Complex metal hydrides

1

Complex metal hydrides Are they possible
option for hydrogen storage? Ph. D.,
Seminar - 1 L. Hima Kumar
2

• Contents
• Need for alternative fuel
• Hydrogen as an alternative
• Hydrogen Storage
• Complex metal hydrides
• Conclusions

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Why alternative fuel ?

• Growing demand
• Depletion in fossil fuels
• Environmental concerns
• Awareness for equidistribution
• Economy and processibility

S. Dunn, Tech Monitor, Nov-Dec (2001) 14
4

• Hydrogen as an energy carrier
• C (coal) ? -CH2- (oil) ? CH4 (natural gas) ? H2
  (hydrogen)
• High energy per unit mass
• Most abundant
• Renewable sources
• Eco-friendly

M. Conte, A. Jacobazi, M. Ronchetti and R.
Vellone, J. Power Sources, 100 (2001) 171
5
Comparison of fuel properties
Properties Unit Hydrogen H2 Methane CH4 Gasoline -(CH2)n-
Lower heating value kWh kg-1 33.33 13.9 12.4
Self ignition temperature K 858 813 498-774
Flame temperature K 2318 2148 2473
Ignition limits in air vol 4 - 75 5.3 - 15 1.0-7.6
Min. ignition energy mW 0.02 0.29 0.24
Flame propagation in air m s-1 0.02 0.4 0.4
Explosion energy kg TNT m-3 2.02 7.03 44.22
Diffusion coefficient in air cm2 s-1 0.61 0.16 0.05
Toxicity No No High
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Various ways of production of hydrogen

• Reforming, partial oxidation
• Electrolysis of water
• Thermochemical dissociation of water
• Photochemical, Photobiological process

7
What options are available for hydrogen storage ?

• High pressure gas cylinders
• Liquid hydrogen in cryogenic tanks
• Porous materials
• Carbon nanomaterials
• Metal hydrides
• Complex metal hydrides

8
Conventional methods
High pressure gas cylinders ? Pressure 20 80
MPa ? The volumetric density increases with
increasing the pressure ? Low gravimetric
density ? Unsafe Liquid hydrogen ? Cryogenic
tanks Temp 21 K Pressure ambient ?
Density 70.8 kg m-3 ? Boil-off losses ? Energy
necessary for liquefaction is high
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Solid-state hydrogen storage
Criteria for a hydrogen storage medium
?Appropriate thermodynamics ? Fast kinetics
(quick uptake and release) ? High storage
capacity ? Effective heat transfer ? High
gravimetric and volumetric densities ? Long
cycle life time for hydrogen absorption/desorption
? High mechanical strength and durability ?
Safety under normal use
10
Hydrogen storage capacity Target ?
(6.5 wt)
DoE Department of Energy
11
Porous materials
Mesoporous silica
Zeolites
Metal-organic framework
Glass microspheres
Boron nitride nanotubes
N. L. Rosi, J. Eckert, M. Eddaoudi, David T.
Vodak, J. Kim, Michael OKeeffe and Omar M.
Yaghi, Science, 300 (2003) 1127
12
Carbon nanostructures

• Graphite nanofibers
• Carbon nanotubes (SWNT, MWNT)
• Fullerenes

13
Reversible amount of hydrogen vs. B.E.T. surface
area
Some of reported results achieved for
hydrogen storage in carbon nanostructures
Carbon nano structures Pressure (MPa) Temp. (K) Wt of hydrogen
Graphitic nano fibers 12 300 10-67
SWNT 0.04 133 5-10
SWNT(50 purity) 10.01 300 4.2
MWNT Ambient 300-700 0.25
Li-CNT 0.1 473-673 20 0.75-4.2
K-CNT 0.1 313 14
SWNT-Fe 0.08 Ambient 0.005
SWNT-TiAl0.1V0.04 0.067 Ambient 1.47
MWNT-NiO-MgO 600 Ambient 0.65
Specific surface area (m2/g)
Li Zhou, Renewable and Sustainable Energy Reviews
(in press)
14
Metal hydrides

• M (x/2)H2 MHx
• Ionic, covalent and metallic
• Non-transition metals ionic, covalent
• Transition metals metallic
• Metal hydrides have high volumetric storage
  densities. The storage density is higher
  than liquid or solid hydrogen.
• Interaction of hydrogen with metal in metallic
  hydrides is absorption process

15
Schematic representation of hydrogen storage in
Metal hydrides
Distance from the metal Å
Potential energy of hydrogen approaching a
metallic surface
Interaction of hydrogen with metal

• Under hydrogen pressure metals absorb hydrogen
• By reducing the pressure and supplying heat,
  hydrogen is released
• H2 molecule is first adsorbed on the surface and
  then dissociated
• as strongly bound individual H atoms

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Pressure-Composition isotherms for hydrogen
absorption in a typical intermetallic compound

H/M
H/M
1000/T K-1
L. Schlapbach and A. Züttel, Nature, 414 (2001)
23
17

• Which hydrides for hydrogen storage?
• Metal hydrides
• Interametallic compound hydrides
• Complex metal hydrides

• Metal hydrides
• - MgH2, BeH2, TiH2
• High storage capacity
• Poor kinetics
• High temperature pressure

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Intermetallic hydrides
Important families of hydride-forming
intermetallic compounds
IMC Prototype Hydride
AB5 LaNi5 LaNi5H6
AB2 ZrV2, TiMn2 ZrV2H5.5
AB3 CeNi3,YFe3 CeNi3H4
A2B7 Y2Ni7, Th2Fe7 Y2Ni7H3
A6B23 Y6Fe23 Y6Fe23H12
AB TiFe, ZrNi TiFeH2
A2B Mg2Ni,Ti2Ni Mg2NiH4
A hydrogen absorber (rare earth or alkaline
earth metal) B hydrogen activator (transition
metal )
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• Conventional methods
• Porous materials
• Carbon nanotubes
• Intermetallics

Storage Capacity lt 3 wt
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Complex metal hydrides General formula -
AyMHxz ? Alanates ?
Borohydrides
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What are alanates?
- Complex metal hydrides containing AlH4-
General formula M(AlH4)n NaAlH4
LiAlH4 Mg(AlH4)2 Ca(AlH4)2 KAlH4 Ti(AlH4)4
? Hydrogen atoms arranged tetrahedrally around
Al ? Hydrogens retain significant hydride or
electron-rich character
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Synthesis

• 4 LiH AlCl3 LiAlH4
  3 LiCl
• NaAlH4 , Ca(AlH4)2 and Mg(AlH4)2
• Direct synthesis
• Na Al 2 H2 NaAlH4
  (545 K, 175 bar )
• Mechanochemical synthesis
• MH AlH3 MAlH4
• Ball to powder weight ratio 201

ether
A. E. Finholt, A. C. Bond Jr. and H. J.
Schlesinger, J. Am. Chem. Soc. 69 (1947) 1199
23
Calculated hydrogen storage capacity
Hydride H2 Content (wt)
LiAlH4 10.5
NaAlH4 7.5
KAlH4 5.7
Be(AlH4)2 11.3
Mg(AlH4)2 9.3
Ca(AlH4)2 7.7
Ti(AlH4)4 9.3
LiBH4 18.0
NaBH4 10.4
Al(BH4)3 17.0
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Decomposition Reaction
Two step process NaAlH4 1/3
Na3AlH6 2/3 Al H2 Na3AlH6
3 NaH Al 3/2 H2
H/M

• Reversibility
• NaAlH4 Ti(OBu)4 Ti doped NaAlH4
• NaAlH4 TiCl3 Ti doped NaAlH4

H/M
B. Bogdanovic and M. Schwickardi, J. Alloys Comp.
257 (1997) 1
25
Thermodynamics
3 NaAlH4 Na3AlH6 2 Al 3 H
2 (3.7 wt) ?H 37 kJ/mol Na3AlH6
3 NaH Al 3/2 H 2 (1.9 wt)
?H 47 kJ/mol
T (ºC)
1000/T K-1
? NaAlH4 is a low temperature hydride ? Na3AlH6
is the medium temperature hydride
B. Bogdanovic, Richard A. Brand, A. Marjanovic,
M. Schwickardi and J. Tölle, J. Alloys Comp.
330-332 (2002) 683
26
Doping of alanates - wet chemical
method - dry method
Advantages

• Reversibility
• Reversible content of doped alanate 3.1 - 4.2
  wt
• undoped alanate
  0.5 0.8 wt
• Improved H2 desorption rate
• Higher cycle stability
• Reduction in dehydriding temperature by 323 K

Difficulties

• Use of alkoxides contaminates desorbed H2
• Weight penalty
• Oxygen from decomposition of alkoxide
  contaminates active material

K. J. Gross, G. J. Thomas and C. M. Jensen, J.
Alloys Comp. 330-332 (2002) 683
27
Kinetics of alanates Factors affecting the
reaction rates Particle size Catalyst ?
type ? method of doping ?
amount
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Dehydrogenation rates for various transition
metal catalysts
Rate k exp(-Q/RT ) Q
activation energy
Catalyst additions and resultant
dehydrogenation rates
D. L. Anton, J. Alloys Comp. 356-357 (2003) 400
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Effect of method of doping
Milling time (hr)
Effect of milling time on dehydrogenation rate
Temperature (C)
Doped with Ti(OBu)4
C. M. Jensen, R. Zidan, N. Mariels, A. Hee and
C. Hagen, Int. J. Hydrogen Energy, 24 (1999)
461
30

• Doping with TiCl4
• TiCl4 4 NaAlH4 Ti 4 NaCl
  4 Al 8 H2

• Doping with Ti
• Ball milling of elemental Ti and NaAlH4
• Kinetics better than ball milling alone
• Rehydrides at 393 K and 55 bar
• Poor kinetics for subsequent dehydriding

• Doping with Carbon
• Kinetics improved over other dopants
• Rate increases with subsequent cycles
• Rehydrogenation occurs under practical
  conditions

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Effect of Catalyst Loading
TiCl3 level (mol)
TiCl3 level (mol)
TiCl3 level (mol)
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Mechanism
a (Å)
NaAlH4
a (Å)
Ti4 - 0.67 Å Ti3 0.76 Å Ti2 0.82 Å
Concentration (mol)
Schematic illustration of the changes in NaAlH4
lattice upon increased level of doping
D. Sun, T. Kiyobayashi, H. T. Takeshita, N.
Kuriyama and C. M. Jensen, J. Alloys Comp. 337
(2002) L8
33
Two Theta

• 3 (NaH)(AlH3) (NaH)3 (AlH3) 2 (AlH3)
  3 (NaH) 3 (AlH3)
• AlH3 (catalyst) Al 3/2
  H2
• ? Catalyst remains on the surface of the NaAlH4
  crystal surface
• ? Phase transformations occur by the long-range
  diffusion of metal species through the
  alanate crystal structure to the catalyst
  on the surface
• ? Catalyst would work on the surface of the
  crystal as a dissociation-recombination
  site

K. J. Gross, G. Sandorck and C. M. Jensen, J.
Alloys Comp. 330-332 (2002) 691
34

• Highlights
• Slow de/rehydriding kinetics remain a
  significant barrier
• Destabilizing the second desorption step is
  necessary to achieve the full theoretical
  capacity of hydrogen available
• Long-term cycling studies are required
• Safety
• Complete understanding of the reaction mechanism
  is still unknown
• Thermodynamic tailoring of alanates
• Extension to other complex metal hydride

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Alkaline earth metal alanates
- Mg(AlH4)2, Ca(AlH4)2 Synthesis MCl2
Li/NaAlH4 M(AlH4)2 2
Li/NaCl Decomposition M(AlH4)2
MH2 Al 3 H2 Reversibility ?
Catalyst
Catalyst
M. Fichtner, J. Engel, O. Fuhr, O. Kircher and O.
Rubner, Mat. Sci. Eng. B 108 (2004) 42
36
Borohydrides ? Compound with highest gravimetric
hydrogen density known today is LiBH4 (18
wt) ? Decomposition is similar to that of
alanates 2 NaBH4 2
NaH B 3 H2 ? Reversible conditions 963 K and
200 bar ? Pyrolysis - high temp, high pressure

?
Other possibility ?
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Hydrogen storage by NaBH4 Hydrogen generation
by the hydrolysis of alkaline sodium borohydride
solution Reaction BH4- 2 H2O
BO2- 4 H2
Catalysts
Pt, Ru, Ni, Co NaBO2 2 MgH2 ? NaBH4 2 MgO
catalyst
S. C. Amendola, S. L. Sharp-Goldman, M.S. Januja,
N.C. Spencer, M. T. Kelly, P. J. Petillo and M.
Binder, Int. J. Hydrogen Energy, 29 ( 2004) 263
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Advantages of NaBH4

• NaBH4 solutions are non-flammable
• NaBH4 solutions are stable in air for months
• H2 generation occurs only in the presence of
  selected catalysts
• Reaction products are environmentally safe
• H2 generation rates are easily controlled
• Volumetric and gravimetric H2 storage
  efficiencies are high
• Reaction products can be recycled

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Volumetric and gravimetric hydrogen density of
some selected hydrides
Gravimetric density mass
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• Conclusions
• 1. The critical components in hydrogen economy
  hydrogen production, hydrogen storage and
  distribution still need technological
  development.
• 2. Todays hydrogen storage technologies do not
  meet the
• vehicle requirements.
• 3. New materials and/or new technical approaches
  are required to meet hydrogen storage targets for
  vehicular applications.
• 4. The possibility of complex metal hydrides as
  storage media
• seems to be promising.

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Thank you
42
References

• Seth Dunn, Tech Monitor, Nov-Dec (2001) 14
• Li Zhou, Renewable and Sustainable Energy Reviews
  (in press)
• Louis Schlapbach and Andreas Züttel, Nature, 414
  (2001) 23
• A. E. Finholt, A. C. Bond Jr. and H. J.
  Schlesinger, J. Am. Chem.
• Soc. 69 (1947) 1199
• B. Bogdanovic, M. Schwickardi, J. Alloys Comp.
  257 (1997) 1
• K. J. Gross,G. J. Thomas and C. M. Jensen, J.
  Alloys Comp. 330-332
• (2002) 683
• D. L. Anton, J. Alloys Comp. 356-357 (2003) 400
• C. M. Jensen, R. A. Zidan, N. Mariels, A.G. Hee
  and C. Hagen,
• Int. J. Hydrogen Energy 24 (1999) 461
• 10. D. Sun, T. Kiyobayashi, H. T. Takeshita, N.
  Kuriyama and
• C. M. Jensen, J. Alloys Comp. 337 (2002)
  L8
• M. Fichtner, J. Engel, O. Fuhr, O. Kircher and O.
  Rubner, Mat. Sci. Eng.
• B 108 (2004) 42