Article pubs.acs.org/JPCC
Hydrogen Sorption in Li12C60 Philippe Mauron,*,† Mattia Gaboardi,‡ Arndt Remhof,† Andreas Bliersbach,† Denis Sheptyakov,§ Matteo Aramini,‡ Gina Vlahopoulou,‡ Fabio Giglio,‡ Daniele Pontiroli,‡ Mauro Riccò,‡ and Andreas Züttel† Division ‘‘Hydrogen and Energy’’, EMPA Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland § Laboratory for Neutron Scattering, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland ‡ Dipartimento di Fisica e Scienze della Terra, Università degli studi di Parma, Via G. P. Usberti 7/a, 43124 Parma, Italy †
ABSTRACT: 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 Li12−xC60D36+y.
1. INTRODUCTION C60, theoretically, has the potential to be a carbonaceous hydrogen storage material. A typical stable molecule is C60H36,1 which has a capacity of 4.8 mass % H2. In C60H60 a capacity of 7.7 mass % H2 is reached at temperatures up to 600 °C and 130 bar,2 but it also has been observed that prolonged hydrogenation (T = 400 °C, p = 120 bar, up to 3000 min) can lead to fragmentation of C60.3,4 Even higher hydrogen absorption amounts have been proposed by theoretical calculations, e.g., Li12C60 as an isolated cluster has been studied by Sun et al.,5 and up to ∼13 mass % H2 storage capacity was calculated from ab initio simulation. Yoshida et al.6 measured the hydrogen absorption of Li9C60 and established that up to ∼2.6 mass % H2 can be stored at 250 °C and 30 bar H2. Reversible hydrogen uptake of alkali metal fullerides (NaxC60, LixC60) synthesized by dehydrogenation of C60 mixed with NaAlH4 or C60 mixed with LiAlH4 each in a molar ratio of 1:6 has been shown by Teprovich et al.7 For lithium-doped fullerenes (Lix-C60-Hy) a reversible uptake of 5 mass % H2 (350 °C, 105 bar H2) and desorption onset temperature of ∼270 °C was observed when the molar ratio between Li and C60 is 6.8 Hydrogen uptake for Li12-C60-Hy was determined to be 3.5 mass % H2 at 250 °C and 105 bar H2. By anelastic spectroscopy measurements Paolone et al.9 suggested that Li6C60Hx and Na6C60Hx are homogeneous materials and cannot be considered as constituted by bulk C60Hn and Li(Na)Hx. In pure sodium-intercalated fulleride Na10C60 3.5 mass % H2 can reversibly be absorbed at a temperature as low as 200 °C and 200 bar.10 The ab/desorption kinetics can be significantly improved compared to pure C60, and the stability for © 2013 American Chemical Society
dehydrogenation has been lowered. During hydrogenation NaH is formed, leading to partial sodium deintercalation in the compound. It was found that part of hydrogen is ionically bound to NaH and part covalently bond to C60. During dehydrogenation sodium reintercalates into the compound, promoting reformation of Na10C60. In this paper we investigate the deuterium absorption and desorption properties of pure Li12C60 without any byproducts by making use of pcT (pressure, composition, and temperature), XRD (X-ray diffraction), FT-IR (Fourier transformed infrared) spectroscopy, and in-situ neutron powder diffraction. Lithium-doped fullerides have a polymeric11 or monomeric structure12 depending on the amount of lithium intercalated. Li12C60 is a highly doped phase of LixC60 in which it is established that Li atoms form an incompletely ionized cluster in the central octahedral void of the fcc fullerite structure,12,13 similarly to Na10C60.14 Lithium atoms are, in principle, able to donate up to 6 electrons to the 3-fold degenerate t1u LUMO of C60, and the remainder are expected to delocalize their charge onto the Li cluster. The high negative charge onto C60 and the presence of the alkaline cluster in the fulleride has proved to be of fundamental importance in decreasing the energy barrier required for dissociation of hydrogen molecules and formation of C60Hy.15 Received: August 29, 2013 Revised: October 2, 2013 Published: October 21, 2013 22598
dx.doi.org/10.1021/jp408652t | J. Phys. Chem. C 2013, 117, 22598−22602
The Journal of Physical Chemistry C
Article
2. EXPERIMENTAL SECTION Lithium fulleride was synthesized by mixing pure C60 (MER Corp., 99.9%) with granular lithium (Sigma-Aldrich 99%) cut in very small flakes. About 500−600 mg of the reagents was ground in 3 spheres agatha ball mill at 30 Hz for 60 min. When the sample became a black and uniform powder it was then pelletized (∼100 mg per pellet), placed in tantalum foil bags, sealed in high vacuum (
