Hydrogen Adsorption on Functionalized Nanoporous Activated Carbons

Abstract

There is considerable interest in hydrogen adsorption on carbon nanotubes and porous carbons as a method\nof storage for transport and related energy applications. This investigation has involved a systematic\ninvestigation of the role of functional groups and porous structure characteristics in determining the hydrogen\nadsorption characteristics of porous carbons. Suites of carbons were prepared with a wide range of nitrogen\nand oxygen contents and types of functional groups to investigate their effect on hydrogen adsorption. The\nporous structures of the carbons were characterized by nitrogen (77 K) and carbon dioxide (273 K) adsorption\nmethods. Hydrogen adsorption isotherms were studied at 77 K and pressure up to 100 kPa. All the isotherms\nwere Type I in the IUPAC classification scheme. Hydrogen isobars indicated that the adsorption of hydrogen\nis very temperature dependent with little or no hydrogen adsorption above 195 K. The isosteric enthalpies of\nadsorption at zero surface coverage were obtained using a virial equation, while the values at various surface\ncoverages were obtained from the van't Hoff isochore. The values were in the range 3.9−5.2 kJ mol^-1 for\nthe carbons studied. The thermodynamics of the adsorption process are discussed in relation to temperature\nlimitations for hydrogen storage applications. The maximum amounts of hydrogen adsorbed correlated with\nthe micropore volume obtained from extrapolation of the Dubinin−Radushkevich equation for carbon dioxide\nadsorption. Functional groups have a small detrimental effect on hydrogen adsorption, and this is related to\ndecreased adsorbate−adsorbent and increased adsorbate−adsorbate interactions.