Synergistic effects played by CMK-3 and NbF5 co-additives on de/re-hydrogenation performances of NaAlH4

In the present work, a strategy for simultaneously reducing the thermal stability of NaAlH4 and enhancing its dehydrogenation kinetics was suggested by means of synergistic effects from co-additives of mesoporous carbon material CMK-3 and NbF5. The ball milled NaAlH4 + 10 wt% (NbF5 + CMK-3) (NbF5: CMK-3 = 1:1 in weight ratio) composite can liberate hydrogen at an onset temperature of 358 K, which was drastically decreased by 93 K from that of pristine NaAlH4. By means of Kissinger's method, the activation energy of NaAlH4 + 10 wt% (NbF5 + CMK-3) can be identified as 99.2 kJ mol−1, which was greatly reduced from that of pristine NaAlH4 (121 kJ mol−1). Investigations on the dehydrogenation process revealed that CMK-3 was beneficial to reducing the particle size of NaAlH4 during ball milling, while NbF5 was actively involved in the decomposition of NaAlH4 and yielded some Nb-relevant intermediate phases NbH0.89 during the heating process. The modified dehydrogenation pathway of NaAlH4 also results in the destabilization of dehydrogenation by 2.13 kJ mol−1 H2 from that of pristine NaAlH4. During the hydrogenation process, the NbH0.89 and the mesoporous carbon material CMK-3 played synergistic roles in improving the dehydrogenation performance of NaAlH4.