Hydrogen adsorption and phase transitions in fullerite

Hydrogen desorption and adsorption properties of the fullerene materials ${\mathrm{C}}_{\mathrm{60}},$ ${\mathrm{C}}_{\mathrm{70}},$ and fullerite (a mixture of ${\mathrm{C}}_{\mathrm{60}}$ and ${\mathrm{C}}_{\mathrm{70}})$ were measured volumetrically using a Sievert’s apparatus. Over several cycles of isotherm measurements at 77 K, the hydrogen storage capacities of one of the fullerite samples increased from an initial value of 0.4 wt % for the first cycle to a capacity of 4.4 wt % for the fourth cycle. Correspondingly, the surface area of this sample increased from 0.9 to $\text{11\hspace{0.05em}}{\mathrm{m}}^{\mathrm{2}}\mathrm{/g},$ and there were changes in its x-ray powder diffraction pattern. In comparison, two other fullerite samples, prepared by a different procedure showed no such behavior. Pure ${\mathrm{C}}_{\mathrm{60}}$ and pure ${\mathrm{C}}_{\mathrm{70}}$ 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 to the presence of ${\mathrm{C}}_{\mathrm{60}}$ oxide.