Hydrogen storage behavior of Mg-based alloy catalyzed by carbon-cobalt composites

The composites comprised of Co nanoparticle and C nanosheet were prepared though a high-temperature carbonization reaction.The catalysis of Co@C composites on the hydrogen storage behavior of Mg_(90)Ce_(5)Y_(5)alloy was investigated in detail by XRD,SEM,TEM,PCI,and DSC method.Because of the synergistic catalytic function of C and Co in C@Co nanocomposites,the Mg_(90)Ce_(5)Y_(5)alloy with 10 wt.%C@Co shows the excellent hydrogen absorption and desorption performances.Time for releasing hydrogen reduces from 150 min to 11 min with the addition of the C@Co composites at the temperature of 300℃.Meanwhile,the dehydrogenation activation energy also declines from 130.3 to 81.9 kJ mol^(-1)H_(2)after the addition of the C@Co composites.This positive effect attributes to the C layer with the high defect density and the Co nanoparticles,which reduces the energy barriers for the nucleation of Mg/MgH_(2)phase and the recombination of hydrogen molecule.Besides,the C@Co composites also improve the activation property of the Mg_(90)Ce_(5)Y_(5)alloy which was folly activated in the first cycle.Moreover,the temperature for initial dehydrogenation and the endothermic peak of the alloy hydride were also decreased.Although the addition of the C@Co composites increases the plateau pressures and decreases the value of the decomposition enthalpy,these differences are so small that the improvement on thermodynamics can hardly be seen.

Hydrogen Absorption and Electrochemical Properties of As-Quenched Nanocrystalline Mg20Ni10 – xCux (x = 0 – 4) Alloys

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

Phase transformation,thermodynamics and kinetics property of Mg90Ce5RE5(RE=La,Ce,Nd)hydrogen storage alloys

The Mg90Ce5 RE5(RE=La,Ce,Nd)alloys were prepared by a vacuum induction furnace and their micro structure,phase transformation,thermodynamics and kinetics property were systematically studied by XRD,SEM,TEM,and PCT characterization methods.The result shows that the activated alloys are composed of Mg/MgH2 and corresponding REH2+x with nanoscale.The REH2+x grain with Ce and La or Nd functional group have lower nucleation potential barriers than CeH2+x grains as the nucleation location,thus improve the hydrogen absorption kinetics of these alloys among which the Mg90Ce5Nd5 alloy can absorb 90%of the hydrogen within 2 min at 320℃.In addition,the Mg90Ce10 alloy has the lowest activation energy with 103.2 kJ mol-1 and the fastest desorption kinetics,which can release 5 wt%of the hydrogen within 20 min at 320℃.This is a correlation with grain size and the in-suit formed CeH2.73/CeO2 interface.Moreover,the co-doping Ce and La or Nd can effectively disorganize the thermodynamic stability of Mg-based hydrogen storage alloys to a certain degree,but the dehydrogenation kinetics of that still is restricted by the recombination energy of hydrogen ions on the surface.

Phase evolution,hydrogen storage thermodynamics and kinetics of ternary Mg90Ce5Sm5 alloy

Greatly stable thermodynamics and sluggish kinetics impede the practical application of Mg-based hydrogen storage alloys.The modifications of composition and structure are important strategies in turning these hydrogen storage properties.In this study,Mg-based Mg90Ce5 Sm5 ternary alloy fabricated by vacuum induction melting was investigated to explore the performance and the reaction mechanism as hydrogen storage material by X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscopy(TEM) and pressure-composition isotherms(PCI) measurements.The results indicate that the Mg-based Mg90Ce5 Sm5 ternary alloy consists of two solid solution phases,including the major phases(Ce,Sm)5 Mg41 and the minor phases(Ce,Sm)Mg12.After hydrogen absorption,these phases transform into the MgH2 and(Ce,Sm)H2.73 phase,while after hydrogen desorption,the MgH2 transforms into the Mg phase,but the(Ce,Sm)H2.73 phases are not changed.The alloy has a reversible hydrogen capacity of about 5.5 wt% H2 and exhibits well isothermal hydrogen absorption kinetics.Activation energy of 106 kJ/mol was obtained from the hydrogen desorption data between 573 and 633 K,which also exhibits the enhanced kinetics compared with the pure MgH2 sample,as a result of bimetallic synergy catalysis function of(Ce,Sm)H2.73 phases.The rate of hydrogen desorption is controlled by the release and recombination of H2 from the Mg surface.Furthermore,the changes of enthalpy and entropy of hydrogen absorption/desorption were calculated to be-80.0 kJ/mol H2,-137.5 J/K/mol H2 and 81.2 kJ/mol H2,139.2 J/K/mol H2,respectively.

Gaseous hydrogen storage properties of Mg-Y-Ni-Cu alloys prepared by melt spinning

For purpose of promoting the hydrogen absorption and desorption thermodynamics and kinetics properties of Mg-Ni-based alloys, partially substituting Y and Cu for Mg and Ni respectively and melt spinning technique were applied for getting Mg25-xYxNi9 Cu(χ = 0-7) alloys. Their microstructures and phases were characterized with the help of X-ray diffraction and transmission electron microscopy. Their hydrogen absorbing and desorbing properties were tested by a Sievert apparatus, DSC, and TGA, which were connected with a H2 detector. In order to estimate the dehydrogenation activation energy of alloy hydride, both Arrhenius and Kissinger methods were applied for calculation. It is found that their hydriding kinetics notably declines, however, their hydrogen desorption kinetics conspicuously improves, with spinning rate and Y content increasing. Their hydrogen desorption activation energy markedly decreases under the same constraint, and it is found that melt spinning and Y substituting Mg improve the real driving force for dehydrogenation. As for the tendency of hydrogen absorption capacity,it presents an elevation firstly and soon after a decline with the rising of spinning rate, however, it always lowers with Y content growing. With Y content and spinning rate increasing, their thermodynamic parameters(△H and △S absolute values) visibly decrease, and the starting hydrogen desorption temperatures of alloy hydrides obviously lower.

Structure and hydrogen storage characteristics of as-spun Mg-Y-Ni-Cu alloys

Experimental alloys with compositions of Mg(25-x)YxNi9Cu(x=0,1,3,5,7)have been successfully prepared through melt spinning method.The phase compositions and microstructures were measured by X-Ray diffraction(XRD)and high-resolution transmission electron microscopy(HRTEM).The de-/hydrogenation properties were measured by utilizing Sievert apparatus,differential scanning calorimetry(DSC)and thermal gravimetric analyzer(TGA)connected with a H2 detector.The Arrhenius and Kissinger methods were adopted to calculate their dehydrogenation activation energies.The results show that hydrogen absorption kinetics of the alloys notably decline while their hydrogen desorption kinetics conspicuously improve with spinning rate increasing.The dehydrogenation activation energy markedly decreases with spinning rate increasing,which makes the hydrogen desorption kinetics improve.The thermodynamic parameters(H and S absolute values)clearly decrease with spinning rate increasing.The hydrogen absorption capacity exhibits different trends with spinning rate rising.Specifically,hydrogen absorption capacity increases at the beginning and declines later for Y1 alloy,but that of Y7 alloy always decreases with spinning rate growing.

Structure and Electrochemical Hydrogen Storage Properties of as-Milled Mg-Ce-Ni-Al-Based Alloys

At room temperature,crystalline Mg-based alloys,including Mg2 Ni,MgNi,REMg12 and La2 Mg17,have been proved with weak electrochemical hydrogen storage performances.For improving their electrochemical property,the Mg is partially substituted by Ce in Mg-Ni-based alloys and the surface modification treatment is performed by mechanical coating Ni.Mechanical milling is utilized to synthesize the amorphous and nanocrystalline Mg1-xCexNi0.9Al0.1(x=0,0.02,0.04,0.06,0.08)+50 wt%Ni hydrogen storage alloys.The effects made by Ce substitution and mechanical milling on the electrochemical hydrogen storage property and structure have been analyzed.It shows that the as-milled alloys electrochemically absorb and desorb hydrogen well at room temperature.The as-milled alloys,without any activation,can reach their maximal discharge capacities during first cycling.The maximal value of the 30-h-milled alloy depending on Ce content is 578.4 mAh/g,while that of the x=0.08 alloy always grows when prolonging milling duration.The maximal discharge capacity augments from337.4 to 521.2 mAh/g when milling duration grows from 5 to 30 h.The cycle stability grows with increasing Ce content and milling duration.Concretely,the S100 value augments from 55 to 82%for the alloy milled for 30 h with Ce content rising from 0 to 0.08 and from 66 to 82%when milling the x=0.08 alloy mechanically from 5 to 30 h.The alloys’electrochemical dynamics parameters were measured as well which have maximum values depending on Ce content and keep growing up with milling duration extending.

Structure and electrochemical hydrogen storage behaviors of Mg-Ce-Ni-Al-based alloys prepared by mechanical milling

The influences of milling time and Ce content on the electrochemical property and micro structure of asmilled Mg1-xCexNi0.9Al0.1(x=0,0.02,0.04,0.06,0.08)+50 wt%Ni alloys were investigated systematically.The as-milled alloys have an outstanding activation property.The cycle stability conspicuously grows up with milling time and Ce proportion increasing.The capacity retention rate at 100 th cycle of x=0.02 alloy augments from 47% to 63% when prolonging milling time from 5 to 30 h and it grows from55% to 82% for the 30 h milled alloy with Ce content growing from 0 to 0.08.The discharge capacity of x=0.02 alloy grows up invariably with milling time prolonging,while that of the 30 h milled alloys has the maximal value of 578.4 mAh/g with Ce content increasing.Moreover,the electrochemical kinetic properties of alloys significantly improve with milling duration extending,while they have the maximal values with Ce proportion varying.

Hydrogen Storage Kinetics of Nanocrystalline and Amorphous LaMg_(12)-Type Alloy–Ni Composites Synthesized by Mechanical Milling

The nanocrystalline and amorphous LaMg11Ni ＋ x wt% Ni （x = 100, 200） composites were synthesized by the mechanical milling, and their gaseous and electrochemical hydrogen storage kinetics performance were systematically investigated, The results indicate that the as-milled composites exhibit excellent hydrogen storage kinetic performances, and increasing Ni content significantly facilitates the improvement of the hydrogen storage kinetics properties of the composites. The gaseous and electrochemical hydrogen storage kinetics of the composites reaches a maximum value with the variation of milling time. Increasing Ni content and milling time both make the hydrogen desorption activation energy lower, which are responsible for the enhancement in the hydrogen storage kinetics properties of the composites. The diffusion coefficient of hydrogen atom and activation enthalpy of charge transfer on the surface of the as-milled composites were also calculated, which are considered to be the dominated factors for the electrochemical high rate discharge ability.

A comparison study of hydrogen storage properties of as-milled Sm_5Mg_(41) alloy catalyzed by CoS_2 and MoS_2 nano-particles

The influences of the catalysts of CoS2 and MoB2 nano-particles on microstructure and hydrogen stor-age behaviors of as-milled SmsMg41 alloy have been compared in this work. The SmsMg41 ＋ 5 wt.% M （M = COS2, MoS2） alloys were prepared by milling the mechanical ground as-cast SmsMg41 alloy powders （particle size ≤75 μm） with 5 wt.% CoS2 or MoS2 nano-particles （particle size ≤ 30 nm）, respectively. The results demonstrate that the CoS2 and MoS2 nanoparticles are embedded into the alloy surface, which is nanostructure containing some crystal defects, such as dislocation, grain boundary and twin etc. Those microstructures play a beneficial role in reducing the total potential barrier that the hydrogen absorption or desorption reactions must overcome, hence improving the hydrogen storage kinetics of the alloys. The as-milled alloys are composed of SmsMg41 and SmMg3 phases, and ball milling refines their crys-tal grains. The MgH2 and Sm3H7 phases appear after hydrogenation, while Mg and Sm3H7 phases exist after dehydrogenation. The dehydriding activation energy of M = CoS2 and MoS2 alloys are 101.67 and 68.25 kJ/mol H2 respectively. The initial hydrogen desorption of M = CoS2 and MoS2 alloys are 252.9 ℃ and 247.8 ℃.The hydrogenation and dehydrogenation enthalpy changes of M = MoS2 alloy are a little smaller than that of MzCoS2 alloy. Therefore, the catalyst MoS2 can improve the as-milled SmsMg41 alloy in hydrogen storage property more effectively than C0S2.

A comparison study of hydrogen storage performances of SmMg_(11)Ni alloys prepared by melt spinning and ball milling

The melt spinning（MS） and ball milling（BM） technologies are thought to be efficient to prepare nanostructured Mg and Mg-based alloys for improving their hydrogen storage performances. In this paper, two technologies, viz. melt spinning and ball milling, were employed to fabricate the SmMg_（11）Ni alloy. The structure and hydrogen storage performance of these two kinds of alloys were researched in detail. The results reveal that the as-spun and milled alloys both contain nanocrystalline and amorphous structures. By means of the measurement of PCT curves, the thermodynamic parameters of the alloys prepared by MS and BM are ΔN_（Ms）（des） = 82.51 kJ/mol and ΔH_（BM）（des） = 81.68 kJ/mol, respectively, viz.ΔH_（MS）（des） 〉 ΔH_（BM）（des）. The as-milled alloy shows a larger hydrogen absorption capacity as compared with the as-spun one. The as-milled alloy exhibits lower onset hydrogen desorption temperature than the as-spun one. As to the as-milled and spun alloys, the onset hydrogen desorption temperatures are557.6 and 565.3 K, respectively. Additionally, the as-milled alloy shows a superior hydrogen desorption property than the as-spun one. On the basis of time that required by desorbing hydrogen of 3 wt% H_2, the as-milled alloy needs 1488.574,390 and 192 s corresponding to hydrogen desorption temperatures 593,613,633 and 653 K, while the as-spun alloy needs 3600,1020,778 and 306 s corresponding to the same temperatures. The dehydrogenation activation energies of the as-milled and spun alloys are 100.31 and105.56 kJ/mol, respectively, the difference of which is responsible for the much faster dehydriding rate of the as-milled alloy.