HYDROGEN STORAGE IN Mg BASED MATERIALS DOPPED WITH TINY AMOUNTS OF LiBH4 AND Fe HALIDES (FeF3 Y FeCl3)

The high thermodynamic stability of MgH2 results in a hydrogen release temperature of 300 ºC at 1 atm. Due to kinetic constraints the dehydriding temperatures of MgH2 in dynamic conditions are even higher. Moreover, long dehydriding times are required. In this work, materials composed of Mg plus tiny amounts of LiBH4 doped with FeF3 and FeCl3 are studied. The addition of tiny amounts of LiBH4 to Mg during the milling process noticeably modifies size and morphology of the Mg agglomerates. On one hand, the sizes of Mg agglomerates are markedly reduced. On the other hand, pits and cracks, which enhance hydrogen diffusion during hydriding, are generated upon the Mg agglomerates. These structural changes give rise to larger hydrogen capacities without modifying the thermodynamic behavior of the Mg–H system. However, the hydrogen release at low temperatures (≤ 300 ºC) is still slow. Thus, it is proposed the FeF3 and FeCl3 doping in order to improve the kinetic behavior of the Mg + tiny amounts of LiBH4 material. Throughout the milling and hydrogen sorption processes Fe halides react with LiBH4 to form FeB species. Experimental results suggest that the in-situ formation of MgH2 during milling allows the further Mg agglomerate size reduction between 5 and 50 μm for the 85mol%Mg–10mol%LiBH4– 5mol%FeCl3 material. The additional Mg agglomerate reduction along with the FeB dispersion, which might have catalytic properties, reduce the activation energy for hydrogen release about 20 kJ/mol H2 in comparison with 90mol%Mg–10mol%LiBH4. This leads to faster hydrogen desorption rate and reversible hydrogen capacities of about 5 wt. % at 275 ºC.