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MgH2-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO2, and Ti4Fe2Ox suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH2 and hydrogenation of nano-Ti and Ti4Fe2Ox. The possible reduction of TiO2 during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO2 compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH2 by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH2 down to 74 and 67 kJ/mol for MgH2 modified with nano-TiO2 and Ti4Fe2O0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH2. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti4Fe2O0.3 nanocomposite, ΔHdes = 76 kJ/mol H2 and ΔSdes = 138 J/K·mol H2, correspond to the reported literature values for pure polycrystalline MgH2. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO2 and Ti4Fe2Ox additives. Cycling stability of Mg/Ti4Fe2Ox was substantially improved by introduction of 3 wt% graphite into the composite.

The work studies influence of yttrium on the crystal structure, hydrogenation properties and electrochemical behaviors of the PuNi3-type La2−xYxMgNi9 (x = 0.25 – 1.00) intermetallic alloys used as anodes of the Ni–MH batteries.

Social and labor relations in the process of market environment development in the agricultural sector of the agro-industrial complex are undergoing significant changes. The study of these changes is necessary to identify trends that characterize all the processes associated with the use of labor, increase its productivity and impact on the efficiency of agricultural production. Miscalculations in socio-economic policy result in negative consequences and loss of human capital. One of the peculiarities of the period of Ukraine's integration with the EU in Ukraine is that the state does not regulate the content of social and labor relations, and other regulators, such as social partnership, are not yet working at full capacity. The most serious problems were in social and labor relations directly on agricultural enterprises and especially in farms and households. There is a need for effective state influence, using the opportunities of OTG, on the formation of a national model of social and labor relations and the effective use of labor potential. Further development of social and labor relations should be carried out in the context of Ukraine's integration with the EU, which raises the need to improve socio-economic policy towards the agricultural sector in terms of its transformation and adaptation to European integration structures. It is advisable to determine the main directions and priorities of socio-economic modernization of OTG in the coming years. The main ones are: and popularization of knowledge, including the reform of rural vocational schools, the search for new forms of adult learning, the development of economic, technical and social infrastructure, support for entrepreneurship and non-agricultural activities in rural areas.

The present work is focused on the studies of the phase-structural transformations in the La3-xMgxNi9 (x ​= ​1.0, 1.1 and 1.2) alloys as active materials of negative electrodes in the Nickel-Metal Hydride (Ni/MH) batteries. The phase equilibria and phase-structural transformations in the alloys were probed by in situ neutron powder diffraction (NPD) at the temperatures ranging from 300 ​K to 1273 ​K using the measurements of the equilibrated alloys at 8 setpoint temperatures of 300, 973, 1073, 1123, 1173, 1223, 1248 and 1273 ​K. Prepared by induction melting initial alloys were found to be multi-phase structured, containing up to 6 individual intermetallic compounds with different stoichiometric compositions. With the increase of the temperature and holding time, various transformations took place in the studied alloys. These included the formations and transformations of super-stacking intermetallics with variable ratios (La ​+ ​Mg)/Ni, 1:3, 2:7 and 5:19. With increasing temperatures, several systematic changes took place. (a) Abundances of (La,Mg)2Ni4 AB2 and (La,Mg)Ni3 AB3 type intermetallics gradually decreased before they melted/decomposed above 1073 ​K; (b) The (La,Mg)2Ni7 A2B7 type intermetallics began to decrease in abundances above 1123 ​K; (c) The transformation in the (La,Mg)5Ni19 intermetallics from 3R to 2H proceeded above 1223 ​K. The increase of Mg content had no obvious influence on (La,Mg)2Ni4 and (La,Mg)2Ni7 phases, and corresponding reactions R1 and R3 took place at the same temperatures as in the La–Ni system. However, with increasing Mg content the melting point of (La,Mg)5Ni19 phase increased while the melting point of the (La,Mg)Ni3 phase it decreased, leading to the variation of the reaction temperatures of the corresponding processes. The present study will assist in optimizing phase-structural composition of the alloys in the La–Mg–Ni system which contain Mg-modified layered structures by tailoring the high temperature annealing conditions.

Artículo escrito por un elevado número de autores, solo se referencian el que aparece en primer lugar, el nombre del grupo de colaboración, si le hubiere, y los autores pertenecientes a la UAM, Hydrides based on magnesium and intermetallic compounds provide a viable solution to the challenge of energy storage from renewable sources, thanks to their ability to absorb and desorb hydrogen in a reversible way with a proper tuning of pressure and temperature conditions. Therefore, they are expected to play an important role in the clean energy transition and in the deployment of hydrogen as an efficient energy vector. This review, by experts of Task 40 'Energy Storage and Conversion based on Hydrogen' of the Hydrogen Technology Collaboration Programme of the International Energy Agency, reports on the latest activities of the working group 'Magnesium- and Intermetallic alloys-based Hydrides for Energy Storage'. The following topics are covered by the review: multiscale modelling of hydrides and hydrogen sorption mechanisms; synthesis and processing techniques; catalysts for hydrogen sorption in Mg; Mg-based nanostructures and new compounds; hydrides based on intermetallic TiFe alloys, high entropy alloys, Laves phases, and Pd-containing alloys. Finally, an outlook is presented on current worldwide investments and future research directions for hydrogen-based energy storage

Hydrides of the AB2 Laves type alloys (A=Zr, Ti; B = transition metal – Fe, Co, Ni, Mn, Cr, V) have been extensively studied as materials for the storage of gaseous hydrogen. They contain up to 4 H atoms/formula unit AB2, thus achieving reversible H storage capacities in the range between 1.5 and 2.0 wt% H and offering high rates of hydrogen charge and discharge, thus making them suitable for designing efficient hydrogen stores operating at ambient conditions. In the present study, we performed an experimental study and modeling of the thermodynamics and the kinetics of interaction in the AB2-hydrogen system. The experimental data was collected by studying a model alloy with a composition Ti0.15Zr0.85La0.03Ni1.126Mn0.657V0.113Fe0.113. Hydrogen absorption and desorption were studied in a volumetric Sieverts type apparatus at isothermal conditions using a single-step change/discharge and stepwise methods. The results obtained from the model simulation show that the reaction follows the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, with the value of exponent n = 1–1.25 for absorption and 1 for desorption. This indicates that the rate-limiting hydrogen absorption and desorption steps are jointly governed by hydrogen diffusion and grain boundary nucleation of alpha-solid solution and beta-hydride. The activation energies for both hydrogen absorption and desorption decrease along with increasing hydrogen content in the hydride.

Understanding an interrelation between the structure, chemical composition and hydrogenation properties of intermetallic hydrides is crucial for the improvement of their hydrogen storage performance. Ability to form the hydrides and to tune the thermodynamics and kinetics of their interaction with hydrogen is related to their chemical composition. Some features of the metal–hydrogen interactions remain however poorly studied, including chemistry of Sc-containing hydrides. ZrNiAl-type ScNiSn-based intermetallic hydride has been probed in the present work using a broad range of experimental techniques including Synchrotron and Neutron Powder Diffraction, $^{119}$Sn Möessbauer Spectroscopy, hydrogenation at pressures reaching several kbar H$_2$ and hydrogen Thermal Desorption Spectroscopy studies. Computational DFT calculations have been furthermore performed. This allowed to establish the mechanism of the phase-structural transformation and electronic structure changes causing a unique contraction of the metal lattice of intermetallic alloy and the formation of the ...H-Ni-H-Ni… chains in the structure with H atoms carrying a partial negative charge. Such hydrogen absorption accompanied by a formation of a covalent Ni-H bonding and causing an unusual behavior contracts to the conventionally observed bonding mechanism of hydrogen in metals as based on the metallic bonding frequently accompanied by a jumping diffusion movement of the inserted H atoms – in contrast to the directional Metal-Hydrogen bonding observed in the present work. At high applied pressures ScNiSnH$_{0.83}$ orthorhombic TiNiSi type hydride is formed with H atoms filling Sc$_3$Ni tetrahedra. Finally, this study shows that scandium closely resembles the behavior of the heavy rare earth metal holmium.

A series of cubic TbMgNi4-xCox (x = 0–4) pseudo-binary compounds have been synthesized by either a solid-state reaction or mechanical alloying with further annealing treatment. Their hydrogenation properties have been studied by pressure–composition–temperature (PCT) measurements, showing the formation of α-, β-, and γ-hydrides. The Co for Ni substitution yields a lowering of the equilibrium pressure and an increase of the hydrogen capacity. An improvement of the kinetics of hydrogen absorption was observed with an increasing Co content. β-TbMgNi4-xCoxH∼4 (x = 0–3) hydrides show an orthorhombic distortion (Pmn21 space group), whereas the β-TbMgCo4H3.3 hydride crystallizes in a monoclinic structure (Pm space group) derived from the orthorhombic structure. Sensitivity of the formation of the β-TbMgCo4H3.3 hydride to temperature has been observed in PCT curves to exist below 50 °C. γ-TbMgNi4-xCoxHy (x = 2–4, y > 5) hydrides preserve the parent cubic structure (F43m space group) with a hydrogen-induced volum...

Titanium hydride is a well characterized binary hydride which is used as a high temperature H storage material. Detailed studies on hydrogen absorption and desorption kinetics in titanium, as well as phase transformations in the Ti-based hydrides are important to achieve reversibility of H storage and their efficient use. As oxygen forms a solid solution in titanium metal, this affects the mechanism of its hydrogenation and dehydrogenation. In the present work, we have studied the hydrogenation and dehydrogenation of titanium containing a variable amount of oxygen, 1.8wt% and 3.0wt%. A detailed study of the phase-structural transformations in O-containing Titanium-Hydrogen system was performed usingin situsynchrotron X-ray diffraction. This study showed that they proceed via the following reaction pathway Ti(O)→δ→β→δ−ΤιΗ2→ε-TiH2. The δ→ε transformation was studied by monitoring the splitting of the (110) peak of the δ phase into the (110) and (002) peaks of the ε phase. This transformation starts at ~25°C with a slight lattice expansion occurring during the cooling and isothermal holding. It is found that after the rehydrogenation, oxygen becomes segregated to form an oxygen-rich α-Ti-based phase and Ti-based hydrides, and the extent of segregation increases with increasing oxygen concentration in the initial titanium metal. A reduction of the lattice parameters of TiH2proceeds after accomplishing one hydrogen absorption-desorption-absorption cycle indicating a decrease in H/Ti ratio occurring within the homogeneity region of the dihydride TiH.

A C15 AB2 Laves-type Ti0.15Zr0.85La0.03Ni1.2Mn0.7V0.12Fe0.12 alloy was prepared by arc melting and annealing. Phase-structural composition, microstructure, hydrogen absorption-desorption properties, thermodynamic and electrochemical performances were characterized by X-ray diffraction, scanning electron microscopy, hydrogen absorption-desorption measurements and electrochemical characterization and were related to the use of the alloys as metal hydride battery anodes. The alloy contains a C15 FCC intermetallic compound as the main phase and a LaNi secondary phase as the minor constituent (∼1 wt%). During the electrochemical tests, the anode electrodes quickly, after just a few activation cycles, reached a maximum discharge capacity. This was related to the catalytic effect of the La-rich secondary phase which acted as a catalyst of hydrogen absorption-desorption. Annealing resulted in increase of the maximum discharge capacity from 345 mAh/g for the as cast alloy to 370 mAh/g. Furthermore, the annealed alloy showed a better high rate dischargeability and a higher cyclic stability. After 100 cycles with 100% DOD at discharge current density of 1C, the discharge capacity of the annealed alloy was very high, at a level of 90% of the initial capacity. The rates of hydrogen diffusion have been characterized by Potentiostatic Intermittent Titration Technique and Electrochemical Impedance Spectroscopy. With increasing an extent of transformation into the hydride, the H diffusion rate in the bulk of the alloy particles decreased. The maximum value of DH measured by PITT for the annealed alloy was observed for the nearly fully discharged electrode, (SOC 2%).

We describe three fundamental problems of hydrogen power engineering, namely, the problems of efficient production, storage, and use of hydrogen. We present a survey of recent works devoted to the investigation of various types of materials used for hydrogen accumulation and, in particular, of magnesium-based nanocomposites as the most promising materials. Our main attention is given to the analysis of the sizes of particles of the components of composite materials and the effect of catalytic admixtures on the parameters of hydrogen sorption and desorption. We present a brief overview of our results on the mechanochemical synthesis of MgH2, as well as of the new results obtained for Mg-based composites with nano-Ti admixtures, including the synthesis and properties of composites produced from Ukrainian raw materials. The prospects of application of magnesium-based hydride composites are also demonstrated.

A multi component AB2type hydrogen storage intermetallic alloy (A=Ti0.85Zr0.15, B2=Mn1.22Ni0.22Cr0.2V0.3Fe0.06; was investigated in this work. The intermetallic specified above was modified by oxygen to yield the composition AB2O0.05. The oxygen was introduced by adding TiO2to the charge, with corresponding decrease of the Ti amount, followed by arc melting and annealing at the same conditions as for the oxygen free AB2-type alloy. The addition of oxygen to the alloy did not change much the PCT properties; the only difference was that the plateau pressure for the oxygen-modified alloy increased slightly. Both alloys have shown to be excellent candidates for H2storage, particularly for utility vehicles, due to their relatively high reversible H2storage capacity (1.6wt%) and low plateau pressure at room temperature (2alloy allowing it to immediately absorb H2without activation while for the non-modified sample an incubation period (30min) was observed at the same conditions.

Performance of the thermally-driven metal hydride hydrogen compressor (MHHC) is defined by (a) its H2 compression ratio and maximum output H2 pressure; (b) throughput productivity/average output flow rate; (c) specific thermal energy consumption which determines H2 compression efficiency. In earlier studies, the focus of the R&D efforts was on the optimisation of the design of the MH containers and heat and mass transfer in the MH storage and compression system aimed at shortening the time of the H2 compression cycle. This work considers an important but insufficiently studied aspect of the development of the industrial-scale thermally driven MHHC's – selection of the materials and optimisation of the materials performance. Further to the operation in the specified pressure/temperature ranges, materials selection should be based on the estimation of the productivity of the compression cycle, and specific heat consumption required for the H2 compression, which together determine the process efficiency. The current work presents a model to determine productivity and heat consumption for a single- and multi-stage MHHC's which is based on use of Pressure – Composition – Temperature (PCT) diagrams of the utilized metal hydrides at defined operating conditions – temperatures and hydrogen pressures – and main operational features of the MHHC (number of stages, amounts of the MH materials used, cycle time). In Part I of this work [Lototskyy, Yartys, et al., Int J Hydrogen Energy, DOI: 10.1016/j.ijhydene.2020.10.090], we showed that the calculated cycle productivities significantly vary for the different materials. Analysis of the system performance carried out in this work (Part II) shows that the throughput productivity and efficiency of a multi-stage MHHC also depends on the types and amounts of the used MH materials in the multi-stage compressor layout. This has been analysed for a number of the most practically important AB5 and Laves type AB2 hydrogen storage alloys integrated into the MHHC's. A comparison of experimentally measured performances of single-, two- and three-stage industrial-scale MHHC's developed by the authors earlier shows their satisfactory agreement with the modelling results thus demonstrating a high value of the presented method for the proper materials selection during development of the MHHC. As an important future development, the work presents a performance evaluation of a two-stage MHHC for H2 compression operating in the pressure range from 30 to 500 atm at operating temperatures between 20 and 150 °C.

International audience; The main objective of the present work is to study the effect of rapid solidification on the electrochemical performance of Zr-based Laves type alloy with a nominal composition Ti12Zr21.5V10Cr7.5Mn8.1Co8Ni32.2Al0.4Sn0.3. The samples were prepared from the as-cast arc melted buttons by melt spinning at different copper wheel rotation speeds of 5, 16.5, 33, and 100 Hz, which are equivalent to linear speeds of 6.3, 21, 41, and 62.8 m s−1 respectively using a cooling wheel with a diameter of 20 cm. The phase composition and morphology of the ribbons were analyzed by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). The microstructural changes of the ribbons induced by the variations in the wheel rotation speed were found to be closely related to the electrochemical performances. High discharge capacities exceeding 400 mAh∙g−1 were achieved for the melt spun samples during the measurements at low current densities. Furthermore, melt spun casting performed at the highest wheel rotation speed of 100 Hz resulted in the best rate performance of the alloy. As this alloy has the smallest crystallite size, this resulted in the shortest H atoms diffusion distances, and thus increased the efficient H diffusion rate and improved the electrochemical performance.

The present work was focused on the studies of the effect of variation of stoichiometric composition of Ti–Zr based AB2±x Laves phase alloys by changing the ratio between A (Ti + Zr) and B (Mn + V + Fe + Ni) components belonging to both hypo-stoichiometric (AB1.90, AB1.95) and over-stoichiometric (AB2.08) alloys further to the stoichiometric AB2.0 composition to optimize their hydrogen storage behaviours and performances as the alloy anodes of nickel metal hydride batteries. AB2-xLa0.03 Laves type alloys (A = Ti0.15Zr0.85; B = Mn0.64–0.69V0.11–0.119Fe0.11–0.119Ni1.097–1.184; x = 0, 0.05 and 0.1) were arc melted and then homogenized by annealing. The studies involved probing of the phase-structural composition by X-Ray diffraction (XRD), together with studies of the microstructural state, hydrogen absorption–desorption and thermodynamic characteristics of gas–solid reactions and electrochemical charge-discharge performance, further to the impedance spectroscopy characterization. The alloys were probed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and XRD. These studies concluded that the alloys contained the main C15 FCC Laves type AB2 intermetallic co-existing with a secondary C14 hexagonal Laves phase and a small amount of LaNi intermetallic. The gaseous H storage capacity and electrochemical performances were found to be closely dependent on the stoichiometric compositions of the alloys. High discharge capacities approaching 500 mAh/g were achieved for the AB1.95 alloy. At 500 mA/g current density, the discharge capacity was maintained as high as 80%, with very low capacity retention after 500 cycles. The alloy which had the highest capacity retention after 500 cycles was found to be AB2.0 alloy. Furthermore, AB2.0 alloy showed an excellent cyclic stability together with a high hydrogen diffusion coefficient. Studies of hydrogen diffusion coefficients showed that annealing enhanced the H diffusion rates allowing advanced performance at high discharge current densities. This has been related to a higher content of C15 Laves compound which allows to reach a higher H mobility in contrast with higher storage capacity observed for the C14 Laves type polymorph.

Intermetallic metal hydrides are critical materials for hydrogen storage applications, however, metal hydrides with greater storage capacities are still needed. Within metal hydrides, the volumetric storage capacities are limited by the number of hydrogen-accommodating interstitial sites which can be simultaneously occupied given a minimum hydride nearest-neighbor distance of ≈ 2.1 A, according to the Switendick-Westlake criterion. To date, violations of this criterion are rare. Perhaps the most well studied compounds violating this criterion are the RNiInHx compounds (R = Ce, La, Nd). Previous neutron diffraction studies on the deuterated species revealed the presence of Ni–D∙∙∙D–Ni–D∙∙∙D–Ni chains with anomalously close D∙∙∙D contacts of ≈ 1.6 A. Yet there are no neutron vibrational spectroscopic investigations reported for these atypical hydrides. Here we use neutron vibrational spectroscopy (NVS) measurements to probe the hydrogen dynamics in LaNiInHx (x = 0.67, 1.6) and CeNiInH1.4. For x > 0.67, the presence of close H∙∙∙H contacts yields two related features in the vibrational spectrum centered near ≈ 90 meV corresponding to the oscillations of paired H atoms simultaneously occupying neighboring R3Ni tetrahedra. Notably, these features are energetically distinct from comparable vibrational motions for “unpaired” H atoms when x ≤ 0.67. To compare, we also present powder neutron diffraction and NVS measurements for the newly characterized, chemically similar Sn compounds CeNiSnH, CeNiSnH2, and CeNiSnD2. These compounds also contain R3Ni tetrahedra, however, the H-occupied tetrahedra are well separated from each other with the closest H∙∙∙H distances exceeding 2.1 A, and the Switendick-Westlake criterion is not violated. Consequently, the spectral signature of the close H∙∙∙H contacts is absent in these hydrides.

TiH2-modified MgH2 was prepared by high energy reactive ball milling (HRBM) of Mg and Ti in hydrogen and showed high weight H storage capacity and fast hydrogenation/dehydrogenation kinetics. However, a decrease in the reversible H storage capacity on cycling at high temperatures takes place and is a major obstacle for its use in hydrogen and heat storage applications. Reversible hydrogen absorption/desorption cycling of the materials requires use of the working temperature ≥330 °C and results in a partial step-by-step loss of the recoverable hydrogen storage capacity, with less significant changes in the rates of hydrogenation/dehydrogenation. After hydrogen desorption at 330–350 °C, hydrogen absorption can proceed at much lower temperatures, down to 24 °C. However, a significant decay in the reversible hydrogen capacity takes place with increasing number of cycles. The observed deterioration is caused by cycling-induced drastic morphological changes in the studied composite material leading to a segregation of TiH2 particles in the cycled samples instead of their initial homogeneous distribution. However, the introduction of 5 wt% of graphite into the MgH2–TiH2 composite system prepared by HRBM leads to an outstanding improvement of the hydrogen storage performance. Indeed, hydrogen absorption and desorption characteristics remain stable through 100 hydrogen absorption/desorption cycles and are related to an effect of the added graphite. The TEM study showed that carbon is uniformly distributed between the MgH2 grains covering segregated TiH2, preventing the grain growth and thus keeping the reversible storage capacity and the rates of hydrogen charge and discharge unchanged. Modelling of the kinetics of hydrogen absorption and desorption in the Mg–Ti and Mg–Ti–C composites showed that the reaction mechanisms significantly change depending on the presence or absence of graphite, the number of absorption–desorption cycles and the operating temperature.

In this work, we report about development of computer model of hydrogen Thermal Desorption Spectra (TDS) using a combination of modified Kissinger approach and combined form of differential kinetic equation. For an individual TDS peak, the developed model allows to adequately fit the experimental TDS data using up to 6 parameters, including the activation energy of desorption and the pre-exponent factor. The fitting procedure also allows visualizing the reaction model function using the experimental TDS data. The model was verified for the nanostructured magnesium hydride prepared by high-energy ball milling of magnesium in hydrogen, including its composites with carbon and titanium.

The structural composition and electrochemical capacity of four AB2 Laves-type intermetallic alloys with various Ti/Zr ratios (TixZr1−xLa0.03Ni1.2Mn0.7V0.12Fe0.12, x = 0.12, 0.15, 0.18, and 0.22) were investigated in this study. The alloys were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. These data revealed the coexistence of the main phase of the C15-type FCC Laves-type AB2 compounds with a secondary La–Ni intermetallic that was present in minor amounts. Increasing the Ti substitution for Zr caused the gradual shrinkage of the unit cells of the C15 phase. The neutron powder diffraction studies demonstrated that in the trihydride (Ti, Zr, V)(Ni, Mn, Fe, V)2D3.2, D atoms filled A2B2 tetrahedra, whereas V atoms occupied not only conventional 16d sites but also partially replaced Zr/Ti at 8a sites. All studied alloys showed similar activation behaviors, wherein four cycles were required to realize the highest electrochemical capacity of the anodes. The Ti12/Zr88 alloy demonstrated excellent full discharge capacity that reached 466 mAh/g. The Ti22/Zr78 alloy electrode exhibited good cycling stability (retention rate of ~71%) after 500 cycles and a superior high-rate discharge capability (retention rate of ~71%) at a discharge current density of 400 mA/g. The cycling of the Ti22/Zr78 alloy electrode was studied by electrochemical impedance spectroscopy (EIS) to understand the reasons for the deterioration of cycling capacity, which was related with the pulverization of the alloys and increase in irreversible capacity.

The isothermal sections of the Nd–Mg–Co phase diagram at 300 and 500 °C have been constructed on the basis of X-ray diffraction (XRD), Wavelength Dispersive Spectrometry (WDS) and Electron Probe Micro Analyzer (EPMA) analysis. The existence of two ternary compounds Nd 4 Mg 3 Co 2 (own structure type) and Nd 4 MgCo (Gd 4 RhIn type) was confirmed. The latter has homogeneity range defined as Nd 4- x Mg 1+ x Co with 0 ≤ x ≤ 0.2 at 300 °С and 0 ≤ x ≤ 0.3 at 500 °С. Four new ternary compounds were identified in this work: Nd 23- x Mg 4+ x Co 7 (Pr 23 Mg 4 Ir 7 type; −1.5 ≤ x ≤ 0.1 at 300 °С and −0.6 ≤ x ≤ 0.3 at 500 °С); NdMg 2 Co 9 (YIn 2 Ni 9 type), ∼Nd 77 Mg 14 Co 9 (cubic) and ∼Nd 56 Mg 6 Co 38 (unknown crystal structure). Two binary compounds, NdCo 2 and NdCo 3 , were found to dissolve significant amount of Mg with the formation of substitutional solid solutions Nd 1- x Mg x Co 2 (MgCu 2 type; x ≤ 0.1 at 300 °С and x ≤ 0.15 at 500 °C) and Nd 1- x Mg x Co 3 (PuNi 3 type; x ≤ 0.33 at 300 °С and x ≤ 0.5 at 500 °C), respectively. The Nd 4 Mg 3 Co 2 , Nd 4- x Mg 1+ x Co, Nd 23- x Mg 4+ x Co 7 and Nd 1- x Mg x Co 2 compounds irreversibly absorb hydrogen at room temperature and pressure 1–10 bar to form crystalline or amorphous hydrides with hydrogen content 1.6–2.1 wt. %. Hydrogen thermal desorption from these hydrides in vacuum results in their disproportionation with formation of binary NdH 2 hydride. The Nd 1- x Mg x Co 3 (0 ≤ x ≤ 0.5) alloys reversibly absorb up to 1.3 wt. % of hydrogen. NdMg 2 Co 9 compound has small hydrogenation capacity, absorbing only 0.5 wt. % at room temperature and hydrogen pressure 185 bar.

La2MgNi9-related alloys are superior metal hydride battery anodes as compared to the commercial AB5 alloys. Nd-substituted La2-yNdyMgNi9 intermetallics are of particular interest because of increased diffusion rate of hydrogen and thus improved performance at high discharge currents. The present work presents in operando characterization of the LaNdMgNi9 intermetallic as anode for the nickel metal hydride (Ni-MH) battery. We have studied the structural evolution of LaNdMgNi9 during its charge and discharge using in situ neutron powder diffraction. The work included experiments using deuterium gas and electrochemical charge-discharge measurements. The alloy exhibited a high electrochemical discharge capacity (373 mAh/g) which is 20% higher than the AB5 type alloys. A saturated β-deuteride synthesized by solid-gas reaction at PD2 = 1.6 MPa contained 12.9 deuterium atoms per formula unit (D/f.u.) which resulted in a volume expansion of 26.1%. During the electrochemical charging, the volume expansion (23.4%) and D-contents were found to be slightly reduced. The reversible electrochemical cycling is performed through the formation of a two-phase mixture of the α-solid solution and β-hydride phases. Nd substitution contributes to the high-rate dischargeability, while maintaining a good cyclic stability. Electrochemical Impedance Spectroscopy (EIS) was used to characterize the anode electrode on cycling. A mathematical model for the impedance response of a porous electrode was utilized. The EIS showed a decreased hydrogen transport rate during the long-term cycling, which indicated a corresponding slowing down of the electrochemical processes at the surface of the metal hydride anode. ©2017 Elsevier B.V. All rights reserved. This is the authors' accepted and refereed manuscript to the article. Locked until 1 March 2019 due to copyright restrictions

New Nd 3− x Mg x Co 9 alloys have been synthesized by powder sintering method and their crystal structure, hydrogen storage and magnetic properties have been systematically studied. X-ray diffraction analysis showed that Nd 3− x Mg x Co 9 ( x ≤ 1.5) alloys belong to the PuNi 3 -type structure and NdMg 2 Co 9 is an individual ternary compound with YIn 2 Ni 9 -type structure. Hydrogen storage capacity of the PuNi 3 -type Nd 3− x Mg x Co 9 alloys is in the range 1.1–1.6 wt % H at room temperature, while for NdMg 2 Co 9 it is below 0.5 wt % H. PСT measurements demonstrated that thermodynamic stability of hydrides decrease with increasing Mg content. Hydrides of the alloys with lower Mg content ( x ≤ 1.0) are stable in air at room temperature. XRD study of these hydrides showed preserved PuNi 3 -type structure with hydrogen-induced volume expansion 16.7–24%. The positions of hydrogen atoms were determined by neutron powder diffraction study of Nd 2 MgCo 9 D 10 . In the Nd 2 MgCo 9 D 10 structure D atoms partially occupy four types of interstices, including octahedral Nd 2 Co 4 sites, tetrahedral Nd 2 Co 2 sites and two types of trigonal bipyramids (Nd,Mg) 3 Co 2 . Electrochemical investigations of the Nd 2 MgCo 9 electrode demonstrated rather low discharge capacity, 100 mAh/g, which is about 30% of its theoretical capacity. Magnetic measurements have shown that Nd 2 MgCo 9 is ferromagnetic with high Curie temperature (T C = 633 K) and spin reorientation at 225 K, its hydrogenation causes a significant decrease of the magnetization.

Hydrogen storage nanocomposites prepared by high energy reactive ball milling of magnesium and vanadium alloys in hydrogen (HRBM) are characterised by exceptionally fast hydrogenation rates and a significantly decreased hydride decomposition temperature. Replacement of vanadium in these materials with vanadium-rich Ferrovanadium (FeV, V80Fe20) is very cost efficient and is suggested as a durable way towards large scale applications of Mg-based hydrogen storage materials. The current work presents the results of the experimental study of Mg–(FeV) hydrogen storage nanocomposites prepared by HRBM of Mg powder and FeV (0–50 mol.%). The additives of FeV were shown to improve hydrogen sorption performance of Mg including facilitation of the hydrogenation during the HRBM and improvements of the dehydrogenation/re-hydrogenation kinetics. The improvements resemble the behaviour of pure vanadium metal, and the Mg–(FeV) nanocomposites exhibited a good stability of the hydrogen sorption performance during hydrogen absorption – desorption cycling at T = 350 °C caused by a stability of the cycling performance of the nanostructured FeV acting as a catalyst. Further improvement of the cycle stability including the increase of the reversible hydrogen storage capacity and acceleration of H2 absorption kinetics during the cycling was observed for the composites containing carbon additives (activated carbon, graphite or multi-walled carbon nanotubes; 5 wt%), with the best performance achieved for activated carbon.

The objective of the present work was to study an interrelation between the conditions of the rapid solidification casting and electrochemical properties of a C15 Laves-type Zr-based metal hydride battery anode alloy. A 7-component overstoichiometric with B/A>2 alloy (Ni + Mn + V + Fe/Zr + Ti + La = 2.078) was Zr- and Ni-rich. Melt spinning was performed using as cast mother ingot at different rotation speeds of 5, 16.5, and 33 Hz with a 20-cm diameter Cu cooling wheel. The phase composition and morphology of the ribbons were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) along with X-ray energy dispersive spectroscopy (EDS). Partial Mn vaporization took place during the melt spinning and became more pronounced with the increase in the quenching rate at higher wheel rotation speeds. The SEM study showed a successful refinement and a more homogeneous distribution of La in the melt-spun alloys. However, the alloys demonstrated a more difficult activation performance as compared to the as-cast sample which was associated with introduction of Zr into the composition of the secondary La-and Ni-rich phase when quenching the melt. Presence of Zr in the secondary phase results in inferior rate of its hydrogenation. The formed during the rapid solidification (La,Zr) (Ni,Mn) secondary phase also contained a significant amount of oxygen additionally worsening the activation performance. The grain size gradually decreased from 3.5 μm in the as-cast alloy to approximately 250 nm for the melt spun sample obtained at the highest cooling rate. The two-phase alloys contained a mixture of two Laves type intermetallics, C15 and C14 ones. The abundance of the secondary C14 phase decreased with increase of the cooling rate as compared to the as-cast sample. The electrochemical performance was significantly affected by the compositional and microstructural changes associated with the variation in the cooling rate. The highest electrochemical discharge capacity of 414 mAh/g was obtained for the 16.5 Hz melt spun rate because of an optimized composition of the Laves phase. Furthermore, the diffusion coefficient of hydrogen in this alloy was the highest in a full state of charge, which indicates that faster hydrogen diffusion and shorter hydrogen diffusion paths were achieved via an application of the rapid solidification process. An optimization of the rapid solidification conditions is required to improve an activation behavior of the studied Laves types metal hydride anode alloys. © 2019 The Authors. Published by Elsevier B.V. Open Access. CC BY-NC-ND 4.0

The present work is focused on the studies of structure, hydrogen storage and electrochemical properties of the La3-xMgxNi9 (x = 1.0, 1.1 and 1.2) alloys as active materials of negative electrodes in the Nickel-Metal Hydride (Ni/MH) batteries. A change of Mg content affects properties of the studied alloys such as the phase homogeneity, hydrogen storage and electrochemical capacities, cycle stability, and high-rate discharge performance. X-ray diffraction study shows that Mg substitution for La and annealing of the La3-xMgxNi9 alloys promotes the formation of more homogeneous materials, with a predominant formation of the target AB3 PuNi3 structure type intermetallics. The electrodes prepared from the annealed alloys show the maximum discharge capacities of ∼400 mAh g−1 at discharge current density of ∼60 mA/g. The high rate discharge-abilities (HRD) at the discharge current density of 350 mA g−1 keep high values of the remaining reversible discharge capacities, ∼86, 85 and 80%, for the La2MgNi9, La1·9Mg1·1Ni9 and La1·8Mg1·2Ni9 alloy electrodes, respectively. After 200 cycles with 100% depth of discharge (DOD), the La1·9Mg1·1Ni9 alloy electrode exhibits a very good cycling stability with its discharge capacity remaining at a level of ∼64% of its initial capacity.

Sainudeen Nazer, Nazia; Yartys, Volodymyr; Azib, Tahar; Latroche, Michel; Cuevas, Fermin; Forseth, Sissel; Vie, Preben Joakim Svela; Denys, Roman Volodymyrovich; Sorby, Magnus Helgerud; Hauback, Bjorn; Arnberg, Lars; Henry, Paul F.. In operando neutron diffraction study of a commercial graphite/(Ni, Mn, Co) oxide-based multi-component lithium ion battery. Journal of Power Sources 2016 ;Volum 326. s. 93-103

Intermetallic compounds La3–xMgxNi9 (x = 1.0, 1.1) were synthesized and their hydrogen sorption and electrochemical properties were studied. The maximum hydrogen storage capacities for La2MgNi9 and La1.9Mg1.1Ni9 were shown to be 1.6±0.1 and 1.5±0.1 wt.%, respectively, and the unit cell volume increased by 24% and 16%, respectively, upon the hydrogenation of the alloys. The maximum specific capacity of the electrodes with the La1.9Mg1.1Ni9 and La2MgNi9 alloys is 390 mA h g–1 at a discharge current density of 60 mA g–1, which is 24% higher as compared to the similar data for the LaNi5 alloy (315 mA h g–1). The electrodes demonstrate high specific capacity and performance at high current densities, as well as good cyclic stability.

This work was focused on studies of temperature-induced phase-structural transformations in the as-cast La 2 MgNi 9 metal hydride battery electrode alloy. The interactions in the alloy were studied by in situ neutron powder diffraction at temperatures ranging from 300 K to 1273 K. Initial alloy is multi-phase structured, containing six different intermetallics with five stoichiometric compositions. These include the targeted rhombohedral La 2 MgNi 9 as the main phase constituent, together with three electrochemically active intermetallics, La 3 MgNi 14 (3R), La 4 MgNi 19 (2H) and La 4 MgNi 19 (3R). Furthermore, the alloy contained two “ballast” intermetallics, LaNi 5 and LaMgNi 4 . Various transformations take place on heating, leading to the significant changes in the contents of the constituent intermetallics, first of all to the disappearance of LaNi 5 and LaMgNi 4 . Thermal volume expansions of the studied intermetallics were well fitted by the linear dependences and resulted in similar values of 4.3–5.5 vol.% at 1223 K as compared to 300 K.

This work was aimed at in situ studies of the phase-structural transformations in an Mg- and Nd-modified LaNi 3 intermetallic having a bulk composition La 1.5 Nd 0.5 MgNi 9 , as related to its performance as battery anode material in the Ni-Metal Hydride batteries. The interactions in the La 1.5 Nd 0.5 MgNi 9 system were studied by in situ neutron diffraction in a temperature range 300–1248 K. Two alternative processes of preparation of the alloy were studied: (a) Synthesis from a mixture of the powders La 0.75 Nd 0.25 Ni 5 + La 0.75 Nd 0.25 MgNi 4 ; and (b) annealing of the alloy prepared by induction melting. Various transformations take place in the samples on heating, leading to the formation of four new phases, all with the stacking structures of the intermetallic alloys having three variable ratios (RE + Mg)/Ni, 1:3, 2:7 and 5:19. Further to (La 0.75 Nd 0.25 ) 2 MgNi 9 with a rhombohedral PuNi 3 type, there were formed (La 0.75 Nd 0.25 ) 3 MgNi 14 with a rhombohedral Gd 2 Co 7 structure and two polymorphic modifications of (La 0.75 Nd 0.25 ) 4 MgNi 19 compound, a high temperature hexagonal Pr 5 Co 19 type and a low temperature rhombohedral Ce 5 Co 19 type. In every case the unit cell parameters showed shrinking as compared to the La-based analogues. Nd substitution for La in La 2 MgNi 9 promotes the formation of a more homogeneous alloy, with a predominant formation of the target AB 3 intermetallic. Properly homogenized La 1.5 Nd 0.5 MgNi 9 alloy shows a more flat and longer plateau, both in the course of hydrogen absorption–desorption and under electrochemical charge–discharge. An apparent hydrogen diffusion coefficient in alloy bulk appears to be higher as compared to Nd-free alloy, contributing to a better reversibility under a high-rate operation at high discharge current densities, while maintaining a good cycle stability.

The isothermal section of the Tb–Mg–Co phase diagram at 500 °C has been built on the basis of XRD analysis of forty samples prepared by powder metallurgy. The existence of two ternary compounds Tb4Mg3Co2 and Tb4MgCo was confirmed. The formation of two solid solutions, Tb1−xMgxCo3 (0≤x≤0.4) and Tb1-−xMgxCo2 (0≤x≤0.6), was found for the first time. It is shown that Tb5Mg24 also dissolves a small amount of Co. Other binary compounds do not dissolve the third component. The Tb4MgCo and TbMgCo4 compounds form hydrides (12.7 and 5.3 at.H/f.u. capacity, respectively) that retain the original structure of metallic matrices. Upon thermal desorption the Tb4MgCoH12.7 hydride was stable up to 300 °C and disproportionated at higher temperature. Two other hydrides, Tb4Mg3Co2H∼4 and Tb2MgCo9H12, are unstable in air and decompose into the initial compounds.

Our earlier study Yartys et al. (2015) showed that at high hydrogen pressures, hexagonal MgNi2 undergoes a hydrogen assisted phase transition into the orthorhombic MoSi2-type structure. Here we report on a combined high resolution synchrotron and neutron diffraction investigation of the crystal structure of MgNi2D3, and ab initio calculation of its electronic structure that revealed the nature of the metal–hydrogen bonding. The diffraction data (293 and 1.8 K) are well described with a Cmca unit cell with H atoms filling the deformed octahedra Mg4Ni2 and the positions within the buckled nets –Ni–H–Ni–H– penetrating through the structure. DFT and phonon calculations showed that the Cmca structure of MgNi2D3 is the most stable, both from the electronic structure and the lattice dynamical arguments. The Bader charge analysis indicates an electronic transfer from Mg (−1.59e−) to Ni (+0.21e−), H1 (+0.55e−) and H2 (+0.31e−). The phonon dispersion curves of MgNi2H3 show positive frequencies, indicating that the structure is mechanically stable. The calculated gross heat of formation for the Cmca phase of MgNi2H3 is −37.3 kJ/mol-H2, which makes it more stable by 3 kJ/mol-H2 than the prototype structures tested in Yartys et al. (2015). The stability of the Cmca crystal structure of MgNi2H3 is enhanced by the formation of the directional Ni–H covalent bonds supplemented by the electron transfer from Mg to both Ni and H. The heat capacity as a function of temperature is obtained by phonon calculation in the quasi-harmonic approximation.

Hydrogen diffusion in the La 1.5 Nd 0.5 MgNi 9 battery electrode material has been studied using low amplitude potentiostatic experiments. Complex diffusion behavior is examined in frames of electroanalytical models proposed for the lithium intercalation materials. Hydrogen diffusion coefficient D H changes with hydrogen content in the metal hydride anode electrode and has a maximum of ca. 2 × 10 −11 cm 2 /s at ca. 85% of discharge. Such a behavior differs from the trends known for the transport in lithium battery materials, but qualitatively agrees with the data for the highly concentrated β-PdH x .

The hydrogen absorption properties of the R 4 MgCo compounds ( R =Y, La, Nd, Tb; str. type Gd 4 RhIn; sp.gr. F 4 ¯ 3 m ) have been studied for the first time. It was shown that their hydrogen storage capacity reaches about 2 wt%. At low pressure hydrogenation and moderately elevated temperatures the formed hydrides preserve the original structure of the metallic matrix. The crystal structure of the R 4 MgCoH x hydrides have been determined by XRD. Experimental hydrogen storage capacity (12 at.H/f.u. for Y 4 MgCo) is in good agreement with the theoretically calculated models, which allow also to estimate the distribution of H-atoms in metal lattice. TDS and DSC experiments demonstrated the multistep desorption process. XRD studies of the Tb 4 MgCoH x sample after TDS demonstrated the formation of TbH 2 as the main phase and disproportionation of the parent compound.

The influence of Co substitution on the structural and magnetic properties of NdMgNi 4− x Co x ( x ⩽ 3) alloys and their hydrides has been investigated. The formation of NdMgCo 4 compound has not been observed. The gas hydrogenation of these alloys resulted in the formation of NdMgNi 4 H 4 , NdMgNi 3 CoH 4.8 , NdMgNi 2 Co 2 H 5.6 and NdMgNiCo 3 H 6.2 hydrides. Transformation from cubic into orthorhombic structure upon hydrogenation has been confirmed for NdMgNi 4 H 4 hydride, whereas it has been found that Co-containing hydrides preserved the parent cubic structure. PC diagrams at 50 °C for NdMgNi 4− x Co x –H 2 ( x = 0, 2, 3) showed only one absorption/desorption plateau, which equilibrium pressure decrease as the cell volume increase. The NdMgCu 4 ternary compound has been synthesized, but it does not interact with hydrogen under normal conditions. Electrochemical investigations demonstrated that substitution of the nickel atoms by the cobalt ones in NdMgNi 4 does not significantly influence on the maximum discharge capacity (244–261 mA h/g) and all alloys are characterized by high cyclic stability. NdMgNi 4 and its hydride display a Pauli paramagnet behavior. NdMgNi 2 Co 2 displays a magnetic order below 50 K whereas its hydride shows a Pauli paramagnet behavior similar to that of NdMgNi 4 H 4 .

MgNi2 intermetallic was synthesized by powder metallurgy and crystallizes with a Laves-type C36 structure (space group P63/mmc (No. 194); a = 4.826; c = 15.832 A). At 300 °C during interaction with hydrogen (deuterium) gas compressed to 2.8–7.4 GPa, a trihydride MgNi2H(D)3.2 was synthesized. The trihydride remained metastable at ambient conditions allowing its structure, stability and magnetic properties to be studied. The formation of MgNi2H3.2 is associated with a complete rebuilding of the initial hexagonal structure into the orthorhombic distorted MoSi2-type sublattice (space group Fmmm (No. 69); a = 4.55; b = 4.69; c = 8.80 A). Neutron diffraction of the MgNi2D3.2 demonstrated that D atoms fill sites having octahedral Mg4Ni2 (D1/4b) and planar Ni2 (D2/8f) coordination. Within the framework of the density functional theory, density of states (DOS) calculations showed the formation of a structure around −10 to −6 eV caused by the chemical bonds of hydrogen and its 1s states mainly via interaction with the 3d states of Ni. Analysis of the electronic structure revealed a charge transfer from Mg to Ni, and to the H atoms. The calculated enthalpy of formation of MgNi2H3 is about −30 kJ/mol-H2, which is consistent with the stability of the hydride at normal conditions. The initial sample contained a small amount of a secondary MgNi3 intermetallic, which has been formed during the equilibrium interaction of magnesium and nickel at 800 °C. Thus this compound should be included in the phase diagram of the Mg–Ni system. MgNi3 decomposes under high-temperature/high-pressure hydrogenation conditions and forms nickel monohydride.

This work focused on the high pressure PCT and in situ neutron powder diffraction studies of the LaMg 2 Ni 9 -H 2 (D 2 ) system at pressures up to 1,000 bar. LaMg 2 Ni 9 alloy was prepared by a powder metallurgy route from the LaNi 9 alloy precursor and Mg powder. Two La 3−x Mg x Ni 9 samples with slightly different La/Mg ratios were studied, La 1.1 Mg 1.9 Ni 9 (sample 1) and La 0.9 Mg 2.1 Ni 9 (sample 2). In situ neutron powder diffraction studies of the La 1.09 Mg 1.91 Ni 9 D 9.5 (1) and La 0.91 Mg 2.09 Ni 9 D 9.4 (2) deuterides were performed at 25 bar D 2 (1) and 918 bar D 2 (2). The hydrogenation properties of the (1) and (2) are dramatically different from those for LaNi 3 . The Mg-containing intermetallics reversibly form hydrides with DH des = 24.0 kJ/mol H2 and an equilibrium pressure of H 2 desorption of 18 bar at 20 °C (La 1.09 Mg 1.91 Ni 9 ). A pronounced hysteresis of H 2 absorption and desorption, ~100 bar, is observed. The studies showed that LaNi 5 -assisted hydrogenation of MgNi 2 in the LaMg 2 Ni 9 hybrid structure takes place. In the La 1.09 Mg 1.91 Ni 9 D 9.5 (1) and La 0.91 Mg 2.09 Ni 9 D 9.4 (2) (a = 5.263/5.212; c = 25.803/25.71 Å) D atoms are accommodated in both Laves and CaCu 5 -type slabs. In the LaNi 5 CaCu 5 -type layer, D atoms fill three types of interstices; a deformed octahedron [La 2 Ni 4 ], and [La(Mg) 2 Ni 2 ] and [Ni 4 ] tetrahedra. The overall chemical compositions can be presented as LaNi 5 H 5.6/5.0 + 2*MgNi 2 H 1.95/2.2 showing that the hydrogenation of the MgNi 2 slab proceeds at mild H 2 /D 2 pressure of just 20 bar. A partial filling by D of the four types of the tetrahedral interstices in the MgNi 2 slab takes place, including [MgNi 3 ] and [Mg 2 Ni 2 ] tetrahedra.

Herein, we present a comparison of the electrochemical hydrogen-storage characteristics of two state-of-art Laves phase-based metal hydride alloys (Zr21.5Ti12.0V10.0Cr7.5Mn8.1Co8.0Ni32.2Sn0.3Al0.4 vs. Zr25.0Ti6.5V3.9Mn22.2Fe3.8Ni38.0La0.3) prepared by induction melting and hydrogen decrepitation. The relatively high contents of lighter transition metals (V and Cr) in the first composition results in an average electron density below the C14/C15 threshold (e/a~6.9) and produces a C14-predominated structure, while the average electron density of the second composition is above the C14/C15 threshold and results in a C15-predominated structure. From a combination of variations in composition, main phase structure, and degree of homogeneity, the C14-predominated alloy exhibits higher storage capacities (in both the gaseous phase and electrochemical environment), a slower activation, inferior high-rate discharge, and low-temperature performances, and a better cycle stability compared to the C15-predominated alloy. The superiority in high-rate dischargeability in the C15-predominated alloy is mainly due to its larger reactive surface area. Annealing of the C15-predominated alloy eliminates the ZrNi secondary phase completely and changes the composition of the La-containing secondary phase. While the former change sacrifices the synergetic effects, and degrades the hydrogen storage performance, the latter may contribute to the unchanged surface catalytic ability, even with a reduction in total volume of metallic nickel clusters embedded in the activated surface oxide layer. In general, the C14-predominated alloy is more suitable for high-capacity and long cycle life applications, and the C15-predominated alloy can be used in areas requiring easy activation, and better high-rate and low-temperature performances. © 2017 The Authors. This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

MgCo 2 and MgNiCo crystallize with hexagonal Laves type intermetallic structures of the C14 type and do not form hydrides at ambient hydrogen pressures. However, applying high hydrogen pressures in the GPa range forces the hydrogen absorption and leads to the formation of multi-phase compositions, which contain approximately 2.5 atoms H per formula unit of MgCo 2 or MgNiCo and remain thermally stable under normal conditions. The hydrogenation of MgCo 2 resulted in its decomposition to a ternary Mg 2 CoD 5 deuteride and metallic cobalt. Phase-structural transformations accompanying the vacuum desorption of deuterium in the temperature range of 27–500 °C were studied using in situ neutron powder diffraction. The investigation showed a complete recovery of the initial MgCo 2 intermetallic via a Hydrogenation-Disproportionation-Desorption-Recombination process. At 300 °C, the Mg 2 CoD 5 deuteride first decomposed to elementary Mg and hexagonal Co. At 400 °C, a MgCo phase was formed by interaction between Mg and Co. At the highest processing temperature of 500 °C, a solid-state interaction of MgCo and Co resulted in the recovery of the initial MgCo 2 . The interaction of MgNiCo with deuterium under the synthesis conditions of 2.8 GPa and 200 °C proceeded in a more complex way. A very stable ternary deuteride MgNi 2 D 3 was leached away while Co was separated in the form of Mg 2 CoD 5 and the remaining nickel formed a solid solution with Co with the approximate composition Ni 0.7 Co 0.3 . The thermal desorption of deuterium from MgCo 2 D 2.5 and from MgNiCoD 2.5 has been studied by Thermal Desorption Spectroscopy with deuterium released into a closed volume. The observed effects nicely correlate with changes in the phase structural composition of the hydrides formed. MgCo 2 is a new example of the hydrogen storage alloy, in which a successful HDDR processing results in the reversible formation of the initial intermetallic at much lower temperatures than in the equilibrium phase diagram of the Mg-Co system.

International audience; Hydrogen interaction with intermetallic compound Nd2Ni2Mg crystallizing in the tetragonal Mo2FeB2 type of structure leads to the formation of hydride/deuteride containing 7.4-8 H(D)/f.u. Hydrogen absorption is accompanied by a monoclinic deformation of the unit cell with the crystal structure data refined from Synchrotron XRD (Sp. gr. P2(1)/c; a = 11.60733(5) angstrom; b = 7.66057(3) angstrom; c = 11.78743(5) angstrom, beta = 92.4126(4)degrees, V = 1047.194(8) angstrom(3)) and high resolution powder XRD data. The formed interstitial hydride disproportionates during heating forming NdH2. Nd2Ni2Mg is an antiferromagnet with the Neel temperature T-N approximate to 19 K. Presence of another magnetic phase transition at 14 K together with two metamagnetic transitions indicates a complicated magnetic phase diagram. The hydrogenation reduces T-N to 1.0 K. (C) 2017 Elsevier Ltd. All rights reserved.

In present work we have developed a theoretical model for the description of the electrochemical discharge process in the metal hydride anodes of the metal hydride batteries at various current densities. The model is based on the description of the assembly of the spherically shaped metal hydride particles where the process of electrochemical discharge is fit using a shrinking core model, with a shell of the H storage alloy growing inside the particle by decreasing volume of the internal metal hydride core. The model accounts results of the D.O.S. calculations for La2MgNi9 intermetallic alloy and has been tested for this metal hydride anode material having an electrochemical discharge capacity of 400 mAh/g and hydrogen storage capacity of 13 at.H/f.u. La2MgNi9. It allows to quantitatively describe kinetic behaviors of the electrode at various applied current densities and to estimate the diffusion coefficient for hydrogen and equilibrium content of H in the solid solution domain of hydrogen in La2MgNi9. This model has a general applicability and can be used for the optimization of the materials and the electrodes of the metal hydride rechargeable batteries at high current densities.

New two ternary YMgCo4 and YMgCu4 and one quaternary YMgCo2Ni2 compounds have been synthesized by mechanical alloying with further annealing. The hydrogenation capacity of YMgCo4 reaches 6.8 at. H/f.u. The Pressure-Composition-Temperature studies of YMgCo4–H2 and YMgNi4–H2 systems revealed that introduction of magnesium, accompanied by shrinking of the unit cell, decreases the stability of hydrides comparing to binary YCo2 and YNi2 compounds. The values of heat and entropy of the YMgCo4H6.8 hydride formation were calculated: ΔH = −27.9 ± 0.8 kJ mol–1 H2 and ΔS = −93.4 ± 2.6 J mol−1 H2 K−1. The YMgCo2Ni2–H2 system shows intermediate thermodynamic properties compared to the ternary hydrides (ΔH = −28.8 ± 0.2 kJ mol–1 H2 and ΔS = −117.6 ± 2.4 J mol–1 H2 K−1). The YMgCo4H6.8 and YMgCo2Ni2H4.9 hydrides keep the cubic structure of the parent compounds with a cell volume expansion of 23 and 14.4% respectively. It is shown that the YMgCu4 compound does not interact with hydrogen under normal conditions.

The intermetallic La2MgNi9 has been investigated as negative electrode material for NiMH battery by means of in situ neutron powder diffraction. This hydride-forming compound exhibits suitable plateau pressures ranging within the practical electrochemical window and leads to significant reversible electrochemical capacities. Charge and discharge of the composite electrode have been performed in beam following various current rates and galvanostatic intermittent titration. From the diffraction data analysis, phase amounts and cell volumes have been extracted, allowing the interpretation of the hydride formation and decomposition. From the evolution of the diffraction line widths, differences are observed between charge and discharge with the possible formation of an intermediate γ phase on charge. The electrode readily responds to current rate variations and does not show any kinetic limitation in the range C/10 and C/5 (C/n: full capacity C in n hours). This material shows excellent properties regarding e...