Many methods have been proposed for efficient storage of molecular hydrogen for fuel cell applications. However, despite intense research efforts, the twin U.S. Department of Energy goals of 6.5% mass ratio and 62 kg͞m 3 volume density has not been achieved either experimentally or via theoretical simulations on reversible model systems. Carbon-based materials, such as carbon nanotubes, have always been regarded as the most attractive physisorption substrates for the storage of hydrogen. Theoretical studies on various model graphitic systems, however, failed to reach the elusive goal. Here, we show that insufficiently accurate carbon-H 2 interaction potentials, together with the neglect and incomplete treatment of the quantum effects in previous theoretical investigations, led to misleading conclusions for the absorption capacity. A proper account of the contribution of quantum effects to the free energy and the equilibrium constant for hydrogen adsorption suggest that the U.S. Department of Energy specification can be approached in a graphite-based physisorption system. The theoretical prediction can be realized by optimizing the structures of nano-graphite platelets (graphene), which are lightweight, cheap, chemically inert, and environmentally benign. equilibrium constants ͉ hydrogen storage ͉ quantum effects A recent report on hydrogen clathrate hydrate (1) shows that under high pressure, molecular hydrogen can be trapped in the clathrate cavities reaching a mass ratio close to that defined by the U.S. Department of Energy (DOE) (2). However, the hydrogen clathrate is only stable under high pressure or at very low temperature. Simple sterical considerations suggest that the use of a ''help gas'' to stabilize the clathrate hydrate under less severe thermodynamic conditions would lead to the deterioration of the hydrogen storage mass ratio and may not be viable for mobile applications. On the other hand, there have also been numerous experimental studies on the binding capacity of molecular hydrogen with graphitic substrates (3, 4). At technologically viable conditions, reliably reproducible results are still far from the DOE goal (3, 4). In the attempt to understand and improve the storage capacity of graphitic materials, calculations have been made on many models. Some of the calculations were based on empirical interaction potentials (5-9), and the others used potentials derived from quantum mechanical calculations (10-16). The role of quantum behavior of molecular hydrogen at low temperatures has also been investigated (6,8,(17)(18)(19). Unfortunately, the binding capacity for hydrogen at near-ambient conditions has not been calculated, including the quantum effects and accurate, ab initio-based interaction potentials. To date, there has not been a reliable theoretical study indicating that the DOE goal of 6.5% mass ratio can or cannot be achieved in pure graphitic materials.The interaction of nonpolar H 2 molecules with physisorption substrates in graphitic system is mainly the London dispersion. Accurate...

The performance of density functional theory (DFT) (VWN-LDA, PBE-GGA, and B3LYP hybrid functionals), density-functional-based tight binding (DFTB), and ab initio methods [HF, MP2, CCSD, and CCSD(T)] for the treatment of London dispersion is investigated. Although highly correlated ab initio methods are capable of describing this phenomenon, if they are used with rather large basis sets, DFT methods are found to be inadequate for the description of H2/PAH (polycyclic aromatic hydrocarbon) interactions. As an alternative approach, an a posteriori addition of a van der Waals term to DFTB is proposed. This method provides results for H2/PAH interactions in close agreement with MP2 and higher-level ab initio methods. Bulk properties of graphite also compare well with the experimental data.