Itoko Saita

Hydriding combustion synthesis of Mg2NiH4

Hydriding combustion synthesis (HCS) can produce full hydrides of alloys simply and in a short time. The conventional process based on ingot metallurgy (IM) cannot produce magnesium-based alloy easily with the desired composition and the cast product needs a long activation process for the practical use of hydrogen storage. The purpose of this study was to investigate the various hydrogen storage properties of HCSed Mg2NiH4 in comparison to the cast Mg2Ni. The results suggest that the HCSed Mg2NiH4 has some advantages over the IM product.

Effect of synthesis temperature on the purity of product in hydriding combustion synthesis of Mg2NiH4

Abstract This paper describes the effect of synthesis temperature on hydriding combustion synthesis of Mg 2 NiH 4 to improve the purity of product. The properties of the products were examined by means of X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD patterns showed that the main phases of the products were Mg 2 NiH 4 independently of the synthesis temperatures. However, when the samples were synthesized over 779 K, the eutectic temperature of magnesium and nickel system, most products contained a little Mg 2 NiH 0.3 . In contrast, in the products synthesized under the eutectic temperature, MgH 2 and Ni existed with no Mg 2 NiH 0.3 : all Mg 2 Ni in the combustion synthesized products fully absorbed hydrogen and transferred to Mg 2 NiH 4 . These results revealed that the hydriding activity of the medium product from combustion synthesis of Mg 2 Ni depended on the synthesis temperature. In conclusion, to increase the activity of hydriding reaction and the storage capacity of products, and to avoid the evaporation loss of magnesium during combustion synthesis, a slightly lower synthesis temperature than the eutectic temperature is quite beneficial and available for hydriding combustion synthesis of Mg 2 NiH 4 .

Hydriding combustion synthesis of hydrogen storage alloys of Mg–Ni–Cu system

Abstract Hydriding combustion synthesis of Mg–Ni–Cu system hydrogen storage alloys from metal powder mixture was reported in this study. For studying this new process, the binary system of Mg2Cu was also synthesized at 753 and 798 K under hydrogen/argon atmosphere. The X-ray diffraction (XRD) patterns of the binary system alloy showed a good yield without any un-reacted Mg and Cu and lower synthesis temperature was preferred for avoiding evaporation loss of magnesium and for promoting the hydriding reaction. The XRD patterns of the ternary system alloys, Mg2Ni0.75Cu0.25 and Mg2Ni0.5Cu0.5, showed the catalytic effect of nickel or copper on hydriding reaction because no intermediate products of Mg2Ni and Mg2Cu existed in final product. The hydriding curves of Mg2Ni0.75Cu0.25 showed the hydrogen storage capacity of 1.7 mass% at 473 K after 120 min, although the hydriding rate was not very high. The images of scanning electron microscopy (SEM) showed very interesting character of particle surface like scales of fish, which gave an evidence of eutectic reaction and disproportionation reaction in synthesizing the ternary alloy of Mg-Ni-Cu system.

Kinetic Improvement of Hydrogen Storage Alloy by Generating Nanofissures

Hydrogen storage alloys are attracting worldwide attention as hydrogen suppliers to fuel cells. However, several kinetic problems remain to be solved before practical use is possible. A methodology to accelerate reactions of hydrogenation and dehydrogenation and to lower the reaction temperature is strongly required. In this study, therefore, to overcome these problems, a production method based on hydriding combustion synthesis (HCS) was applied to produce a metallic hydride directly and the kinetics of the product was microscopically studied and compared to the commercially available product based on ingot metallurgy (IM). The results showed that the HCS product was fully charged by hydrogen in the form of Mg 2 NiH 4 just after synthesis and had a very large reaction rate with no activation treatment; only 5 min for full charge. A most interesting result is the high activity of our product: it stored hydrogen even at room temperature. Moreover, transmission electron microscopy (TEM) observation revealed the mechanism of the improved kinetics of the HCS product. Many tree-like nanofissures emerged inside the HCS product just after the first dehydrogenation; in contrast the IM product has no fissures even after three times activation treatment. In conclusion. a methodology has been introduced to improve a hydrogen storage alloy kinetically by generating nanofissures from inside, not from the surface, which could be applied to other hydrogen storage alloys.