Study of Fe and Fe–based additives upon the sorption properties of the 2LiBH4 + MgH2 = 2LiH + MgB2 + 4H2 reaction
The reaction 2LiBH4 + MgH2 ↔ MgB2 + 2LiH + 4 H2 presents a suitable thermodynamic stability (ΔH = 40.5 kJ.mol-1 H2) and high theoretical hydrogen gravimetric capacity (11.45 wt% H). The thermodynamics of the reaction predicts a desorption temperature of 225 ºC at 1 bar. Nonetheless, kinetic constraints preclude desorption temperatures below 400 ºC. In order to improve the operative conditions at which the 2LiBH4 + MgH2 reactive hydride composite (RHC) uptake/release hydrogen, we add Fe and
... we add Fe and Fe-based additives (Fe-isopropoxide, FeF3 and FeCl3) as catalysts. Techniques such as XRD, DSC, TG and Sieverts apparatus as well as synchrotron techniques (XAFS, ASAXS and IN-SITU XRD) have been used in order to evaluate the effect of such additives upon the sorption properties of the RHC. It was found that the Fe-based additives (Fe-isopropoxide, FeF3 and FeCl3) are reduced to Fe by either LiH or LiBH4 during the milling process and hydrogen cycling. This leads to lower hydrogen capacities and the loss of catalytic effect because of the formation of compounds such as LiF and LiCl. The addition of Fe noticeable reduces mainly the desorption times from 12 to 6 hours. However, such effect does not remain upon further cycling. According to the ASAXS results, this effect is related to changes in the microstructure of the material. Moreover, XAFS results suggest that iron boride species might be formed during hydrogen cycling. The evidence obtained from the several techniques contitutes a key factor for the interpretation of the interactions occurring during both milling and hydriding/dehydriding processes.