Hydrogen Absorption in Alkali-Metal-Graphite Intercalation Compounds and Their Solid State Properties

Abstract

Alkali-metal-graphite intercalation compounds absorb hydrogen between graphitic layers through two kinds of absorption mechanisms; physisorption andchemisorption. Below about 200K, large amount of hydrogen molecules are occluded physisorptively, while, above this temperature, chemisorption takes place where hydrogen is accommodated in the gallaries of graphitic sheets through dissociation of hydrogen molecule and charge transfer from the host material. In this article, we discuss the electronic properties and structures of alkali-metal-hydrogen-graphite intercalation compounds prepared through hydrogen chemisorption. Hydrogen chemisorption behaviors depend on the kinds of alkali-metals among K, Rb and Cs. The introduction of hydrogen induces charge transfer from the host alkali-metal-graphite intercalation compounds to hydrogen leading to the enhancement of ionicity in the intercalate lattice consisting of alkali-metal and hydrogen and the reduction in the number of graphitic π electron carriers. Especially, in potassium-hydrogen-graphite ternary systems, two-dimensional metallic hydrogen lattice with novel electronic structure is found to exist between graphitic layers.