The influence of dispersion interactions on the hydrogen adsorption properties of expanded graphite

We demonstrate the importance of London dispersion forces in defining the adsorption capacity within expanded graphite, a simple model of the more complex experimental geometries of activated carbon, using a combination of the non-local correlation functional of Dion etal paired with a recent exchange functional of Cooper (vdW-DF ^C09x) and a classical continuum model. Our results indicate that longer ranged interactions due to dispersion forces increase the volume over which molecules interact with a porous medium. This significantly enhances the adsorption density within a material, and explains recent experimental work showing that the densification of H ₂ in carbon nanopores is sensitive to the pore size. Remarkably, our slit pore geometries give adsorption densities of up to 3wt% at 298K and 20MPa which correlates well with experimental values for 9pores - a value that could not be predicted using local density approximation (LDA) calculations. In its entirety, this work presents a powerful approach for assessing molecular uptake in porous media and may have serious impacts on efforts to optimize the properties of these materials.