Hydriding/dehydriding properties of Mg-Ni-based ternary alloys synthesized by mechanical grinding

The Mg-Ni-based ternary alloys Mg2-xTixNi(x=0,0.2,0.4)and Mg2Ni1-xZrx(x=0,0.2,0.4)were successfully synthesized by mechanical grinding.The phases in the alloys and the hydriding/dehydriding properties of the alloys were investigated.Mg2Ni and Mg are the main hydrogen absorption phases in the alloys by XRD analysis.Hydriding kinetics curves of the alloys indicate that the hydrogen absorption rate increases after partial substitution of Ti for Mg and Zr for Ni.According to the measurement of pressure-concentration-isotherms and Van't Hoff equation,the relationship between ln p(H2)and 1 000/T was established.It is found that while increasing the content of correspondingly substituted elements at the same temperature,the equilibrium pressure of dehydriding increases,the enthalpy change and the stability of the alloy hydride decrease.

Effect of Preparation Methods on Hydriding Properties of La1.5Mg17 Ni0.5 Composite Materials

La1.5Mg17Ni0.5 hydrogen storage materials were prepared by hydriding combustion synthesis (HCS) and mechanical alloying (MA) method respectively. The experimental results show that the hydrogen absorption properties of La1.5Mg17Nio.5 prepared by MA are better than that by HCS. La1.5Mg17Nio.5 prepared by MA can absorb 6.73 mass% hydrogen at 523 K within 1 min, and 4.92 mass% hydrogen at 423 K. The improvement of hydriding properties of La1.5Mg17Ni0.5alloy prepared by MA can be ascribed to the formation of nano-crystalline and defects during the mechanical alloying.

Hydriding-dehydriding properties of Mg_2Ni alloy modified by ball-milling in tetrahydrofuran

A new approach of ball-milled Mg_2Ni in tetrahydrofuran (THF) to improve thehydriding kinetics of Mg_2Ni alloy is suggested and studied. It is found that the modified alloydisplayed the improved activity for hydriding even at relatively low temperature (e.g., 323-373 K).In the case of the sample milled in THF for 20 h, the hydrogen content (mass fraction) reaches 1.6 %at 323 K, 2.1% at 348 K and 3.4% at 448 K, respectively. The use of THF during grinding led to thechange of the structure, which is reflected by the broadening and weakening of the diffraction peaksin the XRD spectra. The XPS analysis shows that Mg (2s) binding energy peak of Mg_2Ni aftermodification shifted from a lower binding energy to a higher one, indicating the charge transferencebetween Mg and THF and the formation of catalytically active electron donor-acceptor (EDA)complexes on the surface of modified Mg_2Ni alloy.

Characteristics of Hydrogen Storage Alloy Mg_2Ni Produced by Hydriding Combustion Synthesis

A high activity and large capacity of hydrogen storage alloy Mg2Ni by hydriding combustion synthesis was investigated by means of pressure composition isotherms, X-ray diffraction and scanning electron microscopy. The results showed that the maximum hydrogen absorption capacity of Mg2Ni is 3.25 mass fraction at 523 K, just after synthesis without any activation. The relationships between the equilibrium plateau pressure and the temperature for Mg2Ni were lgp (0.1 MPa)=-3026/T+5.814 (523 K≤T≤623 K) for hydriding and Igp (0.1 MPa)=-3613/T+6.715 (523 K≤T ≤623 K) for dehydriding. The kinetic equation is [-ln(1-a)]3/2 = kt and the apparent activation energy for the nucleation and growth-controlled hydrogen absorption and desorption were determined to be 64.3±2.31kJ/(mol.H2) and 59.9±2.99kJ/(mol.H2)respectively.

First-principles study of the elastic and thermodynamic properties of thorium hydrides at high pressure

The high pressure behaviors of Th_4H_（15） and ThH_2 are investigated by using the first-principles calculations based on the density functional theory（DFT）. From the energy–volume relations, the bct phase of ThH_2 is more stable than the fcc phase at ambient conditions. At high pressure, the bct ThH_2 and bcc Th_4H（15） phases are more brittle than they are at ambient pressure from the calculated elastic constants and the Poisson ratio. The thermodynamic stability of the bct phase ThH_2 is determined from the calculated phonon dispersion. In the pressure domain of interest, the phonon dispersions of bcc Th_4H（15） and bct ThH_2 are positive, indicating the dynamical stability of these two phases, while the fcc ThH_2 is unstable. The thermodynamic properties including the lattice vibration energy, entropy, and specific heat are predicted for these stable phases. The vibrational free energy decreases with the increase of the temperature, and the entropy and the heat capacity are proportional to the temperature and inversely proportional to the pressure. As the pressure increases, the resistance to the external pressure is strengthened for Th_4H_（15） and ThH_2.

Structural and elastic properties of Mg_(3)CuH_(0.6)ternary hydride by ab initio study

In the present work,we investigate the structural and elastic properties of a new intermetallic hydride compound Mg 3 CuH 0.6.The theoretical study has been carried out based on density functional theory(DFT),within local density approximation(LDA)and generalized gradient approximation(GGA).The calculated structural parameters of Mg 3 CuH 0.6 compound are consistent with the available experimental data in both approximations.The heat of formation found shows that this compound has a stable structure.According to Hooke’s law which leads to a linear relation between stress and strain,the elastic constants were determined.Once the elastic constants are obtained,the bulk modulus B,shear modulus G,Young’s modulus E,Poisson’s ratioσ,for Mg 3 CuH 0.6 compound were deduced using Voigt-Reuss-Hill(VRH)approximation.The results show that this alloy exhibit ductile property.Finally,the Debye temperature is also calculated and discussed for the first time.This will be considered as the first quantitative theoretical prediction of these properties.

Effect of praseodymium substitution for lanthanum on structure and properties of La_(0.65-x)Pr_xNd_(0.12)Mg_(0.23)Ni_(3.4)Al_(0.1)(x=0.00–0.20) hydrogen storage alloys

In order to investigate the effect of substituting La with Pr on structural and hydrogen storage properties of La-Mg-Ni system （AB3.5-type） hydrogen storage alloys, a series of La0.65-xPrxNd0.12Mg0.23Ni3.4Al0.1（x=0, 0.10, 0.15, 0.2） hydrogen storage alloys were prepared. X-ray diffraction （XRD）, scanning electron microscopy （SEM） and energy dispersive spectrometer （EDS） analyses revealed that two alloys （x=0.0 and 0.10） were composed of （La,Mg）2（Ni,Al）7 phase, La（Ni,A1）5 phase and （La,Mg）Ni2 phase, while other alloys （x=0.15 and 0.20） consisted of （La,Mg）2（Ni,A1）7 phase, La（Ni,A1）5 phase, （La,Mg）Ni2 phase and （La,Mg）（Ni,A1）3 phase. All alloys showed, however, only one pressure plateau in P-C isotherms. The Pr/La ratio in alloy composition influenced hydrogen storage capacity and kinetics properties. Electrochemical studies showed that the discharge capacity decreased from 360 mAh/g （x=-0.00） to 335 mAh/g （x=-0.20） as x increased. But the high-rate dischargeability （HRD） of alloy electrodes increased from 26% （x=0.00） to 56% （x=-0.20） at a discharge current density of Id=1800 mA/g. Anode polarization measurements were done to further understand the electrochemical kinetics properties after Pr substitution.

The effect of K2SiF6 on the MgH2 hydrogen storage properties

The catalytic effect of K2SiF6 on MgH2 was first timely studied.The MgH2+5 wt.%K2SiF6 was prepared via the ball milling technique.The catalyst had lessened the initial decomposition temperature by 134℃ and 48℃ as compared to both pristine and milled MgH2 samples,respectively.In 2 minutes,4.5 wt.%of hydrogen was absorbed(250℃)by the doped composite,which was 0.8 wt.%higher than the milled MgH2.Meanwhile,for the desorption kinetics(320℃,1 atm),the amount of desorbed hydrogen was increased by 2.4 wt.%and 2.3 wt.%for the first 10 and 20 minutes.Besides,contracting volume and Johnson-Mehl-Avrami models were used to analyse the kinetics sorptions.The decomposition activation energy calculated based on Kissinger equation was 114 kJ/mol.As for the active species,Mg2Si,MgF2 and KH were formed during the heating process.These active species are speculated to be responsible for the improvement of the hydrogenation properties of the composite.

Hydrogen Absorption Thermodynamic Properties of Rare Earth Based Hydrogen Storage Alloy in Benzene

The hydriding/dehydriding thermodynamic properties of the slurry system formed by suspending La rich mischmetal nickel hydrogen storage alloy (MlNi 5) in Benzene (C 6H 6) were investigated. The pressure composition isotherms for both the alloy powder and the slurry suspended with MlNi 5 were measured at several temperatures(10, 20, 30, 40 ℃). The standard enthalpy of formation Δ H ° and standard entropy of formation Δ S ° for the alloy powder with and without benzene were determined respectively. The experimental results show that the values of Δ H ° and Δ S ° for the hydriding reaction of hydrogen storage alloy (MlNi 5) of the slurry system and the gas solid system are all very close.

Microstructure and remarkably improved hydrogen storage properties of Mg_2Ni alloys doped with metal elements of Al, Mn and Ti

Mg2Ni0.7M0.3(M=Al,Mn and Ti)alloys were prepared by solid phase sintering process.The phases and microstructure of the alloys were systematically characterized by XRD,SEM and STEM.It was found that Mg3MNi2intermetallic compounds formed in Mg2Ni0.7M0.3alloys and coexisted with Mg and Mg2Ni,and that radius of M atoms closer to that of Mg atom was more beneficial to the formation of Mg3MNi2.The hydrogen storage properties and corrosion resistance of Mg2Ni0.7M0.3alloys were investigated through Sievert and Tafel methods.Mg2Ni0.7M0.3alloys exhibited remarkably improved hydrogen absorption and desorption properties.Significantly reduced apparent dehydriding activation energy values of-46.12,-59.16and-73.15kJ/mol were achieved for Mg2Ni0.7Al0.3,Mg2Ni0.7Mn0.3and Mg2Ni0.7Ti0.3alloys,respectively.The corrosion potential of Mg2Ni0.7M0.3alloys shifted to the positive position compared with Mg2Ni alloy,e.g.there was a corrosion potential difference of0.110V between Mg2Ni0.7Al0.3alloy(-0.529V)and Mg2Ni(-0.639V),showing improved anti-corrosion properties by the addition of Al,Mn and Ti.

Hydrogen storage properties of Mg(Al)solid solution alloy doped with LaF_(3) by ball milling

LaF_(3) was doped to the Mg(Al)solid solution alloy for enhancing the hydrogen absorption and desorption by ball milling.XRD was used to analyze the phases of the samples and the phase transition induced by hydrogenation and dehydrogenation.The microstructure and phase distribution were investigated by SEM and STEM.The hydrogen storage properties were measured by Sieverts method.For Mg_(0.93)Al_(0.07)−5wt.%LaF_(3) nanocomposite,the hydrogen storage kinetic properties were significantly improved by reducing the hydriding and dehydriding activation energies to 65 and 78 kJ/mol,respectively,and the dehydriding enthalpy was calculated to be 69.7 kJ/mol.The improved hydrogen storage properties were mainly attributed to the catalytic effects of the in situ formed nanostructure Al_(11)La_(3) and MgF_(2) together with the dissolving of Al in Mg lattice.

Effect of Gd content on microstructure and dynamic mechanical properties of solution-treated Mg−xGd−3Y−0.5Zr alloy

The effect of Gd content ranging from 6.5 wt.%to 8.5 wt.%on microstructure evolution and dynamic mechanical behavior of Mg−xGd−3Y−0.5Zr alloys was investigated by optical microscopy,X-ray diffraction,scanning electron microscopy and split Hopkinson pressure bar.The microstructure of as-cast Mg−xGd−3Y−0.5Zr alloys indicates that the addition of Gd can promote grain refinement in the casting.Due to the rapid cooling rate during solidification,a large amount of non-equilibrium eutectic phase Mg_(24)(Gd,Y)_(5) appears at the grain boundary of as-cast Mg−xGd−3Y−0.5Zr alloys.After solution treatment at 520℃ for 6 h,the Mg_(24)(Gd,Y)_(5) phase dissolves into the matrix,and the rare earth hydrides(REH)phase appears.The stress−strain curves validate that the solution-treated Mg−xGd−3Y−0.5Zr alloys with optimal Gd contents maintain excellent dynamic properties at different strain rates.It was concluded that the variation of Gd content and the agglomeration of residual REH particles and dynamically precipitated fine particles are key factors affecting dynamic mechanical properties of Mg−xGd−3Y−0.5Zr alloys.

The electrochemical characteristics of AB_(4)-type rare earth-Mg-Ni-based superlattice structure hydrogen storage alloys for nickel metal hydride battery

Rare earth-Mg-Ni-based alloys with superlattice structures are new generation negative electrode materials for the nickel metal hydride batteries.Among them,the novel AB_(4)-type superlattice structure alloy is supposed to have superior cycling stability and rate capability.Yet its preparation is hindered by the crucial requirement of temperature and the special composition which is close to the other superlattice structure.Here,we prepare rare earth-Mg-Ni-based alloy and study the phase transformation of alloys to make clear the formation of AB_(4)-type phase.It is found Pr_(5)Co_(19)-type phase is converted from Ce_(5)Co_(19)-type phase and shows good stability at higher temperature compared to the Ce_(5)Co_(19)-type phase in the range of 930-970℃.Afterwards,with further 5℃increasing,AB_(4)-type superlattice structure forms at a temperature of 975℃by consuming Pr_(5)Co_(19)-type phase.In contrast with A_(5)B_(19)-type alloy,AB_(4)-type alloy has superior rate capability owing to the dominant advantages of charge transfer and hydrogen diffusion.Besides,AB_(4)-type alloy shows long lifespan whose capacity retention rates are 89.2%at the 100;cycle and 82.8%at the 200;cycle,respectively.AB_(4)-type alloy delivers 1.53 wt.%hydrogen storage capacity at room temperature and exhibits higher plateau pressure than Pr_(5)Co_(19)-type alloy.The work provides novel AB_(4)-type alloy with preferable electrochemical performance as negative electrode material to inspire the development of nickel metal hydride batteries.

Effects of annealing on microstructures and electrochemical properties of La_(0.8)Mg_(0.2)Ni_(2.4)Mn_(0.10)Co_(0.55)Al_(0.10) alloy

The La0.8Mg0.2Ni2.4Mn0.10Co0.55Al0.10 alloy was prepared by induction melting. The structural and morphological characterizations were performed by means of X-ray powder diffraction （XRD） and scanning electron microscopy （SEM）. The electrochemical measurements were performed using LAND and CH/660b electrochemical workstation. The main phases of the alloy were LaNi5 and （La,Mg）Ni3. After annealing, the maximum discharge capacity, cycle stability and high rate dischargeability （HRD） were improved obviously. The maximum discharge capacity reached 373.80 mAh/g （T=1173 K）, the C100/Cmax（%） was 72.63% （T=1173 K）, and the value of HRD reached 51.8% at a discharge current density of 1150 mA/g （T=1173 K）. The cyclic voltammetry （CV） and potentiodynamic polarization were also studied.

Structural, magnetic, electronic, and elastic properties of face-centered cubic PuH_x (x= 2, 3):GGA (LSDA) + U + SO

We perform first-principles calculations to investigate the structural, magnetic, electronic, and mechanical properties of face-centered cubic （fcc） Pull2 and fcc Pull3 using the full potential linearized augmented plane wave method （FP- LAPW） with the generalized gradient approximation （GGA） and the local spin density approximation （LSDA） taking account of both relativistic and strong correlation effects. The optimized lattice constant a0 = 5.371 A for fcc Pull2 and a0 = 5.343 A for fcc PuH3 calculated in the GGA ＋ sp （spin polarization） ＋ U （Hubbard parameter） ＋ SO （spin-orbit coupling） scheme are in good agreement with the experimental data. The ground state of fcc PuH3 is found to be slightly ferromagnetic. Our results indicate that fcc PuH2 is a metal while fcc PuH3 is a semiconductor with a band gap about 0.35 eV. We note that the SO and the strong correlation between localized Pu 5f electrons are responsible for the band gap of fcc PuH3. The bonds for PuH2 have mainly covalent character while there are covalent bonds in addition to apparent ionicity bonds for PuH3. We also predict the elastic constants of fcc PuH2 and fcc PuH3, which were not observed in the previous experiments.

HYDROGEN STORAGE PROPERTIES OF Mg MODIFIED WITH TETRAHYDROFURAN

The hydriding and dehydriding behaviors of tetrahydrofuran modified Mg,its electronic struc- ture,crystal structure,micro-morphology as well as its stability have been investigated.The modifield Mg absorbs 3.5 wt-％ hydrogen at 643 K in 3.5 MPa H_2,its hydride gives off 3.2 wt-％ hydrogen at 643 K in a vaccum of 1.3 Pa after 20 cycles of hydriding and dehydriding. Tetrahydrofuran alters the electronic structure of Mg but keeps its crystal strueture unchanged.In hydriding products,a new hydride phase is found in addition to the known MgH_2 phase.The hydride formed from Mg is polv-erystalline.The wide-spreading slip bands and twins within crystals indicate that the transformation during absorption of hydrogen causes serious lattice distortions.

Electronic structure and optical properties of LiXH_3 and XLiH_3 (X = Be, B or C)

The equilibrium lattice constant, the cohesive energy and the electronic properties of light metal hydrides LiXH3 and XLiH3 （X = Be, B or C） with perovskite lattice structures have been investigated by using the pseudopotential plane-wave method. Large energy gap of LiBeH3 indicates that it is insulating, but other investigated hydrides are metallic. The pressure-induced metallization of LiBeH3 is found at about 120 GPa, which is attributed to the increase of Be-p electrons with pressure. The electronegativity of the p electrons of X atom is responsible for the metallicity of the investigated LiXH3 hydrides, but the electronegativity of the s electrons of X atom plays an important role in the metallicity of the investigated XLiH3 hydrides. In order to deeply understand the investigated hydrides, their optical properties have also been investigated. The optical absorption of either LiBeH3 or BeLiH3 has a strong peak at about 5 eV, showing that their optical responses are qualitatively similar. It is also found that the optical responses of other investigated hydrides are stronger than those of LiBeH3 and BeLiH3 in lower energy ranges, especially in the case of CLiH3.

Electrochemical Properties and Crystal Structure of Rare Earth AB_(3.5)-Type Alloy as Negative Electrode Material in MH-Ni Battery

The electrochemical characteristics and crystal structure of metal hydride electrode of AB_(3.5)-type alloy was studied. The electrochemical properties of the metal hydride electrode were investigated at room temperature and -30 ℃. The partial substitution of Ni by Al element causes an expansion of the lattice cell and increases the specific capacity and rate discharge ability of the alloy.

Zr-Cr-Ni BASED MULTICOMPONENT METAL HYDRIDE ELECTRODES

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Characteristics of multi-component Ml-based hydrogen storage alloys and their hydride electrodes

A series of multi-component M1-based hydrogen storage alloys witha cobalt atomic ratio of 0.40-0.75 were prepared. The electrochemicalproperties under different charge-discharge conditions and PCTcharaceristics measured by electrochemical method were investigated.The addition of other alloying elements for partial substitution ofCo low- ers the hydrogen equilibrium pressure and discharge capacity,but improves the cycling stability and makes the alloys keep nearlythe same rate discharge capability and high-temperature dischargecapability as those of the compared alloy. The reasons werediscussed.