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• W1820178547 abstract "Adsorption occurs whenever a solid surface is exposed to a gas or liquid, and is characterized by an increase in fluid density near the interface. Adsorbents have drawn attention in the current effort to engineer materials that store hydrogen at high densities within moderate temperature and pressure regimes. Carbon adsorbents are a logical choice as a storage material due to their low costs and large surface areas. Unfortunately, carbon adsorbents suffer from a low binding enthalpy for H2 (about 5 kJ mol-1), well below the 15 to 18 kJ mol-1 that is considered optimal for hydrogen storage systems. Binding interactions can be increased by the following methods: (1) adjusting the graphite interplanar separation with a pillared structure, and (2) introducing dopant species that interact with H2 molecules by strong electrostatic forces. Graphite intercalation compounds are a class of materials that contain both pillared structures and chemical dopants, making them an excellent model system for studying the fundamentals of hydrogen adsorption in nanostructured carbons. Pressure-composition-temperature diagrams of the MC24(H2)x graphite intercalation compounds were measured for M = (K, Rb, Cs). Adsorption enthalpies were measured as a function of H2 concentration. Notably, CsC24 had an average adsorption enthalpy of 14.9 kJ mol-1, nearly three times larger than that of pristine graphite. The adsorption enthalpies were found to be positively correlated with the alkali metal size. Adsorption capacities were negatively correlated with the size of the alkali metal. The rate of adsorption is reduced at large H2 compositions, due to the effects of site-blocking and correlation on the H2 diffusion. The strong binding interaction and pronounced molecular-sieving behavior of KC24 is likely to obstruct the translational diffusion of adsorbed H2 molecules. In this work, the diffusion of H2 adsorbed in KC24 was studied by quasielastic neutron scattering measurements and molecular dynamics simulations. As predicted, the rate of diffusion in KC24 is over an order of magnitude slower than in other carbon adsorbents (e.g. carbon nanotubes, nanohorns and carbon blacks). It is similar in magnitude to the rate of H2 diffusion in zeolites with molecular-sized cavities. This suggests that H2 diffusion in adsorbents is influenced very strongly by the pore geometry. The H2 diffusion process in KC24 contains at least two distinct jump frequencies. Bound states of adsorbed H2 in KC24 were investigated by inelastic neutron scattering measurements and density functional theory calculations. Spectral peaks in the neutron energy loss range of 5 meV to 45 meV were observed for the first time. These peaks were interpreted as single- and multi-excitation transitions of the H2 phonon and rotational modes. The rotational barrier for H2 molecules is many times larger in KC24 than in other carbon adsorbents, apparently due to the confinement of the molecules between closely-spaced graphitic layers. Evidence was found for the existence of at least three H2 sorption sites in KC24, each with a distinctive rotational barrier." @default.
• W1820178547 created "2016-06-24" @default.
• W1820178547 creator A5024437109 @default.
• W1820178547 date "2010-01-01" @default.
• W1820178547 modified "2023-09-24" @default.
• W1820178547 title "Hydrogen Adsorption by Alkali Metal Graphite Intercalation Compounds" @default.
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