Transmission Electron Microscopy Studies of 5-cycled Na Alanate with Ti Based Additive
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GG2.4.1
Transmission electron microscopy studies of 5-cycled Na alanate with Ti based additive Carmen M. Andrei1, 3, John C. Walmsley2, Randi Holmestad3,*, Gianluigi A. Botton1, Sesha S. Srinivasan4, Craig M. Jensen4, and Bjorn C. Hauback5 1 Department of Materials Science and Engineering, McMaster University, Hamilton, ON, L8S 4M1, Canada 2 SINTEF Materials and Chemistry, NO-7465 Trondheim, Norway 3 Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway 4 Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA 5 Institute for Energy Technology, P.O. Box 40, NO-2007 Kjeller, Norway ABSTRACT Ti doped NaAlH4 hydride is proposed as a reversible hydrogen storage material. In this work, the microstructure of NaAlH4 with 2% TiCl3 additive was studied after 5 hydrogen cycles using a combination of transmission electron microscopy (TEM) techniques including energy dispersive spectroscopy (EDS) X-ray analysis. Selected area diffraction and high-resolution (HR) imaging confirmed the presence of the NaH phase in the material. Electron diffraction was dominated by Al. HRTEM showed the presence of edge dislocations, which might influence the hydrogen diffusivity process in these materials. INTRODUCTION The use of proton exchange membrane (PEM) fuel cell cars, which can significantly reduce emissions and increase energy efficiency, has been a research focus subject in the last few years. Complex hydrides of aluminum-alanates are one of the most promising classes of hydrogen storage materials. Among these materials, sodium alanate, NaAlH4 is the only known complex hydride, which has acceptable gravimetric storage capacity and favorable thermodynamics for PEM fuel cell systems. Sodium alanate releases hydrogen through a series of decomposition-recombination reactions given by: (1) NaAlH4 ↔1/3 Na3AlH6 +2/3 Al +H2 (2) 1/3 Na3AlH6 ↔ NaH +1/3 Al +1/2 H2 (3) NaH ↔Na +1/2 H2 The first two reactions give a theoretical reversible hydrogen storage capacity of 5.6 wt%, although in practice only about 5 wt% has been reported. The third reaction occurs at too high temperature to be considered practical for hydrogen storage applications. Interest in NaAlH4 dramatically increased following the work of Bogdanovic and Schwickardi who reported in 1997 that improved kinetics of the hydrogenation /dehydrogenation processes could be achieved by the addition of a few mol percent of Ti compounds [1]. Mechanical mixing of the titanium catalyst with NaAlH4 improves the hydrogenation/dehydrogenation and cycling processes [2, 3]. Chlorides of vanadium, zirconium and several lanthanide [3, 4], Ti clusters [5] and recently single wall carbon nanotube [6] have been investigated as catalysts on sodium alanates, but there is still no clear understanding regarding the catalyst state upon doping and hydrogen cycling.
GG2.4.2
Several experimental techniques such as X-ray diffraction [7-11, 18], neutron diffraction [7, 12], Mössbauer spectroscopy [4], scanning electron microscopy [4, 13], vibrational spect
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