1. Reversibility in LiBH 4 -MgH 2 -Al composite systems Bjarne R. S. Hansen a, Dorthe B. Ravnsbæk a, Jørgen Skibsted b, Carsten Gundlach c & Torben R. Jensen a a Center for Materials Crystallography and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark b Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark c MAX-II laboratory, Lund University, Ole Römers väg 1, 223 63, S-22100 Lund, Sweden Abstract LiBH 4 is an interesting hydrogen storage material, as it has a high gravimetric hydrogen content of 18.5 wt% [1]. However, the utilization is hampered by lack of reversibility and high decomposition temperatures. One way of improving metalborohydrides, is the utilization of reactive hydride composites, which can be achieved by adding for instance Al [2] or MgH 2 [3]. By adding both MgH 2 and Al the LiBH 4 -MgH 2 -Al composite further improves the hydrogen release and uptake properties of LiBH 4 [4]. In this study the decomposition reactions of LiBH 4 -MgH 2 -Al in molar ratios (4:1:1) and (4:1:5) are investigated. The study shows a complex decomposition and reversibility studies reveal formation of Li 2 B 12 H 12, which decrease the reversible hydrogen storage capacity. Acknowledgements and references Sincere acknowledgements are directed to iNANO, Danscatt, MAX-Lab Bor4Store (EU) and Center for Materials Crystallography (CMC) Mechanochemical Synthesis: Fritsch Pulverisette no. 4 with main disk speed: 400 rpm., Relative ratio: -2.25, Mill time: 5 min, Pause time: 2 min, Ball-to- Powder mass ratio: 35:1, Repetitions: 23 (Total mill time: 120 min). [1] L. Schlapbach, Nature 2009, 460, 809-811; [2] D. B. Ravnsbæk, T. R. Jensen, J. Appl. Phys. 2012, 111, 112621 ; [3] U. Bösenberg, S. Doppiu, L. Mosegaard, G. Barkhordarian, N. Eigen, A. Borgschulte, T. R. Jensen, Y. Cerenius, O. Gutfleisch, T. Klassen, M. Dornheim, Acta Materialia 2007, 55, 3951–3958; [4] Y. Zhang, Q. Tian, H. Chu, J. Zhang, L. Sun, J. Sun, Z. Wen, J. Phys. Chem. C 2009, 113, 21964–21969, [5] Zhong Y., Yang M., and Liu Z.K., CALPHAD: Comput. Coupling Phase Diagrams Thermochem., Vol. 29, 2005, p 303-311 Thermal analysis (comparison) At T = 300 °C the decomposition of MgH 2 is observed in sample LiBH 4 -MgH 2 -Al (4:1:1), but not in the (4:1:5)-sample, although a release of hydrogen is observed in the MS data for both samples. The formation of MgAlB 4 is observed as a broad endotherm and the formation of LiAl is seen at T = 410 °C. In LiBH 4 -MgH 2 -Al (4:1:1) and LiBH 4 - MgH 2 (2:1) the formation of LiMg is observed. Thermal observations follow the in situ SR-PXD measurements precisely. Temperature (°C)T onset T peak T end LiBH 4 -Al (2:1)390425470 LiBH 4 -Al (2:3)325375450 LiBH 4 -MgH 2 (2:1)350420440 (565) LiBH 4 -MgH 2- Al (4:1:1)275415440 (550) LiBH 4 -MgH 2- Al (4:1:5)275350/400485 λ = 1.103671 Å λ = 1.102050 Å * = spinning sidebands from satellite transitions LiBH 4 -MgH 2 -Al (4:1:5) After 3 PCT cycles λ = 1.54056 Å 11 B MAS NMR S2, LiBH 4 -MgH 2 -Al (4:1:5) after 3 PCT cycles Centerband resonance from LiBH 4 (-41.3 ppm) Centerband resonance consistent with Li 2 B 12 H 12 (-9 ppm) * * * * * * * * * ** * Comparison with the LiBH 4 -Al and LiBH 4 - MgH 2 samples show that the onset tempe- rature for hydrogen release is lowered up to ~75 °C. Reversibility PCT desorptions of the LiBH 4 -MgH 2 -Al samples show a decrease in hydrogen storage capacity. The release related to MgAlB 4 is decreasing the most. Hence, the capacity drop is likely related to a boron-containing species. In both samples the 11 B MAS NMR spectra after three hydrogen release and uptake cycles show a centerband resonance from LiBH 4, and a chemical shift consistent with Li 2 B 12 H 12 which was not detected by PXD. Li 2 B 12 H 12 was also observed by FT-IR in the cycled samples. This stable compound is not rehydrogenated at the used conditions. S1: LiBH 4 -MgH 2 -Al (4:1:1) S2: LiBH 4 -MgH 2 -Al (4:1:5) LiBH 4 -MgH 2 -Al (4:1:1) After 3 PCT cycles λ = 1.54056 Å More information: brsh@chem.au.dk λ = 1.54056 Å LiBH 4 -MgH 2 -Al (4:1:1) More Al  lower T onset No B 2 H 6 detected ATR-FTIR Hence, Li 2 B 12 H 12 is likely responsible for the hydrogen storage capacity loss in the samples. λ = 1.54056 Å LiBH 4 -MgH 2 -Al (4:1:5) Decomposition reactions LiBH 4 -MgH 2 -Al (4:1:1, S1)LiBH 4 -MgH 2 -Al (4:1:5, S2) Formation of Mg 17 Al 12 No formation of Mg 17 Al 12 S2 S1 [5] 3 CYCLES