Combined effects of hydrogen back-pressure and NbF_5 addition on the dehydrogenation and rehydrogenation kinetics of the LiBH_4-MgH_2 composite system

摘要

It is well known that the dehydrogenation pathway of the LiBH_4-MgH_2 composite system is highly reliant on whether decomposition is performed under vacuum or a hydrogen back-pressure. In this work, the effects of hydrogen back-pressure and NbF_5 addition on the dehydrogenation kinetics of the LiBH_4-MgH_2 system are studied under either vacuum or hydrogen back-pressure, as well as the subsequent rehydrogenation and cycling. For the pristine sample, faster desorption kinetics was obtained under vacuum, but the performance is compromised by slow absorption kinetics. In contrast, hydrogen back-pressure remarkably promotes the absorption kinetics and increases the reversible hydrogen storage capacity, but with the penalty of much slower desorption kinetics. These drawbacks were overcome after doping with NbF_5, with which the dehydrogenation and rehydrogenation kinetics was significantly improved. In particular, the enhanced kinetics was observed to persist well, even after 9 cycles, in the case of the NbF_5 doped sample under hydrogen back-pressure, as well as the suppression of forming Li_2B_(12)H_(12). Furthermore, the mechanism that is behind these effects of NbF_5 additive on the reversible dehydrogenation reaction of the LiBH_4-MgH_2 system is discussed.