Studies of off-stoichiometric AB2 metal hydride alloy: Part 1. Structural characteristics

In Part 1 of this two-part series of papers, phase abundances, lattice parameters, crystallite sizes, and microstructures of three series of AB₂-based metal hydride alloys were studied. The base alloys with B/A stoichiometry of 2.0 in series 177, 190, and 193 are rich in C14, equal in C14/C15, and rich in C15 phases, respectively. In each series of alloys, the B/A stoichiometry varies from 1.8, 1.9, 2.0, 2.1, to 2.2. The effects of varying B/A stoichiometry to microstructures are the same for these three series of alloys. As the alloy formula changes from AB_1.8, AB_1.9, AB_2.0, AB_2.1, to AB_2.2, the following events occur: C14-to-C15 phase ratio decreases, both C14 and TiNi secondary phase lattice parameters and unit cell volume reduce; the a/c aspect ratio of C14 phase first decreases and then increases; abundances of non-Laves secondary phases decrease; and the Zr/Ti ratio in AB phase decreases. The C14/C15 ratio is closely related to the average electron density with a threshold that first decreases from 7.13 (AB_1.8) to 7.08 (AB_1.9) and to 7.06 (AB_2.0) and then increases to 7.08 (AB_2.1) and 7.09 (AB_2.2) as the stoichiometry increases. The distributions of B-site elements are not uniform with most of the V, Cr, Mn, Co residing in AB₂ phase and Sn in Zr₇Ni₁₀ phase.     

Improvement in the electrochemical properties of gas atomized AB2 metal hydride alloys by hydrogen annealing

The effects of high temperature hydrogen annealing were studied on powders made by gas atomization of both conventional vanadium-containing AB₂ metal hydride alloys and new vanadium-free AB₂ alloys designed for high power and low self-discharge applications. In both alloy systems, annealing in 950 °C hydrogen for 30 min was proven to be effective in improving the capacity, formation, high power, and low temperature performance in the nickel metal hydride battery compared to previous gas atomization trials where each property was reduced. The advantage in improving the cycle life by gas atomization was further extended by the hydrogen annealing process. Reduction in the surface oxide was confirmed by the use of Auger electron spectroscopy depth profiling and magnetic susceptibility. Metallic elements were reduced from the oxide state by hydrogen to react with the metallic nickel particulates originally embedded in the surface oxide in a high temperature environment and created a new surface free of oxygen.     

Studies of off-stoichiometric AB2 metal hydride alloy: Part 2. Hydrogen storage and electrochemical properties

In Part 2 of this two-part series of papers, gaseous hydrogen storage and electrochemical properties of three series of alloys with different combinations of Cr/Mn/Co ratios are studied and compared to the structural properties reported in Part 1. As the B/A stoichiometry in each series of alloys increases from 1.8 to 2.2, systematic trends in certain storage properties are found: the hydrogen dissociation pressure and heat of hydride formation increases; the alloy with a AB_2.0 stoichiometry has the highest electrochemical full capacity; and slightly higher and lower B-contents increase the electrochemical high-rate-dischargeability and gaseous phase maximum storage capacity, respectively. Stoichiometric or slightly hyper-stoichiometric AB₂ alloys have lower PCT hysteresis which are expected to reduce pulverization during cycling. The full and high-rate discharge electrochemical capacities correlate well with the maximum and reversible gaseous hydrogen storages, respectively. Slight hyper-stoichiometry increases the high-rate dischargeability. Open circuit voltage, an important parameter in high-power application, is also found to be more relevant to the surface reaction than to the bulk hydride stability.     

Effects of H2O2 addition to the cell balance and self-discharge of Ni/MH batteries with AB5 and A2B7 alloys

In this paper we have compared nickel/metal hydride batteries made from AB₅ and Nd-only A₂B₇ alloys with or without addition of hydrogen peroxide (H₂O₂). The biggest advantages Nd-only A₂B₇ alloys have over AB₅ alloys are: a higher positive electrode utilization rate, lower initial internal resistance and less resistance increase after a 60 °C storage, and higher capacity and resistance degradation during cycling. The hydrogen peroxide was used as an oxidation agent and was added into the electrolyte before closing the cells. The H₂O₂ can oxidize both Co(OH)₂ in the positive electrode and MH alloy in the negative electrode. From the test results, H₂O₂ oxides the MH alloy preferentially over the Co(OH)₂ in the case of AB₅ alloy. This preferential oxidation is reversed in the case of the A₂B₇ alloy in which Co(OH)₂ is oxidized first. In cells made from both alloys, the addition of H₂O₂ prevented the venting of cells during formation, increased the utilization of positive electrode, improved the 60 °C charge retention, and increased the mid-point voltage after 300 cycles. Additionally the H₂O₂ also improved the cell balance for A₂B₇ alloy by decreasing the over-discharge reservoir in the negative electrode and reducing the capacity degradation in A₂B₇ alloy. However, the addition of H₂O₂ in cells made with AB₅ alloy deteriorated the cell balance by increasing the over-discharge reservoir in the negative electrode. The different cell balance and failure mechanisms for the two alloy compositions and H₂O₂ additive were compared and discussed.     

Synthesis,characterization and hydrogen storage behaviour of AB2 (ZrFe2, Zr(Fe0.75V0.25)2, Zr(Fe0.5V0.5)2 type materials

In this paper, we describe and discuss the synthesis, structural-microstructural and hydrogen storage behaviour of three AB₂ type storage materials namely (a) ZrFe₂, (b) Zr(Fe_0.75V_0.25)₂ and (c) Zr(Fe_0.5V_0.5)₂. These alloys were synthesied by radio frequency induction melting in argon atmosphere. X-ray diffraction and transmission electron microscope have been employed for structural and microstructural characterizations. The XRD study reveals that the lattice constants and the unit cell volume of ZrFe₂, Zr (Fe_0.75V_0.25)₂, Zr(Fe_0.5V_0.5)₂ alloys, which has C14 type hexagonal Laves phase. The Surface morphology and elemental composition of these alloys were investigated by scanning electron microscope and energy dispersive X-ray analysis. The pressure composition isotherms of these alloys were investigated at room temperature and pressure ranges of 0–100 atm respectively, measured through a fully computerized PCI apparatus. As we increase the concentration of V (substituted for Fe), the total hydrogen storage capacities increased up to 1.45 wt%. This capacity is achieved in Zr(Fe_0.5V_0.5)₂ alloy, while the reversible hydrogen storage capacity decreases due to the formation of a stable hydride phase. It has been found that the lattice constants increase with higher vanadium concentration. This is indicating that the majority of vanadium atoms reside in the B-site. The broader X-ray diffraction peaks observed in Zr(Fe_0.5V_0.5)₂ alloy indicates a higher degree of disorder for alloys with the higher V-content. The yet another interesting feature observed in our present study is that the plateau pressure remains well below 1 atm for all the compositions.     

Microstructure and electrochemical hydrogen storage characteristics of (La0.7Mg0.3)1−xCexNi2.8Co0.5 (x = 0-0.20) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of (La_0.7Mg_0.3)_1−xCe_xNi_2.8Co_0.5 (x = 0, 0.05, 0.10, 0.15 and 0.20) alloys have been investigated. The results show that all alloys consist of (La, Mg)Ni₃ and LaNi₅ phases. The cyclic stability (S₁₀₀) of the alloy electrodes increases from 58.7% (x = 0) to 69.8% (x = 0.20) after 100 charge/discharge cycles. The high rate dischargeability (HRD) increases from 66.8% (x = 0) to 69.6% (x = 0.10), then decreases to 65.1% (x = 0.20) at the discharge current density of 1200 mA/g. Moreover, the electrochemical kinetic characteristics of the alloy electrodes are also improved by increasing Ce content.     

Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2

The influence of multiple additions of two oxides, Cr₂O₃ and Nb₂O₅, as additives on the hydrogen sorption kinetics of MgH₂ after milling was investigated. We found that the desorption kinetics of MgH₂ were improved more by multiple oxide addition than by single addition. Even for the milled MgH₂ micrometric size powders, the high hydrogen capacity with fast kinetics were achieved for the powders after addition of 0.2 mol% Cr₂O₃ + 1 mol% Nb₂O₅. For this composition, the hydride desorbed about 5 wt.% hydrogen within 20 min and absorbed about 6 wt.% in 5 min at 300 °C. Furthermore, the desorption temperature was decreased by 100 °C, compared to MgH₂ without any oxide addition, and the activation energy for the hydrogen desorption was estimated to be about 185 kJ mol⁻¹, while that for MgH₂ without oxide was about 206 kJ mol⁻¹.     

The structure, hydrogen storage, and electrochemical properties of Fe-doped C14-predominating AB2 metal hydride alloys

A series of Fe-substituting cobalt C14-predoninating AB₂ alloys with the general formula Ti₁₂Zr_21.5V₁₀Cr_7.5Mn_8.1Fe_xCo_8−xNi_32.2Sn_0.3Al_0.4 (x = 0-5) were studied for the impacts of Fe to structure, gaseous, and electrochemical hydrogen storage properties. All alloys exhibit hyper-stoichiometric C14 main phase due to the formation of A-rich non-Laves secondary phases and the loss of Zr and Ti in the melt. Lattice parameters together with the unit cell volume increases and then decreases with increasing Fe-content which indicates the existence of anti-site defects. The amount of TiNi secondary phase increases with the increase of Fe-content up to 4% and shows a detrimental effect to the high-rate dischargeability of the alloys. Most of the gaseous storage characteristics remain unchanged with the addition of Fe. In the electrochemical properties, Fe-addition in the AB₂ alloys facilitates activation, increases the total electrochemical capacity and effective surface reaction area, decreases the half-cell high-rate dischargeability and bulk hydrogen diffusion, and deteriorates both −10 and −40 °C low-temperature performance. Fe-substituting Co in AB₂ alloys as negative electrode of nickel metal hydride battery can reduce the raw material cost with the trade-off being mainly in the low-temperature performance.     

Effects of SnO2 on hydrogen desorption of MgH2

The hydrogen desorption properties of Magnesium Hydride (MgH₂) ball milled with cassiterite (SnO₂) have been investigated by X-ray powder diffraction and thermal analysis. Milling of pure MgH₂ leads to a reduction of the desorption temperature (up to 60 K) and of the activation energy, but also to a reduction of the quantity of desorbed hydrogen, referred to the total MgH₂ present, from 7.8 down to 4.4 wt%. SnO₂ addition preserves the beneficial effects of grinding on the desorption kinetics and limits the decrease of desorbed hydrogen. Best tradeoff – activation energy lowered from 175 to 148 kJ/mol and desorbed hydrogen, referred to the total MgH₂ present, lowered from 7.8 to 6.8 wt% – was obtained by co-milling MgH₂ with 20 wt% SnO₂.     

Study of the different ZrxNiy phases of Zr-based AB2 materials

Hydride-forming alloys are used as components of the negative electrode of nickel-metal hydride (NiMH) batteries. In previous works, the study of Zr-based AB₂-type alloys indicated that the material without heat treatment (annealing) had better electrochemical characteristics than the annealed one. The effect was attributed to the presence of secondary phases Zr_xNi_y formed during the solidification of the alloy button obtained by arc melting, and to the fact that these phases diminished their concentration or disappeared upon annealing. The main secondary phases formed by microsegregation are Zr₇Ni₁₀, Zr₉Ni₁₁ and Zr₈Ni₂₁.     

MgH2 intermediate scale tank tests under various experimental conditions

Magnesium hydrogenation being an exothermic reaction, the loading time of a tank is limited by heat extraction. The compaction of ball-milled MgH₂ associated with Expanded Natural Graphite was used to improve the thermal conductivity of the resulting compacts. Taking advantage of these compacts, an intermediate scale tank (1.8 kg MgH₂) cooled down by forced air circulation was designed. The absorption is initiated at 90 °C. Since the intrinsic kinetic is not the limiting factor, the hydrogen pressure does not affect the loading process. The loading time is strongly dependent on the cooling efficiency. However, beyond a given air flow rate it doesn’t decrease any more, the heat transfer being limited by the thermal conductivity of the compacted disks. During desorption, the maximum hydrogen flow (25 Nl/mn) is directly proportional to the thermal power of the heating system. The tank absorbs 1170 Nl, has a specific-energy of 270 W h/kg and a system volumetric-density of 42 gr/l.     

Comparative analysis of the hydriding kinetics of LaNi5, La0.8Nd0.2Ni5 and La0.7Ce0.3Ni5 compounds

In two-phase domains, the plateau pressure of hydride forming materials (such as intermetallic compounds or IMCs) depends markedly on the operating temperature (Van’t Hoff relationships). Therefore, for practical applications, it is necessary to select hydrogen storage materials by considering the thermal environment of the hydride tank. The thermodynamic properties (absorption and desorption pressure plateaux) of IMCs can be adjusted to some extend by chemical alloying with foreign metals and substitution on different crystallographic sites. In this paper, we report on the hydriding kinetics of substituted AB₅ compounds. Isotherms have been measured at different temperatures on La_xNd_1−xNi₅ (x ≈ 0.2) and La_xCe_1−xNi₅ (x ≈ 0.3) compounds. Pneumato-chemical impedance spectroscopy has been used to analyze the hydriding kinetics and to determine microscopic rate parameters associated with surface dissociation of molecular hydrogen, diffusion-controlled transport of atomic hydrogen to bulk regions and hydride formation. Results have been compared to those measured on LaNi₅ and the interest of using such substituted compounds for application in auxiliary power units is discussed.     

Effect of molybdenum content on structural, gaseous storage, and electrochemical properties of C14-predominant AB2 metal hydride alloys

The structure, gaseous storage, and electrochemical properties of Mo-modified C14-predominant AB₂ metal hydride alloys were studied. The addition of Mo expands the unit cell volume and stabilizes the metal hydride. This increased metal-to-hydrogen bond strength reduces the equilibrium plateau pressure, reversible hydrogen storage, and the high-rate dischargeability in the flooded cell configuration, but not the high-rate dischargeability in the sealed cell configuration. The low-temperature performance was improved by the addition of Mo through increases in bulk diffusion rate, surface area, and surface catalytic ability. The increase in bulk diffusion is the result of smaller crystallites and larger AB₂-AB₂ grain boundary densities. The increase in surface area is due to the high solubility of Mo in alkaline solution. Even with a higher leaching rate, the Mo-containing alloys still have strong corrosion resistance which contributes positively to both the charge retention and the cycle life performances. As the Mo-content in the alloy increases, the low temperature performance improves at the expense of a lower capacity.     

Hydrogen storage properties of flexible and porous La0.8Mg0.2Ni3.8/PVDF composite

A study on the hydrogen storage properties of flexible and porous La_0.8Mg_0.2Ni_3.8/PVDF (polyvinylidene fluoride) composite was reported. In this composite, PVDF acted as a binder to connect the alloy powders and (NH₄)₂CO₃ as a pore-forming agent to create void space. Increasing PVDF content, the hydrogen absorption kinetics of the composite gradually decreased. Increasing (NH₄)₂CO₃ from 1% to 5%, the capacity firstly increased and then decreased. 0.08–0.13 wt% increased capacity for the composite was observed at 70 °C by comparison with the intrinsic composite (La_0.8Mg_0.2Ni_3.8/1%PVDF). Varying temperature from 0 °C to 100 °C, 0.1–0.15 wt% increased capacity were obtained for the typical porous composite (La_0.8Mg_0.2Ni_3.8/1%PVDF/3%(NH₄)₂CO₃). The PVDF-assisted composite showed the flexible/solidified characteristic in hydriding/dehydriding, which maybe lowed down the oxidation of the alloy powders and preserved the void space. Finally, ∼0.1 wt% increased capacity remained after ten hydriding/dehydriding cycles.     

A study of the ZrF4, NbF5, TaF5, and TiCl3 influences on the MgH2 sorption properties

Magnesium hydride with 7 wt.% of various metal halide additives (ZrF₄, TaF₅, NbF₅ and TiCl₃) were ball milled, and the influence of these dopants on the kinetics of absorption and desorption was studied. The pressure-composition-temperature isotherms (P-C-T) measured by Sieverts’ apparatus did not show thermodynamic changes in the studied materials. Moreover, XPS studies demonstrated that the metal halides used in this study (except ZrF₄) took part in the partial and full disproportionation reactions directly after milling and the first desorption/absorption cycle. The catalytic effect of metal halides on the Mg hydrogenation/dehydrogenation process was caused by the formation of pure transition metal and/or the MgF₂ phase, which led to the influence of two simultaneous factors on the sorption properties of the MgH₂.     

Cycling performance and hydriding kinetics of LaNi5 and LaNi4.73Sn0.27 alloys in the presence of CO

We analyzed the sorption cycling behavior of LaNi₅ and LaNi_4.73Sn_0.27 alloys in H₂ containing 10 and 100 ppm of CO. The effect of temperature was studied for the Sn-containing alloy. When cycling in the presence of CO, we found the reaction was strongly retarded due to surface contamination but no loss of capacity was observed when samples were given enough time for both absorption and desorption. The retardation was stronger at lower temperatures and higher CO concentration. The results also indicate that a fraction of the adsorbed CO is released during the desorption process. For the Sn-containing alloy, a stationary state is met after about 10 cycles, with no further degradation occurring past this point. The retarding factor at 40 °C and 100 ppm in this condition, with respect to the kinetics in pure hydrogen, is of about 600.     

Hydrogen evolution characteristics of Ni–Mn microencapsulated MlNi3.03Si0.85Co0.60Mn0.31Al0.08 alloys in 6 M KOH

Nickel–manganese alloys were coated from sulphate baths by electrodeposition with ‘Packed Bed’ technique on the surface of proprietary lanthanum rich non-stoichiometric MlNi_3.03Si_0.85Co_0.60Mn_0.31Al_0.08 (Ml = lanthanum rich misch metal) hydrogen storage alloy particles. The structure and nature of the microencapsulated alloys were characterized using X-ray diffraction (XRD) and electron paramagnetic resonance (EPR). The hydrogen evolution reaction (HER) was investigated in 6 M KOH at 30 °C by galvnostatic cathodic polarisation technique. The effects of Ni/Mn ratio in the bath and deposition current density were studied. Among the investigated depositions, Ni₁₅₀Mn₁₀₀ (30) and Ni₁₅₀Mn₁₀ (60) (concentration of Ni and Mn salts in electrodeposition bath given in grams per liter; electrodeposition current density (CD) given within brackets in milliamphere per square centimeter) coated samples exhibited the highest activity towards the HER. It can be concluded that disordered paramagnetic coatings with Ni concentrations above 80 at.% exhibit higher catalytic activity towards HER. The Tafel mechanism is the easiest pathway for HER on most of the studied coatings. However, some of the Ni-rich coatings prefer the Volmer–Tafel path and one sample [Ni₁₅₀Mn₁₅₀ (80)] prefers the Heyrovsky–Volmer path.     

Hydrogen storage and release: Kinetic and thermodynamic studies of MgH2 activated by transition metal nanoparticles

Commercial metal nanoparticles of Fe, Co, Ni, Cu, Zn were added to MgH₂ by ball-milling to improve the kinetics of hydrogen release and the reversibility during successive absorption/desorption cycles. metal nanoparticles were well dispersed into the MgH₂ matrix without formation of any ternary metal hydrides, nor binary compounds. Activation energy values were determined for the various samples by temperature programmed desorption experiments while the hydride formation enthalpy was deduced from Van't Hoff equation starting from high pressure volumetric isotherms acquired at different temperatures. The presence of transient effect during the absorption process was excluded by comparing successive hydrogenation/dehydrogenation cycles recorded at 350 °C on Ni and Fe-containing samples. Information about hydrogen absorption kinetics was also obtained. Promisingly, the Ni, Fe, and Co containing samples have shown a good stability, enhanced catalytic performance, and high rate of hydrogen absorption while Zn and Cu nanoparticles worked more like inhibitors than activators.     

Preparation of dense composite membrane with Ba-cerate conducting oxide and rapidly solidified Zr-based alloy

Hydrogen separation with dense ceramic membranes is non-galvanic, i.e. it does not require any electrode or an external power supply to drive the separation, and the hydrogen selectivity is almost 100% because the membrane contains no interconnected porosity. In this study, a mixed proton-electron conducting perovskite made from BaCe_0.9Y_0.1O_3-δ (BCYO) was prepared using a solid-state reaction, whereas a rapidly solidified Zr-based alloy (RSZ) was obtained via a melt-spinning process at a specified cooling rate. Finally, the BCYO/RSZ composite membrane was successfully fabricated by aerosol deposition (AD) at room temperature. The powders and composite membranes were characterized by high-temperature X-ray diffraction (HTXRD), particle size analysis (PSA), scanning electron microscopy (SEM), and X-ray elemental mapping (XRM). The hydrogen permeability of the dense BCYO/RSZ composite membrane was measured with the change of temperature. Under a pure hydrogen atmosphere at 773 K-1073 K, the BCYO/RSZ composite membrane exhibited higher permeability compared with the sole BCYO membrane over the entire investigated temperature range.     

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.