ARTICLE pubs.acs.org/JPCC
Experimental Evidence of Na2[B12H12] and Na Formation in the Desorption Pathway of the 2NaBH4 + MgH2 System Sebastiano Garroni,*,†,‡ Chiara Milanese,§ Daphiny Pottmaier,|| Gabriele Mulas,^ Pau Nolis,# Alessandro Girella,§ Riccarda Caputo,r David Olid,O Francesc Teixdor,O Marcello Baricco,|| Amedeo Marini,§ Santiago Suri~nach,† and M. Dolors Baro† †
Departament de Física, Universitat Autonoma de Barcelona, E-08193 Bellaterra, Spain Pavia H2 Lab, C.SGI & Dipartimento di Chimica, Sezione di Chimica Fisica, Universita di Pavia, Viale Taramelli 16, I-27100 Pavia, Italy Dipartimento di Chimica IFM and NIS Centre of Excellence, Universita di Torino UNITO, INSTM-UdR, V. Pietro Giuria 7, I-10125 Turin, Italy ^ Dipartimento di Chimica, Universita di Sassari and INSTM, Via Vienna 2, I-07100 Sassari, Italy # Servei de Ressonancia Magnetica Nuclear (SeRMN), Universitat Autonoma de Barcelona, E-08193 Bellaterra, Spain r Department of Chemistry and Applied Biosciences, Lab Inorganic Chemistry Wolfgang-Pauli, ETH, SwissFederal Institute of Technology Zurich, Strasse 10, CH-8093 Zurich, Switzerland O Institut de Ciencia de Materials de Barcelona, CSIC, Campus UAB, E-08193, Bellaterra, Spain
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ABSTRACT: In the present work we report the desorption pathway of the 2NaBH4 + MgH2 system. Ex-situ X-ray powder diffraction (XRPD) and solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR) measurements have been performed on samples heat treated up to 450 C for different times. Ex-situ X-ray powder diffraction experiments conducted on fully desorbed samples allowed us to identify nanocrystalline MgB2 and metallic Na as dehydrogenation products. 11B and 23Na NMR analyses have been also carried out in order to evaluate the structural evolution of decomposed materials. Our measurements show that the local structure of MgB2 is influenced by replacement of Mg with Na atoms in the Mg sites. Moreover, amorphous Na2[B12H12] was detected in the partially desorbed sample and in the final products of the decomposition reaction. The presence of the [B12H12]2 anion was confirmed by both direct comparison with the 11B{1H} NMR spectrum of pure Na2[B12H12] and dynamic cross-polarization experiments.
’ INTRODUCTION Light-weight and compact hydrogen storage is both a scientific and a technological challenge. To store hydrogen in metal hydrides, thermodynamic and kinetic limitations associated with the hydrogen sorption process must be overcome.1,2 Complex borohydrides represent a class of promising materials for hydrogen storage applications due to their high volumetric hydrogen density and gravimetric capacity.3 In particular, the alkaline and alkaline-earth borohydrides, such as NaBH4 (10.7 wt %), LiBH4 (18.5 wt %), Mg(BH4)2 (14.9 wt %), and Ca(BH4)2 (11.6 wt %), have recently received considerable attention as potential hydrogen storage materials.47 The drawback is that in these materials B and H form strong covalent bonds: this leads to very thermodynamically stable hydrides (e.g., NaBH4, ΔHf = 191.4 kJ/mol8), and the high negative values of the formation enthalpies result in the high temperatures required for the dehydrogenation step. Thermodynamic destabilization is an approach introduced by Reilly and Wiswall in the 1960s, where the basic concept was to use alloys in order to make the hydrides thermodynamically less stable.9 A classical example is the Mg2Ni alloy, which can be r 2011 American Chemical Society
hydrogenated to Mg2NiH4, with a modest reduction in the thermodynamic stability. Indeed, addition of non-hydrogencontaining elements/compounds produces a significant loss in hydrogen capacity, and this is a typical problem for destabilization of a single-phase hydride. Recently, a large number of very promising mixed systems with suitable thermodynamic properties has been predicted by Alapati et al.10 Multicomponent hydrogen storage systems (also referred to in the literature as reactive hydride composites), for example, comprise more than one phase in the hydrogen-charged state in order to decrease the dehydrogenation and hydrogenation temperature and, at the same time, keep the high gravimetric capacity of the system.10 Typically, one of the phases in the system is a complex borohydride, due to the already quoted high hydrogen gravimetric and volumetric capacity of this class of materials. The other phase is a metal hydride such as MgH2 (e.g., 2NaBH4 + MgH2; LiBH4 + MgH2), but addition of other metal hydride phases was also tried (e.g., TiH2).1214 Received: March 11, 2011 Revised: July 8, 2011 Published: July 11, 2011 16664
dx.doi.org/10.1021/jp202341j | J. Phys. Chem. C 2011, 115, 16664–16671
The Journal of Physical Chemistry C The NaBH4/MgH2 multicomponent system has a high gravimetric capacity, 9.9 wt %, high volumetric hydrogen density, and a rather low dehydrogenation temperature compared to the single borohydride compound.15 Moreover, its resistance to moisture and low price (
