Andr Dtu 260805

Dehydrogenation kinetics ofLithium Aluminum Hydride

Anders Andreasen

[email protected]

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


Motivations




• 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


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


Motivations




• 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


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


Outline

• Mechanism of dehydrogenation• Basic properties• Dehydrogenation of as-received samples• Effect of ball milling• Effect of catalysis by Ti


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


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


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


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


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


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


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


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


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


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


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


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?


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?


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


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


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


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?


Technology

Andr Dtu 260805