Journal of Alloys and Compounds 476 (2009) 500–506 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Hydrogen storage in Pd capped thermally grown Mg ﬁlms: Studies by nuclear resonance reaction analysis Sanjiv Kumar ∗ , G.L.N. Reddy, V.S. Raju National Centre for Compositional Characterization of Materials, Bhabha Atomic Research Centre, ECIL Post, Hyderabad 500062, India article info Article history: Received 18 June 2008 Received in revised form 1 September 2008 Accepted 5 September 2008 Available online 5 November 2008 Keywords: Mg ﬁlms Thermal evaporation Hydrogenation Nuclear resonance reaction analysis abstract The paper reports the studies on reversible hydrogen storage characteristics of Pd/Mg/Si (glass) ﬁlms by depth proﬁling hydrogen using 6.44 MeV resonance of 1 H( 19 F,) 16 O nuclear reaction. These bilayered coatings wherein Mg ﬁlms are prepared by thermal evaporation, exhibit excellent hydrogenation–dehydrogenation characteristics. Mg ﬁlms absorb 6.0–7.0wt.% hydrogen on hydrogena- tion under 0.15 MPa hydrogen gas pressure in 348–423 K temperature range for ≤5 h and undergo complete dehydrogenation in 3 h at 373 K under dynamic vacuum. Pd(40 nm)/Mg(250 nm) ﬁlms perform satisfac- torily up to two cycles of hydrogenation (348 K, 4 h) and dehydrogenation (373 K, 3 h), however, complete hydrogen release is affected in the third cycle. Hydrogen depth proﬁles suggest that hydriding starts at Pd/Mg interface and subsequently proceeds further into the interiors of the ﬁlms with the formation of hydrogen deﬁcient non-stoichiometric magnesium hydride as an intermediate step. The hydrogenated ﬁlms exhibit room-temperature stability that depends on the temperature and duration of hydrogenation. The formation of metastable magnesium hydride may be responsible for the observed (de)hydrogenation characteristics of the ﬁlms. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The current interest in harnessing hydrogen as a renewable and environmental friendly source of energy makes hydrogen storage an active area of research. Hydrogen can be stored as (i) pressurized gas, (ii) cryogenic liquid and (iii) solid state in the form of hydrides or carboneous materials. Solid state storage has high volumetric efﬁciency and is, by far, the safest method of hydrogen storage. David provides an excellent overview of advanced materials, metal hydrides as well as materials based on carbon structure, suitable for storing hydrogen [1]. Similarly in a review article Sakintuna et al. report the recent advances in the development of metal hydrides with improved storage characteristics [2]. Among metal hydrides, magnesium hydride (MgH 2 ) is a promising material in view of its high stoichiometric hydrogen content (∼ 7.6wt.%) and excellent volumetric efﬁciency (∼150 kg/m 3 ). As a result, magnesium in vari- ous physical as well as chemical forms, such as elemental powders, thin ﬁlms, alloys or composites, has been investigated with hydro- genation to the stoichiometric value, complete dehydrogenation and re-cyclability being the major objectives. Further, it is desirable that the storage reactions occur at lower temperatures and have fast kinetics. However due to high enthalpy of formation (H f ) of ∗ Corresponding author. Tel.: +91 40 27123546; fax: +91 40 27125463. E-mail address: sanjucccm@rediffmail.com (S. Kumar). MgH 2 (-78 kJ/mole H 2 ), hydrogenation of Mg and dehydrogenation of MgH 2 occur at elevated temperatures (∼ 573 K) and are slow. Sev- eral studies have been undertaken to alleviate the thermodynamic and kinematic limitations by engineering the microstructure and surface of the materials [2,3]. Most of the investigations on the hydrogen storage in Mg based materials have been carried out on powders. Ball-milled (BM) MgH 2 powders exhibit high storage capacity (7.0wt.%) and fast kinetics [3,4]. The improvements are attributed to nanometric materials with superior powder morphology formed due to high-energy ball- milling. Further enhancement in the hydrogenation performance can be achieved by the modifying powder surfaces by catalysts, for example, Nb 2 O 5 [5]. However, ball-milling or the usage of cat- alysts does not cause any signiﬁcant lowering of the temperatures of hydrogen absorption or desorption which remain as high as 573–623 K [2–5]. Thin ﬁlm preparation offers an alternate approach for producing nano-structured materials with better control over morphology, stoichiometry and contamination as well. Hydrogen storage char- acteristics of thin Mg ﬁlms have been investigated by several researchers [6–10]. For example, Higuchi et al. reported that Pd-capped Mg ﬁlms prepared by RF sputtering on glass sub- strates absorbed, at 373 K, 2.9–6.6 wt.% hydrogen depending on the conditions of sputtering [7]. The hydrogenated ﬁlms underwent complete dehydrogenation at temperatures >463K. In yet another study they investigated the efﬁcacy of three layered Pd/Mg/Pd ﬁlms 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.080