The lightweight compound material NaNH-NaBH is regarded as a promising hydrogen storage composite due to the high hydrogen density. Mechanical ball milling was employed to synthesize the composite NaNH-NaBH (2/1 molar ratio), and the samples were investigated utilizing thermogravimetric-differential thermal analysis-mass spectroscopy (TG-DTA-MS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analyses. The full-spectrum test (range of the ratio of mass to charge: 0-200) shows that the released gaseous species contain H, NH, BH, and N in the heating process from room temperature to 400 °C, and possibly the impurity gas BH also exists. The TG/DTA analyses show that the composite NaNH-NaBH (2/1 molar ratio) is conductive to generate hydrogen so that the dehydrogenation process can be finished before 400 °C. Moreover, the thermal decomposition process from 200 to 400 °C involves two-step dehydrogenation reactions: (1) Na(NH)BH hydride decomposes into NaBN and H (200-350 °C); (2) remaining Na(NH)BH reacts with NaBH and NaBN, generating Na, BN, NH, N, and H (350-400 °C). The better mechanism understanding of the thermal decomposition pathway lays a foundation for tailoring the hydrogen storage performance of the composite complex hydrides system.
The composites of M(BH) -LiNH (1/2 n molar ratio, n = 1 or 2, M = Ca, Mg, Li) were synthesized by liquid ball milling. Samples were characterized by X-ray diffraction, thermogravimetry-differential thermal analysis-mass spectroscopy (TG-DTA-MS), and kinetic models (Achar differential/Coats-Redfern integral method). The higher-electronegativity metal M in M(BH) -4LiNH (M = Ca, Mg) samples not only enables [BH] group to release easily, so as to facilitate the interaction of [BH] and [NH] groups, but also restrains the NH release and slightly decreases the onset dehydrogenation temperature concluded by TG-MS. Moreover, in stage 1 (200-350 °C), the kinetics performances of M(BH) -4LiNH (M = Ca, Mg) samples are distinctly improved, that is, the activation energies of them are reduced by ca. 30% compared to those of sample LiBH-2LiNH. The outstanding contribution of the replacement of M(BH) with high-electronegativity metal ion is to both improve the kinetics performance by changing the kinetics mechanism and decrease the temperature range of the initial dehydrogenation region.
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