hydrogen storage in magnesium based alloys
DESCRIPTION
HYDROGEN STORAGE IN MAGNESIUM BASED ALLOYS. Jasmina Grbovi ć Novakovi ć. 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 - PowerPoint PPT PresentationTRANSCRIPT
HYDROGEN STORAGE IN MAGNESIUM BASED ALLOYS
Jasmina Grbović Novaković
…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
(Why) Do we need alternative fuels and energy carriers?
Alternative fuel
Distributed Generation
TransportationBiomass
HydroWindSolar
Coal
Nuclear
Natural Gas
Oil
Wit
h C
arb
on
Seq
ues
trat
ion
HIGH EFFICIENCY & RELIABILITY
ZERO/NEAR ZEROEMISSIONS
Why Hydrogen?Why Hydrogen? It’s abundant, clean and can be derived from diverse resources.
Alternative fuel
Why not Hydrogen?Why not Hydrogen?
Problem is safe, efficient and cost-effective storage
Alternative fuel
Common requirements for hydrogen storage
• High gravimetric and volumetric storage capabilities
• Cost• Efficiency• Safety• Life cycle• Environmental impact
Hydrogen storage
Hydrogen Storage Options
Mobile applications Volume of 4 kg of hydrogen compacted in different ways, with size relative to the size of the car
Stationary applications
Schlapbach & Züttel, Nature, 15 Nov. 2001
• 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
Hydrogen storage
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.
molKJEphys
10
molKJEphys
50
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
How does solid storage occur?
Solid state storage
Lennard-Jones potential of hydrogen approaching a metallic surface.
The reaction of hydrogen gas with metal can be described in terms of a simplified one-dimensional potential energy curve
Solid state storage
Solid state storage
fcc bcc hcp
In many cases H occupies interstitial sites tetrahedral and octahedral.
A medium value of electronegativity Indicates that H can form various kinds of chemical bonds with
various elements
a) I and II group of elements which has small electronegativity H forms ionic compounds called saline hydrides (M+H- and Mg2
+H2-)
b) Most of Group III–V non-metallic elements form covalently bonded crystals
c) 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
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 Hatoms.
Details of the phase diagram of NbHx. [ Schober and Wenz ]. The full line is a calculation by Kuji and Oates.
Hydrides
T
R
Hydrides
NF 2F is the degree of freedom
is the number of phases
number of chemical species
R
S
RT
Hp
ln
H is enthalpy,
S is entropy
is gas constant
is temperature
N
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
Van’t Hoff plots of some technically important reversible metal hydrides
Hydrides
Hydride Comparison
Classical/ interstitial metal hydrides
No structure changes
Reversible @ ambient T
Tailorable thermodynamic properties
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?!
Hydrides
Complex light metal hydrides
Structure changes
non Reversible @ ambient T
tailorability
Hydrides
• 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
MgH2
• High gravimetric (7.6 wt.%) and volumetric
(130 kg H2/m3) storage capabilities • Endothermic desorption reaction• Low cost
Nanostructuring and nano-scale catalysis through ball-milling
High density of extended defects acting as short circuit path for hydrogen atom diffusion
Can be used to introduce a small amount of catalyst able to support the molecule dissociation
Increase kinetics: diffusion time
Possibility of co-existence of chemi- and physi sorption
Possibility of changing thermodynamic properties
Long H-diffusion distances in bulk materials reduced H-exchange rate
Short H-diffusion distances in
nanoparticle: fast H-exchange rate
Ball milling and catalysis
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
Nanostructuring and nano-scale catalysis through ball-milling
Low energy ball milling to introduce catalyst
Ball milling and catalysis
Ball milling and catalysis
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
X-ray powder diffraction of nanocrystalline MgH2
as a function of the milling time
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
Ball milling and catalysis
J. Phys. Chem. B 2005, 109, 7188-7194
N. Hanada, T.Ichikawa, H. Fujii
a) carbon and carbon containing liquid additives, b) catalytic metals c) intermetallic compounds
Different approaches set up in order to improve the hydruration/dehydruration
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.
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.
Imamura et al.
Improvement of hydrogen storage properties
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.
Improvement of hydrogen storage properties
Montone et al.
H-desorption: DSC scans
300 350 400 450 5000
5
10
15
20 (Mg70 C30) none
(Mg70 C30) 1/3
(Mg70 C30) 3/1
(Mg97 C7) 1/3
(Mg85 C15) 1/6
(Mg85 C15) 1/3
H
eat F
low
(W
/g)
Temperature (°C)
(Mg70 C30)3/1
(Mg85 C15) 1/6
en
do
150 200 250 300 350 400 450 500
0
2
4
6
8
10
12
CFe
=10 wt.%
Hea
t Flo
w (
W/g
)
T (°C)
BPR= 1:1 3:1 10:1 20:1
BPR:20:1BPR:10:1
BPR:3:1 BPR:1:1
CFe=10wt.%
MgH2-Fe
Improvement of hydrogen storage properties
MgH2-intermetallic compounds
450 500 550 600 650 700 750
0
2
4
6
8
10
12
14
BPR 20:1
Milling time: A 1h B 5h C 10h
Hea
t F
low
(W
/g)
Temperature (K)
DSC traces of MgH2 –35 wt.% Mg2NiH4 composite.
Improvement of hydrogen storage properties
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 -240C 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 280C.
Cycling life
1)Hamiltonian of the many-electron system is unique functional of spin densities
NNxcNeeestotEEEETE
)()()()()(
)(
sT
)(
eeE
kinetic energy (of the non-interacting particles,
electron-electron repulsion,
nuclear-electron attraction,)(
NeE
exchange-correlation energy, we do not know this term !
)(
xcE
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:
Theoretical approach
the repulsive Coulomb energy of the fixed nuclei and the electronic contributionsNNE
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.
drExcxc
][*)(
xcii), the particular form chosen for
xcE
xc
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:
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
ik
rrVVVikikikxceeNe
2
So????????
Theoretical approach
Like most “energy-band methods“, the LAPW (linearized augmented plane waves ) method is a procedure for solving the Kohn-Shamequations 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 (centeredat 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
The density of states (DOS)
X-ray absorption and emission spectra
X-ray structure factors
Optical properties
An analysis of the electron density according to Bader’s “atoms in molecules” theory can be made
Theoretical approach
Theoretical approach
What we can obtain using WIEN 2k?
Theoretical approach
Charge densitiesDOS
Predicted values of the formation enthalpy of binary metal hydrides obtained from DFT-GGA calculations vs. experimental values
Thermodynamically Favorable Does Not Mean Kinetically Favorable
Theoretical approach
100 200 300 400 500 600-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
T/T
T
MgH2-Ti
MgH2-Co
Obtained Hf for Ti -60KJ/molH2
for Co -55KJ/molH2
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!!!
Public perceptions
Thank you and see you next year
Siemens Nixdorf Notebook powered by a PEM fuel cell /metal hydride tank
At the Hannover Fair 1998 a Siemens Nixdorf laptopcomputer 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),
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 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
H- has ionic radius 2.1 Å ( halogens has 1.95-2.1 Å)
H has radius of 0.529 Å
Solid state storage
• 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
Problems
MgH2
Ball milling and catalysis
Varin et el.