Résumé:

Carbon-based materials, due to their low cost and weight, have long been considered as suitable phy-sisorption substrates for the reversible storage of hydrogen. In fact, carbons can be engineered to achieve exceptional storage capacities: the ALL-CRAFT1 nanoporous carbon achieves (at T = 80 K, P = 100 bar) gravimetric excess adsorption of 0.073  0.003 kg H2/kg carbon, gravimetric storage capacity of 0.106  0.005 kg H2/kg carbon (including the gas in pore spaces), and volumetric storage capacity of 0.040  0.002 kg H2/l carbon. The nanopores generage high storage capacity by: (i) having a very high surface area (~2,600 m2/g carbon); (ii) by generating a high H2-wall interaction potential; and (iii) by allowing multi-layer adsorption of H2 (at cryogenic temperatures). In this presentation we will show how the experimental characteristics of the ALL-CRAFT1 carbon correlate to the observed H2 storage properties, with help from basic theoretical considerations and computational simulations of the adsorption process (Grand Canonical Monte Carlo). The ALL-CRAFT carbon is composed of a large variety of pore sizes (pore-size distributions from sub-critical N2 adsorption analyzed via quenched solid-state density functional theory, corroborated via small-angle X-ray scattering, SAXS). This generates substantial heterogeneity in the adsorption potentials. Hydrogen adsorption isotherms can be readily simulated (GCMC) at low pressures (< 30 bar) by a two-binding energies (~9 kJ and ~5 kJ/mol), corresponding to narrow (< 1 nm) and wide pores. The H2 adsorption in our samples has two further characteristics that we explain: (i) exceptional low-temperature H2 storage (in excess of the usual Chahine’s rule), and (ii) absence of an excess adsorption peak (for 0 < P < 100 bar). By performing extensive computer simulations that take into account the sample’s pore-size distributions we are able to explain these characteristics in terms of a weak multi-layer adsorption and the existence of lower binding-energy regions. Finally we will discuss the influence of these significant sources of heterogeneity on the adsorption-desorption cycle, in particular in terms of the amount of stored H2 that is deliverable via an isothermal pressure sweep.