New double-cation borohydrides for on-board hydrogen storage Inge Lindemann, Roger Domènech Ferrer, Yaroslav Filinchuk, Radovan Černý, Hans Hagemann, Ludwig Schultz, Oliver Gutfleisch

H 2 as energy carrier? Highly abundant –Electrolysis of water Clean –Oxidation product is H 2 O Longterm energy carrier High energy density –120 MJ/kg vs. 44 MJ/kg petrol 2 no H 2 with H 2

Compaction of H 2 Basically 3 different options: 7.6 wt.% H wt.% H 2 MgH 2 ~35 kg/m 3 Liquid H 2 (23K, opened system) ~70 kg/m 3 >100 kg/m 3 Solid state H 2 storage Density: Pressurized H 2 Schlapbach & Züttel, Nature 414 (2001), 354 4kg H 2 For ~400km 3

Requirements on the material 1)High gravimetric H 2 density > 6 wt.% H 2 2)ΔH ~ kJ/mol, °C at 1bar H 2 (PEM fuel cell) 3)Reversibility 4)Good cycle life 5)Fast kinetics 4

Complex hydrides Light cation for high hydrogen content (> 6 wt.% H 2 ) But either too stable or too unstable Conventional: LiBH 4, Ca(BH 4 ) 2, Al(BH 4 ) 3,... Electronegativity of cation key for borohydride stability 5 Borohydrides + - (BH 4 ) - M+M+ Y. Nakamori et al., Phys. Rev. B 2006, 74, NaAl(BH 4 ) wt.% H 2 LiAl(BH 4 ) wt.% H 2

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 6 Synthesis (2) AlCl NaBH 4 NaAl(BH 4 ) LiCl (1) AlCl LiBH 4 LiAl(BH 4 ) LiCl Hummelshøj et al., J. Chem. Phys. (2009) 131,

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

8 Structure determination Structure analysis from SR-PXD of 1:5 powder showed very different structure than expected Usually exchange of cations: LiAl(BH 4 ) 4 Primitive cubic unit cell Complex framework Al 3 Li 4 (BH 4 ) 13 Ideal stoichiometry for metathesis: 1:4.33 [Al(BH 4 ) 4 ] - [Li 4 (BH 4 )] 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 9 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 H 2 but also B 2 H 6

Decomposition Products? LiBH 4 (h) Al 3 Li 4 (BH 4 ) 13 LiBH 4 (o) Structural analysis by in-situ Raman spectroscopy 10

Na-Al-Borohydride Structure determination revealed an orthorombic structure - very high Cl content NaAl(BH 4 ) x Cl 4-x 1.0<x<1.43 Only stable with Cl incorporation NaSc(BH 4 ) 4 exists Al 3+ might be too small (in comparison to Sc 3+ ) Reduced H 2 density - theoretical: 14.2 wt.% - experiment: ~3 wt.% 11

Conclusion Synthesis of two new double-cation systems 1) Al-Li-Borohydride: Structure determination –Unique framework structure within the borohydrides Al 3 Li 4 (BH 4 ) 13 –primitive cubic unit cell containing complex cations and anions! Low T dec at ~70°C During decomposition formation of LiBH 4 while releasing B 2 H 6 and H 2 2) Na-Al-Borohydride: Structure determination –Only stable by Cl incorporation –Reduces H 2 content (only ~3 wt.%) 12

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) 13