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Investigation of Hydrogen Adsorption in Different Nanoporous Materials over Wide Ranges of Pressure and Temperature

Eric Poirier, General Motors Corporation, Purdue University

Experimental investigations of hydrogen adsorption on different nanoporous materials over wide ranges of pressure and temperature are presented. In a first part, rigorous gravimetric and volumetric methods developed to measure hydrogen adsorption on single-walled carbon nanotubes (SWNTs) are presented. These systems were especially adapted for measurements on small masses and in-situ conditioning. Cross-checked measurements using these methods showed that SWNTs adsorb hydrogen according to mechanisms comparable to other forms of carbons. In a second part, experimental and theoretical methods implemented to study hydrogen adsorption thermodynamics on different metal-organic frameworks (MOFs) are presented. It was found that the best material, MOF-177, can adsorb up to 12.6 wt% at 10 bar and 50 K. This capacity decreases, however, with increasing temperature as a result of weak solid-gas interactions (.H~3-4 kJ/mol). For the lowest measured temperatures, the near-saturation regions of the excess isotherms were used to calculate the adsorbed phase density and volume, two usually eluding quantities. In all tested materials, the adsorbed phase density was found to behave like an incom¬pressible fluid and to reach a density comparable to that of the bulk liquid hydrogen. This conjunction of liquid state properties, exceptional above the critical temperature, could be explained by a traditional micropore filling model. These properties suggest that repulsive interactions and quantum effects could be important limiting factors for the molecular storage of hydrogen under nanoscale confinement.

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