Microstructural modification upon hydrogen cycling of MgH 2 nanocomposites
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Microstructural modification upon hydrogen cycling of MgH2 nanocomposites A. Montone, A. Aurora, D. Mirabile Gattia, M. Vittori Antisari ENEA, Technical Unit “Materials Technology”, Research Center of Casaccia, Via Anguillarese 301, Rome, 00123, Italy ABSTRACT The morphological evolution of Mg based powders during repeated absorptiondesorption reactions with hydrogen has been studied by Scanning Electron Microscopy. The main feature observed is the presence, after several cycles, of surface protrusions probably constituted by the base Mg material without the presence of a catalyst. The effect is present both in catalyzed and non-catalyzed materials and it is considered an indication of the tendency of the base material to exit from the oxide shell surrounding the Mg powder particles. This tendency is confirmed by the observation of empty MgO boxes indicating that the effect can push until a complete expulsion of the base material. This effect can represent the base for an innovative method for cleaning the surface of a tank material by an “in situ” procedure. INTRODUCTION Carbon-free management of energy represents one important challenge for the next future in order to contribute to reduce the emission of greenhouse gases in the atmosphere. In this frame the hydrogen, as energy carrier which is particularly attractive since water is only the reaction product by burning with oxygen. In the case of automotive, hydrogen can feed electrical vehicles equipped with fuel cell. For most applications high storage density both in terms of weight fraction and volume are required and both gaseous hydrogen in high pressure tanks and liquid hydrogen at low temperatures do not fulfill the requirements for technological applications [1]. On the other hand, hydrogen can be stored in a compact and safe way as metal hydrides. However, despite strong research efforts a material fully tailored to this purpose is still missing and further research is required. Among the requirements of this material, besides the storage capacity are the thermodynamic characteristics influencing the equilibrium temperature with the hydrogen gas and the kinetics of reaction with hydrogen. Among the more studied materials, MgH2 shows interesting properties for what concerns the gravimetric storage capacity which is about 7.6 wt%. In the last years also the reaction speeds with hydrogen has been strongly fastened mainly by microstructural refining generally performed by ball milling and by the addition of catalyst particle supporting the phase transformation reaction [2,3]. We have recently studied this effect and we have been able to demonstrate that the addition of catalyst particles affects deeply the reaction path since the nucleation site is moved from the material surface to the interface between the catalyst particle and the matrix. In this way a diffuse nucleation through the material bulk is achieved, the reaction front is widened and the just formed product phase is not anymore acting as a diffusion barrier for the reacting species. [4,5].
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