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We report on the C-60, Na, and Li dynamics in NaxLi6xC60 fullerides (x = 0, 1, 5, and 6) in the temperature range 80-550 K by using C-13, Na-23, and Li-7 solid state NMR. The results show that the C-60 reorientation dynamics is hindered at room temperature for the Li-enriched fullerides, but it is active for the Na rich ones with a rate of the order of few kilohertz. Na-23 and Li-7 NMR measurements show the presence of two dominant thermally activated dynamics that can be associated with Li/Na ionic motions within the octahedral sites (intrasite motion) and between the octahedral and tetrahedral sites (intersite motion). The substitution of one Na or one Li ion in the end members Li6C60 and Na6C60, respectively, yields to an increase of the hopping rate of the intersite motion, which is necessary for the ionic diffusion in possible fulleride-based ionic conductors.

Li6C60 is so far the best performing fullerene-based hydrogen-storage material. The mechanism of its reversible hydrogen absorption is not, however, totally known. Here, we report the thermal evolution of Li6C60 upon deuterium absorption up to 330 °C and under 60 bar deuterium gas pressure using in situ high-intensity neutron powder diffraction. The temporal resolution of the collected data allowed the hydrogenation of Li6C60 and the segregation of lithium hydride (here LiD) processes to be distinguished from a mechanistic point of view. During absorption, Li6C60 undergoes several phase transitions, involving the partial segregation of Li in the form of hydride and the expansion of the face-centered cubic (fcc) lattice, followed by a body-centered cubic (bcc) rearrangement of the deuterated fullerenes. The amount of absorbed deuterium was determined by the analysis of the variation in the scattered neutron intensity and confirmed by an ex situ desorption measurement. This analysis clarifies the complex ph...

For different hydrogenated metal intercalated fullerides (Na10C60-H, Li12C60-H, and Li28C60-H) the activation energies for hydrogen desorption were determined by DSC. The Vyazovkin advanced method (VA) was used for the calculation of the reaction model free activation energy as a function of the extent of conversion a. Activation energies are highest for low a and decrease for increasing alpha, between around 200-145 and 245-175 kJ/mol for the Na and Li compounds, respectively. The decrease of activation energy as a function of the extent of conversion can be explained by an increasing charge transfer to the C60H36+y cage during desorption. Na intercalation leads to a significant thermodynamic destabilization for hydrogen desorption. Dehydrogenation enthalpies of 52 (Na10C60-H), 66 (Li12C60-H), and 69 kJ/mol H-2 (Li28C60-H) were determined. These values are lower compared to literature values for desorption of pure C60H36 (74 kJ/mol H-2). The onsets of hydrogen desorption are 185 degrees C (Na10C60-H), 260 degrees C (Li12C60-H), and 250 degrees C (Li28C60-H) compared to >400 degrees C for pure C60H36.

The lithium-intercalated fulleride Li12C60 was investigated in view of a lightweight hydrogen storage material due to the low molecular weight of its constituents. Deuterium (D2) absorption in Li12C60 shows an uptake of up to 9.5 mass % D2 (equivalent to ∼5 mass % H2 for the same stoichiometry). Under a pressure of 190 bar the onset of absorption was observed at a temperature below 100 C, which is 200 C lower than that for pure C60. Deuterium desorption was investigated by in-situ neutron powder diffraction, and at a pressure of 1 bar desorption was observed above 300 C. The ab/desorption is accompanied by a partial de/reintercalation of lithium, observed by the appearance and disappearance of LiD reflections after absorption and during desorption, respectively. A minor part of deuterium is present in ionic form in LiD, and the major part is covalently bound in a Li-depleted compound Li 12-xC60D36+y. © 2013 American Chemical Society.