L Journal of Alloys and Compounds 305 (2000) 264–271 www.elsevier.com / locate / jallcom Influence of cycling on the thermodynamic and structure properties of nanocrystalline magnesium based hydride a, a b b a a b * Z. Dehouche , R. Djaozandry , J. Huot , S. Boily , J. Goyette , T.K. Bose , R. Schulz a ` ´ ´ ` ` ` ´ Institut de recherche sur l’ hydrogene, Universite du Quebec a Trois-Rivieres, C.P . 500, Trois-Rivieres, Quebec, Canada G9A 5H7 b ´ ´ ´ Technologies emergentes de production et stockage, Institut de recherche d’ Hydro-Quebec 1800, boulevard Lionel-Boulet, Varennes, Quebec, Canada J3X 1S1 Received 3 January 2000; accepted 14 January 2000 Abstract We have investigated the effect of prolonged cycling on the hydriding/dehydriding properties and on the structure of nanocrystalline MgH –V composite produced by high-energy ball milling. The hydrogen charge and discharge kinetics of the nanocomposite hydride 2 were tested at 3008C using up to 2000 cycles. Pressure composition isotherms at 3008C were also carried out. The nanocomposite exhibits good reversibility in its hydrogenation / dehydrogenation curves after 2000 cycles. The results show some improvements in hydrogen capacity during cycling; this enhanced H-solubility is believed to be the result of structural relaxation. The sample resistance to hydrogen decrepitation was also evaluated via additional experiments involving SEM, BET specific surface area and X-ray crystal structure characterisations. These observations indicate that the nanostructured Mg-based composite does not decrepitate much upon cycling. However, a slight deterioration in the discharge rate of the nanocrystalline magnesium hydride is observed, apparently related to the crystal growth during cycling.  2000 Elsevier Science S.A. All rights reserved. Keywords: Nanocrystalline composite material; Cycling stability; Magnesium hydride 1. Introduction composite produced by mechanical alloying presents fast sorption kinetics [1]. The improved kinetics is due to the Hydrogen is considered as a future fuel source because catalytic effect of vanadium and the microstructure of the of its clean-burning and renewable character. However, composite. In fact, vanadium exhibits the highest hydrogen prior to its wide scale use, a safe and efficient way of volumetric density and diffusivity of all metals [2]; this storing hydrogen needs to be developed. Nanocrystalline may be attributed to the particularly small lattice parame- magnesium based hydrides are of growing interest as ters of vanadium hydride. Although numerous papers [3– useful materials for transient hydrogen storage. Because of 14] have shown that hydriding / dehydriding kinetics of the extremely small dimension of the nanocrystalline nanocrystalline hydrides prepared by mechanical alloying material, a large fraction of the atoms are located at the were significantly improved by the small crystallite size as grain boundaries inducing short hydrogen atom diffusion well as by adding catalyst, important issues still remain, in distances inside the crystallite which confers to this particular the effect of long term cycling conditions on the material enhanced kinetic properties. Moreover, the H-storage capacity and microstructure of the sorbing nanocrystalline magnesium hydrides have under cycling metal. Also, a clear understanding of the synergetic experiments improved resistance to decrepitation compared interactions between magnesium nanocrystals and the to polycrystalline materials. attached vanadium catalyst clusters that may occur during It has recently been shown that MgH 15 at.% V cycling is essential for controlling the activity of hydrid- 2 ing / dehydriding kinetics. The purpose of the present work is to investigate the effects of a large number of hydriding / dehydriding cycles on the kinetics and the hydrogen storage capacity of nanocrystalline magnesium hydrides *Corresponding author. E-mail address: zahir dehouche@uqtr.uquebec.ca (Z. Dehouche) containing vanadium catalyst. ] 0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388(00)00718-0