HYDROGEN STORAGE IN MAGNESIUM BASED ALLOYS

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HYDROGEN STORAGE IN MAGNESIUM BASED ALLOYS
Jasmina Grbovic Novakovic
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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
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Why Hydrogen? Its abundant, clean and can be
derived from diverse resources.
Alternative fuel
Biomass
Transportation
Hydro Wind Solar
HIGH EFFICIENCY RELIABILITY
Nuclear
Oil
Distributed Generation
ZERO/NEAR ZEROEMISSIONS
Coal
With Carbon Sequestration
Natural Gas
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Why not Hydrogen?
Alternative fuel
Problem is safe, efficient and cost-effective
storage
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Common requirements for hydrogen storage
Hydrogen storage

• High gravimetric and volumetric storage
  capabilities
• Cost
• Efficiency
• Safety
• Life cycle
• Environmental impact

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Hydrogen storage
Hydrogen Storage Options
Schlapbach Züttel, Nature, 15 Nov. 2001
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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
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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.
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Solid state storage
In many cases H occupies interstitial sites
tetrahedral and octahedral.
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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 compounds called
  saline hydrides (MH- and Mg2H2-)
• Most of Group IIIV 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

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Hydrides
The TaHx phase diagram according to Schober. a
and a are disordered BCC solutions of H in Ta. e
is a tetragonal phase and ß, d, ? and ? are
orthorhombic. The a-ß 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.
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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
number of chemical species
is enthalpy,
is gas constant
is entropy
is temperature
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Hydrides
Vant Hoff plots of some technically important
reversible metal hydrides
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Hydrides
Complex light metal hydrides Structure changes
non Reversible _at_ ambient T tailorability
Hydride Comparison
Classical/ interstitial metal hydrides No
structure changes Reversible _at_ ambient
T Tailorable thermodynamic properties
Chemical hydrides Structure changes Reversible _at_
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?!
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Hydrides
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MgH2

• High gravimetric (7.6 wt.) and volumetric
• (130 kg H2/m3) storage capabilities
• Endothermic desorption reaction
• Low cost

• Rutile-type structure (H/M2)
• 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

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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
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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 great influence on morphology
time of milling atmosphere Ar or H2
Low energy ball milling to introduce catalyst
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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
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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
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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
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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.
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Improvement of hydrogen storage properties
H-desorption DSC scans
endo
Montone et al.
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Improvement of hydrogen storage properties
MgH2-Fe
CFe10wt.
BPR201
BPR101
BPR31
BPR11
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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.
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Cycling life
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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-electron attraction,
exchange-correlation energy, we do not know this
term !
the repulsive Coulomb energy of the fixed nuclei
and the electronic contributions
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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.
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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
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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????????
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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
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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
Baders atoms in molecules theory can be made
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Theoretical approach
What we can obtain using WIEN 2k?
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Theoretical approach
Charge densities
DOS
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Predicted values of the formation enthalpy of
binary metal hydrides obtained from DFT-GGA
calculations vs. experimental values
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Theoretical approach
Obtained Hf for Ti -60KJ/molH2 for Co
-55KJ/molH2
Thermodynamically Favorable Does Not Mean
Kinetically Favorable
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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!!!
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Public perceptions
Thank you and see you next year
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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
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Solid state storage
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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

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Ball milling and catalysis
Varin et el.