Ab-initio modelling of atomic and molecular Hydrogen adsorption in graphite
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Ab-initio modelling of atomic and molecular Hydrogen adsorption in graphite Sara Letardi1, Massimo Celino1,3, Fabrizio Cleri2,3, Vittorio Rosato1,3 and Manuela Volpe2,4 1
Ente Nazionale per le Nuove Tecnologie, Energia e Ambiente (ENEA), Centro Ricerche Casaccia, HPCN Project, C.P. 2400, I-00100 Roma, Italy 2 Ente Nazionale per le Nuove Tecnologie, Energia e Ambiente (ENEA), Centro Ricerche Casaccia, Divisione Materiali, C.P. 2400, I-00100 Roma, Italy 3 Istituto Nazionale di Fisica della Materia (INFM), Unità di Ricerca Roma 1, Italy 4 Dipartimento Scienze e Tecnologie Chimiche, Università Tor Vergata, 00158 Roma, Italy ABSTRACT Ab-initio electronic structure calculations have been used to evaluate the binding energy of atomic and molecular hydrogen to graphite lattice defects. Results show that graphite defects (StoneWales, vacancy) are preferred binding sites with respect to regular lattice sites. We find that molecular hydrogen can be physisorbed between the graphite planes, but cannot diffuse across a graphitic plane. INTRODUCTION Low-density carbon nanostructures, such as nanofibers, nanotubes, amorphous or porous carbon, have been suggested as potential candidates for hydrogen storage[1,2]. All such structures have in common the feature of a huge specific surface, (up to 104 cm2/g) if compared to conventional, bulk-like counterparts such as activated carbon [3,4]. Moreover, nanostructured carbon could insure a high mechanical stability versus repeated gas loading-unloading cycles. Consequently, a considerable effort is currently being devoted to the investigation of both fundamental and technological issues related to the definition of an optimal, carbon-based material to be used, e.g., as hydrogen getter for energetic applications. Molecular hydrogen can be either physisorbed bot at free surfaces and on defects of a graphitic nanostructure, or can be dissociated into atomic hydrogen and then chemisorbed. Moreover, such carbon nanostructures exhibit structural properties that make them particularly interesting for hydrogen adsorption. The large surface-to-volume ratio translates into a large density of adsorption sites. Furthermore, as low- density structures are mostly sp2-bonded, they show a large chemical affinity with hydrogen. In fact, low-density structures such as nanofibers or amorphous carbon can be described as disordered graphitic structures with density values ranging from about 0.3 to 1.0 the actual graphite density. On the other hand, carbon nanotubes, fullerenes, onions and polyhedral particles are more ordered structures based on a rather regular folding of graphite fragments (or graphenes). In general, all such graphitic nanostructures will necessarily contain a large concentration of bonding defects. The different environments which can occur in a disordered graphitic nanostructure, such as a curved graphene, a bonding defect, dangling bond, vacancy and so on may, in principle, allow a continuum spectrum of binding energies, going from the very small (e.g., H2 physisorption on a perfect grap
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