Molecular simulation of hydrogen adsorption in graphitic nanofibres

Rodriguez, Baker and co-workers (A. Chambers, C. Park, R. T. K. Baker and N. M. Rodriguez, J. Phys. Chem. B, 1998, 102, 4253; C. Park, C. D. Tan, R. Hidalgo, R. T. K. Baker and N. M. Rodriguez, Proc. 1998 US DOE Hydrogen Program Review, (http://www.eren.doe.gov/hydrogen/docs/25315toc.html); C. Park, P. E. Anderson, A. Chambers, C. D. Tan, R. Hidalgo and N. M. Rodriguez, J. Phys. Chem. B, 1999, 103, 10572) have reported uptake of hydrogen in graphitic nanofibres (GNFs) of 40% by weight. If these results are confirmed, then this class of material could be a suitable storage medium for hydrogen for use in fuel cell vehicles. In order to test whether these results are feasible, we report results for grand canonical Monte Carlo simulation of hydrogen adsorption in graphitic pores. A classical technique was employed but the results obtained were shown to be consistent with previous path integral Monte Carlo calculations of Wang and Johnson (Q. Wang and J. K. Johnson, J. Chem. Phys., 1999, 110, 577; Q. Wang and J. K. Johnson, J. Phys. Chem. B, 1999, 103, 277). The interaction between hydrogen and the graphitic surface was modelled initially by dispersion forces. The predicted uptake (up to 1.5%) was much lower than the Baker-Rodriguez results. The results were found to be fairly insensitive as to whether the hydrogen molecule was modelled as a Lennard-Jones sphere or a dumbbell fluid with two Lennard-Jones sites. Two models for a hypothetical potential for chemisorption were also used in the simulation. The potential was based on calculation of the interaction between atomic hydrogen and a graphitic surface. Adsorption of up to 17 wt.% was measured with the stronger model potential but there was negligible desorption at ambient pressure, making it impractical. A more plausible, though still hypothetical, potential gave loadings of up to 8 wt.% in the model system. These results are still much lower than the Baker-Rodriguez data in spite of the fact that there is no evidence to suggest that chemisorption actually occurs in a real system.