Dehydrogenation kinetics and long term cycling behavior of Titanium doped NaAlH 4
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Dehydrogenation kinetics and long term cycling behavior of Titanium doped NaAlH4 Sesha S. Srinivasan1, 2 and Craig M. Jensen1 1 Clean Energy Research Center, College of Engineering, University of South Florida, Tampa, Florida 33620, USA 2 Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA ABSTRACT The development of light weight hydrogen storage systems with high volumetric and gravimetric hydrogen densities is indeed essential for the on-board fuel cell vehicular applications. Titanium doped NaAlH4 is right now considered as the potential hydrogen storage system, which satisfies the said criteria. The dehydrogenation of NaAlH4 consists of two consecutive steps of decomposition at 220 and 250o C with the total hydrogen release of 5.6 wt.%. However, doping a few mole concentrations of selected transition metal complexes to the host hydride reduces significantly the decomposition temperatures to 100 and 185o C (equilibrium H2 pressure ~1 MPa) respectively. This breakthrough has been followed by a great deal of effort to develop NaAlH4 as a practical hydrogen storage material. For an ideal hydrogen storage material, the dehydrogenation kinetics and the cycling stability are important properties to be evaluated. Keeping these points to ponder, we have studied the dehydriding kinetics of the Ti-doped NaAlH4 over a number of dehydrogenation and rehydrogenation cycles. Besides, the Ti-doped NaAlH4 has been prepared from the hydrogenation of NaH and Al using the solvent mediated milling method. Comparing the initial and final cycling stages of Ti doped (NaH + Al), the synchrotron powder x-ray diffraction profiles exhibit, a growing resistance to the hydrogenation of Na3AlH6 to NaAlH4. INTRODUCTION Alkali and alkaline earth metal based complex aluminum hydrides, MAlH4 [M = Na, Li, K] and Mg(AlH4)2, have been found to have great potential as viable modes of storing hydrogen at moderate temperatures and pressures. These hydrides have been demonstrated to have higher hydrogen storage capacities at moderate temperatures and lower cost than conventional intermetallic metal hydride systems such as AB5H6, ABH2, AB2H3, A2BH4-6 [1, 2]. Among the various alkali based complex hydrides investigated in the recent years [3-10], titanium doped sodium aluminum hydride; NaAlH4 has shown the greatest promise as a reversible hydrogen storage material. The decomposition reactions of Ti- doped NaAlH4 to NaH + Al + 3/2 H2 with intermediate stage 1/3 Na3AlH6 + 2/3 Al + H2 have been investigated extensively on the basis of structural phase determination and the release of hydrogen [11]. However, fewer studies have focused on the hydrogenation of mixtures of Na/Al or NaH/Al to NaAlH4. Ashby [12] accomplished the synthesis of NaAlH4 by reacting sodium under high hydrogen pressure (10-35 MPa) and temperature (140-160 °C) in the solvent, tetrahydrofuron (THF), for several hours. Dymova et al [13] found that, in absence of solvent medium, a temperature of at least 280 °C and a hydrogen pressure of 17-18 MPa are necessary to
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