Reversible Hydrogen Uptake in Carbon-Based Materials
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C K H K C Figure 1: Schematic diagram of a ternary first stage C4 KHx compound [9]. capacity of hydrogen molecules above a plane of graphite, which has a honeycomb structure
(also P6 3 /mmc), with lattice parameters a = 2.46 A and c = 6.71 A. Since the value of a for graphite is larger than the diameter of the hydrogen molecule, the closest packing of an adsorbed hydrogen layer would have to be incommensurate with the graphite planar surface. An atomic ratio of [H/C] -1 is considered to be the minimum uptake of practical interest for energy related applications [5]. Because of the 12:1 ratio between the masses of carbon and hydrogen, an atomic ratio of [H/C] = 1 corresponds to 8.3 wt% of hydrogen to carbon. GRAPHITE INTERCALATION COMPOUNDS The intercalation process involves the insertion of atoms or molecules of various guest species between planes of graphite which acts as a host material. The stage index (n) is used to describe the graphite intercalation compounds (GIC), where n is the number of planes of graphite separating one intercalate layer from another [6]. As a result of intercalation, the graphite sample expands along the c-axis to accommodate the planes of intercalant lying between two graphene layers. This increase in elastic energy is balanced by the attractive electrostatic interaction between the intercalate and graphite layers because of charge transfer that occurs during the intercalation process [6]. The greatest intercalant uptake occurs at stage n = 1. Intercalation of fullerene films [7] and carbon nanotubes [8] have also been
reported, and hydrogen uptake in fullerenes and carbon nanotubes are discussed below. Unfortunately, hydrogen does not seem to intercalate directly into a graphite host material. Hydrogen can, however, be co-intercalated with an alkali-metal in a chemical intercalation process to obtain stage n = 1 or stage n = 2 compounds, while a physisorption process can be used for hydrogen uptake on a second stage alkali-metal intercalated graphite host. Chemisorption of hydrogen by graphite is apparently accompanied by a back-donation of electrons from the alkali metal to the graphene and hydrogen layers in an alkali-metal GIC (see Fig. 1). There are two ways of introducing hydrogen into graphite in a chemisorption process: intercalation of the alkali-metal hydride and adsorption of hydrogen gas into an alkali-metal GIC. The resulting intercalation compound has the stoichiometry C4 nKH, (n is the stage index) [9,10], where the value of x varies from 0.67 to 0.8, depending on the synthesis method. Regarding chemisorption, many studies have been conducted on potassium hydride (KH) intercalated into GICs [9-12]. KH is an ionic insulator, with the hydrogen existing as an anion (H-). The resulting graphite intercalation compound can be stage n = 1 (C4 KH0.s) or n = 2 (CgKHO.s), depending on the experimental growth conditions (i.e., temperature or exposure time). C4 KH0 .8 yields a [H/C] ratio of 20 atom%, while C8 KH0 .8 yields 10 atom%. 158
The structural model (see Fig. 1) for th
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