Methane Storage in Carbonaceous Adsorbents

Nanoporous carbonaceous materials are appropriate containers for storing molecules under strong confinement [196-207], In the case of the physical adsorption of methane with carbonaceous nanoporous materials, the attractive adsorption forces are only dispersive, therefore very weak hence, efficient carbonaceous materials for methane storage should have the highest possible specific surface areas and pore volume. However, the factor area is just not enough to get an efficient material, that is, the sizes of the pores are significant, especially when adsorption of a supercritical gas is concerned 

Indeed, if the methane molecule is supposed to act similar to a spherical molecule, its diameter is 0.381 nm hence, pores that have widths, L, of less than approximately 1 nm can house only one monolayer, while those narrower than around 0.4nm cannot store methane at all [186], If wider pores are considered, the attractive potential produced by the facing pore walls diminishes very fast with the value of the pore width, L, so that, if L 3-4 nm, methane is weakly adsorbed, and its density is comparable to that of the gas phase in equilibrium with it. Subsequently, it is obvious that the maximum adsorption capacities are achieved with materials for which the volume of pores that have the pertinent width is the maximum [200,202], As a result, active carbons having slit-shaped pores may be the best material for methane adsorption [187], 

The study of methane adsorption on activated carbon fibers has demonstrated, as was previously explained, that these carbonaceous materials, because of their cylindrical morphology and smaller diameter, have higher packing density than activated carbons with similar micropore volumes [191]. Subsequently, the higher adsorption capacity for the powdered activated carbons against the higher packing density for the fibers helps both kinds of materials reach similar, maximum adsorption values [191]. 

To conclude this section, widespread consensus exist that the adsorbents with the highest methane capacity are the carbonaceous materials, especially activated carbons [192], where storage values up to 200 v/v have been reported [213], 

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