andr dtu 260805
TRANSCRIPT
Dehydrogenation kinetics ofLithium Aluminum Hydride
Anders Andreasen
Materials Research Department, Risø National Laboratory, Roskilde, Denmark
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 1
Motivations
• Complex hydrides shows great potential as a solidstate hydrogen storage solution
• However, NaAlH4 is too stable and stores too littlehydrogen
• LiAlH4 is less stable and stores more hydrogen
• LiAlH4 has not been investigated to the same extent
Ca(AlH4)2
Mg(AlH4)2
Be(AlH4)2
KAlH4
KBH4
NaAlH4
NaBH4
LiAlH4
LiBH4
0 2.5 5 7.5 10 12.5 15 17.5 20
Hydrogen density [wt. %]
2 2.5 3 3.51000/T [K
-1]
-2
0
2
4ln
(pH
2/po )
NaH
Na3AlH6
NaAlH4
150o C
100o C
50o C
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 2
Motivations
• Complex hydrides shows great potential as a solidstate hydrogen storage solution
• However, NaAlH4 is too stable and stores too littlehydrogen
• LiAlH4 is less stable and stores more hydrogen
• LiAlH4 has not been investigated to the same extentCa(AlH4)2
Mg(AlH4)2
Be(AlH4)2
KAlH4
KBH4
NaAlH4
NaBH4
LiAlH4
LiBH4
0 2.5 5 7.5 10 12.5 15 17.5 20
Hydrogen density [wt. %]
2 2.5 3 3.51000/T [K
-1]
-2
0
2
4ln
(pH
2/po )
NaH
Na3AlH6
NaAlH4
150o C
100o C
50o C
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 2
Motivations
• Complex hydrides shows great potential as a solidstate hydrogen storage solution
• However, NaAlH4 is too stable and stores too littlehydrogen
• LiAlH4 is less stable and stores more hydrogen
• LiAlH4 has not been investigated to the same extent
Ca(AlH4)2
Mg(AlH4)2
Be(AlH4)2
KAlH4
KBH4
NaAlH4
NaBH4
LiAlH4
LiBH4
0 2.5 5 7.5 10 12.5 15 17.5 20
Hydrogen density [wt. %]
2 2.5 3 3.51000/T [K
-1]
-2
0
2
4
ln(p
H2/p
o )
NaH
Na3AlH6
NaAlH4
150o C
100o C
50o C
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 2
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. 3
Reaction mechanism
Step 1: LiAlH4 → 1/3Li3AlH6 + 2/3Al + H2
Step 2: 1/3Li3AlH6 → LiH + 1/3Al + 1/2H2
Step 3: LiH → Li + 1/2H2
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 4
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. 5
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. 5
As-received samples
Constant heating rate DSC experiments
140 160 180 200 220 240 260Temperature [
oC]
-3
-2
-1
0
1
2
Hea
t flu
x dQ
/dt [
mW
/mg]
β = 2oC/min
β = 3oC/min
β = 4oC/min
β = 5oC/min
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 6
As-received samples
Kissinger analysis of DSC experiments
1.90 1.95 2.00 2.05 2.10 2.15 2.20 2.251000/T [K
-1]
-12.0
-11.5
-11.0
-10.5
-10.0
ln(β
/T2 )
[--]
EA = 276 kJ/molLiAlH4(s) -> LiAlH4(l)
EA = 81 kJ/molLiAlH4(l) -> Li3AlH6(s) + Al(s) + H2(g)
Li3AlH6(s) -> LiH(s) + Al(s) + H2(g)EA = 107 kJ/mol
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 7
As-received samples
Isothermal kinetics from in situ gravimetry
0 2 4 6 8 10 12 14 16 18 20Time [h]
0
1
2
3
4
5
6
7
Hyd
roge
n re
leas
e [w
t. %
]
0 0.5 1 1.5 20
1
2
3
4
115 oC
132 oC140
oC
152 oC
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 8
As-received samples
Kinetic analysis of isothermal experimentsSimple two-step kinetic model
Wtot = W1 exp (1 − (k1t)η1) + W2 exp (1 − (k2t)
η2)
0 5 10 15 20Time [h]
0
1
2
3
4
5
6
7
Hyd
roge
n re
leas
e [w
t. %
H2]
Exp.Model fit
Activation energies fromArrhenius analysisEA1 = 82 kJ/molEA2 = 90 kJ/mol
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 9
Ball milled samples
Line broadening in XRPD patterns
15 20 25 30 35 40 45 50Diffraction angle 2θ [ ο]
0
200
400
600
800
1000
1200
1400
Inte
nsity
[cou
nts/
s]
BM 2 h 400 rpm
BM 1 h 150 rpm
BM 1 h 400 rpm
BM 6 h 400 rpm
BM 10 h 400 rpm
*
*
Scherrer equation: β ∝1
B
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 10
Ball milled samples
Isothermal dehydrogenation kinetics
0 1 2 3 4 5Time [h]
0
1
2
3
4
5
Hyd
roge
n re
leas
e [w
t %]
BM 10 h 400 rpmBM 6 h 400 rpmBM 2 h 400 rpmBM 1 h 400 rpm BM 1 h 150 rpmAs recieved
0 5 10 15 200
1
2
3
4
5
6
7
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. 11
Ball milled samples
Isothermal dehydrogenation kinetics
0 1 2 3 4 5Time [h]
0
1
2
3
4
5
Hyd
roge
n re
leas
e [w
t %]
BM 10 h 400 rpmBM 6 h 400 rpmBM 2 h 400 rpmBM 1 h 400 rpm BM 1 h 150 rpmAs recieved
0 5 10 15 200
1
2
3
4
5
6
7
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. 11
Ball milled samples
Rate constant of step 1 vs. crystallite size
50 75 100 125 150Crystallite size [nm]
0
1
2
3
4
5
Rat
e co
nsta
nt, k
1 [h-1
]
As-received1 h 150 rpm
2 h 400 rpm
1 h 400 rpm
6 h 400 rpm
10 h 400 rpm
Dependency: k1 ∝1
β2.3
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 12
Ball milled samples
Step 1: Explanation of the k1 ∝1
β2.3 relationship
• Mass transfer limited kinetics?• Nabarro-Herring theory (β2): lattice diffusion?
• Coble theory (β3): grain boundary diffusion?
Step 2: Explanation of the missing k2 vs. βrelationship
• Mass transfer limited kinetics? No!• “Intristic” kinetics is limiting the process?
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 13
Ball milled samples
Step 1: Explanation of the k1 ∝1
β2.3 relationship
• Mass transfer limited kinetics?• Nabarro-Herring theory (β2): lattice diffusion?
• Coble theory (β3): grain boundary diffusion?
Step 2: Explanation of the missing k2 vs. βrelationship
• Mass transfer limited kinetics? No!• “Intristic” kinetics is limiting the process?
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 13
Ti-doped samples
3LiAlH4 + TiCl3 + ball milling → Ti + 3Al + 3LiCl +6H2
15 20 25 30 35 40 45 50Diffraction angle 2θ [
o]
0
500
1000
1500
2000
2500
Inte
nsity
[a.u
.]
BM 1 min@100 rpm
BM 5 min@400 rpm
BM 1 h@400 rpm
Al/LIH
Al/LIH
Li3AlH6
100 120 140 160 180 200 220 240Temperature [
oC]
-2000
-1500
-1000
-500
0
500
1000
Hea
t flu
x [a
.u.]
BM 1 min@100 rpm
BM 1 h@400 rpm
1
2
2
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 14
Ti-doped samples
3LiAlH4 + TiCl3 + ball milling → Ti + 3Al + 3LiCl +6H2
15 20 25 30 35 40 45 50Diffraction angle 2θ [
o]
0
500
1000
1500
2000
2500
Inte
nsity
[a.u
.]
BM 1 min@100 rpm
BM 5 min@400 rpm
BM 1 h@400 rpm
Al/LIH
Al/LIH
Li3AlH6
100 120 140 160 180 200 220 240Temperature [
oC]
-2000
-1500
-1000
-500
0
500
1000
Hea
t flu
x [a
.u.]
BM 1 min@100 rpm
BM 1 h@400 rpm
1
2
2
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 14
Ti-doped samples
Kissinger analysis of DSC experiments
2 2.1 2.2 2.3 2.4 2.51000/T [K
-1]
-11.2
-11
-10.8
-10.6
-10.4
ln(β
/T2 )
[--]
Li3AlH6 -> LiH + Al + H2
EA = 103 kJ/mol
LiAlH4 -> Li3AlH6 + Al + H2
EA = 89 kJ/mol
Dehydrogenation kinetics of Lithium Aluminum Hydride – p. 15