Journal of Alloys and Compounds 440 (2007) L18–L21 Letter Unexpected kinetic effect of MgB 2 in reactive hydride composites containing complex borohydrides Gagik Barkhordarian ∗ , Thomas Klassen, Martin Dornheim, R ¨ udiger Bormann Institute for Materials Research, GKSS Research Center Geesthacht GmbH, D-21502 Geesthacht, Germany Received 31 August 2006; received in revised form 12 September 2006; accepted 12 September 2006 Available online 3 November 2006 Abstract Complex borohydrides of light metals are promising hydrogen storage materials due to their high hydrogen capacity. However, they exhibit two main drawbacks: their high thermodynamic stability and their slow kinetics. In the present work, the effect of various reactants on the formation kinetics of complex borohydrides is investigated. It is found that the kinetic barriers for the formation of LiBH 4 , NaBH 4 and Ca(BH 4 ) 2 are drastically reduced when MgB 2 is used instead of B as starting material. Since this kinetic enhancement is observed in all borohydride studied so far, the observed effect is attributed to the higher reactivity of B in MgB 2 to form [BH 4 ] - complexes. In addition, by using MgB 2 instead of elemental B, the corresponding reaction enthalpies are reduced by about 10 kJ/mol H, while the high gravimetric hydrogen capacities are largely preserved, i.e. LiBH 4 + MgH 2 with 11.4 wt%, Ca(BH 4 ) 2 + MgH 2 with 8.3 wt%, and NaBH 4 + MgH 2 with 7.8 wt%. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; Gas–solid reactions; Chemical synthesis; X-ray diffraction 1. Introduction Considering increasing CO 2 pollution and exploitation of fossil energy resources, new energy concepts are essential to the future industrial society. Renewable sources have to replace fossil fuels that produce carbon dioxide upon combustion, which—as a green-house gas – is largely responsible for global warming. Hydrogen is the ideal means of energy storage for transportation and conversion of energy within a comprehensive clean-energy concept. Storage of hydrogen for mobile applica- tions is a major concern to the implementation of a hydrogen based economy. In the last decades, metal hydrides have there- fore been extensively investigated for their hydrogen storage properties. Because of thermodynamic and kinetic constraints, the essential properties needed for metal hydride hydrogen stor- age material, i.e. high hydrogen capacity, low reaction enthalpy, reversibility and low desorption temperature, are very difficult to satisfy simultaneously. ∗ Corresponding author. Tel.: +49 4152 87 2616; fax: +49 4152 87 2636. E-mail addresses: Gagik.Barkhordarian@gkss.de (G. Barkhordarian), Thomas.Klassen@gkss.de (T. Klassen), Martin.Dornheim@gkss.de (M. Dornheim), Rudiger.Bormann@gkss.de (R. Bormann). For example, among single binary hydrides, MgH 2 has been one of the most promising materials. It shows reversible hydro- gen content and fast kinetics at moderate conditions and has relatively high hydrogen capacity of 7.6 wt% [1]. However, it reaches a hydrogen equilibrium pressure of 1 bar at an elevated temperature of 300 ◦ C, which is too high for mobile appli- cations, according to the technical targets of U.S. department of energy for onboard hydrogen storage systems for example, which demands operating temperatures lower than 100 ◦ C [2]. On the other hand, NaAlH 4 , which is the only known com- plex hydride reversible at moderate pressures and temperatures, exhibits a desorption temperature of about 130 ◦ C. However, the hydrogen capacity of this hydride is practically less than 5.0 wt%, due to necessary dopants [3]. Other hydrides based on [AlH 4 ] - complex (alanates), like LiAlH 4 , are considered to be thermodynamically too unstable for hydrogen storage [4]. Con- versely, light-metal complex hydrides based on [BH 4 ] - complex (borohydrides) have high desorption temperatures, and cannot be formed from elements at moderate pressures and tempera- tures [5]. Obviously, to overcome the above mentioned prob- lems, it is essential to be able to tune the thermodynamics and kinetics of hydriding reactions. This can be achieved by sev- eral approaches. One approach is to substitute specific atoms in the structure of a hydride by a dopant atom. For example, 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.048