Improved Hydrogen Storage Properties of NaBH4 Destabilized by CaH2 and Ca(BH4)2
NaBH4, with a 10.6 wt % theoretical H2 capacity, is a promising hydrogen storage candidate material. However, the high thermodynamic stability and slow H-exchange kinetics have to be improved for its practical use. In this study, NaBH4 was destabilized by using CaH2 or Ca(BH4)2. Temperatureprogrammed desorption experiments revealed a significant improvement in the decomposition temperature and the rate of desorption, as well as the weight percentage of hydrogen released from composites with the additive upon heating to 5000C. X-ray diffraction has confirmed the formation of CaB6 upon dehydrogenation, which stabilizes the dehydrogenated state of boron and, therefore, destabilizes the NaBH4. Interestingly, as a source of CaH2, the addition of Ca(BH4)2 results in superior hydrogen desorption performance compared with that of CaH2. To fully characterize this composite system, xNaBH4 þ Ca(BH4)2 composites, for several x values between 1 and 20, were systematically investigated by a series of dehydrogenation and structural analyses. Furthermore, partial reversibility of the NaBH4/Ca(BH4)2 composite with and without NbF5 catalyst was also confirmed under moderate conditions. The mechanism underlying the characteristic enhancement in the NaBH4/CaH2 and NaBH4/Ca(BH4)2 is also discussed.