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 NbH_0.89 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 NbH_0.89 and the mesoporous carbon material CMK-3 played synergistic roles in improving the dehydrogenation performance of NaAlH₄.