| Abstract
| - The storage of gases in porous adsorbents, such as activated carbon and carbon nanotubes, is examined herethermodynamically from a systems viewpoint, considering the entire adsorption−desorption cycle. The results provideconcrete objective criteria to guide the search for the “Holy Grail” adsorbent, for which the adsorptive delivery ismaximized. It is shown that, for ambient temperature storage of hydrogen and delivery between 30 and 1.5 bar pressure,for the optimum adsorbent the adsorption enthalpy change is 15.1 kJ/mol. For carbons, for which the average enthalpychange is typically 5.8 kJ/mol, an optimum operating temperature of about 115 K is predicted. For methane, anoptimum enthalpy change of 18.8 kJ/mol is found, with the optimum temperature for carbons being 254 K. It is alsodemonstrated that for maximum delivery of the gas the optimum adsorbent must be homogeneous, and that introductionof heterogeneity, such as by ball milling, irradiation, and other means, can only provide small increases in physisorption-related delivery for hydrogen. For methane, heterogeneity is always detrimental, at any value of average adsorptionenthalpy change. These results are confirmed with the help of experimental data from the literature, as well as extensiveMonte Carlo simulations conducted here using slit pore models of activated carbons as well as atomistic models ofcarbon nanotubes. The simulations also demonstrate that carbon nanotubes offer little or no advantage over activatedcarbons in terms of enhanced delivery, when used as storage media for either hydrogen or methane.
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