Hydrogen storage thermodynamics and dynamics of La-Mg-Ni-based LaMg_(12)-type alloys synthesized by mechanical milling

Nanocrystalline/amorphous LaMg_(12)-type alloyNi composites with a nominal composition of LaMg_(11)Ni+x wt% Ni(x=100,200) were synthesized by mechanical milling.Effects of Ni content and milling time on the gaseous hydrogen storage thermodynamics and dynamics of alloys were systematically investigated.The hydrogen desorption properties were studied by Sievert apparatus and a differential scanning calorimeter(DSC).Thermodynamic parameters(△H and ΔS) for the hydrogen absorption and desorption of alloys were calculated by Van't Hoff equation.Hydrogen desorption activation energy of alloy hydride was estimated by Arrhenius and Kissinger methods.The increase in Ni content has a slight effect on the thermodynamic properties of alloys,but it significantly enhances the hydrogen absorption and desorption kinetics performance of alloys.Moreover,variation of milling time clearly affects the hydrogen storage properties of alloys.Hydrogen absorption capacity(C_(100)~a) and hydrogen absorption saturation ratio(R_(10)~a)(a ratio of the hydrogen absorption capacity at 10 min to the saturated hydrogen absorption capacity) have maximum values with milling time varying.But hydrogen desorption ratio(R_(20)~d)(a ratio of the hydrogen desorption capacity at 20 min to the saturated hydrogen absorption capacity) always increases with milling time prolonging.Particularly,prolonging milling time from 5 to 60 h makes R_(20)~d increase from 10.89% to 16.36% for the x=100 alloy and from 13.93% to 21.68% for the x=200 alloy,respectively.     

Hydrogen storage properties of nanocomposite Mg-Ni-MnO2 made by mechanical milling

The hydrogen storage properties of the nanocomposite MggsNi3(MnO2)2(maas fraction, % ) were studied.The temperature changes in hydriding/dehydriding process were investigated. The nanocomposite was fabricated by ball milling process of mixed demental Mg, Ni and oxide maganese MnCh under hydrogen pressure (approximately 0.6 MPa).The hydrogen absorption and desorption properties of the samples milled for various times were investigated. A remarkable enhancement of hydrogen absorption kinetics and low operational desorption temperature have been.found after the sample milled for over 57h. For example, this nanocomposite can absorb hydrogen more than 6.0% (mass fraction) in 60s at 200℃ under 2.0 MPa, and desorption capacity also exceeds 6.0 % (mass fraction) in 400 s at 310℃ under 0.1 MPa.The storage properties of samp1es milled for various times were studied and the kinetics of the samples were analyzed.     

A Comparison Study of Hydrogen Storage Thermodynamics and Kinetics of YMg_(11)Ni Alloy Prepared by Melt Spinning and Ball Milling

Melt spinning(MS)and ball milling(BM)were employed to fabricate YMg_(11)Ni alloy,and their structures and hydrogen storage performances were examined.The results reveal that the as-spun and as-milled alloys both exhibit the nanocrystalline and amorphous structure.The as-milled alloy shows a larger hydrogen absorption capacity as compared with the as-spun alloy.More than that,the as-milled alloy exhibits lower onset hydrogen desorption temperature than the as-spun one,which are 549.8 and 560.9 K,respectively.Additionally,the as-milled alloy shows a superior hydrogen desorption property to the as-spun one.On the basis of the time needed by desorbing hydrogen of 3 wt%H_2,for the asmilled alloy,it needs 1106,456,343,and 180 s corresponding to hydrogen desorption temperatures of 593,613,633,and653 K.However,for the as-spun alloy,the time needed is greater than 2928,842,356,and 197 s corresponding to the same temperatures.Hydrogen desorption activation energies of as-milled and as-spun alloys are 98.01 and 105.49 k J/mol,respectively,which is responsible for that the as-milled alloy possesses a much faster dehydriding rate.By means of the measurement of pressure–composition–temperature(P–C–T)curves,the dehydrogenation enthalpy change of the alloys prepared by MS(DH_(de)(MS))and BM(DH_(de)(BM))is 81.84 and 79.46 k J/mol,respectively,viz.DH_(de)(MS)[DH_(de)(BM).     

Influence of rapid quenching on hydrogen storage characteristics of nanocrystalline Mg2Ni-type alloys

Nanocrystalline Mg2Ni-type alloys with nominal compositions of Mg20Ni10–xCux(x=0,1,2,3,4,mass fraction,%) were synthesized by rapid quenching technique.The microstructures of the as-cast and quenched alloys were characterized by XRD,SEM and HRTEM.The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system.The hydriding and dehydriding kinetics of the alloys were measured using an automatically controlled Sieverts apparatus.The results show that all the as-quenched alloys hold the typical nanocrystalline structure and the rapid quenching does not change the major phase Mg2Ni.The rapid quenching significantly improves the electrochemical hydrogen storage capacity of the alloys,whereas it slightly impairs the cycling stability of the alloys.Additionally,the hydrogen absorption and desorption capacities of the alloys significantly increase with rising quenching rate.     

Effects of Al partial substitution for Ni on properties of LaNi5-xAlx

The hydrogen storage properties of four LaNi5-xAlx （x=0.25, 0.50, 0.75, 1.00） pseudobinary alloys were systematically studied. The characteristics of microstructure before and after hydrogenation, activation, kinetics and thermodynamics properties, as well the anti-combustibility properties of the four pseudobinary alloys were investigated. The results reveal that the alloys have excellent activation properties and kinetics properties. X-ray diffraction analysis of crystal lattice of the alloys show that the crystal structures of alloys do not change with the addition of AI in the range of 0≤x≤ 1, but the lattice constants slightly increase. It is found by measuring thermodynamic properties of the alloy-H2 systems that with increasing x value the equilibrium pressure, hydrogen-absorbing ability and hysteresis decrease, whereas the absolute value of the enthalpy increases. It is also found that the hydrogen absorbing velocity of the alloys decreases with increasing x value.     

Degradation behaviors of La–Mg–Ni-based metal hydride alloys:structural stability and influence on hydrogen storage performances

The present work focuses on the structural stability upon hydrogenation of three typical La–Mg–Ni-based alloys:La_(2-)Mg Ni_9,La_3Mg Ni_(14)and La_4Mg Ni_(19).Structural changes during gaseous and electrochemical cycles were characterized,and the influence of the structure distortion on the hydrogen storage properties was concerned.Hydrogen-induced amorphization(HIA)and disproportionation of the three alloys have occurred during both the gaseous and electrochemical cycles.Structural stability of the phase structures in the La–Mg–Ni system is found to follow the order:La Ni_(5-)[(La,Mg)_5Ni_(19)[(La,Mg)_2Ni_7[(La,Mg)Ni_3[(La,Mg)Ni_2.HIA increases thermal stability of the metal hydrides and difficulty to dehydrogenation and leads to degradation of both the gaseous and electrochemical capacities.Interestingly,La_2Mg Ni_9with poor stability presents elevated discharge capability even at 60°C which can be attributed to increase in the hydrogen desorption capability and inhibition of the self-discharge induced by severe HIA at higher temperatures.In addition,HIA in the electrochemical reactions is obviously weaker than the extent during the gaseous cycles,which is mainly due to the slower hydrogenation speed.The development of HIA in the gaseous and electrochemical process is considered to follow the direct and gradual modes,respectively.     

Hydrogen absorption of slurry system composed of different MgNi alloy and benzene

The hydrogen absorption amount and kinetics of the slurry formed by suspending the MgNi alloy powder in liquid benzene were studied.It is discovered that hydrogen is absorbed by both the solid phase(alloy) and liquid phase(C6H6)and the hydrogen absorption rate varies with the temperature and the content of the Mg-Ni in the slurry.Most hydrogen absorption curves of the slurry fall into two regions.in which the mechanism of hydriding reaction in the slurry system is different.In the former region,the hydriding of the alloy proceeds with hydrogen diffusing through C6H6.The part in the second region is the outcome of the hydrogenation of C6H6.At 548K and under the hydrogen pressure of 4.5MPa saturation capacity for the slurry of 80% C6H69mass fraction) 20%Mg Ni(mass fraction)is 5.9%(mass fraction)hydrogen,which is 97% of the theoretic capacity of the slurry system. The hydride of the alloy MgNi,which is only the hydride of Mg2 Ni phase,Mg2NiH4,is an efficent catalyst for the hydrogenation of C6H6into C6H12(C6H6 3H2→C6H12)in the slurry system.     

Surface analysis, hydrogen adsorption and electrochemical performance of alkali-reduce treated hydrogen storage alloy

The hydrogen storage alloy powders (M1Ni4.0Co0.6Al0.4, M1 = rich-La mischmetal) were treated in a hot 6 mol/L KOH 0.02 mol/L KBH4 solution, the surface compositions and chemical states of the treated and untreated alloys were analyzed by XPS and EDX, the hydrogen adsorption on the surface of these alloys was evaluated by thermal desorption spectroscopy (TDS), the effects of the surface treatment on the electrochemical performances of the alloy electrodes were investigated. The results show that the hydrogen adsorption is greatly strengthened by the surface modification, and hence leads to marked improvement in the electrocatalytic activity, the treated alloy exhibits higher exchange current density and lower apparent activation energy for the hydrogen electrode reaction than the untreated alloy.     

Hydrogen sorption properties of Ti-Zr hydride doped NaAlH4

The as-prepared Ti-Zr hydride powder is used as dopant to improve hydrogen storage properties of NaAlH4 upon mechanical milling under argon atmosphere. The as-milled sample is investigated by X-ray diffraction（XRD）, scanning electron microscopy（SEM） and Sievert＇s technology test. It is observed that Ti-Zr hydride doped NaAlH4 discharges 2.7% and 4.0% （mass fraction） of hydrogen in 40 min and 11 h at 160℃, respectively, and keeps its reversible dehydrogenation capacity at 4.0% （mass fraction） after 10 hydrogenation/dehydrogenation cycles, These results show the Ti-Zr hydride doped NaAIH4 has good reversible hydrogen storage capacity and kinetics. XRD and SEM investigations also show that the doped Ti-Zr hydride uniformly distributes in NaAIH4 substrate and keeps stable during the hydrogenation/dehydrogenation cycle, indicating that Ti-Zr hydride plays the main surface-catalytic role on improving reversible hydrogen storage properties of NaAlH4,     

Synthesis of nanostructured Mg2FeH6 hydride and hydrogen sorption properties of complex

Reactive mechanical alloying(RMA)was carried out in a planetary ball mill for the synthesis of ternary hydride Mg2FeH6 for hydrogen storage.The formation mechanism of Mg2FeH6 in RMA process and the sorption properties of the products were investigated.The results show that Mg2FeH6 has a yield ratio around 80%,and a grain size below 10 nm in the powder synthesized by milling 3Mg+Fe mixture for 150 h under the hydrogen pressure of 1 MPa.The synthesized powder possesses a high hydrogen capacity and good sorption kinetics,and absorbs 4.42%(mass fraction)of hydrogen within 200 s at 623 K under the hydrogen pressure of 4.0 MPa.In releasing hydrogen at 653 K under 0.1 MPa,it desorbs 4.43%(mass fraction)of hydrogen within 2 000 s.The addition of Ti increases the hydrogen desorption rate of the complex in the initial 120 s of the desorption process.     

Hydriding kinetics of the La1.5Ni0.5Mg17–H system prepared by hydriding combustion synthesis

An experimental investigation of the hydrogen absorption rate in the two-phase (–β) region of La_1.5Ni_0.5Mg₁₇ powder under the condition of various pressures and temperatures is presented. The results are well interpreted using the Jander diffusion model, [1−(1−ξ)^1/3]²=k(T,P)t, which suggests that the rate-controlling step of hydrogen absorption in La_1.5Ni_0.5Mg₁₇ is three-dimensional diffusion. An apparent activation energy for such diffusion process of 90±1 kJ/mol H₂ has been obtained from the absorption data.     

Effect of Co and Mn on the electrochemical properties of La0.7Mg0.3Ni2(Co+Mn) alloys

The effects of the relative Co and Mn content on the electrochemical performance of La_0.7Mg_0.3Ni₂(Co+Mn) hydrogen storage alloys were investigated. The crystal structure, discharge capacity and cycle life of the alloys were evaluated. For all alloys, we found that the higher the Co content, the larger is discharge capacity. The appropriate amount of Mn in La_0.7Mg_0.3Ni₂(Co+Mn) alloys can extend the cycle life of the hydrogen storage alloys although the alloys have less discharge capacity than those with higher Co content. In addition, the LaNi_3.87Mn_1.13 phase appears and the LaNi₅ phase disappears with replacement of Co by Mn.     

Stability of AB5-type hydrogen storage alloys

In this paper, we deduced an empirical equation for predicting the stabilities of the hydrides of AB₅-type compounds with the consideration of the effects of geometrical factor and electric factor. The theoretical curves derived from this equation are in good agreement with the test results of La_1−xCa_xNi₅, Ml_1−xCa_xNi₅ and La_1−xCe_xNi₅ compounds. With this model, we can easily interpret the phenomenon that the initial partial substitution of R by Ca in R_1−xCa_xNi₅ (R is rare earth metal) compounds cause an increase of hydrogen desorption pressure to a maximum value, whereas in the range of larger Ca content x the hydrogen desorption pressure decreases with increasing x.     

Mechanically milled Mg composites for hydrogen storage the transition to a steady state composition

Magnesium alloys are potentially the best materials for gaseous hydrogen storage. However, their practical use is limited by poor hydrogen absorption and desorption kinetics. This problem can be resolved by mixing Mg alloys with other materials to form composites. We present an investigation of the initial hydriding characteristics, as well as the compositional transformation of composites made of La₂Mg₁₇ + LaNi₅ mechanically milled in a 2:1 weight ratio. Composites produced with varying durations and intensities of milling were tested. Those milled to the greatest extent proved to have the best initial hydrogen absorption and desorption kinetics. The kinetics of the most heavily milled composite were superior to those of La₂Mg₁₇. This composite absorbed 90% of its full hydrogen capacity (3.5 wt.% H₂) in less than 1 min at 250°C and desorbed the same quantity of hydrogen in 6 min. Under the same conditions pure La₂Mg₁₇ took 2.5 h to absorb and 3 h to desorb 90% of its full hydrogen capacity (4.9 wt.% H₂). Scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction were used to characterize the mechanically milled powders before and after hydriding. The unhydrided powders consisted of LaNi₅ grains surrounded by a fractured LaMg₁₇ matrix. Hydrogen cycling, at temperatures up to 350°C, induced phase changes, segregation, and disintegration of the composites. The resulting fine powder (less than 1 μm) consisted primarily of Mg, Mg₂Ni, and La phases.     

The effect of Nd content on the electrochemical properties of low-Co La–Mg–Ni-based hydrogen storage alloys

The low-Co content La_0.80−xNd_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) alloys were prepared by inductive melting and the effect of Nd content on the electrochemical properties was investigated. XRD shows that the alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and minor LaNi₃ phase. The electrochemical P–C–T test shows hydrogen storage capacity increases first and then decreases with increasing x, which is also testified by the electrochemical measurement that the maximum discharge capacity increases from 290 mAh/g (x = 0.20) to 374 mAh/g (x = 0.30), and then decreases to 338 mAh/g (x = 0.60). The electrochemical kinetics test shows exchange current density I₀ increases with x increasing from 0.20 to 0.50 followed by a decrease for x = 0.60, and hydrogen diffusion coefficient D increases with increasing x. Accordingly high rate dischargeability increases with a slight decrease at x = 0.60 and the low temperature dischargeability increases with increase in Nd content. When x is 0.50, the alloy exhibits a better cycling stability.     

High-Temperature Structural Stabilities of Ni-Based SingleCrystal Superalloys Ni–Co–Cr–Mo–W–Al–Ti–Ta with Varying Co Contents

It has been recently pointed out that the compositions of industrial alloys are originated from cluster-plus-glueatom structure units in solid solutions. Specifically for Ni-based superalloys, after properly grouping the alloying elements into Al, Ni-like(■), r-forming Cr-like(■) and c-forming Cr-like(■), the optimal formula for single-crystal superalloys is established [Al–Ni_(12)](Al_1■~_(0:5) ■_(1:5)). The Co substitutions for Ni at the shell sites are conducted on the basis of the first-generation single-crystal superalloy AM3, formulated as [Al–■_(12)Co_x](Al_1Ti_(0.25)Ta_(0.25)Cr_1W_(0.25)Mo_(0.25)), with x = 1.5, 1.75, 2 and 2.5(the corresponding weight percents of Co are 9.43, 11.0, 12.57 and 15.71, respectively). The900 ℃ long-term aging follows the Lifshitz–Slyozov–Wagner theory(LSW theory), and the Co content does not have noticeable influence on the coarsening rate of c0. The microstructure and creep behavior of the four(001) single-crystal alloys are investigated. The creep rupture lifetime is reduced as Co increases. The alloy with the lowest Co(9.43 Co) shows the longest lifetime of about 350 h at 1050 ℃/120 MPa, and all the samples show N-type rafting after creep tests.     

Electrochemical hydrogen storage characteristics of La0.75-xMxMg0.25Ni3.2Co0.2Al0.1 (M =Zr, Pr; x =0, 0.1) alloys prepared by melt spinning

In order to ameliorate the electrochemical hydrogen storage performances of La-Mg Ni system A2B7-type electrode alloys,the partial substitution ofM (M =Zr,Pr) for La was performed.The melt spinning technology was used to fabricate the La0.75-xMxMg0.25Ni3.2Co0.2Al0.1 (M =Zr,Pr; x =0,0.1) electrode alloys.The influences of the melt spinning and substituting La with M (M =Zr,Pr) on the structures and the electrochemical hydrogen storage characteristics of the alloys were investigated.The analysis of XRD,SEM,and TEM reveals that the as-cast and spun alloys have a multiphase structure composed of two main phases (La,Mg)2Ni7 and LaNi5 as well as a residual phase LaNi2.The as-spun (M =Pr) alloy displays an entire nanocrystalline structure,while an amorphous-like structure is detected in the as-spun (M =Zr) alloy,implying that the substitution of Zr for La facilitates the amorphous formation.The electrochemical measurements exhibit that the substitution of Pr for La clearly increases the discharge capacity of the alloys; however,the Zr substitution brings on an adverse impact.Meanwhile,the M (M =Zr,Pr) substitution significantly enhances its cycle stability.The melt spinning exerts an evident effect on the electrochemical performances of the alloys,whose discharge capacity and high rate discharge ability (HRD) first mount up and then fall with the growing spinning rate,whereas their cycle stabilities monotonously augment as the spinning rate increases.     

Detailed Structures and Formation Mechanisms of Well-Known Al_(10)RE_2Mn_7 Phase in Die-Cast Mg-4Al-4RE-0.3Mn Alloy

The detailed structures and the corresponding formation mechanisms of the well-known Al_(10)RE_2Mn_7 phase in the conventional die-cast Mg–4Al–4RE–0.3Mn alloy were thoroughly investigated using transmission electron microscopy(TEM). The results indicate that the Al_(10)RE_2Mn_7 phase ordinarily contains both normal (111) twins and orientation twins.Both detailed TEM observations and density functional theory calculations indicate that the Al_(10)RE_2Mn_7 phase is transferred from the Al_8REMn_4 phase following an orientation relationship as [010]_(Al_8REMn_4)//[101]_(Al_(10)RE_2Mn_7) and (101)_(Al_8REMn_4)//(110)_(Al_(10)RE_2Mn_7). Moreover, forming orientation twins in the Al_(10)RE_2Mn_7 phase is attributed to the blurry regions at incoherent twin boundaries in the Al_8REMn_4 phase. Finally, these formed orientation twins result in the (111) twins in the Al_(10)RE_2Mn_7 phase.     

Structure and hydrogen storage properties of MgH2 catalysed with La2O3

The catalytic effect of the addition of lanthanum oxide (La₂O₃), in the range 0.5–2.0 mol%, on the hydrogen storage properties of MgH₂ prepared by ball milling has been studied. The addition of La₂O₃ reduces the formation during milling of the metastable orthorhombic γ-MgH₂ phase. The desorption rate of samples with 1 and 2 mol% La₂O₃ comes out to be about 0.010 wt% per second at 573 K under an hydrogen pressure of 0.3 bar, better than for sample with 0.5 mol% La₂O₃. The presence of LaH₃ after hydrogenation/dehydrogenation cycles has been observed in all samples. The sample with 1 mol% of La₂O₃ gives a lower hysteresis factor compared with sample with 2 mol%.     

Surface Structures and Their Relative Stabilities of Orthorhombic YAlO3: A First-Principles Study

First-principles energetics calculations were performed to investigate the structures and relative stabilities of six low miller-index surfaces of orthorhombic YAlO₃ (YAP). The stoichiometric YAP (100) and (001) were predicted to have the lowest surface energies of 1.91 and 1.96 J/m², respectively. Using a thermodynamic defect model, non-stoichiometric YAP surface energies were further predicted as a function of ${P}_{{\text{O}}_{2}}$(${P}_{{\text{O}}_{2}}<1\ \mathrm{atm}$) and temperature (T). All the results were combined to construct the surface phase diagrams at T = 300 and 1400 K, revealing the strong correlation of the surface stabilities of YAP with its surface stoichiometry.