Hydrogen-alkali-metal-graphite ternary intercalation compounds

Abstract

Alkali-metal-graphite intercalation compounds (alkali-metal-GIC's) absorb hydrogen in two ways: physisorption and chemisorption. Hydrogen uptake through the physisorption process occurs at low temperatures below about 200 K in higher stage alkali-metal-GIC's, where hydrogen molecules are stabilized to form a two-dimensional condensed phase in the galleries of the graphite sheets. The concentration of absorbed hydrogen molecules is saturated at a rate of H2/alkali metal atom ∼2. The hydrogen physisorption shows a strong isotope effect and a swelling effect on c-axis lattice expansion. In the case of hydrogen uptake through the chemisorption process, dissociated hydrogen species are stabilized in the intercalate spaces. The activity of the chemisorption increases in the order Cs < Rb < K. The introduction of hydrogen generates a charge transfer from the host alkali metal GIC's to the hydrogen since hydrogen has strong electron affinity. The hydrogenated potassium-GIC's have intercalates consisting of K+-H−-K+ triple atomic layer sandwiches which are inserted between metallic graphite sheets. The inserted two-dimensional hydrogen layer is suggested to consist of H ions with a weakly metallic nature. The superconductivity of the hydrogenated potassium-GIC is also discussed in terms of the change in the electronic and lattice dynamical properties by hydrogen uptake. The hydrogen-absorption in alkali-metal-GIC's is an interesting phenomenon in comparison with that in transition metal hydrides from the point of hydrogen storage. The hydrogen-alkali-metal-ternary GIC's obtained from hydrogen absorption have novel electronic properties and lattice structures which provide attractive problems for GIC research. The studies of hydrogen-alkali-metal ternary GIC's are reviewed in this article.

Enhanced Hydrogen Dipole Physisorption, Final Report

“…The reported H2 isosteric heat on KC24 is 8.4 kJ mol -1 , about twice as large as for adsorption on bare graphite 87 . It is also known that the in-plane resistivity in KC24(H2)x is larger than in KC24 94 . This is explained by the observed c-axis expansion, which effectively reduces the in-plane carrier density.…”

“…They were explained in terms of a one-dimensional hindered diatomic rotor model 91,92 . By the late 1980's, a substantial body of research existed on hydrogen-alkali-metal graphite intercalation systems 93,94 . Interest in the area dwindled during the 1990's, as reflected in the small number of publications during the period.…”

Quasi-Two Dimensional Metallic Hydride State and the Soliton-Catalytic Effect in Alkali Metal Hydrogen-Graphite Intercalation Compounds

“…It has been known that AGICs have catalytic activity for hydrogen and absorb hydrogen chemisorptively to form alkali metal hydride GICs through the dissociation of hydrogen molecules and charge transfer from AGICs to hydrogen [2,3]. The stage-1 graphite-potassium compound C 8 K, with a golden loster, is known to fix hydrogen and to become a dark blue compound C 8 KH 2/3 , which have the second stage structure [3,4]. The behaviors of hydrogen absorption and catalytic activities of AGICs have been known to depend on the structure, sort of metals and temperature.…”

“…Enoki et al [3] found out an interesting magnetic effect * This paper was presented at the 6th International Symposium on Surface Science -Towards Nano, Bio and Green Innovation-, Tower Hall Funabori, Tokyo, Japan, December 11-15, 2011.…”

Hydrogen diffusion in potassium intercalated graphite studied by quasielastic neutron scattering

“…The environment surrounding the H 2 sorption sites in KC 24 is impacted by a strongly polarized electric field between graphite layers, generated by charge transfer from potassium to graphite. 13,14 Overlapping graphite corrugation potentials from the host GIC structure are another significant feature. Hydrogen adsorption in KC 24 has an isosteric heat 9,10,15 of approximately −9 kJ mol −1 , in comparison to −4 kJ mol −1 for adsorption on graphite.…”