Pd/Mg/Pd thin films prepared by pulsed laser deposition under different helium pressures: Structure and electrochemical hydriding properties S. Bouhtiyya, L. Roue ´ * INRS-E ´ nergie, Mate ´riaux et Te ´le ´communications, 1650 boulevard Lionel Boulet, Varennes (Que ´bec) J3X 1S2, Canada article info Article history: Received 6 January 2009 Received in revised form 2 May 2009 Accepted 21 May 2009 Available online 18 June 2009 Keywords: Metal hydride Magnesium Palladium Thin film Pulsed laser deposition Hydrogen electrosorption abstract Three-layered Pd/Mg/Pd thin films were prepared by pulsed laser deposition in the pres- ence of helium gas. For Pd layer deposition, the He pressure was fixed at 200 mTorr whereas different pressures of He were used for Mg layer deposition (50, 200 and 600 mTorr). The degree of crystallinity and of (001) texture in the Mg layer increase with increasing He pressure. In addition, the increase in He pressure upon Mg deposition greatly accentuates the roughness of the Mg layer, which induces an extension of the outer Pd/Mg interface region. In contrast, the inner Pd/Mg interface is sharp for all the Pd/Mg/Pd films. The electrochemical hydrogen sorption properties of the Pd/Mg/Pd films are improved by increasing the He pressure for Mg layer deposition. However, the maximum H-solubility in the Mg layer remains low (H/Mg w0.26) and is not significantly increased by the presence of the inner Pd layer, indicating that Mg hydride phase is confined in the outer Pd/Mg inter- face region. ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction Much attention has been paid to the magnesium–hydrogen system from both fundamental and practical viewpoints [1]. Magnesium is a very interesting hydrogen storage material because of its high hydrogen storage capacity (7.6 wt.% for MgH 2 ), low density and low cost. However, its operating temperature is typically over 600 K, because of the presence of a native surface oxide preventing the access to metallic magnesium, the high energy barrier for H 2 dissociation on Mg and the slow hydrogen diffusion through the MgH 2 phase. Moreover, the high thermodynamic stability of MgH 2 (enthalpy of decomposition of about 70 kJ mol 1 H 2 ) results in a low partial hydrogen pressure at ambient temperature. This strongly restricts the practical applicability of magnesium as H-storage material. However, it was shown that hydrogen uptake by Mg can occur at room temperature for Pd-capped Mg thin films aided by the high H 2 dissociation rate and high hydrogen diffusivity of the Pd outermost layer [2–5]. Unfortunately the saturation level of hydrogen in the Mg film is low and decreases with increasing H 2 pressure. This is attributed to the formation of a Mg hydride layer at the Mg/Pd interface that blocks further hydrogen uptake due to the slow hydrogen diffusion in the magnesium hydride (b) phase. This kinetic limitation can be circumvented by working at low H 2 pressure (typically, lower than 0.1 MPa) in the temperature range 330–370 K, leading to an average H/Mg atomic ratio in the Mg layer as high as 0.8 [3,4]. At higher temperatures, H content in the Mg layer decreases due to thermodynamic considerations. Significant Pd–Mg intermixing or alloying can also occur at temperatures above 390 K. X-ray photoelectron spectroscopy studies on Mg * Corresponding author. Tel.: þ1 450 929 8185; fax: þ1 450 929 8102. E-mail address: roue@emt.inrs.ca (L. Roue ´). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.05.094 international journal of hydrogen energy 34 (2009) 5778–5784