Mechano-Chemical Synthesis and Characterization of New Complex Hydrides for Hydrogen Storage
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0927-EE02-06
Mechano-Chemical Synthesis and Characterization of New Complex Hydrides for Hydrogen Storage Sesha Srinivasan, Luis Rivera, Elias Stefanakos, and Yogi Goswami Clean Energy Research Center, College of Engineering, University of South Florida, Tampa, FL, 33620 Abstract Mechano-chemical synthesis procedure has been adopted to prepare new light weight complex borohydrides. The thermal calorimetric and gravimetric analysis of these hydrides has shown a hydrogen decomposition temperature (Tdec) below 150° C with theoretical capacity of ~8.0-10.0 wt%. The catalysts (e.g. ZnCl2, TiF3) doping and destabilization by reacting with binary hydride (MgH2) reveal the enhancement of decomposition kinetics and reversible dehydrogenation-rehydrogenation behavior of these complex borohydrides. Introduction Light weight, low cost, high reversible hydrogen storage systems are indeed essential for realizing the low temperature PEM fuel cells powered vehicles [1, 2]. The breakthrough discovery of Ti- catalyzed NaAlH4 [3, 4] for the reversible onboard hydrogen storage seems may not be the ideal system to attain the DOE 2010 and FreedomCAR technical targets [5, 6]. This is because for NaAlH4, the usable hydrogen storage capacity achievable is 5.6 wt.%, which is well below the set DOE goals. On the other hand, the borohydride complexes such as NaBH4 and LiBH4 possess high hydrogen storage capacity of 13.0 wt.% and 19.6 wt.% respectively [7]. However, the release of hydrogen from NaBH4 is possible only by hydrolysis (reaction with H2O) and this process is irreversible [8]. For the case of LiBH4, the catalytic addition of SiO2, significantly enhances its thermal desorption at 200° C [9]. The incorporation of various metal oxides such as TiO2, V2O5 etc. have been shown to improve the reversible hydrogen sorption characteristics of LiBH4 [10]. In any case, the thermal decomposition and/or recombination of hydrogen either from NaBH4 or LiBH4 accompanies with the thermodynamic and kinetic limitations due to strong B-H interactions [11-14]. A new transition metal assisted complex borohydrides, Zn(BH4)2 is recently reported with the high theoretical hydrogen storage capacity of 8.4 wt.% at the thermal decomposition temperature of 85° C [14]. The preparation of Zn(BH4)2 via wet chemical synthesis route [15] or mechanical milling [16, 17] is being investigated according to the stoichiometric reaction, 2NaBH4 + ZnCl2
Zn(BH4)2 + 2NaCl
(1)
Zn(BH4)2 thermally decomposes into the constituent elements and releases hydrogen, Zn(BH4)2
Zn + 2B + 4H2
(2)
The present paper aims to systematically study the synthesis and characterization of new Zn- based complex borohydrides, Zn(BH4)2 for the efficient hydrogen storage. The active
hydrogen content of 8.4 wt.% have been obtained at 100° C for the hydride milled for only 15 minutes; this capacity however decreases with increasing milling duration. For the case of complex mixture, (2LiBH4+ZnCl2), the gravimetric capacity of 10.8 wt.% is achieved at temperature below 120° C. We have also demonstr
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