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Relaxation Processes and Structural Changes in Li and Na Doped Fulleranes for Hydrogen Storage Annalisa Paolone, Federica Vico, Francesca Teocoli, Simone Sanna, Oriele Palumbo, Rosario Cantelli, Douglas Knight, Joseph Anthony Teprovich Jr., and Ragaiy Zidan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp304552r • Publication Date (Web): 18 Jul 2012 Downloaded from http://pubs.acs.org on July 21, 2012
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The Journal of Physical Chemistry
Relaxation processes and structural changes in Li and Na doped fulleranes for hydrogen storage
A. Paolone1,2, F. Vico1, F. Teocoli3, S. Sanna1, O. Palumbo1,2, R. Cantelli1, D.A. Knight4, J.A. Teprovich Jr.4, R. Zidan4 1
Department of Physics, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Rome, Italy
2
CNR-ISC, UOS Sapienza, Piazzale A. Moro 5, 00185 Rome, Italy
3
Research Center Hydro-ECO, Sapienza University of Rome, Piazzale A. Moro 5, 00185 Rome,
Italy 4
Clean Energy Directorate, Savannah River National Laboratory, Aiken, SC 29801 USA
ABSTRACT The anelastic spectra of Li6C60Hx and Na6C60Hx have been measured for the first time. The spectra of composite materials obtained by mixing of LiH and NaH with C60 in a solvent still show the structural phase transition around 260 K and preserve the buckyball dynamics of pure fullerenes, but do not show the spectral features of the hydrides. This observation suggests that the composite materials are homogeneous samples in which the hydrides are finely dispersed in the fullerene crystal and therefore lose their features. However, after dehydrogenation and after rehydrogenation both the phase transition and the dynamic process are completely suppressed, indicating a deep modification of the lattice structure, possibly due to polymerization. The present study indicates that Li6C60Hx and Na6C60Hx are homogenous materials and cannot be considered as constituted by bulk C60Hn and Li(Na)Hx, as suggested by a previous study.
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The Journal of Physical Chemistry
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INTRODUCTION The development of hydrogen absorbing storage media fulfilling international targets is currently considered one of the most technologically challenging obstacles in the way of achieving a hydrogen-based economy. The main requirements for such hydrogen absorbers are a high storage capacity, practical working temperatures and pressures, fast kinetics of hydrogenation and dehydrogenation and a high resistance to degradation and contamination during hydrogen absorption-desorption cycles. In recent years many materials, such as alanates [1-3], amides [4, 5], aluminium hydride [6-9], ammonia borane [10-12], and borohydrides [13] have been considered as promising storage media. However, none of these compounds can fulfil the strict international requirements for mobile applications of hydrogen. Consequently, new, efficient and non-traditional manipulation methods of traditional hydrides have been recently proposed in order to obtain composite systems with improved storage properties [14, 15]. Among these methods, showing good performances materials, are nanoinfiltration and hybridization with carbon based systems. In order to optimize such nano and hybridized systems for hydrogen storage a fundamental understanding of hydrogen interaction with these systems is needed. In the framework of hybridization, the mixing of C60 [16] and alkali hydrides (NaH and LiH) has been considered [17, 18]. Both fullerene and NaH/LiH are able to store hydrogen, but their major drawback is the extremely high desorption temperatures (Td >500 °C for hydrofullerene, C60Hx; Td > 450 °C for NaH; Td> 720 °C for LiH). However, it has been reported that composite samples can reversibly store 5.0wt% and 2.5 wt% hydrogen in the case of Li6C60Hx and Na6C60Hx, respectively; in addition, the desorption temperatures are reduced to
