Preparación, estudio y optimización de hidruros complejos para almacenamiento de hidrógeno. / Preparation, study and optimization of complex hidrides for hydrogen storage
Creators: name: Puszkiel, Julián
Publisher: name: Zenodo
Year: 2012
Subjects: hydrogen storage
Hydrides
Mechanical milling
Thermodynamics
Kinetics

description: Hydrogen is regarded as a promising energy vector because it is abundant, its oxidation produces water and releases huge amounts of energy. Despite the fact that its practical application is not far away, there are many technological constraints yet to solve. One of the main constraints is the lack of a safe and efficient method to store hydrogen. In the present thesis, hydride forming materials for hydrogen storage were investigated. The materials were prepared by mechanical milling procedures (MM and MMR). Several techniques were applied to evaluate their microstructural characteristics (XRD ex–situ and in–situ, SEM, EDS, PSD, XAFS and ASAXS), thermal behavior (DSC, HP-DSC and TG) and thermodynamic and kinetic properties (Sieverts technique). At the beginning, different stoichiometric compositions of Mg–Fe materials were studied. The thermodynamic behavior and hydrogen absorption mechanisms of the Mg–Fe–H system were characterized. The absorption enthalpy of the Mg–Fe–H hydride system belongs to the MgH<sub>2</sub>–Mg<sub>2</sub>FeH<sub>6</sub> mixture (~ 70 kJ/mol H<sub>2</sub>). On the other hand, the Mg–Fe–H hydride system exhibits two desorption enthalpies. One corresponds to the MgH<sub>2</sub> (~ 70 kJ/mol H<sub>2</sub>) and the other to the Mg<sub>2</sub>FeH<sub>6</sub> (~ 90 kJ/mol H<sub>2</sub>). It was found that a material composed of 15Mg–Fe (Mg + 6.3 mol % Fe) prepared by reactive mechanical milling (H<sub>2</sub>) exhibits capacities of about 5.0 wt% H at 350 ºC and 2.0 MPa and cycling stability. In order to avoid the H<sub>2</sub> atmosphere during milling, the 15Mg–Fe material was prepared by mechanical milling under inert atmosphere (Ar). The obtained material just reached 1.5 wt% H at 350 ºC and 2.0 MPa owing to its poor microstructural characteristics. However, adding tiny amounts of LiBH<sub>4</sub> to the 15Mg–Fe material led to a noticeable improvement in the degree of microstructural refinement. Hence, through the addition of iron halides capacities of about 6.5 wt% H at 350 ºC (2.0 MPa) and 5.0 wt% H at 275 ºC (2.5 MPa) were reached. Based on experimental results and equilibrium calculations, the LiBH<sub>4</sub> and Fe halides (FeF<sub>3</sub> and FeCl<sub>3</sub>) resulted in the formation of a Fe boride compound. Despite the fact that Mg based materials with interesting hydrogen storage characteristics were obtained, their thermodynamic behavior still belongs to MgH<sub>2</sub>. The MgH<sub>2</sub> thermodynamic stability is conferred by the strong Mg–H bond and it does not allow reaching reversible hydrogen capacities of 7.6 wt% H (theoretical capacity of MgH<sub>2</sub>) at temperatures below 300 ºC. For this reason and due to the proper theoretical thermodynamic stability (ΔH = 40.5 kJ.mol<sup>-1</sup> H<sub>2</sub>, Td= 225 ºC at atmospheric pressure, 101.3 kPa) and the high theoretical gravimetric capacity (11.45 wt % H) of the reaction 2LiBH<sub>4</sub>+MgH<sub>2</sub> ↔ 2LiH + MgB<sub>2</sub> + 4H<sub>2</sub>, materials composed of 2LiBH<sub>4</sub> + MgH<sub>2</sub> (2LiH + MgB<sub>2</sub>) plus Fe–based additives were studied (Fe, Fe-isopropoxide, FeF<sub>3</sub> and FeCl<sub>3</sub>). It was found that the Fe-based additives are reduced to metallic Fe by either LiH or LiBH<sub>4</sub> during the milling process and hydrogen cycling. This led 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 the desorption times from 12 to 6 hours. However, such effect does not remain upon further cycling. XAFS results confirm that FeB is formed during hydrogen cycling. ASAXS results suggest that upon cycling the iron rich nanoscale particles exhibit poor microstructural characteristics because of agglomeration. Thus, the presence of FeB, which is regarded as chemically inert, and the poor microstructural characteristics of the material result in slower hydrogen absorption–desorption rates. Hydrogen storage capacities of about 7.0 wt% H (absorption at 350 ºC and 5.0 MPa; desorption at 400 ºC and 0.55 MPa) were reached with a material composed of 2LiH + MgB<sub>2 </sub>+ 5 mol % Fe. The study of different hydride forming materials (Mg–Fe, Mg and 2LiBH<sub>4</sub>+MgH<sub>2</sub> based materials) provides knowledge about their microstructural characteristics, thermodynamic and kinetic behavior and their potential properties for mobile hydrogen storage applications.