Substitution of nickel in Mg2Ni and its hydride with elements from groups XIII and XIV: An ab initio study

We have performed ab initio calculations with equilibrium supercells of the Mg₂Ni compound and its hydride Mg₂NiH₄ doped with elements X = Al, Ga, In, Si, Ge and Sn. Two concentrations of X in both structures have been set: (1) every 16th, and (2) every fourth Ni atom has been substituted by X. Total energy calculations yielded the Mg₂NiH₄ hydrogen absorption enthalpy ΔH_abs according to the chemical reaction Mg₂Ni + 2H₂ → Mg₂NiH₄. Reduction of the hydrogen absorption enthalpy was reported for both concentrations of X. When doping the Mg₂NiH₄ hydride with X = In in a low concentration (1), the value of hydrogen desorption enthalpy decreases from 68.22 to 55.96 kJ(mol H₂)^?1. Doping with X = In in a high concentration (2) further decreases the hydrogen desorption enthalpy to 5.50 kJ(mol H₂)^?1. Further, the electronic structure of Mg₂(Ni–In)H₄ hydride with a low In concentration indicates weaker Ni–H bonds in comparison with the pristine Mg₂NiH₄. Attraction between H and In atoms induced enhanced bonding between Mg and H atoms compared to the pristine Mg₂NiH₄.     

First-principles studies of the structures and properties of Al- and Ag-substituted Mg2Ni alloys and their hydrides

The structures and properties of hydrogen storage alloy Mg₂Ni, of aluminum and silver substituted alloys Mg_2−xM_xNi (M = Al and Ag, x = 0.16667), and of their hydrides Mg₂NiH₄, Mg_2−xM_xNiH₄ (M = Al and Ag, x = 0.125) have been calculated from first-principles. Results show that the primitive cell sizes of the intermetallic alloys and hydrides were reduced by substitution of Mg with Al or Ag. Also, the interaction of Ni–Ni was weakened by the substitution. A strong covalent interaction between H and Ni atoms forms tetrahedral NiH₄ units in Mg₂NiH₄. The NiH₄ unit near the Al/Ag atom became tripod-like NiH₃ in Mg_2−xM_xNiH₄ (M = Al, Ag), indicating that the hydrogen storage capacity was decreased by the substitution. The calculated enthalpies of hydrogenation for Mg₂Ni, Mg_2−xAl_xNi and Mg_2−xAg_xNi are −65.14, −51.56 and −53.63 kJ/mol H₂, respectively, implying that the substitution destabilizes the hydrides. Therefore, the substitution is an effective technique for improving the thermodynamic behavior of hydrogenation/dehydrogenation in magnesium-based hydrogen storage materials.     

Ab initio calculations on energetics and electronic structures of cubic Mg3MNi2 (M = Al, Ti, Mn) hydrogen storage alloys

Mg₃MNi₂ (M = Al, Ti, Mn) ternary intermetallic compounds with cubic structure are a new type of potential hydrogen storage alloys. Using ab initio density functional theory (DFT) calculations, the energetics and electronic structures of Mg₃MNi₂ (M = Al, Ti, Mn) compounds are systematically investigated. The optimized structural parameters including lattice constants and internal atomic positions are close to experimental data determined from X-ray powder diffraction. The calculated results of formation enthalpy ΔH_form show that the stabilities of cubic Mg₃MNi₂ (M = Al, Ti, Mn) compound, compared with hexagonal Mg₂Ni, increase in the order of Mg₃MnNi₂, Mg₂Ni, Mg₃TiNi₂ and Mg₃AlNi₂, whereas the stabilities of their saturated Mg₃MNi₂H₃ (M = Al, Ti, Mn) hydrides, compared with monoclinic Mg₂NiH₄, decrease in the order of Mg₂NiH₄, Mg₃AlNi₂H₃, Mg₃TiNi₂H₃ and Mg₃MnNi₂H₃. Further calculations of hydrogen desorption enthalpy ΔH_des indicate that these cubic Mg₃MNi₂ (M = Al, Ti, Mn) compounds possess promising dehydrogenation properties for their relatively lower ΔH_des values. Among of them, the dehydrogenation ability of Mg₃TiNi₂ is the most pronounced. Analysis of electronic structures suggests that the strong covalent bonding interactions between Ni and M within cubic Mg₃MNi₂ (M = Al, Ti, Mn) are dominant and directly control the structural stabilities of these compounds.     

Catalytic effect of Ni, Mg2Ni and Mg2NiH4 upon hydrogen desorption from MgH2

MgH₂ is a perspective hydrogen storage material whose main advantage is a relatively high hydrogen storage capacity (theoretically, 7.6 wt.% H₂). This compound, however, shows poor hydrogen desorption kinetics. Much effort was devoted in the past to finding possible ways of enhancing hydrogen desorption rate from MgH₂, which would bring this material closer to technical applications. One possible way is catalysis of hydrogen desorption. This paper investigates separate catalytic effects of Ni, Mg₂Ni and Mg₂NiH₄ on the hydrogen desorption characteristics of MgH₂. It was observed that the catalytic efficiency of Mg₂NiH₄ was considerably higher than that of pure Ni and non-hydrated intermetallic Mg₂Ni. The Mg₂NiH₄ phase has two low-temperature modifications below 508 K: un-twinned phase LT1 and micro-twinned phase LT2. LT1 was observed to have significantly higher catalytic efficiency than LT2.     

Effects of nonmetal element (B,C and Si) additives in Mg2Ni hydrogen storage alloy: A first-principles study

Nonmetal atoms (B, C and Si) are designed to add into Mg₂Ni hydrogen storage alloy and its hydride. First-principles density-functional theory calculations have been performed to investigate their crystal structures, electronic and thermodynamic properties. The calculation results present that nonmetal additions in Mg₂NiH₄ show more effective destabilization than metal additions. Especially for B and C, the decreases of formation enthalpies of Mg₂NiH₄ reach 0.19 and 0.21 eV/atom. The NiH₄ structure near B or C in Mg₂NiH₄ hydride becomes the tripod-like NiH₃ structure. The results show that the thermodynamic stabilities of Mg₂Ni and Mg₂NiH₄ exhibit a nearly linear decrease with the increasing content of nonmetal atoms. The calculated dehydrogenation energies are 59.39, 58.12, 55.84 and 55.30 kJ/mol H₂ for Mg₂NiH₄, Mg₂NiB_0.5H₄, Mg₂NiC_0.5H₄ and Mg₂NiSi_0.5H_4, respectively. It is found that the addition of nonmetal atoms favors the dehydrogenation reaction for Mg₂Ni hydrogen storage material. In addition, the effects of nonmetals to the heat capacities and vibrational entropies of Mg₂Ni and Mg₂NiH₄ are also analyzed.     

Hydrogen desorption performance of high-energy ball milled Mg2NiH4 catalyzed by multi-walled carbon nanotubes coupling with TiF3

The Mg₂NiH₄ complex hydrides were synthesized by high-energy ball milling (HEBM) MgH₂ + Ni mixtures. Multi-walled carbon nanotubes (MWCNTs) or TiF₃ as catalysts were added and the catalytic-dehydrogenation behaviors were investigated. All prepared samples are characterized by X-ray diffraction (XRD) spectroscopy, scanning electron microscope (SEM) and differential scanning calorimetry (DSC) to acquire information of microstructure, phase compositions, surface and dehydrogenation properties. The results indicate that the method of adding catalysts by HEBM is reasonable and the hydrogen desorption property of Mg₂NiH₄ is improved by catalysts. It is worth noting that the dehydrogenation temperature (T_D) and the activation energy (E_a) of Mg₂NiH₄ catalyzed by MWCNTs coupling with TiF₃ are reduced to 230 °C (243.6 °C of Mg₂NiH₄) and 53.24 kJ/mol (90.13 kJ/mol of Mg₂NiH₄), respectively. The addition of proper catalysts is proved to be an effective strategy to decrease T_D and E_a of Mg₂NiH₄ hydrides.     

Effect of in-situ formed Mg2Ni/Mg2NiH4 compounds on hydrogen storage performance of MgH2

Magnesium-based hydrogen storage materials (MgH₂) are promising hydrogen carrier due to the high gravimetric hydrogen density; however, the undesirable thermodynamic stability and slow kinetics restrict its utilization. In this work, we assist the de/hydrogenation of MgH₂ via in situ formed additives from the conversion of an MgNi₂ alloy upon de/hydrogenation. The MgH₂–16.7 wt%MgNi₂ composite was synthesized by ball milling of Mg powder and MgNi₂ alloy followed by a hydrogen combustion synthesis method, where most of the Mg converted to MgH₂, and the others reacted with the MgNi₂ generating Mg₂NiH₄, which produced in situ Mg₂Ni during dehydrogenation. Results showed that the Mg₂Ni and Mg₂NiH₄ could induce hydrogen absorption and desorption of the MgH₂, that it absorbed 2.5 wt% H₂ at 473 K, much higher than that of pure Mg, and the dehydrogenation capacity increased by 2.6 wt% at 573 K. Besides, the initial dehydrogenation temperature of the composite under the promotion of Mg₂NiH₄ decreased greatly by 100 K, whereas it is 623 K for MgH₂. Furthermore, benefiting from the catalyst effect of Mg₂NiH₄ during dehydrogenation, the apparent activation energy of the composite reduced to 73.2 kJ mol⁻¹ H₂ from 129.5 kJ mol⁻¹ H₂.     

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.     

Microstructural evolution and hydrogen storage proprieties of melt-spun eutectic Mg76.87Ni12.78Y10.35 alloy with low hydrides formation/decomposition enthalpy

Ternary eutectic Mg_76.87Ni_12.78Y_10.35 (at. %) ribbons with mixed amorphous and nanocrystalline phases were prepared by melt spinning. The microstructures of the melt-spun, hydrogenated and dehydrogenated samples were examined and compared by X-ray diffraction and transmission electron microscopy. The amorphous structure transforms into a thermally stable nanocrystalline structure with a grain size of about 5 nm during hydrogen ab/desorption cycles. The Mg, Mg₂Ni and phases with Y in the melt-spun state transform into MgH₂, Mg₂NiH₄, Mg₂NiH_0.3, YH₂ and YH₃ after hydrogenation, and transform back to Mg, Mg₂Ni and YH₂ upon subsequent dehydrogenation. The reaction enthalpy (ΔH) and entropy (ΔS) of the higher plateau pressure corresponding to Mg₂Ni hydride formation are −53.25 kJ mol⁻¹ and −107.74 J K⁻¹ mol⁻¹, respectively. The amorphous/nanocrystalline structure effectively reduces the enthalpy and entropy of Mg₂Ni hydride formation, but has little effect on Mg. The activation energy for dehydrogenation of the hydrogenated ribbons is 69 kJ mol⁻¹. This suggests that Mg–Ni–Y with ternary eutectic composition can form an amorphous/nanocrystalline structure by melt spinning, and this nanostructure efficiently improves the thermodynamics and kinetics for hydrogen storage.     

Structure and hydrogen storage properties of Mg2Cu1−xNix (x = 0–1) alloys

The effect of Ni-substitution on the structure and hydrogen storage properties of Mg₂Cu_1−xNi_x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloys prepared by a method combining electric resistance melting with isothermal evaporation casting process (IECP) has been studied. The X-ray single-crystal diffraction analysis results showed that the cell volume decreases with increasing Ni concentration, and crystal structure transforms Mg₂Cu with face-centered orthorhombic into Ni-containing alloys with hexagonal structure. The Ni-substitution effects on the hydriding reaction indicated that absorption kinetics and hydrogen storage capacity increase in proportion to the concentration of the substitutional Ni. The activated Mg₂Cu and Mg₂Ni alloys absorbed 2.54 and 3.58 wt% H, respectively, at 573 K under 50 bar H₂. After a combined high temperature and pressure activation cycle, the charged samples were composed of MgH₂, MgCu₂ and Mg₂NiH₄ while the discharged samples contained ternary alloys of Mg–Cu–Ni system with the helpful effect of rising the desorption plateau pressures compared with binary Mg–Cu and Mg–Ni alloys. With increasing nickel content, the effect of Ni is actually effective in MgH₂ and Mg₂NiH₄ destabilization, leading to a decrease of the desorption temperature of these two phases.     

Reactivity of complex hydrides Mg2FeH6, Mg2CoH5 and Mg2NiH4 with lithium ion: Far from equilibrium electrochemically driven conversion reactions

The electrochemical reaction of lithium ion with Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄ complex hydrides prepared by reactive grinding is studied here. Plateaus at an average potential of 0.25 V, 0.24 V and 0.27 V corresponding to discharge capacities of 6.6, 5.5 and 3.6 Li can be achieved respectively for Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄. From in situ X-ray diffraction (XRD) characterizations of complex hydride based electrodes, dehydrogenation leads to a decrease of the intensities of the diffraction peaks suggesting a strong loss of crystallinity since formation of Mg and M (M = Fe, Co, Ni) peaks is not observed. ⁵⁷Fe Mössbauer spectroscopy confirms the formation of nanoscale Fe or an amorphous Mg–Fe alloy during the decomposition of Mg₂FeH_6. Interestingly, lattice parameter variations suggest phase transitions in the Mg₂NiH₄ system involving the formation of low hydrogen content hydride Mg₂NiH, while an increase of lattice parameters of Mg₂CoH₅ hydride could be attributed to the formation of a Mg₂CoH₅Li_x solid solution compound up to x = 1.     

Effect of Ni/tubular g-C3N4 on hydrogen storage properties of MgH2

Additive doping is one of the effective methods to overcome the shortcomings of MgH₂ on the aspect of relatively high operating temperatures and slow desorption kinetics. In this paper, hollow g-C₃N₄ (TCN) tubes with a diameter of 2 μm are synthesized through the hydrothermal and high-temperature pyrolysis methods, and then nickel is chemically reduced onto TCN to form Ni/TCN composite at 278 K. Ni/TCN is then introduced into the MgH₂/Mg system by means of hydriding combustion and ball milling. The MgH₂–Ni/TCN composite starts to release hydrogen at 535 K, which is 116 K lower than the as-milled MgH₂ (651 K). The MgH₂–Ni/TCN composite absorbs 5.24 wt% H₂ within 3500 s at 423 K, and takes up 3.56 wt% H₂ within 3500 s, even at a temperature as low as 373 K. The apparent activation energy (E_a) of the MgH₂ decreases from 161.1 to 82.6 kJ/mol by the addition of Ni/TCN. Moreover, the MgH₂–Ni/TCN sample shows excellent cycle stability, with a dehydrogenation capacity retention rate of 98.0% after 10 cycles. The carbon material enhances sorption kinetics by dispersing and stabilizating MgH₂. Otherwise, the phase transformation between Mg₂NiH₄ and Mg₂NiH_0.3 accelerates the re/dehydrogenation reaction of the composite.     

Dual-tuning effect of In on the thermodynamic and kinetic properties of Mg2Ni dehydrogenation

Mg₂In_0.1Ni solid solution with an Mg₂Ni-type structure has been synthesized and its hydrogen storage properties have been investigated. The results showed that the introduction of In into Mg₂Ni not only significantly improved the dehydrogenation kinetics but also greatly lowered the thermodynamic stability. The dehydrogenation activation energy (E_a) and enthalpy change (ΔH) decreased from 80 kJ/mol and 64.5 kJ/mol H₂ to 28.9 kJ/mol and 38.4 kJ/mol H₂, respectively. The obtained results point to a method for improving not only the thermodynamic but also the kinetic properties of hydrogen storage materials.     

Dual-tuning of de/hydrogenation kinetic properties of Mg-based hydrogen storage alloy by building a Ni-/Co-multi-platform collaborative system

The Mg-based hydrogen storage alloy with multiple platforms is successfully prepared by ball milling Co powder and Mg-RE-Ni precursor alloy, and its hydrogen storage behavior was investigated in detail by XRD, EDS, TEM, PCI, and DSC methods. The ball-milled alloy consists of the main phase Mg, the catalytic phases Mg₂Ni, Mg₂Co as well as a small amount of Mg₁₂Ce, and convert into the MgH₂–CeH_2.73-Mg₂NiH₄–Mg₂CoH₅ composite after hydrogenation. The composite has three PCI platforms corresponding to the reversible de/hydrogenation reaction of Mg/MgH₂, Mg₂Ni/Mg₂NiH₄ and Mg₆Co₂H₁₁/Mg₂CoH₅. Among them, the transformation between Mg₂Ni and Mg₂NiH₄ triggers the “spill-over” effect which promote the decomposition of MgH₂ phases and enhances the hydrogen desorption kinetics. Meanwhile, the conversion of the Mg₆Co₂H₁₁ to Mg₂CoH₅ phase induces the “chain reaction” effect, which leads to preferential nucleation of Mg phase and improves the hydrogen absorption kinetics. Therefore, the Mg-RE-Ni-Co alloy has a double improvement on hydrogen absorption and desorption kinetics. Concretely, the alloy has an optimal hydrogen absorption temperature of 200 °C, at which it can absorb 5.5 wt. % H₂ within 40 s. Under the conditions, the capacity of absorption almost reaches the maximum reversible value (about 5.6 wt. %). Besides, the alloy has a dehydrogenation activation energy of 67.9 kJ/mol and can desorb 5.0 wt. % H₂ within 60 min at the temperature of 260 °C.     

Microstructure characteristics,hydrogen storage kinetic and thermodynamic properties of Mg80–xNi20Yx (x = 0–7) alloys

Mg_80–xNi₂₀Y_x (x = 0–7) alloys were successfully prepared by using the vacuum induction melting method. The structural characterizations of the alloys were performed by using X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy measurements. The effects of yttrium content on the microstructure and hydrogen storage properties of the as-cast alloys were investigated. The alloys are composed of a primary phase of Mg₂Ni, lamella eutectic composites of Mg + Mg₂Ni, and some amount of YNi₃. The YNi₃ has completely transformed into in situ formed nanoscale YH₂ and YH₃, with the formation of Mg₂NiH₄. Pulverization of the alloy particles was observed during the hydrogen absorption and desorption cycles. However, the phase composition remains unchanged even after 20 hydrogenation cycles. Yttrium addition significantly improved the hydrogen-absorption kinetic performance of the alloy. In addition, pressure-composition-temperature measurements indicated that the entropy and enthalpy changes for the formation and decomposition of MgH₂ and Mg₂NiH₄ gradually decreased with the increase of yttrium content.     

Enhanced dehydrogenation properties of LiBH4 compositing with hydrogenated magnesium-rare earth compounds

LiBH₄ is regarded as a promising hydrogen storage material due to its high hydrogen density. In this study, the dehydrogenation properties of LiBH₄ were remarkably enhanced by doping hydrogenated Mg₃RE compounds (RE denotes La, Ce, Nd rare earth metals), which are composed of nanostructured MgH₂ and REH_2+x (denoted as H − Mg₃RE). For the LiBH₄ + H − Mg₃La mixture, the component LiBH₄ desorbed 6 wt.% hydrogen even at a relatively low temperature of 340 °C, far lower than the desorption temperature of pure LiBH₄ or the 2LiBH₄ + MgH₂ system. This kinetic improvement is attributed to the hydrogen exchange mechanism between the H − Mg₃La and LiBH₄, in the sense that the decomposition of MgH₂ and LaH_2+x catalyzed the dehydrogenation of LiBH₄ through hydrogen exchange effect rather than mutual chemical reaction requiring higher temperature and hydrogen pressure. However, prior to fast hydrogen release, the hydrogen exchange effect suppressed the dehydriding of MgH₂ and elevated its desorption temperature. It is expected to strengthen the hydrogen exchange effect by compositing the LiBH₄ with other nanosized metal hydrides and to obtain better dehydrogenation properties.     

Hydrogenation and dehydrogenation behaviours of nanocrystalline Mg20Ni10−xCux (x = 0−4) alloys prepared by melt spinning

In order to improve the hydriding and dehydriding kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Cu, and the nanocrystalline Mg₂Ni-type Mg₂₀Ni_10−xCu_x (x = 0, 1, 2, 3, 4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results show that the substitution of Cu for Ni does not change the major phase Mg₂Ni. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hydrogen desorption capacity of the alloys grows with increasing Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacity and kinetics of the alloys.     

Remarkable irreversible and reversible dehydrogenation of LiBH4 by doping with nanosized cobalt metalloid compounds

Nanosized cobalt sulfide and cobalt boride were synthesized and doped into LiBH₄ to improve the dehydrogenation properties of this important candidate for hydrogen storage. With respect to CoS_x doping, the dehydrogenation temperature (peak temperature observed by mass spectrometry) of pristine LiBH₄ can be reduced from 440 °C to 175 °C with a maximum capacity of 6.7 wt% at 50% doping. Unfortunately, B₂H₆ is liberated and the process is not reversible because the CoS_x dopant reacts with LiBH₄ to form more stable compounds. By changing CoS_x to CoB_x, a reversible dehydrogenation was realized with greatly improved reversibility. The dehydrogenation temperature was reduced to 350 °C with a maximum capacity of 8.4 wt% at 50% doping amount. It is very significant that CoB_x is stable and the release of B₂H₆ is eliminated. A reversible hydrogen desorption of about 5.3 wt% can be achieved with a LiBH₄ + 50% CoB_x mixture under a mild rehydrogenation condition of 400 °C at 10 MPa H₂. It is obvious that CoS_x acts as a reactant even though the dehydrogenation is greatly enhanced, while CoB_x behaves as a catalyst significantly promoting the dehydrogenation and reversibility of LiBH₄.     

Mg2NiH4 synthesis and decomposition reactions

A ternary Mg₂NiH₄ hydride was synthesized using method that relies on a relatively short mechanical milling time (one hour) of a 2:1 MgH₂–Ni powder mixture followed by sintering at a sufficiently high hydrogen pressure (>85 bar) and temperature (>400 °C). The ternary hydride forms in less than 2.5 h (including the milling time) with a yield of ∼90% as a mixture of two polymorphic forms. The mechanisms of formation and decomposition of ternary Mg₂NiH₄ under different hydrogen pressures were studied in detail using an in situ synchrotron radiation powder X-ray diffraction (SR-PXD) and high pressure DSC. The obtained experimental results are supported by morphological and microstructural investigations performed using SEM and high resolution STEM. Additionally, effects occurring during the desorption reaction were studied using DSC coupled with mass spectrometry.