Intensive investigation on hydrogen storage properties and reaction mechanism of the NaBH4-Li3AlH6 destabilized system

Abstract

Notable effects of Li3AlH6 on the hydrogen storage properties of the NaBH4 are studied intensively. Li3AlH6 is synthesized by milling 2LiH-LiAlH4 mixture for 12 h. The best molar ratio of the NaBH4- Li3AlH6 destabilized system is 1:1 which has decomposed at two stages; Li3AlH6 decomposition stage at 170 °C and NaBH4decomposition stage at 400 °C. As no significant effect on the decomposition temperature between 1 h and 24 h of milling time can be observed, the 1-hour milling preparation method is selected for the characterization. Isothermal absorption has shown that the system is able to absorb 4.2 wt% and 6.1 wt% of hydrogen in 60 min at 330 °C and 420 °C under 30 atm of hydrogen pressure. In contrast, only about 3.4 wt% and 3.7 wt% of hydrogen can be absorbed by the milled NaBH4 under a similar condition. Meanwhile, the system is able to desorb 2.0 wt% and 4.1 wt% of hydrogen in 60 min at 330 °C and 420 °C in isothermal desorption while only 0.3 wt% and 2.1 wt% can be released by the milled NaBH4 under the similar condition. The decomposition activation energy and enthalpy of the NaBH4 stage are calculated to be 162.1 kJ/mol and 68.1 kJ/mol H2. Based on the X-ray diffraction analysis, Na, Al and AlB2 are formed during the dehydrogenation process. The formation of Al and AlB2 are the keys to the improvement of hydrogenation properties. It is concluded that Li3AlH6 is a good destabilizing agent for the NaBH4 system.