| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 101 === In this study, the dehydrogenation performance of the synthesized magnesium aluminum hydride (Mg(AlH4)2) was explored. Besides, the dehydrogenation temperature and the amount of H2 released of Mg(AlH4)2 were improved by incorporating additives. Mg(AlH4)2 was investigated because of its high gravimetric H2 density of 9.3 wt%, initial dehydrogenation temperature of 140–200 °C and high-purity emmsion, which made it a potential application in the mobile device or vehicle. Mg(AlH4)2 and by-product NaCl can be mechano-chemically synthesized by milling NaAlH4 and MgCl2. The microstructure of the synthesized Mg(AlH4)2 was examined using scanning electron microscope. Ex-situ X-ray and in-situ synchrotron X-ray diffraction analyses were employed to identify the crystal structure of synthesized Mg(AlH4)2 after synthesis and during dehydrogenation process, respectively. Futhermore, the corresponding dehydrogenation performance was evaluated using thermogravimetric analyzer.
The experimental results showed that the milling time and ball-to powder weight ratio during the milling process should be properly controlled to avoid incomplete synthesis or premature dehydrogenation. Specifically, the residual NaAlH4 would be present due to the incomplete synthesis, or the dehydrogenated products would form caused by the premature dehydrogenation of the synthesized Mg(AlH4)2 for the prolonged milling time. After 0.5–2 h milling, the highest purity of the synthesized Mg(AlH4)2 experienced a two-step dehydrogenation including Mg(AlH4)2 → β-MgH2 + 2 Al + 3 H2 from 170 °C and β-MgH2 + 2 Al → 0.5 Mg2Al3 + 0.5 Al(Mg) + H2 from 305 °C, respectively. The largest amount of H2 released was 2.75 wt%. The initial dehydrogenation temperature was lowered by metathesizing NaAlH4 into Mg(AlH4)2, namely from 210 °C to 170 °C.
To explore the synergistic effect of NaAlH4 on the dehydrogenation behavior of Mg(AlH4)2, the various mole ratio of NaAlH4–Mg(AlH4)2 mixtures were also fabricated by MCAS. The results showed that NaAlH4 not only facilitates the first step dehydrogenation of Mg(AlH4)2 in lowering its initial dehydrogenation temperature but also increases the total amount of H2 released. Besides, MgH2 and/or Al phases, the products of the first step dehydrogenation reaction, play a catalytic role in lowering the initial dehydrogenation temperature of NaAlH4. The synthesized NaAlH4–Mg(AlH4)2 mixture has an initial dehydrogenation temperature as low as 125 °C, and is able to release 3.55 wt% H2 below 350 °C. The formation of NaMgH3 suggests the changed reaction pathways. The self-catalytic dehydrogenation behavior of the NaAlH4–Mg(AlH4)2 mixture was elaborated in this study with the aid of in-situ synchrotron XRD.
Multi-walled carbon nanotubes (MWCNTs) were also admixed into the 0.5 h-synthesized Mg(AlH4)2 by mechanical milling, and the effects on dehydrogenation properties were also explored. The addition of 5 wt% MWCNTs demonstrated a significant reduction of the initial dehydrogenation temperature from 170 °C to 70 °C. However, with an increasing addition of MWCNTs into the synthesized Mg(AlH4)2, the hydrogen desorption capacity diminished due to the partial hydrogen release induced by the catalytic decomposition during mechanical milling. The well-dispersed hydride particles, shortened hydrogen gas diffusion path, and electronegative characteristic of carbon nanotubes by adding MWCNTs are responsible for the lowered dehydrogenation temperature of Mg(AlH4)2.
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