Hydrogen isotope adsorption on nano-carbons

Hydrogen adsorption on carbonaceous materials has received considerable attention in recent decades, because physi-sorption of hydrogen was considered to be the most promising hydrogen storage technology to achieve the US Department of Energy (DOE) target for fuel cell powered vehicles. Many simulation studies of hydrogen adsorption on single-wall carbon nano-tubes (SWNTs) and graphitic slit pores have been performed by assuming that hydrogen can be modeled as a classical fluid above 77 K, to predict their hydrogen storage capacities; however, Wang et al. recently developed path integral grand canonical Monte Carlo (PI-GCMC) technique to explore statistical properties of quantum fluids and then they applied the PI-GCMC simulation to a study of hydrogen adsorption on SWNTs including quantum effects. Surprisingly, they showed that quantum effects are very important even at 298 K for adsorption in interstices of SWNT bundles: the interstitial adsorption of hydrogen from the quantum simulations is quite smaller than that from classical simulations. Recently, we also showed that quantum effects on adsorption of hydrogen isotopes on single-wall carbon nano-horn (SWNH) are significant at 77 K by comparing experiment and simulations. We have thus measured adsorption isotherms of H2 and D2 on nano-carbons [activated carbon fibers (ACFs) and single-wall carbon nano-tubes (SWNTs)] to evaluate quantum effects on adsorption at low temperatures, and found that, for example, adsorption of H2 on ACFs are about 10% larger than D2 at 77 K and 0.1 MPa. We have also performed grand canonical Monte Carlo (GCMC) simulations for hydrogen isotope adsorption on graphitic slit pore, SWNT and SWNT bundle models. Quantum effects were incorporated in the simulations through the Feynman-Hibbs (FH) effective potential based on the classical Lennard-Jones (LJ) potential. Fig. 1 shows simulated hydrogen isotope adsorption isotherms on the (10,10) nano-tube bundle at 77 K together with a configurational snapshot of adsorbed H2 collected from the simulation. The quantum effective solid-fluid interaction potentials of the hydrogen isotopes in the interstitial channel of the (10,10) nano-tube bundle show a smearing of the classical potential depending on the mass of the hydrogen isotopes, that is, an absolute magnitude of the effective potential minimum for the SWNH-H2 interaction is always smaller than that for SWNH-D2 interaction at low temperatures. Therefore, simulated adsorption isotherms of H2 on the SWNT bundle model exhibited smaller adsorption than D2 as the results of quantum effects at 77 K. Comparison between the experiments and simulations on the nano-carbons will be shown in detail. Selectivities of D2 over H2 at 77 K were also estimated by ideal adsorption solution theory (IAST) with single component adsorption isotherms from the experiment and simulations. The average selectivity of D2 from the experiments for the respective samples is about 1.2 in a limit of low pressure. However, the selectivity for the (10,10) nano-tube from the simulations was ca. 2.8 at 77 K. This value is relatively high compared with zeolites like 3A and 13X. (authors)