New double-cation

borohydrides for

on-board hydrogen storage

Inge Lindemann, Roger Domènech Ferrer, Yaroslav

Filinchuk, Radovan Černý, Hans Hagemann, Ludwig Schultz,

Oliver Gutfleisch

H2 as energy carrier?

• Highly abundant– Electrolysis of water

• Clean– Oxidation product is H2O

• Longterm energy carrier

• High energy density– 120 MJ/kg vs. 44 MJ/kg petrol

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no H2 with H2

Compaction of H2

• Basically 3 different options:

7.6 wt.% H2 1.4 wt.% H2

MgH2

~35 kg/m3

Liquid H2

(23K, opened system)

~70 kg/m3>100 kg/m3

Solid state H2 storage

Density:

Pressurized H2

Schlapbach & Züttel, Nature 414 (2001), 354

4kg H2

For ~400km

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Requirements on the material

1) High gravimetric H2 density > 6 wt.% H2

2) ΔH ~ 20-40 kJ/mol, 60-120°C at 1bar H2 (PEM fuel

cell)

3) Reversibility

4) Good cycle life

5) Fast kinetics

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• Complex hydrides• Light cation for high hydrogen

content (> 6 wt.% H2)

• But either too stable or too unstable

Conventional:LiBH4, Ca(BH4)2, Al(BH4)3,...

• Electronegativity of cation key for borohydride stability

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Borohydrides

+ -

(BH4)-

M+

Y. Nakamori et al., Phys. Rev. B 2006, 74, 045126

NaAl(BH4)4 14.2 wt.% H2

LiAl(BH4)4 17.2 wt.% H2

• Metathesis by high energy ball milling – 1bar Ar (Fritsch P6)– Different molar ratios: 1:2, 1:3, 1:4, 1:5

– Monitoring of p & T

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Synthesis

(2) AlCl3 + 4 NaBH4 NaAl(BH4)4 + 3 LiCl

(1) AlCl3 + 4 LiBH4 LiAl(BH4)4 + 3 LiCl

Hummelshøj et al., J. Chem. Phys. (2009) 131, 014101

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Al-Li-Borohydride • X-ray diffraction analysis after the milling process

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Structure determination• Structure analysis from SR-PXD of 1:5 powder showed

very different structure than expected

• Usually exchange of cations: LiAl(BH4)4

• Primitive cubic unit cell• Complex framework

Al3Li4(BH4)13

Ideal stoichiometry for metathesis:

1:4.33

[Al(BH4)4]-

[Li4(BH4)]3+

Lindemann et al., Chem. Eur. J. 16 (2010), Issue 29,8707–8712.

• Thermogravimetric and desorption analysis- Desorption analysis in static vacuum- Gravimetric analysis in 1bar Ar

Decomposition analysis

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• Weight loss and gas

desorption at ~70°C

• Powder with ideal stochiometry (1:4.33) shows highest weight loss of about 25 %

• desorption of not only H2 but also B2H6

Decomposition Products?

LiBH4 (h)

Al3Li4(BH4)13

LiBH4 (o)

LiBH4 (o)

• Structural analysis by in-situ Raman spectroscopy

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Na-Al-Borohydride

• Structure determination revealed an orthorombic structure- very high Cl content

NaAl(BH4)xCl4-x

1.0<x<1.43

• Only stable with Cl incorporation

• NaSc(BH4)4 exists

• Al3+ might be too small (in comparison to Sc3+)

• Reduced H2 density- theoretical: 14.2 wt.%- experiment: ~3 wt.%

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Conclusion

• Synthesis of two new double-cation systems

1) Al-Li-Borohydride:

• Structure determination – Unique framework structure within the borohydrides Al3Li4(BH4)13

– primitive cubic unit cell containing complex cations and anions!

• Low Tdec at ~70°C

• During decomposition formation of LiBH4 while releasing B2H6 and

H2

2) Na-Al-Borohydride:

• Structure determination– Only stable by Cl incorporation

– Reduces H2 content (only ~3 wt.%)

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Acknowledgement

IFW Dresden

Dept. 21

especially B.Gebel, M.Herrich, C. Rongeat, C.Geipel

University of Geneva

Radovan Černý (Laboratory of Crystallography)

Hans Hagemann (Dept. of Physical Chemistry)

Catholic University of Leuven Yaroslav Filinchuk (Institute of Condensed Matter and

Nanosciences)

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