Abstract

1. MechanochemicalSynthesis: FritschPulverisette no. 4 with main disk speed: 400 rpm., Relative ratio: -2.25, Mill time: 5 min, Pause time: 2 min, Ball-to-Powdermass ratio: 35:1, Repetitions: 23 (Total mill time: 120 min). Decomposition reactions LiBH4-MgH2-Al (4:1:1, S1) LiBH4-MgH2-Al (4:1:5, S2) λ = 1.102050 Å λ = 1.103671 Å • LiBH4 + MgH2 + Al  •  xLi0.3Mg0.7 + yMg17Al12+ zH2 • MgAlB4 + 4LiH + 7H2(LiAl) 4 LiBH4 + MgH2 + Al  MgAlB4 + 4 LiH + 7 H2 4 LiH + 4 Al  4 LiAl + 2 H2 S2 S1 Reversibility in LiBH4-MgH2-Al composite systemsBjarne R. S. Hansena, Dorthe B. Ravnsbæka, JørgenSkibstedb, CarstenGundlachc &Torben R. Jensenaa Center for Materials Crystallography and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, DenmarkbInstrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmarkc MAX-II laboratory, Lund University, Ole Römersväg 1, 223 63, S-22100 Lund, Sweden Formation of Mg17Al12 No formation of Mg17Al12 [5] Thermal analysis (comparison) At T = 300 °C the decomposition of MgH2 is observed in sample LiBH4-MgH2-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 MgAlB4 is observed as a broad endotherm and the formation of LiAl is seen at T = 410 °C. In LiBH4-MgH2-Al (4:1:1) and LiBH4-MgH2 (2:1) the formation of LiMg is observed. Thermal observations follow the in situ SR-PXD measurements precisely. Comparison with the LiBH4-Al and LiBH4-MgH2 samples show that the onset tempe-raturefor hydrogen release is lowered up to ~75 °C. More Al  lower Tonset Abstract LiBH4 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 MgH2 [3]. By adding both MgH2 and Al the LiBH4-MgH2-Al composite further improves the hydrogen release and uptake properties of LiBH4 [4]. In this study the decomposition reactions of LiBH4-MgH2-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 Li2B12H12, which decrease the reversible hydrogen storage capacity. No B2H6 detected Reversibility PCT desorptions of the LiBH4-MgH2-Al samples show a decrease in hydrogen storage capacity. The release related to MgAlB4is decreasing the most. Hence, the capacity drop is likely related to a boron-containing species. S1: LiBH4-MgH2-Al (4:1:1) S2: LiBH4-MgH2-Al (4:1:5) ATR-FTIR 4 LiBH4 + Mg+ Al  MgAlB4 + 4 LiH + 6 H2 11B MAS NMR Centerband resonance from LiBH4 (-41.3 ppm) S2, LiBH4-MgH2-Al (4:1:5) after 3 PCT cycles MgH2 Mg + H2 4 LiH + 4 Al  4 LiAl + 2 H2 Centerband resonance consistent with Li2B12H12 (-9 ppm) * = spinning sidebands from satellite transitions In both samples the 11B MAS NMR spectra after three hydrogen release and uptake cycles show a centerband resonance from LiBH4, and a chemical shift consistent with Li2B12H12which was not detected by PXD. Li2B12H12was also observed by FT-IR in the cycled samples. This stable compound is not rehydrogenated at the used conditions. * * * * * * * * * * * * 3 CYCLES LiBH4-MgH2-Al (4:1:1) LiBH4-MgH2-Al (4:1:1) After 3 PCT cycles λ = 1.54056 Å LiBH4-MgH2-Al (4:1:5) λ = 1.54056 Å LiBH4-MgH2-Al (4:1:5) After 3 PCT cycles λ = 1.54056 Å λ = 1.54056 Å Hence, Li2B12H12 is likely responsible for the hydrogen storage capacity loss in the samples. 3 CYCLES More information: brsh@chem.au.dk Acknowledgements and references Sincereacknowledgementsare directed to iNANO, Danscatt, MAX-Lab Bor4Store (EU) and Center for Materials Crystallography (CMC) [1] L. Schlapbach, Nature2009, 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 Materialia2007, 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