Hydrogen adsorption and phase transitions in fullerite

Hydrogen desorption and adsorption properties of the fullerene materials C- 60, C-70, and fullerite (a mixture of C-60 and C-70) were measured volumetr ically using a Sievert's apparatus. Over several cycles of isotherm measure ments at 77 K, the hydrogen storage capacities of one of the fullerite samp les increased from an initial value of 0.4 wt % for the first cycle to a ca pacity of 4.4 wt % for the fourth cycle. Correspondingly, the surface area of this sample increased from 0.9 to 11 m(2)/g, and there were changes in i ts x-ray powder diffraction pattern. In comparison, two other fullerite sam ples, prepared by a different procedure showed no such behavior. Pure C-60 and pure C-70 were also cycled and exhibited small and constant capacities of 0.7 and 0.33 wt %, respectively, as a function of number of cycles. The enhanced storage capacity of fullerite material is tentatively attributed t o the presence of C-60 oxide. (C) 2000 American Institute of Physics. [S000 3-6951(00)03039-4].