quan Li

Activation behaviors of Mg2NiH4 at different hydrogen pressures in hydriding combustion synthesis

Abstract For the industrialization of the process of hydriding combustion synthesis of Mg2NiH4, the activation behaviors of Mg2NiH4 at different hydrogen pressures were measured by means of DSC (differential scanning calorimeter) and XRD (X-ray diffraction). The profiles of heat flow of DSC were very different at 0.5, 1.0, 2.0 and 4.0MPa hydrogen pressures within three cycles of temperature scanning at a rate of 0.1 K / s in hydriding combustion synthesis of Mg2NiH4. It is apparent that an activation behavior existed in the hydriding combustion synthesis since the height of DSC peak from the hydriding reaction of Mg2NiH4 increased with the cycling of temperature scanning. The pressure of hydrogen strongly affected this kind of an activation. The peak height increased as hydrogen pressure increased. It reached maximum values after the second cycle of temperature scanning at 4.0MPa of hydrogen pressure and after the third cycle at 2.0MPa. The patterns of XRD revealed that pure products of Mg2NiH4 were obtained after the third cycle of temperature scanning at 2.0MPa of hydrogen pressure and after the second cycle at 4.0MPa. These results are significant for developing the industrial process of hydriding combustion synthesis of hydrogen storage alloy Mg2NiH4 within only one temperature scanning without any activation process.

Reaction mechanism of hydriding combustion synthesis of Mg2NiH4

Abstract A previous study suggested that the process of combustion synthesis of Mg 2 NiH 4 from the compact of magnesium and nickel mixture in pressurized hydrogen cannot be expressed by a single reaction formula such as 2Mg+Ni+2H 2 =Mg 2 NiH 4 . In this paper, our attention is addressed to studying the reaction mechanism during the process of hydriding combustion synthesis of Mg 2 NiH 4 by means of a differential scanning calorimeter (DSC) and an X-ray diffractometer (XRD). The results show that this process is consisted of seven distinguished reactions: (1) Mg+H 2 →MgH 2 ; (2) MgH 2 →Mg+H 2 ; (3) 2Mg+Ni→Mg 2 Ni; (4) 2Mg+Ni→Mg 2 Ni (L); (5) Mg 2 Ni+0.15H 2 →Mg 2 NiH 0.3 ; (6) Mg 2 Ni+2H 2 →Mg 2 NiH 4 (HT); and (7) Mg 2 NiH 4 (HT)→Mg 2 NiH 4 (LT).

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 .

Activity and capacity of hydrogen storage alloy Mg2NiH4 produced by hydriding combustion synthesis

Abstract A high activity and a large capacity of hydrogen storage alloy Mg 2 NiH 4 produced by hydriding combustion synthesis were investigated by means of differential scanning calorimeter (DSC) and pressure-composition isotherms (PCT). The results showed that the product of Mg 2 NiH 4 from the hydriding combustion synthesis has enough activity to the hydriding reaction and the amount of hydrogen absorbed by the product reached the maximum (3.4–3.6%) mass near the theoretical value just after synthesis without any activation process. This kind of activity and capacity of hydrogen storage are not only stable, but also tolerant of high temperature of 850 K. PCT results give some phenomenon of three phases existing in isotherms. The relationships between the equilibrium plateau pressure and the temperature were Log P (0.1 MPa)=−3525/ T +6.667 for hydriding and Log P (0.1 MPa)=−3724/ T +6.883 for dehydriding.

Hydrogen storage alloy of Mg2NiH4 hydride produced by hydriding combustion synthesis from powder of mixture metal

As previous results, we reported that hydrogen storage alloy of Mg2NiH4 was produced by process of hydriding combustion synthesis in laboratory scale and regardless of the compressive pressure of the compacts from 140 MPa, 280 MPa, 550 MPa to 1.10 GPa. In present work, at first, we study the effect of the hydrogen atmosphere on the synthesis of Mg2NiH4 by this process. It is confirmed that the hydrogen atmosphere in the heating period plays an important role to the synthesis of Mg2NiH4 in the cooling period due to the expansion of the compact in the heating period resulted from the hydriding reaction of Mg+H2→MgH2. Then, we use powder of the mixture metal of magnesium and nickel as raw material directly and use lower pressures of hydrogen from 2.0 MPa, 1.0 MPa and to 0.5 MPa to produce Mg2NiH4. It is found that pure Mg2NiH4 from X-ray diffraction (XRD) analysis results can be produced at only 1.0 MPa hydrogen atmosphere when powder of mixture metal was used, which is much lower than that of 4.0 MPa when compact of mixture metal was used.

Nickel-decorated graphene nanoplates for enhanced H2 sorption properties of magnesium hydride at moderate temperatures

Magnesium hydride is considered as an ideal candidate for effective hydrogen storage due to its high gravimetric hydrogen capacity and accessibility. But its use as a commercial material is hindered by its relatively high operating temperatures and slow release/uptake kinetics. To solve this, we first synthesized Ni decorated graphene nanoplate (Ni/Gn) catalysts with highly dispersed metal nano-particles (NPs) via a facile method, then the as-prepared Ni/Gn catalysts were introduced by using the hydriding combustion synthesis and mechanical milling (HCS + MM) method to obtain Mg-based composites. Remarkable enhancement of hydrogen sorption rates has been found for these composites in the presence of Ni/Gn additives, especially for the Mg@Ni8Gn2 sample: a hydrogen absorption amount of 6.28 wt% within 100 s at 373 K and a hydrogen desorption amount of 5.73 wt% within 1800 s at 523 K. A rather low activation energy (71.8 kJ mol−1) for the dehydrogenation of MgH2 was determined in the same sample, indicating that relatively moderate temperatures are required to absorb/desorb hydrogen. The excellent hydrogen sorption rates of the composites are thought to be associated with the high dispersity of in situ formed nanometric Mg2NiH4 particles during the HCS + MM process. In addition, a microstrain-induced synergetic hydrogen sorption mechanism is proposed, being correlated by the local introduction of a Mg2Ni nano-catalyst into the Mg matrix.

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.

Hydriding and dehydriding behavior of the product in hydriding combustion synthesis of Mg2NiH4

Abstract The hydriding and dehydriding behavior of the product of hydriding combustion synthesis of Mg 2 NiH 4 was investigated by TG-DTA. During seven heating (dehydriding) and cooling (hydriding) cycles at a rate of 0.1 K/s, the TG-DTA curves of the combustion synthesis product were measured in 1.0 MPa hydrogen atmosphere. Behavior such as activation within the first four hydriding and dehydriding cycles was observed. The hydriding and dehydriding reactions gradually became more rapid from the first to the fourth cycle, and became almost constant after the fourth cycle. This behavior is likely an activation process. It is much easier with the present hydriding combustion synthesis than with the conventional ingot process. The splitting and overlapping of the peaks of the DTA curves for the dehydriding reaction provide information on the existence of two pairs of modifications of Mg 2 NiH 4 .

In situ X-ray diffraction study of the hydriding combustion synthesis of Mg2NiH4

Abstract An in situ X-ray diffraction study of the hydriding combustion synthesis of Mg 2 NiH 4 directly from the compact of magnesium and nickel mixture in a pressurized hydrogen atmosphere has been conducted. During two cycles of heating (hydriding) and cooling (dehydriding) in a temperature range from 300 to 823 K, the X-ray diffraction patterns were obtained in a pressure of 1.0 MPa at eleven points of the temperature. The obtained X-ray diffraction patterns reveal nine reactions existing in two cycles of heating and cooling. In the first cycle, (1) Mg+H 2 →MgH 2 , partially, (2) MgH 2 →Mg+H 2 , (3) 2Mg+Ni→Mg 2 Ni, in the heating period, and (4) Mg 2 Ni+2H 2 →Mg 2 NiH 4 (HT), partially, (5) Mg 2 NiH 4 (HT)→Mg 2 NiH 4 (LT), in the cooling period. In the second cycle, (6) Mg 2 NiH 4 (LT)→Mg 2 NiH 4 (HT), (7) Mg 2 NiH 4 (HT)→Mg 2 Ni+2H 2 , in the heating period, and (8) Mg 2 Ni+2H 2 →Mg 2 NiH 4 (HT), partially, (9) Mg 2 NiH 4 (HT)→Mg 2 NiH 4 (LT), in the cooling period. The X-ray diffraction intensity of Mg 2 Ni after the second cycle of hydriding decreases to 1/6 times of that after the first cycle of hydriding. In contrast, the X-ray diffraction intensity of Mg 2 NiH 4 after the second cycle of hydriding is six times of that after the first cycle of hydriding, although only one single phase of Mg 2 Ni exists at 823 K before hydriding either the first cycle or the second cycle.