Old stuff from my Ph.d.

1. Dehydrogenation kinetics of
Lithium Aluminum Hydride
Anders Andreasen
anders.andreasen@risoe.dk
Materials Research Department, Risø National Laboratory, Roskilde, Denmark
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

2. Motivations
• Complex hydrides shows great potential as a solid
state hydrogen storage solution
LiBH4
LiAlH4
NaBH4
NaAlH4
KBH4
KAlH4
Be(AlH4)2
Mg(AlH4)2
Ca(AlH4)2
0 2.5 5 7.5 10 12.5 15 17.5 20
Hydrogen density [wt. %]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

3. Motivations
• Complex hydrides shows great potential as a solid
state hydrogen storage solution
• However, NaAlH4 is too stable and stores too little
hydrogen
4
NaAlH4
2 Na3AlH6
ln(pH /p )
o
2
0
NaH
100 C
150 C
50 C
o
o
o
-2
2 2.5 3 3.5
-1
1000/T [K ]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

4. Motivations
• Complex hydrides shows great potential as a solid
state hydrogen storage solution
• However, NaAlH4 is too stable and stores too little
hydrogen
• LiAlH4 is less stable and stores more hydrogen
• LiAlH4 has not been investigated to the same extent
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

5. Outline
• Mechanism of dehydrogenation
• Basic properties
• Dehydrogenation of as-received samples
• Effect of ball milling
• Effect of catalysis by Ti
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

6. Reaction mechanism
Step 1: LiAlH4 → 1/3Li3 AlH6 + 2/3Al + H2
Step 2: 1/3Li3 AlH6 → LiH + 1/3Al + 1/2H2
Step 3: LiH → Li + 1/2H2
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

7. Basic properties
ρm =5.3 wt% H2 , ∆Hf = -15 kJ/mol H2 ,
Step 1:
T(p=1 bar) = -150 ◦ C
ρm =2.6 wt% H2 , ∆Hf = -35 kJ/mol H2 ,
Step 2:
T(p=1 bar) = 0 ◦ C
Step 3: ρm =2.6 wt% H2 , Tdec = 450 ◦ C
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

8. Basic properties
Step 1: ρm =5.3 wt% H2 , ∆Hf = -15 kJ/mol H2 ,
T(p=1 bar) = -150 ◦ C
Step 2: ρm =2.6 wt% H2 , ∆Hf = -35 kJ/mol H2 ,
T(p=1 bar) = 0 ◦ C
Step 3: ρm =2.6 wt% H2 , Tdec = 450 ◦ C
• In TA LiAlH4 melts before releasing hydrogen
• Ball milling and catalytic doping improves
kinetics
• Reversibility only observed after doping
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

9. As-received samples
Constant heating rate DSC experiments
2
Heat flux dQ/dt [mW/mg]
1
0
o
β = 2 C/min
-1 o
β = 3 C/min
o
β = 4 C/min
o
-2 β = 5 C/min
-3
140 160 180 200 220 240 260
o
Temperature [ C]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

10. As-received samples
Kissinger analysis of DSC experiments
-10.0
LiAlH4(s) -> LiAlH4(l)
EA = 276 kJ/mol
-10.5 Li AlH (s) -> LiH(s) + Al(s) + H (g)
3 6 2
EA = 107 kJ/mol
ln(β/T ) [--]
2
-11.0
-11.5 EA = 81 kJ/mol
LiAlH4(l) -> Li3AlH6(s) + Al(s) + H2(g)
-12.0
1.90 1.95 2.00 2.05 2.10 2.15 2.20 2.25
-1
1000/T [K ]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

11. As-received samples
Isothermal kinetics from in situ gravimetry
7
o
152 C o
6 140 C o
Hydrogen release [wt. %]
132 C
o
5 115 C
4
4
3 3
2
2
1
1
0
0 0.5 1 1.5 2
0
0 2 4 6 8 10 12 14 16 18 20
Time [h]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

12. As-received samples
Kinetic analysis of isothermal experiments
Simple two-step kinetic model
Wtot = W1 exp (1 − (k1 t)η1 ) + W2 exp (1 − (k2 t)η2 )
Activation energies from 7
6
Hydrogen release [wt. % H2]
Arrhenius analysis 5
EA1 = 82 kJ/mol 4
Exp.
Model fit
EA2 = 90 kJ/mol 3
2
1
0
0 5 10 15 20
Time [h]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p.

13. Ball milled samples
Line broadening in XRPD patterns
1400
1200 BM 10 h 400 rpm
*
Intensity [counts/s]
1000
BM 6 h 400 rpm
800
* BM 2 h 400 rpm
600
400
BM 1 h 400 rpm
200 BM 1 h 150 rpm
0
15 20 25 30 35 40 45 50
ο
Diffraction angle 2θ [ ]
1
Scherrer equation: β ∝ B
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1

14. Ball milled samples
Isothermal dehydrogenation kinetics
5
Hydrogen release [wt %]
4
7
3
6
5
BM 10 h 400 rpm
2 4
BM 6 h 400 rpm
3 BM 2 h 400 rpm
2 BM 1 h 400 rpm
1 BM 1 h 150 rpm
1 As recieved
0
0 5 10 15 20
0
0 1 2 3 4 5
Time [h]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1

15. Ball milled samples
Isothermal dehydrogenation kinetics
Model fit:
Time [h] Intensity [rpm] W1 [wt.% H2 ] W2 [wt.% H2 ] k1 [h−1 ] k2 [h−1 ]
1 150 3.85 2.17 0.751 0.180
1 400 4.12 2.07 1.567 0.168
2 400 3.57 3.26 1.305 0.190
6 400 3.39 2.04 3.272 0.216
10 400 2.81 1.97 3.817 0.163
• Step 1 depends strongly on applied ball milling time
• Step 2 is independent of ball milling time
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1

16. Ball milled samples
Rate constant of step 1 vs. crystallite size
5
4 10 h 400 rpm
Rate constant, k1 [h ]
-1
6 h 400 rpm
3
2 1 h 400 rpm
2 h 400 rpm
1 1 h 150 rpm As-received
0
50 75 100 125 150
Crystallite size [nm]
1
Dependency: k1 ∝ β 2.3
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1

17. Ball milled samples
1
Step 1: Explanation of the k1 ∝ β 2.3 relationship
• Mass transfer limited kinetics?
• Nabarro-Herring theory (β 2 ): lattice diffusion?
• Coble theory (β 3 ): grain boundary diffusion?
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1

18. Ball milled samples
1
Step 1: Explanation of the k1 ∝ β 2.3 relationship
• Mass transfer limited kinetics?
• Nabarro-Herring theory (β 2 ): lattice diffusion?
• Coble theory (β 3 ): grain boundary diffusion?
Step 2: Explanation of the missing k2 vs. β
relationship
• Mass transfer limited kinetics? No!
• “Intristic” kinetics is limiting the process?
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1

19. Ti-doped samples
3LiAlH4 + TiCl3 + ball milling → Ti + 3Al + 3LiCl +
6H2
2500
Al/LIH
2000 Li AlH
3 6
BM 1 h@400 rpm
Intensity [a.u.]
1500 Al/LIH
BM 5 min@400 rpm
1000
BM 1 min@100 rpm
500
0
15 20 25 30 35 40 45 50
o
Diffraction angle 2θ [ ]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1

20. Ti-doped samples
3LiAlH4 + TiCl3 + ball milling → Ti + 3Al + 3LiCl +
6H2
1000
BM 1 h@400 rpm
500
2
Heat flux [a.u.]
0
1
-500 BM 1 min@100 rpm
-1000
-1500
2
-2000
100 120 140 160 180 200 220 240
o
Temperature [ C]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1

21. Ti-doped samples
Kissinger analysis of DSC experiments
-10.4
LiAlH4 -> Li3AlH6 + Al + H2
EA = 89 kJ/mol
-10.6
ln(β/T ) [--]
-10.8
2
-11
Li3AlH6 -> LiH + Al + H2
-11.2 EA = 103 kJ/mol
2 2.1 2.2 2.3 2.4 2.5
-1
1000/T [K ]
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1

22. Summary
• Ball milling improves kinetics of step 1
• Diffusional limitations?
• Ti-doping improves both step 1 and 2
• Apparent activation energies seems
insensitive to the reaction path
• Ti-doping: A prefactor effect?
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1