Microstructural evolution during low temperature sorption cycling of Mg-AlTi multilayer nanocomposites Ramin Zahiri a,b , Beniamin Zahiri a,b , Alan Kubis a,b , Peter Kalisvaart a,b , Babak Shalchi Amirkhiz a,b, *, David Mitlin a,b a Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada b National Institute for Nanotechnology, NRC, Edmonton, Alberta, Canada article info Article history: Received 14 September 2011 Received in revised form 17 November 2011 Accepted 21 November 2011 Available online 23 December 2011 Keywords: Magnesium hydride Catalyst dispersion Z-contrast STEM Cycling stability Sputtering Multilayers Activation abstract We studied the hydrogen storage behavior of sputtered Mg-AlTi multilayers where nano- metric Mg or MgeAleTi layers were confined by 2 nm thick layers of AlTi. By decreasing the thickness of Mg layers, we were able to achieve 5.1 wt.% H capacity without significant degradation in over 200 cycles at 473 K. However, for the samples with pure Mg layers degradation eventually occurred at higher cycle numbers. In multilayers of 34 nm thick Mg degradation was followed by disintegration of the films into sponge-like flakes. Alloying Mg layers with Al and Ti through cosputtering improved the performance of the multilayer composites. Through cycling, Al and Ti segregated out of Mg matrix and formed a nano- crystalline/amorphous AlTi phase as observed by X-ray diffraction and electron micros- copy. This improved resistance of the microstructure against coarsening while a well dispersion of AlTi particles was achieved. Moreover, the stability of multilayers enhanced to an extent that they not only preserved their physical integrity, also did they maintain their superior kinetics up to over 250 cycles. Pressure - composition isotherms showed no significance change in thermodynamics of MgH 2 formation. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Magnesium with a high gravimetric capacity (7.6 wt.%) has been considered as a strong candidate for hydrogen storage. However, because of the sluggish sorption kinetics and high thermodynamic stability of magnesium hydride, high temperatures of around 573 K are needed for sorption of magnesium (hydride). Attempts to alter the thermodynamics of magnesium hydride by forming intermetallic compounds with lower enthalpy of hydride formation with elements such as Ni, Cu, Al and Si [1e6], have not resulted in any viable way to produce large quantities of a practical hydrogen storage material. As a consequence, most research efforts have been directed at improving the sorption kinetics of magnesium hydride. Transition metals in particular have been intensively studied theoretically and experimentally as suitable catalysts for sorption of magnesium (hydride) [7e13]. Also use of carbon materials in combination with transition metals toward a better dispersion of catalyst along with controlling the size of the particles and the grains has been proposed [14e16]. Most of these studies in enhancing the sorption kinetics of magnesium has been aimed at reducing the particle size or grain size and incorporating well dispersed catalyst distribu- tion using ball milling technique. In practice, the * Corresponding author. University of Alberta, Electrical & Computer Engineering Research Facility (ECERF), 7th Floor, 9107 - 116 Street, Edmonton, Alberta, Canada T6G 2V4. Tel.: þ1 780 492 8775; fax: þ1 780 492 2881. E-mail addresses: shalchia@ualbaerta.ca, shalchi@gmail.com (B. Shalchi Amirkhiz). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 37 (2012) 4215 e4226 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.11.113