Reversible hydrogen storage in metal-doped Mg–LiBH 4 composites J.A. Puszkiel * and F.C. Gennari Consejo Nacional de Investigaciones Cientı ´ficas y Te ´cnicas, CONICET, Instituto Balseiro (UNCuyo and CNEA), Centro Ato ´ mico Bariloche (CNEA), R8402AGP, S. C. de Bariloche, Argentina Received 22 October 2008; revised 12 December 2008; accepted 22 December 2008 Composite powders of Mg 50 Ni and Mg 15 Fe with LiBH 4 have been produced by short-time milling under an argon atmosphere. These composites show enhanced reversible hydrogen capacities of 7 wt.% (Ni) and 5.5 wt.% (Fe) at 573 K in just 300 s. The pres- ence of LiBH 4 does not modify the thermodynamic of the composites, but does account for their improved kinetic behavior. As evidence of a new kind of Mg–B interaction, MgNi 3 B 2 is clearly identified after successive hydrogen sorption cycles below 623 K. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage; Borohydrides; Composite powders; Mechanical milling The design of adequate materials for hydrogen storage is the main constraint for the development of a ‘‘hydrogen economy” in the transport sector. Metal hy- drides, complex hydrides and nanotubes, among others, have been investigated as hydrogen carriers [1,2]. De- spite the effort put into improving such solid materials, none of the available candidate storage materials yet meet the DOE technical targets, which involve reversible hydrogen capacity of >6 wt.% H and >45 kg H 2 m 3 (by year 2010) at moderate pressures and temperatures with fast de/hydrogenation kinetics, high cycle stability, low enthalpy effects and low cost [3]. Although MgH 2 has high reversible storage capacity (7.6 wt.% H) and low cost, its main drawback is that its high enthalpy does not allow a low desorption tempera- ture (>573 K). Several investigations have focused on improving the hydrogen absorption/desorption times of MgH 2 , using nanocrystalline hydrides prepared by high-energy milling and/or suitable catalysts [4–9]. Moreover, in last five years new hydrogen storage sys- tems have been proposed based on the interaction be- tween MgH 2 and different light complex hydrides, mainly LiNH 2 and LiBH 4 [10–16]. Among these com- plex hydrides, lithium borohydride (LiBH 4 ) has promis- ing prospects for on-board applications as the full dehydrogenation of LiBH 4 yields 18.3 wt.% H; however, complete hydrogen release requires a temperature as high as 900 K [17]. Previous results have demonstrated that by adding Mg into the LiBH 4 structure through ball milling the onset dehydriding temperature can be re- duced from 700 to 630 K [11]. Vajo et al. [12] found that LiBH 4 may be reversibly dehydrogenated and rehydro- genated with a reduced reaction enthalpy upon MgH 2 addition. On the other hand, Johnson et al. [13] reported an enhancement in the dehydriding/hydriding reaction kinetics of MgH 2 through the addition of small amounts of LiBH 4 at 573 K. Similarly, Mao et al. [16] observed an improvement in the absorption/desorption from Mg–LiBH 4 produced by milling under 3 MPa of hydro- gen. In the two latter works, the presence of LiBH 4 was crucial; however, its precise role is unknown. In this work we show a superior enhancement of hydrogen absorption/desorption properties of metal- doped Mg by preparing composites with LiBH 4 via short-time milling under argon atmosphere. A study of the hydrogen sorption mechanism of the synthesized hy- dride composites by microstructural, structural and thermal characterization is also presented. Ball-milled samples of Mg 50 Ni and Mg 15 Fe were pre- pared using planetary ball milling (Fritsch P-6) with a ball-to-powder ratio of 40:1 in a hardened steel vial (80 cm 3 ) under 0.1 MPa Ar for 10 h. Subsequently, 10 mol.% of LiBH 4 (purchased from Sigma–Aldrich, purity P 90%) was added to the as-milled metal-doped Mg samples by ball milling for 2 h (Mg 50 Ni) or 5 h (Mg 15 Fe). All sample handling was done in an MBraum Unilab argon-filled glove box, with oxygen and moisture levels lower than 1 ppm. 1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.12.038 * Corresponding author. Tel.: +54 02944 445118; fax: +54 02944 45190; e-mail: jpuszkiel@cab.cnea.gov.ar Available online at www.sciencedirect.com Scripta Materialia xxx (2009) xxx–xxx www.elsevier.com/locate/scriptamat ARTICLE IN PRESS Please cite this article in press as: J.A. Puszkiel, F.C. Gennari, Scripta Mater. (2009), doi:10.1016/j.scriptamat.2008.12.038