Electrical resistivity and hydrogen-physisorption behavior of potassium-graphite intercalation compounds in the course of reactions with ammonia, water, and oxygen

Abstract

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)_xC₃₁ (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)_xC₂₅ (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.