Group Activities and Facilities
← back to Main Page / Hydrogen Storage
Metal hydrides and complex hydrides
In order to understand how the storage properties of Metal Hydrides and Complex Hydrides can be improved to fulfil technological requirements, the Solid State Hydrogen Storage group at the IFW Dresden has been working on the synthesis and characterisation of nano-crystalline Mg-based alloys (e.g. MgH2, Mg2NiH4, Mg2FeH6 and Mg2CoH5) and the effects induced by platinum group metals (PGMs) and graphite in their kinetic properties (EU 5th framework project FUCHSIA). More recently, (EU 6th framework projects NESSHY (IP) and COSY (RTN) and Helmholtz-initiative project FuncHy) the scope of the group has been extended to include complex hydrides such as aluminium-based hydrides (e.g. Mg(AlH4)2, LiAlH4, and NaAlH4), and borohydrides (e.g. LiBH4). In particular, hydrogen-induced phase transformations have been extensively studied.
Experimental Techniques
Our investigations range from the synthesis of metal hydrides and complex hydrides by different techniques (e.g. powder metallurgical routes) to solid state analysis by a variety of methods. Among the analytical methods, X-ray diffraction (XRD; also in-situ at elevated temperatures and hydrogen pressures), high resolution scanning electron microscopy (HR-SEM), transmission electron microscopy (TEM) and Raman spectroscopy are used for the characterisation of our samples. Thermal analyses, on the other hand, are carried out by pressure-composition-temperature-analysis (pcT) differential thermal analysis (DTA), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Surface analyses are carried out by X-Ray Photoelectron Spectroscopy (XPS) and Auger methods. Additional analyses of the samples include chemical characterisation of the hydride and dope composition, measurement of specific surface area (BET) and hot extraction (Leco).
• Synthesis
The Solid State Hydrogen Storage group at the IFW Dresden has recently presented a new and powerful method for the synthesis of hydrogen storage materials that includes direct monitoring of hydrogenation reactions during reactive ball milling at high hydrogen atmospheres. The method allows the various phases involved in the reaction to be identified and monitored during the sorption process. The milling process is carried out in a especially designed bowl that allows in situ monitoring of temperature and hydrogen pressure in the 1–150 bar range (Figures 1 and 2).
Figure 1 (left): Monitoring of the temperature (red curve) and pressure (black curve) during milling.
Figure 2 (right): Schematic view of the high pressure milling vial with gas-temperature monitoring system (see www.evico-magnetics.de).
• Scanning Electron Microscopy (SEM)
High resolution SEM is availlable at IFW Dresden to characterise the morphology of our samples. As an example, Figure 3 shows the evolution of grain size in a Mg99Ni1 sample as a function of milling time (3h and 18h) and pressure (10 bar and 90 bar) observed by SEM.
A transfer module can be adapted to the microscope to image air-sensitive samples.
Figure 3: SEM images corresponding to Mg99Ni1 samples milled during 3h and 18 h at 10 bar (left column) and 90 bar (right column) of hydrogen, respectively.
• in situ Raman Spectroscopy
We have developed within IFW a in-situ Raman cell in collaboration with the group of Prof. Dunsch in IFF and the Research Technology Division.
The cell is designed to study reactions occurring in the samples as a function of pressure and temperature.
The cell can be heated up to 400°C with 200 bar H2 maximum.
Figure 4: Comparison of the Raman spectra corresponding to LiBH4 pure (black curve) and a mixture of LiBH4 + MgCl2 (red curve). The spectra show the presence of Mg(BH4)2 after milling.
• XRD Characterisation
Different diffraction geometries are available for the determination of the composition and structure of the samples.
Figure 5 shows the XRD patterns measured in reflection (Co Kα, flat sample) for a mixture of of NaH, Al and TiCl3 (4% mol) as catalyst before and after reactive ball milling in hydrogen. A complete transformation into NaAlH4 is achieved after 5 hours of high pressure milling under 100 bar hydrogen.
Figure 5: XRD analysis of the sample before (upper curve) and after (lower curve) milling.
XRD can be also carried out in transmission geometry (Mo Kα, capillary) for more accurate analysis at low angles interesting for characterisation of borohydrides.
• Thermodynamic Characterisation
The measurement of thermodynamic properties determines the energy involved in the absorption/desorption processes taking place at different temperatures and the amount of hydrogen contained in the sample. In the case of the synthesised NaAlH4, it is found that doped NaAlH4 leads to a thermally reversible product at high pressure (120 bar), whereas at lower pressures (e.g. 20 bar) the reversibility is not seen (see DSC cycles in Figure 6). On the other hand, PCT curves alow the study of the equilibrium pressure at different temperatures (Figure 7).
Figure 6: DSC curves corresponding to NaAlH4 at different hydrogen pressures with heating rate of 5 K/min. The different peaks are related to the multi-step reaction: 1 desorption of NaAlH4 into Na3AlH6; 2 & 5 structural transformation of Na3AlH6; 3 desorption of Na3AlH6 into NaH; 4 absorption of NaH into Na3AlH6; 6 absorption of Na3AlH6 into NaAlH4
Figure 7: PCT equilibrium curve corresponding to a Mg-hydride at 300 °C. Each portion of the curve represents the different phases of the absorption/desorption process. On the left-hand side, the α-phase represents the solid-state solution of hydrogen inside the metal. On the right-hand side, the β-phase represents the metal hydride. A mixture of the two phases (α+β) is found in the equilibrium plateau.
• Gravimetric and Kinetic analyses
To describe sorption kinetics, various sorption experiments were performed on MgH2 using Ni as catalyst. In Figure 8, the Hydrogen Concentration (wt.%) is plotted versus Time (min).
Figure 8: Kinetic curves corresponding to hydrogen absorption at 200°C for Mg alloys milled under different conditions.