Maximum Hydrogen Absorption Capacity Research Articles

Understanding the effect of TiO 2, VCl 3, and HfCl 4 on hydrogen desorption/absorption of NaAlH 4

The main objective of this work was to investigate the different effects of transition metals (TiO 2, VCl 3, HfCl 4) on the hydrogen desorption/absorption of NaAlH 4. The HfCl 4 doped NaAlH 4 showed the lowest temperature of the first desorption at 85 °C, while the one doped with VCl 3 or TiO 2 desorbed at 135 °C and 155 °C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH 4 doped with TiO 2 reduced to 140 °C. On the contrary, in the case of NaAlH 4 doped with HfCl 4 or VCl 3, the temperature of desorption increased to 150 °C and 175 °C, respectively. This may be because Ti can disperse in NaAlH 4 better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5 wt% by doping with TiO 2, while the amount of restored hydrogen was lower for HfCl 4 and VCl 3 doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO 2 doped samples. In contrast, there was evidence of Al–V alloy in the VCl 3 doped sample and Al–Hf alloy in the HfCl 4 doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH 4 showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions.

Thermal stabilization and hydrogen storage properties of Mg–40 wt% [formula omitted] composite prepared by mechanical milling

The thermal stability and hydrogen storage properties of Mg–40wt% Ti 0.28 Cr 0.50 V 0.22 prepared by mechanical milled for 40 h under 2.0 hydrogen pressure were investigated. The experimental results showed that the composite is easily activated, and exhibits good hydrogen sorption properties. It can absorb nearly 90% hydrogen of the total capacity in 1 min, and the maximum hydrogen absorption capacity reaches to 3.2, 3.5 and 4.0 wt% at 298, 368 and 548 K in 10 min, respectively. DSC measurement results showed that hydrogen desorption temperature of composite is depressed about 150 K compared with that of pure MgH 2 . However, if the composite was heated over 620 K, the hydrogen desorption temperature was elevated to the same temperature as that of pure MgH 2 due to the re-crystallization or growth of the nano-structure Mg, although the hydrogen adsorption kinetic were not obviously affected. Therefore, It can be deduced that nano-structure Mg phase is the main reason for decreasing the hydrogen desoprtion temperature. The nano-structure Ti 0.28 Cr 0.50 V 0.22 phase plays a more important role in improving hydriding kinetics of composite.

Solid-State Synthesis of Hydrogen Storage Mg<SUB>2</SUB>Co Alloys <I>via</I> Bulk Mechanical Alloying

Mg-Co system is employed to investigate the solid-state reactivity of Mg 2 Co from the elemental powder mixture of magnesium and cobalt and to describe the hydrogenation and de-hydrogenation processes of Mg 2 Co. Mg 2 Co is successfully solid-state synthesized as a non-equilibrium phase compound by the bulk mechanical alloying. This solid-state reactivity is controlled by the repeated plastic flow of ductile elements in the powder mixture. This non-equilibrium phase, Mg 2 Co is stable up to 833 K. Its hydrogenation and de-hydrogenation processes are described by DSC, PCT and XRD. These processes take place in multi-step with strong dependency on the holding temperature. Hydrogenation process from Mg 2 Co is composed of two steps to Mg 2 CoH 5 and dehydrogenation from Mg 2 CoH 5 , one step to Mg 2 Co, at 633 K. Maximum hydrogen absorption capacity at 633 K is still limited to 2.03 mass% since hydrogenation and de-hydrogenation steps, working at higher temperature, are never activated without chemical modification to original Mg 2 Co.

Synthesis and hydrogenation properties of Mg–La–Ni–H system by reactive mechanical alloying

We have synthesized Mg–30 mass%LaNi 2.28 composite material and investigated its hydrogenation behaviour. The reactive mechanical alloying process of the mixture of Mg and LaNi 2.28 was studied. It is found that a composite of MgH 2 , La 4 H 12.9 and Mg 2 NiH 4 formed after 80 h ball-milling under 3.0 MPa hydrogen. Scanning electron microscopic analysis indicated that these new phases are distributed homogeneously. This composite shows excellent hydriding properties even at moderate temperature. Under 3.0 MPa hydrogen pressure it absorbed more than 80% of its full capacity in the temperature range of 473–553 K within less than 1 min. The maximum hydrogen absorption capacity at 553 K is 5.4 mass%. The enhanced hydriding properties could be attributed to the fine and uniform particles and a synergeticly catalytic effect generated by mechanical milling.

Hydrogenation properties of mechanically milled Mg2Ni0.8Cr0.2-CoO/Al2O3 composites

Mg2Ni0.8Cr0.2-x wt.% CoO/Al2O3 (x=0.5, 1, 2 and 3) composites were prepared by mechanically milling sintered Mg2Ni0.8Cr0.2 alloy and CoO/Al2O3 compound for 45 h. The addition of CoO/Al2O3 compound resulted in the good kinetics properties of hydriding/dehydriding reaction of the composites. The composite with 1.0 wt.% CoO/Al2O3 catalyst could reach the maximum hydrogen absorption capacity (2.9 wt.%) within 5 min at 393 K under H2 pressure of 4 MPa, and can desorb rapidly at 493 K. The decomposition and synthesis of hydrogen molecule on Mg2Ni0.8Cr0.2 alloy surface was promoted by addition of CoO/Al2O3 catalyst. In addition, the formation of metallic Ni particles, strain and defects during the ball milling process also resulted in the improved hydrogenation performance of Mg2Ni-based alloys.

Effect of ternary Al, Ni additions on hydrogen absorption behaviour of Ti–45Nb alloys

Abstract The present study investigates the hydrogen absorption behaviour of a binary Ti–45Nb alloy and the effect of ternary additions of Ni and Al on the pressure–composition–temperature (i.e. p – c – T ) behaviour of this alloy. It is found that at a given temperature and partial pressure, the Al-containing alloy has the lowest hydrogen content, followed by the Ni-containing alloy, and that the binary alloy exhibits maximum hydrogen absorption capacity. A comparison of the enthalpy of solution as a function of hydrogen content of the three alloys shows that the enthalpy minimum for the Al-containing alloy occurs at the lowest hydrogen content, followed by that for the binary alloy, and the enthalpy minimum for the Ni-containing alloy occurs at the highest hydrogen content. Enthalpy of solution at infinite dilution of hydrogen shows a linear dependence on the lattice parameter of the host alloys. X-ray analysis of the hydrogen containing samples reveals suppression of the α-phase and the Ti 2 Ni-phase in the presence of hydrogen.

Properties of hydrogen storage alloy Mg 2− xAg xNi ( x=0.05, 0.1, 0.5) by hydriding combustion synthesis

Mg 2− x Ag x Ni ( x=0.05, 0.1, 0.5) prepared by hydriding combustion synthesis were investigated by means of pressure–composition isotherms, X-ray diffraction and scanning electron microscopy. The results showed that the maximum hydrogen absorption capacity of Mg 1.95Ag 0.05Ni is 3.49 mass% and of Mg 1.9Ag 0.1Ni 3.43 mass% at 553 K, just after synthesis without any activation; their desorption capacities are 3.31 mass% and 3.18 mass%, respectively. The XRD results explain that it is reasonable to add a little Ag in Mg 2Ni and the kinetics of the Mg 2− x Ag x Ni ( x=0.05, 0.1) hydrogen storage alloy can be improved. The relationships between the equilibrium plateau pressure and temperature for Mg 1.9Ag 0.1Ni were log P(0.1 MPa)=−2827/ T+5.677 (523 K ≤ T ≤ 573 K) for hydriding and log P(0.1 MPa)=−3581/ T+6.818 (523 K ≤ T ≤ 573 K) for dehydriding. That of Mg 1.95Ag 0.05Ni were log P(0.1 MPa)=−2725/ T+5.354 (523 K ≤ T ≤ 573 K) for hydriding and log P(0.1 MPa)=−3444/ T+6.538 (523 ≤ T ≤ 573 K) for dehydriding.

The effect of Zr substitution for Ti on the microstructures and electrochemical properties of electrode alloys Ti 1− xZr xV 1.6Mn 0.32Cr 0.48Ni 0.6

The effect of Zr substitution for Ti on the microstructures and electrochemical properties of the electrode alloys Ti 1− x Zr x V 1.6Mn 0.32Cr 0.48Ni 0.6 ( x=0.2,0.3,0.4,0.5) has been studied. It is found by X-ray powder diffraction and energy dispersive X-ray spectroscope analyses that all the alloys consist of a C14 Laves main phase with hexagonal structure and a V-based solid solution secondary phase with b.c.c. structure. A small amount of TiNi-based third phase with b.c.c. structure has been found precipitated in the C14 Laves main phase in addition. With the increase in the amount of Zr substitution, the lattice parameters of the main phase and secondary phase are found increased and decreased, respectively. The electrochemical PCT curves indicate that the maximum hydrogen absorption capacity [H/M] max decreases with increasing Zr substitution, which may be attributed to the decrease in the content of V-based solid solution. The maximum discharge capacity and high rate dischargeability of the alloy electrodes both decrease, while the cyclic durability increases with the increasing amount of Zr substitution.

Absorption of hydrogen by sintered intermetallics of the Sc — Ni system

The absorption of hydrogen by the intermetallics Sc2Ni, ScNi, ScNi2, Sc2Ni7, and ScNi5 produced by sintering was investigated. Hydrogen absorption isobars at the pressures 0.1 and 3 MPa were obtained. The maximum hydrogen absorption capacity of the compounds was determined. The absorption properties were compared with those of specimens produced by arc melting.

Hydrogen absorption properties of amorphous and crystalline alloys in the pseudobinary ZrCo–TiNi system

The single phase amorphous alloys were prepared by rapid quenching of the pseudobinary (ZrCo)100−x (TiNi)x alloys. The amorphous a-(ZrCo)70(NiTi)30 alloy crystallized into the ordered B2 compound, while the others did into the disordered bcc alloys. The PCT curve of the crystalline compounds showed a plateau pressure, while that of the amorphous one did not. Nearly the same amount of hydrogen was absorbed in the amorphous and crystalline phases. Both the amorphous and crystalline alloys showed the maximum hydrogen absorption capacity of 1.6 (H/M) at the composition of x=30, i.e. at the composition where the alloys crystallized into the B2 structure. The present work indicates that the ordered (B2) alloy absorbs much more hydrogen rather than the corresponding disordered (bcc) ones.

Thermal stability and hydriding properties of nanocrystalline melt-spun Mg 63Ni 30Y 7 alloy

Nanocrystalline Mg 2(Ni,Y) hydrogen storage alloy (with exact composition Mg 63Ni 30Y 7) was prepared by rapid solidification, using a melt-spinning technique. Thermal stability and phase transition in the as-quenched alloy were studied by TEM, DSC, X-ray and electron diffraction. It was found that the as-cast material consists of mainly hexagonal Mg 2(Ni,Y) nanocrystals with an average size of 2–3 nm and a significant amount of amorphous phase with similar composition located between them. During heating the alloy crystallizes completely by three dimensional nanocrystal growth, with an activation energy of 140±7 kJ mol −1. The hydriding properties of the as-quenched nanocrystalline alloy were studied as well. The maximum hydrogen absorption capacity (about 3.0 wt.%) and hydrogenation kinetics of the melt-spun Mg 2(Ni,Y) were found to exceed those of the conventionally prepared polycrystalline Mg 2Ni alloys and to be comparable to the hydrogen absorption characteristics of nanocrystalline ball-milled Mg 2Ni. Hydrogenation of the as-cast alloy causes a change in the crystallization mechanism during annealing, as the microstructure remains nanocrystalline (15–20 nm) even after complete crystallization of the alloy.

Hydrogen-induced amorphisation in the Ce(Fe 1− xAl x) 2 system

Hydriding behaviour of cubic Laves phase pseudo-binary Ce(Fe 1− x Al x ) 2 system has been studied for the iron-rich (0 ⩽ x ⩽ 0.1) as well as for the aluminium-rich compositions (0.7 ⩽ x ⩽ 0.85) at temperatures in the range of 223–298 K, and hydrogen pressure (≲ 0.5 atm), ensuring fast withdrawal of exothermic heat. The maximum hydrogen absorption capacity is found to decrease with increasing aluminium concentration. The results of X-ray diffraction and Mossbauer studies are reported. This system shows the unique feature of stabilising the amorphous ternary, for the Fe-rich region and the crystalline ternary hydride phases for the Al-rich compositions. The role of the maximum hydrogen absorption capacity, the rate and sequence of H atom occupancy and the volume associated with each hydrogen atom is discussed in relation to the amorphous ternary vis-a-vis the crystalline ternary hydride phase formation. It is shown that by restricting the maximum hydrogen absorption capacity such that the H-atom occupancy is confined to the g-type interstitial sites only, the crystalline ternary hydride phase can be stabilised in this system.

Electronic properties and the maximum hydrogen absorption capacity of the novel hydride Na2PdH2

The authors have investigated the electronic structure of the recently synthesized hydride Na2PdH2 by means of a calculation of the energy bands and densities of states using the linear muffin-tin orbital method. The metal-hydrogen bonding features, the Fermi-energy-related properties and the maximum hydrogen content of the system are discussed.

Hydrides of metallic glass alloys

The hydrogen absorption properties of several metallic glass alloys of the general formulas Ti 1− x Cu x and Zr 1− x Cu x ( x = 0.3–0.7) were investigated and were compared with the absorption properties of the corresponding crystalline intermetallic compounds. Under similar conditions of temperature and pressure the metallic glass alloys had larger absorption capacities than their crystalline counterparts. “Amorphous” TiCu, for example, absorbed 35% more hydrogen than crystalline TiCu (TiCuH 1.35 compared with TiCuH). These results indicate the relative importance of electronic structure and crystal structure in hydrogen absorption; it appears that the maximum hydrogen absorption capacity is determined by the electronic structure but that the crystal structure, i.e. the type and size of interstitial sites in the lattice, may not always permit maximum absorption to take place readily.