HYDROGEN STORAGE IN MAGNESIUM BASED ALLOYS

1. HYDROGEN STORAGE IN MAGNESIUM BASED ALLOYS Jasmina Grbović Novaković

2. Alternative fuel (Why) Do we need alternative fuels and energy carriers? …because reserves of fossil fuels are limited??? European economy depends on petroleum exporting countries we need to secure future individual mobility we want to reduce greenhouse gases we aspire to protect our environment by using clean forms of energy

3. Alternative fuel Why Hydrogen? It’s abundant, clean and can be derived from diverse resources. Biomass Transportation Hydro Wind Solar HIGH EFFICIENCY & RELIABILITY Nuclear Oil DistributedGeneration ZERO/NEAR ZEROEMISSIONS Coal WithCarbonSequestration Natural Gas

4. Alternative fuel Why not Hydrogen? Problem is safe, efficient and cost-effective storage

5. Hydrogen storage Common requirements for hydrogen storage • High gravimetric and volumetric storage capabilities • Cost • Efficiency • Safety • Life cycle • Environmental impact

6. high pressure cylinders ( up to 35MPa consumes 20% of its total energy content • cryogenic storage of liquid hydrogenat low temperature (consumes nearly 30% of total energy content • metal hydride, where hydrogen is chemically bound to a metallic material • complex hydride • metal-organic framework materials • zeolites • carbon fibres and nanotubes Solid-state Storage : safer and more efficient Mobile applications Volume of 4 kg of hydrogen compacted in different ways, with size relative to the size of the car Stationary applications Hydrogen storage Hydrogen Storage Options Schlapbach & Züttel, Nature, 15 Nov. 2001

7. Solid state storage How does solid storage occur? The first attractive interaction of hydrogen molecule approaching the metal surface is Van der Waals force, leading to a physisorbed state. The physisorption energy is typically of order. In the next step the hydrogen has to overcome an activation barrier for dissociation and for the formation of the hydrogen metal bond. This process is called chemisorption. The chemisorption energy is typically of order . After dissociation on the metal surface, the H atoms generally diffuse rapidly through the bulk metal even at room T to form M-H solid solution

8. Solid state storage The reaction of hydrogen gas with metal can be described in terms of a simplified one-dimensional potential energy curve Lennard-Jones potential of hydrogen approaching a metallic surface.

9. fcc bcc hcp Solid state storage In many cases H occupies interstitial sites tetrahedral and octahedral.

10. Hydrides • A medium value of electronegativity • Indicates that H can form various kinds of chemical bonds with various elements • I and II group of elements which has small electronegativity H forms ionic compoundscalled saline hydrides (M+H- and Mg2+H2-) • Most of Group III–V non-metallic elements form covalently bonded crystals • But there is still a large number of elements having comparable electronegativites, namely, d-band metals, lanthanides and actinides, which form metallic hydrides. • Metallic hydrides, by nature of metallic bonding, commonly exist over extended ranges of nonstoichiometric compositions. These hydrides can be called interstitial alloys, where interstitials sites of metal lattice are occupied by H atom, randomly at high temperatures and in some regular ways at lower T

11. Hydrides The TaHx phase diagram according to Schober. α and α‘ are disordered BCC solutions of H in Ta. ε is a tetragonal phase and β, δ, ζ and γ are orthorhombic. The α‘-β is a disorder-order transformation for the H atoms. Details of the phase diagram of NbHx. [ Schober and Wenz ]. The full line is a calculation by Kuji and Oates.

12. Hydrides We mast make a series of isothermal measurements of the equilibrium composition of a specimen as a function of the pressure of surrounding gas e.t PCI is the degree of freedom is the number of phases numberofchemicalspecies is enthalpy, is gas constant istemperature is entropy

13. Hydrides Van’t Hoff plots of some technically important reversible metal hydrides

14. Hydrides Complex light metal hydrides Structure changes non Reversible @ ambient T tailorability Hydride Comparison Classical/ interstitial metal hydrides No structure changes Reversible @ ambient T Tailorable thermodynamicproperties Chemical hydrides Structure changes Reversible @ ambient T or irreversible No tailorability Hydrogen storage system challenge: Pack H as close as possible to reach high volumetric densities and use as little additional materials as possible …we need materials satisfying simultaneously all these requirements?!

15. Hydrides

16. MgH2 • High gravimetric (7.6 wt.%) and volumetric • (130 kg H2/m3) storage capabilities • Endothermic desorption reaction • Low cost • Rutile-type structure (H/M=2) • Unit cell volume : 33% larger • than metallic Mg  large nucleation energy barrier  high temperature and pressure for activation • Mixture of covalent and ionic • bonds • Heat of formation(-75 kJ/mol H2) • : high dissociation temperature • Severe surface oxidation and pyrophoricity

17. Short H-diffusion distances in nanoparticle: fast H-exchange rate Long H-diffusion distances in bulk materials reduced H-exchange rate Ball milling and catalysis Nanostructuring and nano-scale catalysis through ball-milling High density of extended defects acting as short circuit path for hydrogen atom diffusion Increase kinetics: diffusion time Possibility of co-existence of chemi- and physi sorption Possibility of changing thermodynamic properties Can be used to introduce a small amount of catalyst able to support the molecule dissociation

18. Ball milling and catalysis Nanostructuring and nano-scale catalysis through ball-milling high energy ball milling to achieve nanostructure- Spex mixer/mill 8000 with hardened steel vials and balls ball-to-powder weight ratio: has greatinfluence on morphology time of milling atmosphere: Ar or H2 Low energy ball milling to introduce catalyst

19. Ball milling and catalysis X-ray powder diffraction of nanocrystalline MgH2 as a function of the milling time DSC trace of MgH2 before and after 20 h of milling. J. Huot, G. Liang, S. Boily, A. V. Neste R. Schulz, J. Alloys Comp. 1999,293-295, p.495

20. Ball milling and catalysis Thermal desorption mass spectra (TDMS) of hydrogen for pure MgH2 milled for 2 h and catalyzed MgH2 with 1 mol % ,Cu, Fe, Co and Ni N. Hanada, T.Ichikawa, H. Fujii J. Phys. Chem. B 2005, 109, 7188-7194

21. Improvement of hydrogen storage properties Different approaches set up in order to improve the hydruration/dehydruration a) carbon and carbon containing liquid additives, b) catalytic metals c) intermetallic compounds

22. Improvement of hydrogen storage properties Mg -C and MgH2- C composites It has been shown that mechanical milling of magnesium and carbon, in the presence of organic additives (tetrahydrofuran, cyclohexan, benzene, etc), results in material, which has enhance absorption/desorption kinetics. Imamura et al. DSC traces for various (Mg/G)BN , (Mg/G)none and Mg samples. The (Mg/G) composites were prepared by grinding with benzene (8.0 cm3 BN ) for (a) 4 h, (b) 10 h, (c) 20 h, (d) 30 h and (e) 40 h. (Mg/G) wasprepared by grinding without benzene for 15 h. By addition of C, the time of first hydrogen uptake can be significantly reduced. There is completely transformation of Mg to MgH2. Therefore, a minimal amount of graphite has to be added in order to have synergetic effect.

23. (Mg85 C15) 1/6 (Mg70 C30)3/1 Improvement of hydrogen storage properties H-desorption: DSC scans endo Montone et al.

24. Improvement of hydrogen storage properties MgH2-Fe CFe=10wt.% BPR:20:1 BPR:10:1 BPR:3:1 BPR:1:1

25. Improvement of hydrogen storage properties MgH2-intermetallic compounds Ball-milled mixtures of MgH2 and Mg2NiH4 exhibit a synergetic effect of hydrogen sorption that results in excellent kinetic properties of the composite material. Sample desorbs hydrogen quickly at temperatures around 220 -240C with hydrogen capacity exceeding 5 wt.%. This result is remarkable in that the dissociation of magnesium hydride does not normally occur at temperatures below at least 280C. DSC traces of MgH2 –35 wt.% Mg2NiH4 composite.

26. Cycling life

27. Theoretical approach An efficient way for solving the many-electron problem of a crystal (with nuclei at fixed positions) are the calculations based on density functional theory. DFT is based on following assumptions: 1)Hamiltonian of the many-electron system is unique functional of spin densities kinetic energy (of the non-interacting particles, electron-electron repulsion, nuclear-electronattraction, exchange-correlationenergy, we do not know this term ! the repulsive Coulomb energy of the fixed nuclei and the electronic contributions

28. Theoretical approach 2)Minimal energy obtained through variation principle corresponds to spin densities of basic state of system. Everything works fine if one knows all terms of Hamiltonian. However this is not the case. We need the way to describe exchange-correlation part of interaction.

29. ii), the particular form chosen for Theoretical approach Two approximations comprise the LSDA, i), the assumption that can be written in terms of a local exchange-correlation energy density times the total (spin-up plus spin-down) electron density as:

30. Theoretical approach The most effective way known to minimize Etot by means of the variational principle is to introduce orbitals constrained to construct the spin densities and then solve Kohn -Sham equation So????????

31. Theoretical approach Like most “energy-band methods“, the LAPW (linearized augmented plane waves ) method is a procedure for solving the Kohn-Sham equations for the ground state density, total energy, and (Kohn-Sham) eigenvalues (energy bands) of a crystal by introducing a basis set which is especially adapted to the problem. We dividing the unit cell into: (I) non-overlapping atomic spheres (centered at the atomic sites. The sphere could be described by linearization of radial function in order to exclude energy dependence) and (II) an interstitial region. The interstitial region could be described by plane waves

32. Theoretical approach The density of states (DOS) X-ray absorption and emission spectra Optical properties X-ray structure factors An analysis of the electron density according to Bader’s “atoms in molecules” theory can be made

33. Theoretical approach What we can obtain using WIEN 2k?

34. Theoretical approach Charge densities DOS

35. Predicted values of the formation enthalpy of binary metal hydrides obtained from DFT-GGA calculations vs. experimental values

36. Theoretical approach Obtained Hf for Ti -60KJ/molH2 for Co -55KJ/molH2 Thermodynamically Favorable Does Not Mean Kinetically Favorable

37. Summary • The challenge is clear and fascinating: supplying more and more abundant and clean energy, consuming less and less natural resources and finding the appropriate solutions for any corner of the planet. • Fundamental theoretical and experimental research is needed to understand the interaction of hydrogen in solid-state materials in order to realize the potential of these materials for hydrogen storage. The challenge still remains!!!

38. Publicperceptions Thank you and see you next year

39. At the Hannover Fair 1998 a Siemens Nixdorf laptop computer was demonstrated , which was powered by a laboratory PEM fuel cell (FhG ISE Freiburg, Germany) and a commercial metal hydride tank SL002(GfE Metalle und Materialien GmbH, Germany), Siemens Nixdorf Notebook powered by a PEM fuel cell /metal hydride tank

40. A small atomic size of hydrogen: One might consider intuitively that a hydrogen atom should be small in size because it has one e-. The situation in fact is not so simple: H has radius of 0.529 Å H- has ionic radius 2.1 Å ( halogens has 1.95-2.1 Å) H+ has ionic radia from (0.18-0.38 Å) depending on the number of surrounding anions. So what that actually implies is that bare proton causes contractions of the neighboring bonds by the effect of hydrogen bonding Solid state storage

41. MgH2 Problems • Severe surface oxidation and pyrophoricity • Sluggish hydrogen diffusion kinetics • Metal-Hydride volume mismatch  large nucleation energy barrier  high temperature and pressure for activation • Large enthalpy of hydride formation

42. Ball milling and catalysis Varin et el.