Synergetic effect of C (graphite) and Nb 2 O 5 on the H 2 sorption properties of the MgeMgH 2 system C. Milanese a, *, A. Girella a , S. Garroni b , G. Bruni a , V. Berbenni a , P. Matteazzi c , A. Marini a a C.S.G.I. e Dipartimento di Chimica Fisica “M. Rolla”, Universita ` degli Studi di Pavia, Viale Taramelli 16, 27100 Pavia, Italy b Dept. Fı´sica, Universitat Autonoma de Barcelona, 08193 Bellaterra, Spain c C.S.G.I. & MBN Nanomaterialia SpA, Via G. Bortolan 42, 31050 Vascon di Carbonera (TV), Italy article info Article history: Received 10 March 2010 Received in revised form 9 June 2010 Accepted 16 June 2010 Available online 16 July 2010 Keywords: Hydrogen storage Mg-based materials Sorption kinetics High-energy ball milling X-ray powder diffraction Absorption/desorption activation energy Absorption/desorption enthalpy abstract Ternary MgeNb 2 O 5 e graphitic C mixtures (molar ratio % ¼ 97.5:0.5:2.0) were prepared by high-energy ball milling (BM) under Ar for different times (from 0.25 h to 4 h) and thor- oughly characterized by manometric, calorimetric, X-ray powder diffraction, and scanning electron microscopy analyses. The aims of the work were: - to assess the effect of the simultaneous presence of the two dopants on the reactivity and the sorption properties of the MgeMgH 2 system; - to study the influence of the milling time on the performance of the mixtures. Neither milling nor the high temperature/high pressure treatments led to reactions among the components of the mixtures, and Mg was the only hydrogen active phase. After 4 activation cycles at 623 K and 35 bar/1 bar charging/discharging pressure, the mixture milled for 1 h was the best performing one: it reversibly charged up to 6.8 wt% H 2 with absorption/desorption rates 64/4.5 times higher than those of a pure Mg sample BM for the same time and activation energies 3.6/2 times lower. The desorption temperature and the dehydrogenation enthalpy of the ternary mixture were respectively 40 K and 4 kJ/ mol H 2 lower than those of pure MgH 2 . ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. 1. Introduction Among the metallic hydrides, MgH 2 is the most promising H 2 storage system, thanks to its high theoretical gravimetric and volumetric capacity (7.6 wt% H 2 and 110 g H 2 /l respectively) and the full reversibility of its sorption reactions combined with low cost, low density and environmental acceptance [1e3]. Unfortunately, the practical uses of this hydride are prevented by its high stability (the dissociation enthalpy is 74.5 kJ/mol H 2 [4]), high decomposition temperature (>573 K at H 2 pressure ¼ 1 bar), and sluggish sorption kinetics at temperatures below 623 K [2,5,6]. The preparation of the hydride in nanocrystalline form by high-energy ball milling [7e11] and the addition of transition metals [12e16] and transition metal oxides [17e21] during the mechanical treatment have allowed large improvements in the sorption kinetics. In particular, the transition metal oxides work very well both as dispersing agents, preventing the agglomeration and cold-welding of MgH 2 and facilitating the refinement of the hydride particles during milling [22e25], and as catalysts, enhancing the hydrogen diffusion throughout the Mg matrix [17]. Moreover, they are cheaper than the corresponding transition metals and effective at lower level of addition, thus leading to only a slight reduction in the gravimetric capacity of the storage system [20,24]. Barkhordarian et al. demonstrated the superior effect of Nb 2 O 5 among a large range of metal oxides [19]: the addition of * Corresponding author. Tel./fax: þ39 382 987670. E-mail address: chiara.milanese@unipv.it (C. Milanese). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 35 (2010) 9027 e9037 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.06.037