the gravimetric and volumetric technological target values. For larger interlayer distances, the H2 storage capacities decrease continuously, and for the limit of a single graphene sheet (c=${\infty}$), only insignificant uptake of H2 on the layer surface is suggested. Thus, a well designed material with appropriate pore sizes is suggested to have reasonable storage capacities. In the mono- and bi-layer model calculations, the graphene layers have arbitrary fixed interlayer distances. The discussion about the energetic stability of the systems was completely omitted considering no mutual attraction between the graphene layers. A more realistic model would include these factors. Therefore, in this chapter attention will be paid to the spatial separation of the graphene layers by means of H2-gas or C60 as spacers.