ETUDE DES REACTIONS COMPLEXES EN PHASE SOLIDE POUR LE STOCKAGE D'HYDROGENE

Hydrides for solid-state hydrogen storage are one of the future solutions - pollutant free - for the storage and the transport of energy. Among the candidates, LiBH4 was selected considering its high gravimetric hydrogen capacity (up to 13.6 wt.% H2). This material has thermodynamic and kinetic properties insufficiently established to be included in future applications. Its decomposition can be facilitated within the presence of the hydride MgH2. Thus, the composite MgH2-xLiBH4 (0< x< 3.5) reactivated by high energy ball-milling, was studied regarding its microstructural properties and stability of the phases. The desorption reaction of hydrogen, with or without additives, shows the appearance of additional phases accompanying the principal reaction. Heat capacity measurements of LiBH4 revealed the presence of an abnormal behaviour before the polymorphous transition (Ttrs = 386 K), attributed to the increase of crystal defects in agreement with the existence of a hypo-stoichiometric domaine LiBH4- observed at higher temperatures. The stability of the three-phase system LiBH4-LiH-B was studied resulting to the principal reaction of decomposition: LiBH4(s,l) LiH(s) + B(s) + 1,5H2(g). Vapour pressure measurements of LiBH4 showed that H2 is the major component of decomposition with minor species such as B2H6 and BH3. The thermodynamic properties of LiBH4 were critically assessed, gathering the new data with those existing in the literature, in the aim of modelling of reactions occurring in hydride mixtures.Le stockage d'hydrogène en phase solide sous forme d'hydrures, est l'une des solutions nonpolluantes futures pour le stockage et le transport de l'énergie. Parmi les matériaux candidats, LiBH4 a été sélectionné vu sa capacité gravimétrique élevée en hydrogène (jusqu'à 13,6 % H2 en masse). Ce matériaux possède des propriétés thermodynamiques et cinétiques insuffisamment établies pour comprendre son comportement dans les applications futures. Sa décomposition peut être facilitée en présence de l'hydrure MgH2. Ainsi, le composite MgH2-xLiBH4 (