Metal Oxide Modified Lithium Borohydrides for Reversible Hydrogen Storage*
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Metal Oxide Modified Lithium Borohydrides for Reversible Hydrogen Storage* Ming Au** and Arthur Jurgensen Savannah River National Laboratory, Aiken, SC 29808, USA Abstract The lithium borohydride has been modified by ball milling with metal oxides and metal chlorides as the additives. The modified lithium borohydrides released 9 wt% hydrogen starting from 473K. The dehydrided modified lithium borohydrides absorbed 7-9 wt% hydrogen at 873K and 7 MPa. The modification with additives reduced the dehydriding starting temperature from 673K to 473K and moderated the rehydrogenation conditions from 923K/15 MPa to 873K/7 MPa. XRD and SEM analysis revealed the formation of the intermediate compound that may play a key role in changing the reaction path resulting in the lower dehydriding temperature and reversibility. The additives reduced the dehydriding temperature and improve the reversibility, but it also reduced the hydrogen storage capacity. Key Words Lithium borohydride, Hydrogen, Storage, Reversible, Hydrogenation, Dehydrogenation 1.
Introduction
Alkali metal borohydrides such as LiBH4 and NaBH4 are lightweight materials that contain large amounts of hydrogen (18.5wt%, 121 kg/m3 and 10.6wt%, 98.7 kg/m3, respectively). The metal borohydrides have great potential to meet or exceed US DOE’s bench mark for transportation applications (7.5 wt% and 65 kg/m3 [1]). Borohydrides are also relatively inexpensive and can be produced from abundant natural borax deposits [2,3]. Unfortunately, these materials are very stable. For example, heating above 673 K that is above its melting point, is required to release the majority of the hydrogen from LiBH4 [4]. Also, the rehydrogenation conditions are too harsh for practical application. However, the solid hydrogen storage potential of these materials warrants investigation into possible methods to resolve or alleviate these two major problems. Fedneva et al [4] investigated LiBH4 by DTA (Differential Thermal Analysis). The thermogram of LiBH4 showed three endothermic effects: at 381-385 K, 541-559K and 756-765K. The endothermic effect at 381-385K is reversible and corresponds to the LiBH4 polymorphic transformation. The second peak at 541-559K corresponds to the LiBH4 fusion, accompanied by a slight decomposition, which liberates approximately 2% of the hydrogen in the compound. The main evolution of gas starts at 653K and liberates 80% in the hydrogen in LiBH4. In 1980, Alain Muller[5] reported that 5 gram of LiBH4 decomposed and released 13.8wt% hydrogen at 723K and 10-2 mmHg (1.3 Pa) pressure within 24 hours by the reaction LiBH4 LiH + B + 3/2 H2 with reaction enthalpy of – * Full paper has been published in Journal of Physical Chemistry B, 110, 7062-7067 ** Corresponding author: (T)803-819-8442, (F)803-819-8432, [email protected]
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24.8 kcal/mol H2. After dehydrogenation, the sample was able to absorb 11.8wt% (6.65 liter) of hydrogen at 923K and 15MPa within 48 hours. To improve the dehydrogenation and subsequent rehydrogenation processes, 10wt% a
