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1.
Mg2NiH4, with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg2NiH4, and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg2Ni and a destabilization of the hydride Mg2NiH4, which significantly alters the hydrogen desorption enthalpy ΔHdes for the reaction Mg2NiH4 → Mg2Ni + 2H2. A desirable enthalpy value of ∼0.4 eV/H2 for application can be obtained for a doping level of x ≥ 0.35 in Mg2−xAlxNi alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg2Ni and Mg2NiH4 exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg2NiH4 is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg2Ni and Mg3AlNi2 as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg2NiH4 can be attributed to the weakened Mg-Ni and Ni-H interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.  相似文献   

2.
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 ΔHform 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 ΔHdes indicate that these cubic Mg3MNi2 (M = Al, Ti, Mn) compounds possess promising dehydrogenation properties for their relatively lower ΔHdes 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.  相似文献   

3.
The hydrogenation characteristics and hydrogen storage kinetics of the melt-spun Mg10NiR (R = La, Nd and Sm) alloys have been studied comparatively. It is found that the Mg10NiNd and Mg10NiSm alloys are in amorphous state but the Mg10NiLa alloy is composed of an amorphous phase and minor crystalline La2Mg17 after melt-spinning. The alloys can be hydrogenated into MgH2, Mg2NiH4 and a rare earth metal hydride RHx. The rare earth metal hydride and Mg2NiH4 synergistically provide a catalytic effect on the hydrogen absorption–desorption reactions in the Mg−H2 system. The hydrogen storage kinetics is not influenced by different rare earth metal hydrides but by the particle size of the rare earth metal hydrides.  相似文献   

4.
Nanostructured materials for hydrogen storage with a composition of Mg85Ni15−xMx (M = Y or La, x = 0 or 5) are formed by devitrification of amorphous and amorphous-nanocrystalline precursors produced by melt-spinning. All three compositions exhibit a maximum storage capacity of about 5 mass % H at 573 K. When ball-milled for 30 min in hexanes, the binary alloy can be activated (first-cycle hydrogen absorption) at 473 K. DSC experiments indicate that desorption in this sample begins at 525 K, compared to 560 K when the material is activated at 573 K; which indicates an improvement in the hydride reaction thermodynamics due to capillarity effects. Additions of Y and La improve the degradation in storage capacity observed during cycling of the binary alloy by slowing microstructural coarsening. Alloying with La also shows a decrease of about 8 kJ/mol and 5 kJ/mol in the enthalpy of reaction for MgH2 and Mg2NiH4 formation, respectively, compared to the binary alloy; resulting in some desorption of H2 at 473 K. The improved thermodynamics are discussed in terms of destabilization of the hydrides relative to new equilibrium phases introduced by alloying additions. The proposed hydriding reaction for the La-containing material is in agreement with previously reported experimental results.  相似文献   

5.
The formation of the Ti substituted Mg2Ni alloys, a promising hydrogen storage material for various applications is studied in detail. Mg1.95Ti0.05Ni alloy and ribbons are successfully prepared by vacuum arc melting and melt spinning methods. The phases, microstructures, and thermal behavior of the alloys and ribbons are characterized by XRD, SEM, TEM, DTA/TG. Sievert-type apparatus is used to study hydrogen sorption properties. Apart from the dominant Mg2Ni phase, the formation of MgNi2, Mg, and Ni3Ti phases is seen in both Mg1·95Ti0·05Ni alloy and ribbons. During the initial three cycles, Mg1·95Ti0·05Ni ribbons showed 2 wt % hydrogen storage capacity. To explain the atomic-scale influence of Ti dopant in the studied alloys and hydrides, FP(L)APW + lo method based on Density Functional Theory (DFT) is applied to Mg2-xTixNi (x = 0.25 and 0.5) alloys and Mg2-xTixNiH4 (x = 0.25 and 0.5) hydrides. An increase in the Ti dopant on the Mg site leads to the hydrides destabilization. Bader's charge density topology analysis provides insight into the charge transfer and bonding between the constituent atoms.  相似文献   

6.
The nanocrystalline and amorphous Mg2Ni-type alloys with nominal compositions of Mg2Ni1−xMnx (x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by melt spinning technique. The structures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system. The results show that the as-spun (x = 0) alloy holds a typical nanocrystalline structure, whereas the as-spun (x = 0.4) alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni facilitates the glass formation in the Mg2Ni-type alloy. The hydrogen absorption capacity of the alloys first increases then decreases with rising Mn content, but the hydrogen desorption capacity of the alloys grows with increasing Mn content. Furthermore, the substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving both the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn from 0 to 0.4, the discharge capacity of as-spun (30 m/s) alloy grows from 116.7 to 311.5 mAh/g, and its capacity retaining rate at 20th charging and discharging cycle rises from 36.7 to 78.7%.  相似文献   

7.
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 Mg2FeH6. 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 Mg2CoH5Lix solid solution compound up to x = 1.  相似文献   

8.
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 ΔHabs 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.  相似文献   

9.
10.
The effect of Ni-substitution on the structure and hydrogen storage properties of Mg2Cu1−xNix (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.  相似文献   

11.
Mg2−xAlxNi (x = 0, 0.25) electrode alloys with and without multiwalled carbon nanotubes (MWCNTs) have been prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures of synthesized alloys are characterized by XRD, SEM and TEM. XRD analysis results indicate that Al substitution results in the formation of AlNi-type solid solution that can interstitially dissolve hydrogen atoms. In contrast, the addition of MWCNTs hardly affects the XRD patterns. SEM observations show that after co-milling with 5 wt. % MWCNTs, the particle sizes of both Mg2Ni and Mg1.75Al0.25Ni milled alloys are decreased explicitly. The TEM images reveal that ball milling is a good method to cut long MWCNTs into short ones. These MWCNTs aggregate along the boundaries and surfaces of milled alloy particles and play a role of lubricant to weaken the adhesion of alloy particles. The majority of MWCNTs retain their tubular structure after ball milling except a few MWCNTs whose tubular structure is destroyed. Electrochemical measurements indicate that all milled alloys have excellent activation properties. The Mg1.75Al0.25Ni-MWCNTs composite shows the highest discharge capacity due to the synergistic effects of MWCNTs and Al on the electrochemical hydrogen storage properties of Mg2Ni-type alloy. However, the improvement on the electrode cycle stability by adding MWCNTs is unsatisfactory.  相似文献   

12.
Amorphous Mgx(LaNi3)100−x (x = 40, 50, 60, 70) alloys with ribbon shape (5 mm wide, 0.2 mm thick) have been prepared by rapid solidification, using a melt-spinning technique. Their microstructure, hydrogen storage properties and thermal stability were studied by means of XRD, SEM, PCTPro2000 and DSC analysis, respectively. The results indicated that when Mgx(LaNi3)100−x alloys have been hydrogenated at 573 K under 2 MPa hydrogen pressure, LaH3 phase is formed in the case of x (x = 40, 50, 60, 70), Mg2NiH4 phase formed in the case of x (x = 40, 50, 60, 70), Mg2NiH0.3 phase formed in the case of x (x = 40, 50), and MgH2 phase formed in the case of x = 70. Experimental data of hydrogen desorption kinetics, tested at 523 K, 573 K and 623 K, are in good agreement with Avrami–Erofeev equation. The maximum hydrogen absorption capacity is 2.71 wt.% for Mg70(LaNi3)30 and 2.35 wt.% for Mg70(LaNi3)30, the increase of hydrogen desorption capacity is in the order of x = 70 > x = 60 > x = 50 > x = 40. Based on DSC analysis, the activation energies for dehydrogenation of these samples are calculated to be 122 ± 2 kJ/mol (x = 40) > 101 ± 3 kJ/mol (x = 50) > 84 ± 5 kJ/mol (x = 60) > 64 ± 3 kJ/mol (x = 70), which are in agreement with the results of hydrogen desorption kinetics.  相似文献   

13.
Mg2−xAlxNi (x = 0, 0.3, 0.5, 0.7) hydrogen storage alloys used as the negative electrode in a nickel–metal hydride (Ni–MH) battery were successfully prepared by means of hydriding combustion synthesis (HCS) and the selected alloy Mg1.5Al0.5Ni was further modified by mechanical milling (MM). The structural and electrochemical hydrogen storage properties of Mg2−xAlxNi alloys have been investigated in detail. XRD results show that a new phase Mg3AlNi2 that possesses an excellent cycling stability is observed with the substitution of Al for Mg. A short-time mechanical milling has a significant effect on improving the discharge capacity of the HCS product of Mg1.5Al0.5Ni. The maximum discharge capacity of Mg1.5Al0.5Ni ascends with increasing mechanical milling time and reaches the maximum 245.5 mAh/g when milled for 10 h. The alloy milled for 5 h shows the best electrochemical kinetics, which is due to its smaller mean particle size and uniform distribution of the particles. Further increasing in mechanical milling time could not bring about better electrochemical kinetics, which might be attributed to the agglomeration of the alloy particles and thus the charge-transfer reaction and hydrogen diffusion are restrained. It is suggested that the novel method of HCS + MM is promising to prepare ternary Mg-based intermetallic compound for electrochemical hydrogen storage.  相似文献   

14.
The solid solutions of Mg2Ni1?xMx (when M = V, Cr, Fe, Co, Cu and Zn) have a Mg2Ni-type structure with a large homogeneity range. A comparative study of the action of hydrogen has been carried out on all alloys corresponding to an x = 0.25 formulation. The absorption-desorption process of hydrogen is reversible and after dissociation of the hydride the starting material is regenerated except for copper which has not been examined here. Hydriding leads for all other alloys to formation of quaternary hydrides. The thermal stability is very close to that of Mg2NiH4 stability: partial substitution of nickel by cobalt in Mg2Ni leads at given temperature to a lower dehydriding rate.  相似文献   

15.
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.  相似文献   

16.
This paper describes the hydrogen storage properties of Magnesium rich ternary Mg-Ni-Cu films of 1.5 μm thickness using binary Mg-Ni and Mg-Cu as baselines, and aims to elucidate the precise influences of alloying element Cu on the hydrogen sorption kinetics, thermodynamics and cycleability. Mg-rich Mg-Ni-(Cu) alloys show two stages during absorption. The first stage due to the absorption of Mg not alloyed in the form of Mg2Ni and Mg2Cu, hereafter denoted as free-Mg, is very quick, but the second one due to the absorption of intermetallic Mg2Ni and/or Mg2Cu is significantly slower. This sequence is confirmed by XRD characterizations at different absorption stages. The rapid first stage absorption is mainly catalyzed by the intermetallic phase, Mg2Ni. Cu substitution improves the desorption kinetics, but severely decreases the kinetics of the second absorption stage. Failure to completely absorb Mg2Cu to MgH2 and MgCu2 in consecutive absorption cycles leads to complete loss of desorption-ability in binary Mg-15 at.%Cu. XRD combined with TEM shows that segregation of Mg2Cu towards the grain boundaries is responsible for this. Pressure-Composition Isotherms are used to examine the thermodynamic properties of the alloys. The thermodynamic properties of the Low-Temperature (LT-) Mg2NiH4 are determined for the first time experimentally, and are found to be ΔH = −78.6 kJ/mol H2 and ΔS = −147.83 J/K-mol H2. It is found that the Cu substitution has no influence on the plateau pressure of MgH2 from free-Mg phase, but slightly increases the plateau pressure of LT-Mg2NiH4.  相似文献   

17.
In order to enhance the glass forming ability of the Mn2Ni-type electrode alloy, Ni in the Mg2Ni compound is partially substituted by Mn. The nanocrystalline and amorphous Mg2Ni-type alloys with nominal compositions of Mg2Ni1−xMnx (x = 0, 0.1, 0.2, 0.3, 0.4) are fabricated by melt-spinning technique. The spun alloy ribbons with a continuous length, a thickness of about 30 μm and a width of about 25 mm are successfully obtained. The microstructures of the as-spun alloy ribbons are characterized by XRD, SEM and TEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were tested by an automatic galvanostatic system. The electrochemical impedance spectra (EIS) are plotted by an electrochemical workstation (PARSTAT 2273). The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The results show that no amorphous structure is detected in the as-spun Mn-free alloy, whereas the as-spun alloys containing Mn display a nanocrystalline and amorphous structure. The amorphization degree of the alloy increases with rising spinning rate, suggesting that the substitution of Mn for Ni facilitates the glass formation in the Mg2Ni-type alloy. The substitution of Mn for Ni markedly improves the electrochemical hydrogen storage performances of the Mg2Ni-type alloy, enhancing both the discharge capacity and the electrochemical cycle stability. Furthermore, the high rate dischargeability (HRD), electrochemical impedance spectrum and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases then decreases with increasing amount of Mn substitution.  相似文献   

18.
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 Mg20Ni10−xCux (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.  相似文献   

19.
Mg-based hydrogen storage alloys are a type of promising cathode material of Nickel-Metal Hydride (Ni-MH) batteries. But inferior cycle life is their major shortcoming. Many methods, such as element substitution, have been attempted to enhance its life. However, these methods usually require time-consuming charge–discharge cycle experiments to obtain a result. In this work, we suggested a cycle life prediction method of Mg-based hydrogen storage alloys based on artificial neural network, which can be used to predict its cycle life rapidly with high precision. As a result, the network can accurately estimate the normalized discharge capacities vs. cycles (after the fifth cycle) for Mg0.8Ti0.1M0.1Ni (M = Ti, Al, Cr, etc.) and Mg0.9  xTi0.1PdxNi (x = 0.04–0.1) alloys in the training and test process, respectively. The applicability of the model was further validated by estimating the cycle life of Mg0.9Al0.08Ce0.02Ni alloys and Nd5Mg41–Ni composites. The predicted results agreed well with experimental values, which verified the applicability of the network model in the estimation of discharge cycle life of Mg-based hydrogen storage alloys.  相似文献   

20.
The partial replacement of La by M (M = Pr, Zr) has been performed in order to ameliorate the electrochemical hydrogen storage performances of La–Mg–Ni-based A2B7-type electrode alloys. For this purpose, we adopt melt spinning technology to prepare the La0.75−xMxMg0.25Ni3.2Co0.2Al0.1 (M = Pr, Zr; x = 0, 0.2) electrode alloys. Then systemically investigate the effects that the preparation methods and M (M = Pr, Zr) substitution have on the structures and electrochemical hydrogen storage characteristics of the alloys. The analysis of XRD and TEM reveals that the as-cast and spun alloys hold a multiphase structure, containing two main phases (La, Mg)2Ni7 and LaNi5 as well as a trace of residual phase LaNi2. Besides, the as-spun (M = Pr) alloy displays an entire crystalline structure, while an amorphous-like structure is detected in the as-spun (M = Zr) alloy, implying the replacement of La by Zr facilitates forming amorphous phase. Based upon electrochemical measurements, an impact engendered by melt spinning on the electrochemical performances of the alloys appears to be evident. The cycle stabilities monotonously augment with the growing of the spinning rate. The discharge capacity and high rate discharge ability (HRD), however, exhibit difference. For the (M = Pr) alloy, they first mount up and then fall with the rising of the spinning rate, whereas for the (M = Zr) alloy, they always decline as the spinning rate elevates. Furthermore, the replacement of La by M (M = Pr, Zr) considerably enhances the cycle stability of the alloys and the replacement of La by Pr clearly increases the discharge capacity, but the Zr replacement results in an adverse impact.  相似文献   

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