Influence of Ti/Hf doping on hydrogen storage performance and mechanical properties of ZrCo compounds: A first principle study

Element doping is an effective way to improve the performance of hydrogen storage. The influences of Ti- and Hf-substituted dopants on the hydrogen storage and mechanical properties in a Zirconium-Cobalt alloy have been investigated by the first principle method. The results show that the Ti and Hf atoms preferentially occupy the Zr atoms to form new Zr₇Co₈Ti and Zr₇Co₈Hf compounds. The cohesive energy of Zr₇Co₈Ti and Zr₇Co₈Hf are ?7.518 eV/atom and ?7.531 eV/atom, thus Zr₇Co₈Hf shows a better structural stability than Zr₇Co₈Ti. The B/G ratio of Zr₇Co₈Ti and Zr₇Co₈Hf are 3.42 and 3.89, respectively, which indicates that Ti doping could increase the ductility of the alloy, thus improving the recycling performance of the alloy. The calculation for the electronic structure, mechanical property and structural stability may provide an effective way and theoretical evidence for the better design and optimization of hydrogen storage materials.     

The effect of hydrogen on the electronic,mechanical and phonon properties of LaMgNi4 and its hydrides for hydrogen storage applications

Density functional theory calculations are used herein to explore the effect of hydrogen on the electronic, mechanical and phonon properties of LaMgNi₄ and its hydrides. The polycrystalline elastic moduli, Poisson's ratios and Debye temperatures are calculated based on the single-crystal elastic constants and Voigt-Reuss-Hill approximations. It is also found that all these materials are metallic behavior, ductile and anisotropic in nature. The mechanical anisotropy is discussed via several anisotropy indices and three-dimensional (3D) surface constructions. The effect of high temperature on the free energy, entropy, and heat capacity are also studied by using the quasi-harmonic Debye model. LaMgNi₄ and its hydrides are found to be energetically, mechanically and dynamically stable. Also, they are thermodynamically stable and the order of phase stability is LaMgNi₄H₇ > LaMgNi₄H₄ > LaMgNi₄H > LaMgNi₄. In addition, the highest gravimetric hydrogen storage capacity is found to be 1.74 wt% for LaMgNi4H₇.     

An alternative route to produce easily activated nanocrystalline TiFe powder

In this paper, an alternative process route to produce active nanocrystalline TiFe compound was investigated. First, TiH₂ and Fe powders were dry co-milled in a planetary ball mill for 5–40 h. TiH₂ was selected as precursor powder, instead of Ti powder, due its fragility, which has proved to be beneficial to decrease powders adherence on milling tools. In terms of loose powder mass, milling yields ranged from 90 to 95 wt.%. Next, milled powders were post-heated at 873 K under dynamic high-vacuum for TiFe synthesis reaction. First hydrogen absorption was verified in situ during the cooling process of samples (until the room temperature), being the amount of hydrogen absorbed and desorbed by this samples measured by automated Sievert's apparatus, under constant hydrogen flow rate of 9 cm³. min^-1 (dynamic measurements). Besides to allowing the first absorption in situ, the investigated process route also allowed the production of the non-stoichiometric TiFe compound (rich in Ti) in samples milled for shorter times (5 and 10 h), both characteristics associated with maintaining the mechanical compound activity. Each sample absorbed hydrogen at 2 MPa during the cooling process, requiring no additional thermal activation cycles, since the samples milled for shorter times (mainly for 10 h) could absorb hydrogen for the first time more easily. However, the samples milled for longer times (25 and 40 h) shown better results in terms of reversible and storage capacities (0.73 and 0.94 wt.%, respectively).     

A 70 MPa hydrogen-compression system using metal hydrides

The objective of this work was to develop a 70 MPa hydride-based hydrogen compression system. Two-stage compression was adopted with AB₂ type alloys as the compression alloys. Ti_0.95Zr_0.05Cr_0.8Mn_0.8V_0.2Ni_0.2 and Ti_0.8Zr_0.2Cr_0.95Fe_0.95V_0.1 alloys were developed for the compression system. With these two alloys, a 70 MPa two-stage hydride-based hydrogen compression system was designed and built with hot oil as the heat source, and composite materials formed by mixing hydrogen storage alloys with Al fiber were used to prevent hydride bed compaction and to prevent strain accumulation. The experimental results showed that Ti_0.95Zr_0.05Cr_0.8Mn_0.8V_0.2Ni_0.2 and Ti_0.8Zr_0.2Cr_0.95Fe_0.95V_0.1 alloys could well meet the requirements of compression system. Composite materials formed by mixing hydrogen storage alloys with Al fiber were an effective way to prevent strain accumulation for hydride compression. With cold oil (298 K) and hot oil (423 K) as the cooling and heating sources, the built compression system could convert hydrogen pressure from around 4.0 MPa to over 70 MPa.     

Hydrogen storage in a Li–Al–N ternary system

Hydrogen-storage properties and mechanisms of a novel Li–Al–N ternary system were systematically investigated by a series of performance evaluation and structural examinations. It is found that ca. 5.2 wt% of hydrogen is reversibly stored in a Li₃N–AlN (1:1) system, and the hydrogenated product is composed of LiNH₂, LiH, and AlN. A stepwise reaction is ascertained for the dehydrogenation of the hydrogenated Li₃N–AlN sample, and AlN is found reacting only in the second step to form the final product Li₃AlN₂. The calculation of the reaction enthalpy change indicates that the two-step dehydrogenation reaction is more thermodynamically favorable than any one-step reaction. Further investigations exhibit that the presence of AlN in the LiNH₂–2LiH system enhances the kinetics of its first-step dehydrogenation with a 10% reduction in the activation energy due likely to the higher diffusivity of lithium and hydrogen within AlN.     

Improvement of as-milled properties of mechanically alloyed LaNi5 and application to hydrogen thermal compression

LaNi₅ was obtained from raw materials by low energy mechanical alloying. Hydriding properties of as-milled intermetallic were improved by annealing. Pressure-composition isotherms showed flat plateaus when annealing temperature was 600 °C, this value is at least 300 °C lower than the synthesis and annealing temperature of standard equilibrium methods. This low energy mechanical alloying - low temperature annealing procedure reduces the number of intermediate stages needed to scale up the fabrication of the intermetallic. After cycling this material in hydrogen, its hydriding properties were studied in the 25-90 °C range. From these results, we propose a one-stage hydrogen thermal compression scheme working between 25 °C (absorption) and 90 °C (desorption) with a compression ratio of 2.5 and a useful capacity of 1.0 mass %.     

Vibrational and thermodynamic properties of LiBH4 polymorphs from first-principles calculations

The study of phonons describes the thermodynamic properties behavior of compounds with small atoms because phonons have an important influence on its properties. Lithium borohydride, LiBH₄, is one of the suitable materials for hydrogen storage solid state. Although the transformations of Lithium borohydride LiBH₄ were repeatedly studied by experiments and fundamental side, these transformations are still under discussion. In the present work, the mode vibrational analysis of orthorhombic and hexagonal LiBH₄ structures were considered with ab initio lattice-dynamics based on the quasi-harmonic approximation approach as implemented in Phonopy code. The results show that the orthorhombic structure is thermodynamically stable, while the hexagonal structure is unstable owing to the presence of negative mode frequency. The thermal expansion behavior and various thermodynamic properties stability like heat capacity, entropy and Helmholtz energy were also studied and the obtained results are in good agreement with experiments. This shows a deep connection between stability and strength and helps researchers to estimate accurately the thermodynamic performance of LiBH₄ materials.     

Enhanced dehydrogenation/hydrogenation kinetics of the Mg(NH2)2-2LiH system with NaOH additive

The effects of NaOH addition on hydrogen absorption/desorption properties of the Mg(NH₂)₂-2LiH system were investigated systematically by means of dehydrogenation/hydrogenation measurements and structural analyses. It is found that the NaOH-added Mg(NH₂)₂-2LiH samples exhibit an enhanced dehydrogenation/hydrogenation kinetics. In particular, a ∼36 °C reduction in the peak temperature for dehydrogenation is achieved for the Mg(NH₂)₂-2LiH-0.5NaOH sample with respect to the pristine sample. Structural examinations reveal that NaOH reacts with Mg(NH₂)₂ and LiH to convert to NaH, LiNH₂ and MgO during ball milling. Then, their co-catalytic effects result in a significant improvement in the dehydrogenation/hydrogenation kinetics of the Mg(NH₂)₂-2LiH system. This finding will help in designing and optimizing the novel high-performance catalysts to further improve hydrogen storage in the amide-hydride combined systems.     

Automation and analysis of the operation of (La0.85Ce0.15)Ni5 in energy storage plants

The hydrogen storage phase of an energy storage plant based on metallic hydrides has a strong influence on the total efficiency of storage power plants as well as on their response time. The technique presented in this paper uses a hydrogen-metal chemical bond during its storage. This paper describes a metal hydride cylinder modeled and simulated by using the main quantities involved in the adsorption and desorption processes as well as in an analysis of the influence of thermal quantities involved in these processes. As a result, a proposal for automation of the thermal exchange of the modeled cylinder is presented and the possibilities of evaluation of this technique.     

Effect of Al and Mo substitution on the structural and hydrogen storage properties of CaNi5

A simple mechanical milling and annealing process has been used to synthesize CaNi₅-based hydrogen storage alloys. Heat treatment at 800 °C under vacuum results in the formation of a crystalline CaNi₅ phase. Secondary phases, including Ca₂Ni₇ and Mo–Ni, are formed when substituting Mo for Ni. Replacement of Ni by Al or Mo leads to an increase in the unit cell volume of the CaNi₅ phase. The hydrogen storage capacity of all substituted alloys is reduced and the plateau pressures are lower than those of pure CaNi₅. Fairly flat plateau regions are retained for all compositions except the CaNi_4.8Mo_0.2 composition where a Ca₂Ni₇ phase is dominant. The incorporation of Mo also causes slow sorption kinetics for the CaNi_4.9Mo_0.1 alloy. CaNi_4.9Al_0.1 maintains its initial hydrogen absorption capacity for 20 cycles performed at 85 °C but the other substituted alloys lose their capacity rapidly, especially the CaNi_4.8Mo_0.2 composition.     

First-principles studies of K1−xMxMgH3 (M = Li,Na, Rb,or Cs) perovskite hydrides for hydrogen storage

The structural stability and hydrogen release properties of M-doped KMgH₃ (M = Li, Na, Rb, or Cs) were examined using density functional theory (DFT) calculations. The reaction enthalpies (ΔH) of the four possible dehydrogenation reaction pathways were calculated using the doped structures with different phases (, Pnma, and R3c). The most favorable reaction pathway among these four pathways was found. Among the dopants investigated, the most promising dopant for this reaction was Li. In addition, the application of pressure was found to be useful for tuning the reaction enthalpies of the dehydrogenation reactions. Overall, the results present an efficient means of designing new promising perovskite-type hydrides for hydrogen storage.     

Structural and hydrogenation properties of SrAl2−xNix alloys

In this paper, Al was partially substituted by Ni in the Zintl phase alloy SrAl₂ and the structural and hydrogenation characteristics of the SrAl_2−xNi_x (0 ≤ x ≤ 0.4) alloys were studied by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and hydrogenation measurements. The alloy consisted of a single Zintl phase of SrAl₂ when x = 0. However, partial substitution of Al by Ni resulted in multiphase structure of the alloys. When x = 0.1, the alloy was composed of SrAl₂, Sr₅Al₉, SrAl and AlNi phases. With the increase of x, the amount of SrAl₂ and Sr₅Al₉ phases decreased, while the amount of SrAl and AlNi phases increased. Hydrogenation measurements were made at 473 K under 3 MPa hydrogen pressure. It was interesting to find that the hydriding kinetics of the alloy was improved greatly after Ni substitution, which could be attributed to the catalysis of AlNi phase.