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

Abstract

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 et al 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 3 wt% at 298 K and 20 MPa which correlates well with experimental values for 9 Å pores-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.

Supercritical Fluids in Confined Geometries

“…At the same time, H 2 density in UMC mesopores is relatively low, close to the density of bulk gas, and undergoes only subtle changes with pressure. These findings were supported by theoretical calculations, using first-principle methods of treating the van der Waals interactions within density functional theory, which confirmed that H 2 density in subnanometer pores indeed could reach a liquidlike density even at 50 bar and 298 K [63,64]. The described SAS results demonstrate the advantage of adsorptive storage over compressed gas storage and emphasize the greater efficiency of micropores over mesopores in the adsorption process [65].…”

Classical density functional approach to adsorption of hydrogen in carbon materials

“…We also tried to use some ab initio data for the H 2 -graphene interaction to obtain corresponding adsorption potential. However, as one can see in figure 7, different ILJ with ab initio data [36][37][38]. and semi-empirical potentials obtained via integration of H 2 − C pair potentials [39,40].…”

From determination of the fugacity coefficients to estimation of hydrogen storage capacity: A convenient theoretical method

“…(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 0 nanoporous carbons and the expanded bilayer graphene with a series of the interlayer distances have already been reported with the similar strategy, but under the assumption that the fugacity coefficient is equal to unity or a empirical value [17,19,20]. It is of course not very precise.…”

“…Obviously, the fugacity coefficients could be totally different, especially at low temperature. Although the empirical value for the hydrogen fluid in their work [19] seems to be close to the experiment, the old fitting process is not reliable. Thus, we conclude four points which should be considered: i).…”

“…The uptake rates of the hydrogen and methane fluids in the nanoporous carbons predicted by Morris' group [19,20] with suitable fugacity coefficients show the acceptable accuracy with this strategy. The predicted results are around 20% less than the experiments.…”