Adsorption and melting of hydrogen in potassium-intercalated graphite

Abstract

Volumetric adsorption and quasielastic neutron scattering are used to study the diffusion and thermodynamics of sorbed H2 in the graphite intercalation compound KC24. A sorption enthalpy of 8.5 kJ/mol at zero coverage is determined from H2 adsorption isotherms. From measurements of total elastic-neutron- scattering intensity as a function of temperature, a melting transition of the H2 adsorbate is observed at 35 K for KC24 (H2) 1. Quasielastic-neutron- scattering (QENS) spectra reveal distinct slow- and fast- H2 -diffusion processes which exist simultaneously at temperatures above the transition point. The temperature dependence of the characteristic diffusion times follows an Arrhenius relation τ= τ0 exp (Ea /T), where τ0 fast =1.0±0.1 ps, τ0 slow =21±2 ps, Ea fast =156±5 K, and Ea slow =189±5 K. The fast-diffusion process is attributable to individual motions of H2 molecules in a static potassium structure, and the slow-diffusion process could be attributable to fluctuations in H2 particle density correlated with jumps of potassium atoms. The QENS spectra at low Q are used to discuss the dimensionality of the diffusion process. © 2009 The American Physical Society.