Carbon nanomaterial-assisted morphological tuning for thermodynamic and kinetic destabilization in sodium alanates

In the present work, we develop an effective strategy, i.e., carbon nanomaterial-assisted morphological tuning, for both thermodynamic and kinetic destabilization in complex hydrides based on the interaction between the complex anion and the carbon matrix. The NaAlH4/carbon nanomaterials of graphene nanosheets (GNs), fullerene (C-60) and mesoporous carbon (MC) were selected as model systems for illustrating the positive effect of carbon nanomaterial-assisted morphological tuning. It is demonstrated that through the dissolution-recrystallization process, the morphologies of NaAlH4 can be altered from the scale-like continuous structure for the GN-assisted sample, to flower-like structures with diameters ranging from 5 to 10 mu m for the C-60-assisted sample and to uniform particles with an average diameter of about 2 mu m for the MC-assisted sample. Correspondingly, the onset temperature for dehydrogenation of NaAlH4 is reduced to about 188, 185 and 160 degrees C for the samples assisted with GNs, C-60 and MC, respectively, much lower than 210 degrees C for the pristine sample. A remarkable reduction in activation energy for three-step dehydrogenation is also obtained in NaAlH4/carbon nanomaterial composites relative to the pristine sample, and the improved efficiency of carbon nanomaterials for kinetics is found to be in the order of MC > C-60 > GNs. These positive improvements can be attributed to both the particle refinement and interaction between NaAlH4 and the carbon nanomaterial that are in intimate contact with each other, which are not only evidenced by FE-SEM observation, but also supported by Al-27 solid-state NMR characterization.