Electrical resistivity and hydrogen-physisorption behavior of potassium-graphite intercalation compounds in the course o
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Y. Takahashi Department of Nuclear Engineering, Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan (Received 22 May 1990; accepted 23 August 1990) The electrical resistivity of potassium-graphite intercalation compounds (K-GICs) was measured in the course of reactions with ammonia, oxygen, water, etc. The hydrogen absorption behavior at 77 K was also investigated on K-GICs before and after the reactions. The electrical resistivity of KC8 increased by reactions with ammonia, furan, and water vapor, whereas almost no change was observed in the case of the reaction with oxygen. Molecules of ammonia, furan, and water are considered to penetrate into the KC8 interlayers, while oxygen draws potassium from interlayer spaces toward the surface with resultant potassium-diluted mixed stage compounds. The hydrogen absorption isotherms of K(NH3),C3i (0 ^ x ^ 2.65) at 77 K showed that the saturated amount of absorbed hydrogen, (H2/K)sat, decreased linearly with increasing ammonia content, x. When x went up to 2, (H2/K)sat became zero. Similar behavior in the degradation of the hydrogen absorption capacity of K(H2O)jC25 (0 ^ x ^ 1.3) was observed. Contrary to such behavior, partially oxidized KC24 could not absorb hydrogen gas. These facts are also explained by taking into account the fact that ammonia and water molecules penetrate K-GICs, while oxygen draws potassium atoms toward the surface, as predicted from the electrical resistivity measurements. Successive oxidation and heat-treatment processes made KC8 more able to absorb hydrogen, while similar processes of ammoniation and hydration followed by heat-treatment did not.
Possible applications of graphite intercalation compounds include highly conductive materials, electrode materials in batteries, catalyzers for organic synthesis, materials for hydrogen storage or its isotope separation, exfoliation of graphite, etc.1 For instance, MC24 (M = K, Rb, Cs) absorbs hydrogen gas at temperatures around 80 K.2'3 In addition, this absorption shows a large isotopic effect, being advantageous for the enrichment of deuterium and tritium.4'5 For the application of alkali metal-graphite intercalation compounds (hereafter abbreviated as AM-GICs), however, we must also understand the reaction behavior of AM-GICs with oxygen, water, etc. A detailed investigation of such reactions was first carried out by Daumas and Herold.6 They examined the reactions of KC8 with O2, N2O, NO, NO2, SO2, CO2, and CO, by measuring the Debye-Scherrer patterns of the sample in the course of the reactions with these substances. According to them, KC8 reacted violently with oxygen when these substances were brought into contact. Slow oxidation was performed by careful introduction of oxygen at a few Torr onto KC8; then the KC8 was deJ. Mater. Res., Vol. 5, No. 12, Dec 1990
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composed gradually. For the completion of the reaction, the sample must be heated up to 100 °C. Graphite and a-KO2 were formed as the f