Graphene nanostructures as tunable storage media for molecular hydrogen

Abstract

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 kg/m3 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-H2 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 light-weight, cheap, chemically inert, and environmentally benign.

Fig. 1.

H₂/benzene ab initio interaction potentials and fitted Lennard–Jones (LJ) potential. Potential energy E (in kJ/mol) is shown as a function of the distance of the centres of mass of H₂ and benzene (in Å). CP, results with the counterpoise correction applied.

Fig. 2.

Gravimetric (Upper) and volumetric (Lower) H₂ storage capacities of layered graphite structures, calculated from the real gas equation of state, as functions of the interlayer separation (see Table 1 and text). The DOE targets for automotive applications (2) (w = 6.5%, v = 31.2 cm³/mol) are indicated by solid horizontal lines.

Fig. 3.

Probability densities for selected lowest eigenstates of the translational nuclear Hamiltonian. The lowest in-phase (Top to Bottom: first, second, and fifth) eigenstates for the double-layer structure are shown (d = 8 Å).

Fig. 4.

Equilibrium constants K_eq (Upper) and reaction free energies ΔF (Lower; in kJ/mol) are plotted as a function of the interlayer distance d (Left; in Å, at T = 300 K) and temperature T (Right; in K, at d = 8 Å), respectively.