Preparation and study of the Mg2FeH6 complex hydride for hydrogen storage (in Spanish)

With the increasing problems in energy supply and the growing environmental concerns of greenhouse gas emission from the burning of fossil fuels, the demand for an efficient and clean fuel alternative has increased in the recent years. Hydrogen has the best potential of becoming the fuel of the future because it is lightweight, highly abundant and its oxidation product (water) is environmentally benign. Moreover hydrogen can be produced from sustainable, renewable sources and may contribute to meet the growth in world energy demand. In spite of its potential, the efficient use of hydrogen as a fuel, compacting it for mobile applications is a key issue. Several alternatives have been investigated, such as compressed gas, condensation into liquid or chemically-bonded in solid hydrides. The last option seems to be the safest, most compact and economic alternative, but the quest of a material that gathers all the technological requirements is still unfinished. Complex hydrides are a good alternative as reversible hydrogen storage. In this group, the ternary hydride Mg2FeH6 shows the highest known volumetric hydrogen density of 150 Kg/m3 and its gravimetric hydrogen density exceeds 5 wt. %. Nevertheless, its main drawbacks are its high stability and the difficulties that its production represents. In the present work the synthesis of the complex hydride Mg2FeH6 has been first synthesized by mechanical milling from 2Mg - Fe powder under argon atmosphere followed by sintering under high pressure and temperature conditions. Afterwards, the direct synthesis of Mg2FeH6 was attempted via reactive mechanical milling from 2Mg–Fe powder under hydrogen atmosphere. The complex could be successfully produced by mechanical milling followed by sintering and the highest yield was about 50 wt. %. On the other hand, no formation of Mg2FeH6 has been observed even after log hours (265 hours) of reactive mechanical milling. Only β – MgH2 was formed. The equilibrium properties of different Mg – H - Fe systems were characterized by absorption and desorption pressure-composition-temperature curves (PCT) measured using a Sieverts - type volumetric equipment. One plateau was observed in the absorption process associated to the Mg2FeH6 formation, and two during the process of desorption, one for MgH2 decomposition and the other one for Mg2FeH6 decomposition. Enthalpy and entropy of decomposition were determined from the Van’t Hoff plot constructed with the PCT measurements. The values obtained were compared with those found in publications and books and used in the calculus. The results from the thermodynamic studies have shown that the systems characterized had a similar behavior. The kinetic of the systems were characterized by absorption and desorption kinetic curves using the same Sieverts-type volumetric equipment used for the thermodynamic characterization. The measurements were carried out to study the influence of the production method on the kinetic behavior, absorption/desorption kinetic variations as a function of temperature and the number of absorption/desorption cycles. Finally, a comparative analysis of the results obtained was carried out and the conclusions were exposed.