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

Abstract

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 (C₆₀) 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 μm for the C₆₀-assisted sample and to uniform particles with an average diameter of about 2 μm for the MC-assisted sample. Correspondingly, the onset temperature for dehydrogenation of NaAlH₄ is reduced to about 188, 185 and 160 °C for the samples assisted with GNs, C₆₀ and MC, respectively, much lower than 210 °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 > C₆₀ > 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 ²⁷Al solid-state NMR characterization.