Hydrogen-alkali-metal-graphite ternary intercalation compounds

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.