Theoretical model on the electronic properties of multi-element AB5-type metal hydride

Nowadays, multi-element alloys are preferred over binary alloys for application point of view. The hydrogenation properties strongly depend on the thermodynamic, structural and electronic properties of the alloys. At present, no model is available which can predict the hydrogen storage properties of the multi-element alloy, before actual synthesis of the alloy. In the present investigation, efforts are made to develop a theoretical mathematical model to predict the hydrogenation properties of multi-element AB₅-type metal hydride. The present investigation deals with the various electronic parameters which may affect the hydrogenation characteristics of the metal hydride. Based on all such parameters, an electronic factor has been proposed for AB₅-type alloys. Electronic factor has been combined with the structural and thermodynamical factor to propose a new combined factor, which was further correlated with the hydrogen storage capacity of the alloy. Atomic radius and electronic configuration of substituted elements in the multi-element AB₅-type hydrogen storage alloy have been found as key players to predict the hydrogenation properties of the alloys before synthesis. It has been shown that in the case of alloy series with multiple substitutions, the combined factor is more relevant in deciding the hydrogen storage capacity in comparison to electronic factor alone. Combined factor is directly proportional to the hydrogen storage capacity. All the three factors thermodynamic, structural and electronic together may lead to the prediction of pressure-composition isotherm of the multi-element AB₅-type hydrogen storage alloy.     

Hydrogen storage properties of new A3B2-type TiZrNbCrFe high-entropy alloy

Crystal structure and hydrogen storage properties of a novel equiatomic TiZrNbCrFe high-entropy alloy (HEA) were studied. The selected alloy, which had a A₃B₂-type configuration (A: elements forming hydride, B: elements with low chemical affinity with hydrogen) was designed to produce a hydride with a hydrogen-to-metal atomic ratio (H/M) higher than those for the AB₂- and AB-type alloys. The phase stability of alloy was investigated through thermodynamic calculations by the CALPHAD method. The alloy after arc melting showed the dominant presence of a solid solution C14 Laves phase (98.4%) with a minor proportion of a disordered BCC phase (1.6%). Hydrogen storage properties investigated at different temperatures revealed that the alloy was able to reversibly absorb and fully desorb 1.9 wt% of hydrogen at 473 K. During the hydrogenation, the initial C14 and BCC crystal structures were fully converted into the C14 and FCC hydrides, respectively. The H/M value was 1.32 which is higher than the value of 1 reported for the AB₂- and AB-type HEAs. The present results show that good hydrogen storage capacity and reversibility at moderate temperatures can be attained in HEAs with new configurations such as A₃B₂/A₃B₂H₇.     

Hydrogen storage properties of multi-principal-component CoFeMnTixVyZrz alloys

This study presents an innovative multi-principal-element CoFeMnTiVZr alloy system for the absorption and desorption of hydrogen. Pressure-composition-isotherms (PCIs) demonstrate that CoFeMnTi_xVZr, CoFeMnTiV_yZr, and CoFeMnTiVZr_z can absorb and desorb hydrogen for x, y, and z that satisfy 0.5 ≤ x ≤ 2.5, 0.4 ≤ y ≤ 3.0, and 0.4 ≤ z ≤ 3.0, respectively. X-ray diffraction (XRD) reveals that CoFeMnTi_xV_yZr_z alloys have a simple C14 Laves phase with a single set of lattice parameters before and after PCI tests. The distributions of each element in CoFeMnTi_xV_yZr_z alloys are roughly equal, as revealed by SEM/EDS mapping. The effects of values x, y, and z on the hydrogen storage properties are elucidated in terms of lattice constant, element segregation, hydride formation enthalpies of the alloy components and hydrogen, and the averaged formation enthalpy. The high-entropy effect promotes the formation of a single C14 Laves phase, and the maximum hydrogen storage capacity is strongly related to the hydride formation enthalpy of the alloy and hydrogen.     

Rapid activation of MmNi5−xMx based MH alloy through Pd nanoparticle impregnation

Faster activation of a multi-component AB₅ based alloy metal hydride electrode through Pd nanoparticle (NP) impregnation is demonstrated. Pd nanoparticle impregnated MmNi_5−xM_x based alloy was prepared and characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and elemental mapping techniques. Electro-catalytic activity of laminar metal hydride electrodes containing Pd nanoparticles and micrometer size Ni particles was studied. Hydrogen absorption efficiency of the nanocomposite electrodes was compared with the metal hydride electrodes without Pd nanoparticles. The incorporation of nanostructured materials in the metal hydride alloy increased its hydrogen absorption capacity at the initial stage and activated much faster, indicating its good prospect for energy storage applications.     

Machine learning analysis of alloying element effects on hydrogen storage properties of AB2 metal hydrides

Zirconium-titanium-based AB₂ is a potential candidate for hydrogen storage alloys and NiMH battery electrodes. Machine learning (ML) has been used to discover and optimize the properties of energy-related materials, including hydrogen storage alloys. This study used ML approaches to analyze the AB₂ metal hydrides dataset. The AB₂ alloy is considered promising owing to its slightly high hydrogen density and commerciality. This study investigates the effect of the alloying elements on the hydrogen storage properties of the AB₂ alloys, i.e., the heat of formation (ΔH), phase abundance, and hydrogen capacity. ML analysis was performed on the 314 pairs collected and data curated from the literature published during 1998–2019, comprising the chemical compositions of alloys and their hydrogen storage properties. The random forest model excellently predicts all hydrogen storage properties for the dataset. Ni provided the most contribution to the change in the enthalpy of the hydride formation but reduced the hydrogen content. Other elements, such as Cr, contribute strongly to the formation of the C14-type Laves phase. Mn significantly affects the hydrogen storage capacity. This study is expected to guide further experimental work to optimize the phase structure of AB₂ and its hydrogen sorption properties.     

The development of hydrogen storage electrode alloys for nickel hydride batteries

The development of hydrogen storage electrode alloys in the 1980s resulted in the birth and growth of the rechargeable nickel hydride (Ni/MH) battery. In this paper we describe briefly a semi-empirical electrochemical/thermodynamic approach to develop/screen a hydrogen storage alloy for electrochemical application. More specifically we will discuss the AB_x Ti/Zr-based alloys. Finally, the current state of the Ni/MH batteries including commercial manufacture processes, cell performance and applications is given.     

Substitutional effect of Ti-based AB2 hydrogen storage alloys: A density functional theory study

Stability of AB₂ alloy in Laves phases C14 and C15 were studied by first-principle density functional theory simulations. A range of different combinations of B and C elements in the Ti_1−xC_xB₂ alloys were considered. The formation energies of these alloys generally increase with the unit cell volumes of alloys. The volume also affects the stability of the corresponding metal hydride. We find that the formation energies and the hydrogenation enthalpies of AB₂ alloys are likely to be determined by at least three factors: electronegativity, atomic radius and covalent radius. The enthalpies of AB₂ hydrides increase with increasing compositionally-averaged electronegativity and volume change upon hydrogenation. However, the enthalpies of AB₂ hydrides decrease with increasing compositionally-averaged atomic and covalent radii. This study provides useful insights for future exploration of AB₂-type alloys for hydrogen storage applications.     

Metal hydride hydrogen storage and compression systems for energy storage technologies

Along with a brief overview of literature data on energy storage technologies utilising hydrogen and metal hydrides, this article presents results of the related R&D activities carried out by the authors. The focus is put on proper selection of metal hydride materials on the basis of AB₅- and AB₂-type intermetallic compounds for hydrogen storage and compression applications, based on the analysis of PCT properties of the materials in systems with H₂ gas. The article also presents features of integrated energy storage systems utilising metal hydride hydrogen storage and compression, as well as their metal hydride based components developed at IPCP and HySA Systems.     

Characteristics of A2B7-type LaYNi-based hydrogen storage alloys modified by partially substituting Ni with Mn

LaY₂Ni_10.5?xMn_x (x = 0.0, 0.5, 1.0, 2.0) alloys are prepared by a vacuum induction-quenching process followed by annealing. The structure, as well as the hydriding/dehydriding and charging/discharging characteristics, of the alloys are investigated via X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), pressure-composition isotherms (PCI), and electrochemical measurement. The alloys have multiphase structures mainly composed of Gd₂Co₇-type (3R) and Ce₂Ni₇-type (2H) phases. Partial substitution of Ni by Mn clearly increases the hydrogen storage capacity of the alloys. The x = 0.5 alloy exhibits a maximum hydrogen storage capacity of 1.40 wt % and a discharge capacity of 392.9 mAh g^?1, which are approximately 1.5 and 1.9 times greater than those of the x = 0.0 alloy, respectively. The high-rate dischargeability (HRD) of the x = 0.5 alloy is higher than that of the other alloys because of its large hydrogen diffusion coefficient D, which is a controlling factor in the electrochemical kinetic performance of alloy electrodes at high discharge current densities. Although the cyclic stability of the x = 0.5 alloy is not as high as that of the other alloys, its capacity retention ratio is as high as 56.3% after the 400th cycle. The thermodynamic characteristics of the x = 0.5 alloy satisfy the requirements of the hydride electrode of metal hydride–nickel (MH–Ni) batteries.     

Influence of intrinsic hydrogenation/dehydrogenation kinetics on the dynamic behaviour of metal hydrides: A semi-empirical model and its verification

Metal hydrides can store hydrogen at low pressures and with high volumetric capacity. For the possible application as storage medium in hydrogen stand-alone power systems, large metal hydride hydrogen storage units are usually required. A reliable and verified kinetic correlation is an important tool in the designing process of a larger storage unit. This paper describes kinetic investigation of a AB₅-type alloy and its corresponding hydride, with the purpose of finding a semi-empirical correlation suitable for use in heat and mass transfer modelling and engineering design of metal hydride storage units.     

Novel hydrogen generator/storage based on metal hydrides

A novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and higher efficiency than a classical electrolyser. The prototype comprises a six-electrode cell assembly using an AB₅ type metal hydride and Ni plates as counter electrodes, in a KOH solution. Metal hydride electrodes with chemical composition LaNi₄.₃Co_0.4Al₀.₃ have been prepared by high frequency vacuum melting followed by high temperature annealing. X-ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La–Ni phase diagram. Thermodynamic study of the alloy has been performed in a Sievert-type apparatus produced by Labtech Ltd. In the present prototype during charging, hydrogen is absorbed in the metal hydride and corresponding oxygen is conveyed out of the system. Conversely, in the case of discharging the hydrogen stored in the metal hydride it is released to an external H₂ storage. Released hydrogen is delivered into the hydrogen storage up to a pressure of 15 bar. It is anticipated that the device will be integrated as a combined hydrogen generator in a stand-alone system associated to a 1 kW fuel cell.     

Effect of measurement parameters on thermodynamic properties of La-based metal hydrides

Pressure–concentration isotherms (PCIs) of LaNi_5−xAl_x (x = 0.3 and 0.4) hydrides were measured using a volumetric method. Two important thermodynamic properties, enthalpy of formation (ΔH) and entropy of formation (ΔS), were calculated using the van't Hoff equation. The effects of the Al content on the hydrogen storage capacity, plateau pressure and thermodynamic properties were studied. Additionally, the effects of the charging/discharging pressure difference (ΔP_s) during each step of the absorption/desorption PCI measurement on the hydrogen storage capacity (wt%), equilibrium pressure (P_e), plateau slope, reaction enthalpy (ΔH) and entropy (ΔS) were studied for LaNi_4.6Al_0.4 hydride. All of these properties (P_e, ΔH, ΔS, etc.) showed a significant variation with ΔP_s. The effect of the temperature range on the estimation of the enthalpy of formation was investigated. It was observed that ΔH depends on the experimental temperature range.     

Effect of La–Mg-based alloy addition on structure and electrochemical characteristics of Ti0.10 Zr0.15V0.35Cr0.10Ni0.30 hydrogen storage alloy

Effect of La–Mg-based alloy (AB₅) addition on Structure and electrochemical characteristics of Ti_0.10Zr_0.15V_0.35Cr_0.10Ni_0.30 hydrogen storage alloy has been investigated systematically. XRD shows that the matrix phase structure is not changed after adding AB₅ alloy, however, the amount of the secondary phase increases with increasing AB₅ alloy content. The electrochemical measurements show that the plateau pressure Ti_0.10Zr_0.15V_0.35Cr_0.10Ni_0.30 + x% La_0.85Mg_0.25Ni_4.5Co_0.35Al_0.15 (x = 0, 1, 5, 10, 20) hydrogen storage alloys increase with increasing x, and the width of the pressure plateau first increases when x increases from 0 to 5 and then decreases as x increases further, and the maximum discharge capacity changes in the same trend. The activation performance, the low temperature dischargeabilities, high-rate dischargeability and cyclic stability of composite alloy electrodes increase greatly with increasing x. The improvement of the electrochemical characteristics caused by adding AB₅ alloy seems to be related to formation of the secondary phase.     

Design and optimization of single particle electrodes for the kinetic analysis of hydrogen evolution and absorption on hydrogen storage alloys

In the literature, there is a large discrepancy between reported values of electrochemical, kinetic and transport parameters of hydrogen storage alloys. These discrepancies arise, because in most cases, electrodes are prepared with the powdered alloy supported within a porous matrix, constituted by carbon and additive binders such as PTFE. The main drawback, of this preparation technique, for the identification of kinetic parameters, is the uncertainty in the specific active area value, where the hydrogen evolution and absorption processes take place. To overcome the disadvantages described, a new type of electrode, was designed, using a single particle of AB₅ and AB₂ hydride forming alloys. The data obtained from electrochemical impedance measurements were adjusted in terms of a physicochemical model that takes into account the processes of hydrogen evolution and absorption coupled to hydrogen diffusion. From the study it can be concluded that the differences in the behavior of the AB₅ and AB₂ alloys, presenting the first best performance during the activation and operation at high discharge currents, are mainly due to higher values of the exchange current density and the diffusion coefficient of H for the AB₅ alloy.     

Interactions of Y and Cu on Mg2Ni type hydrogen storage alloys: A study based on experiments and density functional theory calculation

Evolution of microstructure and hydrogen storage performances were studied in a Y substituted Mg₂₄Ni₁₀Cu₂ hydrogen storage alloy. Interactions of Y and Cu on the phase structure and hydrogen storage properties were explore. Substitution by Y refined the microstructure and yield existence of YMgNi₄. Furthermore, Y addition promoted the replacement of Cu for Ni in the Mg₂Ni.The study of the alloy's dehydrogenation performance and mechanism showed that the addition of Y did not alter the mechanism of random nucleation and subsequent growth, but reduced the activation energy of the dehydrogenation of the alloy from 77.4 kJ/mol to 67.6 kJ/mol. The thermodynamic energy of the dehydrogenation was also improved, and the enthalpy change (ΔH) and entropy change (ΔS) of the Mg₂NiH₄ phase decreased from 67.1 J/K/mol H₂ and 123.1 J/K/mol H₂ to 61.1 J/K/mol H₂ and 115.4 J/K/mol H₂, respectively. Furthermore, the density functional theory calculation showed that the addition of Y promoted the substitution of Cu for Ni, further reduced the stability of the main hydride Mg₂NiH₄, facilitated the release of hydrogen, and reduced the ΔH and ΔS of the hydride dehydrogenation.     

Study of hydrogen storage properties of oxygen modified Ti- based AB2 type metal hydride alloy

A multi component AB₂ type hydrogen storage intermetallic alloy (A = Ti_0.85Zr_0.15, B₂ = Mn_1.22Ni_0.22Cr_0.2V_0.3Fe_0.06; was investigated in this work. The intermetallic specified above was modified by oxygen to yield the composition AB₂O_0.05. The oxygen was introduced by adding TiO₂ to 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 H₂ storage, particularly for utility vehicles, due to their relatively high reversible H₂ storage capacity (1.6 wt%) and low plateau pressure at room temperature (<5 bar). The addition of oxygen improved hydrogen absorption kinetics in the AB₂ alloy allowing it to immediately absorb H₂ without activation while for the non-modified sample an incubation period (30 min) was observed at the same conditions.     

Hybrid nickel-metal hydride/hydrogen battery

High capacity, high efficiency and resource-rich energy storage systems are required to store large scale excess electrical energy from renewable energy. We proposed “Hybrid Nickel-Metal Hydride/Hydrogen (Ni-MH/H₂) Battery” using high capacity AB₅-type hydrogen storage alloy and high-pressure H₂ gas as negative electrode active materials. It was experimentally confirmed that hydrogen gas can be utilized as an active material of negative electrode by the presence of the AB₅-type hydrogen storage alloy. The experimental average cell voltage suggested that H₂ gas passed through the alloy in the form of atoms. The calculated gravimetric energy density of this hybrid battery increased up to 1.5 times of the conventional Ni-MH battery with low content of rare-earth element which is 32 wt% of the Ni-MH battery.     

Mg@C60 nano-lamellae and its 12.50 wt% hydrogen storage capacity

The design and synthesis of new hydrogen storage materials with high capacity are the prerequisite for extensive hydrogen energy application which can be achieved by multi-site hydrogen storage. Herein, a Mg@C₆₀ nano-lamellae structure with multiple hydrogen storage sites has been prepared through a simple ball-milling process in which Mg nanoparticles (∼5 nm) are homogeneously dispersed on C₆₀ nano-lamellae. The as-obtained C₆₀/Mg nano-lamellae displays an excess hydrogen uptake of 12.50 wt% at 45 bar, which is far higher than the theoretical value (7.60 wt%) of metal Mg and the US Department of Energy (DOE) target (5.50 wt%, 2020 year), also the experimental values reported by now. The enhanced hydrogen storage mainly comes from several storage sites: MgH₂, H_x–C₆₀ (CH chemical bonding), H₂@C₆₀ (the endohedral H₂ in C₆₀). Interestingly, the hybridization of Mg and C₆₀ not only facilitate the dissociation of H₂ molecules to form CH bonding with C₆₀, but also promote the deformation of C₆₀ and access H₂ molecules into the cavity of C₆₀. This work provides new insight into the underlying chemistry behind the high hydrogen storage capacities of a new class of hydrogen storage materials, fullerene/alkaline-earth metals nanocomposites.     

Effect of rare earth composition on the high-rate capability and low-temperature capacity of AB5-type hydrogen storage alloys

Effects of rare earth composition on the high-rate capability and low-temperature capacity of AB₅-type hydrogen storage alloys have been studied and analyzed with pattern recognition methods. The results show that the increase of Ce and Pr and the decrease of La and Nd concentration improve the high-rate capability and low-temperature capacity of AB₅-type hydrogen storage alloys, Ce exhibiting better favorable influences than Pr. The improvement of both high-rate capability and low-temperature capacity are mainly ascribed to the lower stability of the hydride. The alloy with the rare earth composition of La_0.1645Ce_0.7277Pr_0.0234Nd_0.0845 shows very good high-rate capability and low-temperature capacity.     

Nanoscale composition modulations in MgyTi1−yHx thin film alloys for hydrogen storage

A detailed structural analysis of Mg–Ti–H thin films reveals the presence of a chemically partially segregated but structurally coherent metastable phase. By combining X-Ray Diffraction and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy on Mg_yTi_1−yH_x thin films we find non-zero Chemical Short-Range Order (CSRO) parameters for all the compositions measured. Despite the positive enthalpy of mixing of Mg and Ti the degree of ordering does not increase upon loading and unloading with hydrogen. The robustness of this system and the fast and reversible kinetics of hydrogen loading and unloading are caused by the formation of nanoscale compositional modulations in the intermetallic alloy. This microstructure is responsible for the exceptional properties of Mg_yTi_1−yH_x thin films. It also shows that reversible metastable metal-hydrides offer new possibilities for hydrogen storage, beyond the limits imposed by thermodynamic equilibrium.