Complex metal hydrides – Are they possible
option for hydrogen storage?
Ph. D., Seminar - 1
L. Hima Kumar
1
Contents
Need for alternative fuel
Hydrogen as an alternative
Hydrogen Storage
Complex metal hydrides
Conclusions
2
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
3
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
4
Comparison of fuel properties
Hydrogen
H2
Methane
CH4
Gasoline
-(CH2)n-
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
kg TNT m-3
2.02
7.03
44.22
cm2 s-1
0.61
0.16
0.05
No
No
High
Properties
Lower heating value
Explosion energy
Diffusion coefficient in air
Toxicity
Unit
5
Various ways of production of hydrogen
Reforming, partial oxidation
Electrolysis of water
Thermochemical dissociation of water
Photochemical, Photobiological process
6
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
7
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
8
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
9
Hydrogen storage capacity : Target ?
(6.5 wt%)
DoE : Department of Energy
10
Porous materials
Zeolites
Mesoporous silica
Metal-organic
Glass microspheres
framework
N. L. Rosi, J. Eckert, M. Eddaoudi, David T. Vodak, J. Kim, Michael O’Keeffe and
Omar M. Yaghi, Science, 300 (2003) 1127
11
Boron nitride nanotubes
Carbon nanostructures
 Graphite nanofibers
 Carbon nanotubes (SWNT, MWNT)
 Fullerenes
12
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
Reversible amount of hydrogen vs.
B.E.T. surface area
Specific surface area (m2/g)
Li Zhou, Renewable and Sustainable Energy Reviews (in press)
13
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
14
Schematic representation of hydrogen storage in Metal hydrides
Interaction of hydrogen with metal
Distance from the metal [Å]
Potential energy of hydrogen approaching a
metallic surface
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
15
Pressure-Composition isotherms for hydrogen absorption in a typical
intermetallic compound
H/M
1000/T [K-1]
H/M
H S
ln p 

RT
R
L. Schlapbach and A. Züttel, Nature, 414 (2001) 23
16
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
17
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 )
18
Conventional methods
Porous materials
Carbon nanotubes
Intermetallics
Storage Capacity < 3 wt%
19
Complex metal hydrides
General formula - Ay[MHx]z
 Alanates
 Borohydrides
20
What are alanates?
- Complex metal hydrides containing AlH4General 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
21
Synthesis
4 LiH + AlCl3
ether
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 20:1
A. E. Finholt, A. C. Bond Jr. and H. J. Schlesinger, J. Am. Chem. Soc.
69 (1947) 1199
22
Calculated hydrogen storage capacity
Hydride
LiAlH4
H2 Content (wt%)
10.5
NaAlH4
KAlH4
Be(AlH4)2
7.5
5.7
11.3
Mg(AlH4)2
Ca(AlH4)2
Ti(AlH4)4
9.3
7.7
9.3
LiBH4
NaBH4
18.0
10.4
Al(BH4)3
17.0
23
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
24
Thermodynamics
3 NaAlH4
Na3AlH6
Na3AlH6 + 2 Al + 3 H 2 (3.7 wt%) ; H = +37 kJ/mol
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
25
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
26
(2002) 683
Kinetics of alanates
Factors affecting the reaction rates
Particle size
Catalyst
 type
 method of doping
 amount
27
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
28
Effect of method of doping
Temperature (°C)
Milling time (hr)
Effect of milling time on
dehydrogenation rate
Doped with Ti(OBu)4
C. M. Jensen, R. Zidan, N. Mariels, A. Hee and C. Hagen,
Int. J. Hydrogen Energy, 24 (1999) 461
29
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
30
Effect of Catalyst Loading
TiCl3 level (mol%)
TiCl3 level (mol%)
TiCl3 level (mol%)
31
Mechanism
a (Å)
NaAlH4
Ti+4 - 0.67 Å
Ti+3 – 0.76 Å
Ti+2 – 0.82 Å
a (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
32
Two Theta [˚]
3 (NaH)(AlH3)
AlH3
(NaH)3 (AlH3) + 2 (AlH3)
(catalyst)
3 (NaH) + 3 (AlH3)
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
33
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
34
Alkaline earth metal alanates
- Mg(AlH4)2, Ca(AlH4)2
Synthesis
MCl2 + Li/NaAlH4
Catalyst
M(AlH4)2 + 2 Li/NaCl
Decomposition
M(AlH4)2
Catalyst
MH2 + Al + 3 H2
Reversibility ?
M. Fichtner, J. Engel, O. Fuhr, O. Kircher and O. Rubner, Mat. Sci. & Eng. B
108 (2004) 42
35
Borohydrides
 Compound with highest gravimetric hydrogen density
known today is LiBH4 (18 wt%)
 Decomposition is similar to that of alanates
2 NaBH4
Δ
2 NaH + B + 3 H2
 Reversible conditions 963 K and 200 bar
 Pyrolysis - high temp, high pressure
Other possibility ?
36
Hydrogen storage by NaBH4
Hydrogen generation by the hydrolysis of alkaline sodium
borohydride solution
Reaction:
BH4
-+
2 H2O
catalyst
BO2- + 4 H2
Catalysts: Pt, Ru, Ni, Co
NaBO2 + 2 MgH2 → NaBH4 + 2 MgO
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
37
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
38
Volumetric and gravimetric hydrogen density of some selected hydrides
Gravimetric density [mass%]
39
Conclusions
1. The critical components in hydrogen economy hydrogen
production, hydrogen storage and distribution still need
technological development.
2. Today’s 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.
40
41
References:
1. Seth Dunn, Tech Monitor, Nov-Dec (2001) 14
2. Li Zhou, Renewable and Sustainable Energy Reviews (in press)
3. Louis Schlapbach and Andreas Züttel, Nature, 414 (2001) 23
4. A. E. Finholt, A. C. Bond Jr. and H. J. Schlesinger, J. Am. Chem.
Soc. 69 (1947) 1199
6. B. Bogdanovic, M. Schwickardi, J. Alloys Comp. 257 (1997) 1
7. K. J. Gross,G. J. Thomas and C. M. Jensen, J. Alloys Comp. 330-332
(2002) 683
6. D. L. Anton, J. Alloys Comp. 356-357 (2003) 400
7. 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
11. M. Fichtner, J. Engel, O. Fuhr, O. Kircher and O. Rubner, Mat. Sci. & Eng.
42
B 108 (2004) 42