Feasibility of tailoring for high isosteric heat to improve effectiveness of hydrogen storage in carbons

The idea that increasing the enthalpy of adsorption increases the adsorptive capacity of carbon and makes it a better storage material for hydrogen is examined here considering the entire adsorption-desorption cycle. Structural modifications of carbon are examined to reveal the complex relationships between the enthalpy of adsorption, the pore volume, and the amount of hydrogen delivered over the course of a single cycle. The results provide an understanding of the connection between enthalpy and effective storage capacity in carbon materials and serve as a guide toward the search for an adsorbent which satisfies the DOE targets. Extensive GCMC simulations show that carbons having single graphene walls are optimal for hydrogen storage and that attempts to increase the enthalpy of adsorption either by increasing the wall thickness or by decreasing the pore size are detrimental to adsorptive capacity over a complete cycle from charging to exhaustion. It is found that carbon nanotubes display the same trend as slit pore carbons. The search for an adsorbent suitable for hydrogen storage should be aimed at the discovery of an entirely new high-capacity adsorbent with an enthalpy of adsorption of 15 kJ/mol, intermediate between that of carbon (4-6 kJ/mol) and metal hydrides (30-75 kJ/mol).