Effect of Carbon Addition on Hydrogen Storage Performances of Magnesium-based Alloys: from Bulk Powders to Thin Films
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Effect of carbon addition on hydrogen storage performances of magnesium-based alloys : from bulk powders to thin films A. Rougier, X. Darok, V.V. Bhat, L. Aymard, G.A. Nazri#, and J.-M. Tarascon LRCS, 33 rue St. Leu, 80039 Amiens, France. # General Motors R&D, Warren Michigan, USA. ABSTRACT Mg-based thin films were successfully grown by Pulsed Laser Deposition. Dramatic optical changes were observed depending on the conditions of deposition. Films grown in vacuum were shiny metallic whereas the ones grown in a Ar/H2 gas mixture were highly transparent. Ex situ hydrogenation, by annealing the films at 200 °C in 15 bars of hydrogen, led to similar metallictransparent transformation. Mg-Cx (x < 20 %) films show a faster hydrogenation associated with a significant decrease in oxygen content by carbon addition. INTRODUCTION Metal hydrides are the subject of very intense research activity for energy storage applications [1-2]. However current alloys present either too low hydrogen capacity for suitable sorption temperatures or suitable high hydrogen capacity but too high desorption temperatures, limiting their use for practical applications. In this respect, among intermetallics, Mg-based alloys exhibit a high hydrogen storage capacities (up to 7.6 wt. % for pure Mg) unfortunately, their sorption kinetics remain slow below 250 °C. More recently metal hydrides have received an increase of interest in respect of the dramatic change in optical properties upon hydrogen absorption/desorption that was firstly reported in rare earth thin films protected by a Pd layer [3]. Described as the third generation of switchable mirrors, Mg transition metal thin films have been largely studied since their discovery by Richardson et al. [4-6]. If the limitations of the two systems (powder and thin film) depend on their own characteristics, both suffer from the high reactivity of magnesium compounds towards oxygen forming oxides of which presence impedes the hydrogen sorption performances. In our group, an earlier study showed improved electrochemical properties (specifically capacity and cyclability) for AB5 alloys, used in Ni-MH batteries, by ball-milling with graphite [7]. Using a similar approach, hydrogen sorption properties of Mg2Ni alloys were successfully enhanced by ball-milling with carbon leading to the preparation of Mg2Ni-C10,320 composite, presenting a reversible sorption capacity of 2.6 wt.% at 150 °C, as compared to 1.5 wt.% for pure Mg2Ni [8]. Herein, after a brief discussion on the origin of such enhanced sorption performances on bulk system, the influence of carbon addition on the morphology, structural and optical properties of Mg-based thin films, grown using pulsed laser deposition, will be addressed. EXPERIMENTAL DETAILS The Mg-Ni alloys were prepared from Mg and Ni powders (< 40 µm, 99.9 %, Alfa Aesar) by mechanical milling under argon atmosphere using a SPEX 8000 mixer-mill. The 50 cc stainless
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steel milling container was filled with a mixture of stainless steel balls of 6 and 12 mm diameter. The
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