Influence of substituting Ni with Co on hydriding and dehydriding kinetics of melt spun nanocrystalline and amorphous Mg_2Ni-type alloys

In order to improve the hydriding and dehydriding kinetics of the Mg2Ni-type alloys,Ni in the alloy is substituted by element Co. The nanocrystalline and amorphous Mg2Ni-type Mg2Ni1-xCox (x=0,0.1,0.2,0.3,0.4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied with an X-ray diffractometer (XRD) and a high resolution transmission electronic microscope (HRTEM). An investigation on the thermal stability of the as-spun alloys was carried out with a differential scanning calorimeter (DSC). The hydrogen absorption and desorption kinetics of the alloys were measured with an automatically controlled Sieverts apparatus. The results demonstrate that the substitution of Co for Ni does not alter the major phase of Mg2Ni but results in the formation of secondary phase MgCo2. No amorphous phase is detected in the as-spun Co-free alloy,but a certain amount of amorphous phase is clearly found in the as-spun Co-containing alloys. The substitution of Co for Ni exerts a slight influence on the hydriding kinetics of the as-spun alloy. However,it dramatically enhances the dehydriding kinetics of the as-cast and spun alloys. As Co content (x) increases from 0 to 0.4,the hydrogen desorption capacity increases from 0.19% to 1.39% (mass fraction) in 20 min for the as-cast alloy,and from 0.89% to 2.18% (mass fraction) for the as-spun alloy (30 m/s).

Hydriding and dehydriding kinetics of nanocrystalline and amorphous Mg_2Ni_(1-x)Mn_x(x=0-0.4) alloys prepared by melt spinning

A partial substitution of Ni by Mn was implemented in order to improve the hydriding and dehydriding kinetics of the Mg2Ni-type alloys. The nanocrystalline and amorphous MgzNi-type Mg2Nil-xMnx （x=0, 0. 1, 0.2, 0.3, 0.4） alloys were synthesized by the melt-spinning technique. The structures of the as-cast and spun alloys were studied by X-ray diffractometry （XRD）, scanning electron microscopy （SEM） and high resolution transmission electron microscopy （HRTEM）. The hydrogen absorption and desorption kinetics of the alloys were measured with an automatically controlled Sieverts apparatus. The results show that the as-spun Mn-free alloy holds a typical nanocrystalline structure, whereas the as-spun alloys containing Mn display a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni intensifies the glass forming ability of the Mg2Ni-type alloy. The hydrogen absorption and desorption capacities and kinetics of the alloys increase with increasing the spinning rate, for which the nanocrystalline and amorphous structure produced by the melt spinning is mainly responsible. The substitution of Mn for Ni evidently improves the hydrogen desorption performance. The hydrogen desorption capacities of the as-cast and spun alloys rise with the increase in the percentage of Mn substitution.

Hydriding and dehydriding kinetics of melt spun nanocrystalline Mg20Ni10-xCux (x = 0-4) alloys

The nanocrystalline Mg2Ni-type electrode alloys with nominal compositions of Mg20Ni10-xCux (x = 0, 1, 2, 3, 4) were synthesized by melt-spinning technique. The microstructures of the alloys were characterized by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinet-ics of the alloys were measured using an auto-matically controlled Sieverts apparatus. The re- sults show that all the as-spun alloys hold ty- pical nanocrystalline structure. The substitution of Cu for Ni does not change the major phase Mg2Ni but it leads to the formation of the sec-ondary phase Mg2Cu. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hy-drogen desorption capacity of the alloys mono- tonously grows with increasing Cu content. The melt spinning significantly improves the hydro- genation and dehydrogenation capacities and kinetics of the alloys.

Thermodynamics and kinetics of hydriding and dehydriding reactions in Mg-based hydrogen storage materials

Mg-based materials are one of the most promising hydrogen storage candidates due to their high hydrogen storage capacity,environmental benignity,and high Clarke number characteristics.However,the limited thermodynamics and kinetic properties pose major challenges for their engineering applications.Herein,we review the recent progress in improving their thermodynamics and kinetics,with an emphasis on the models and the influence of various parameters in the calculated models.Subsequently,the impact of alloying,composite,and nanocrystallization on both thermodynamics and dynamics are discussed in detail.In particular,the correlation between various modification strategies and the hydrogen capacity,dehydrogenation enthalpy and temperature,hydriding/dehydriding rates are summarized.In addition,the mechanism of hydrogen storage processes of Mg-based materials is discussed from the aspect of classical kinetic theories and microscope hydrogen transferring behavior.This review concludes with an outlook on the remaining challenge issues and prospects.

Electrochemical hydriding performance of Mg-TM-Mm (TM=transition metals,Mm=mischmetal) alloys for hydrogen storage

Eighteen as-cast binary Mg-Ni, Mg-Mm and ternary Mg-Ni-Mm and Mg-Ni-TM （TM=transition metals （Cu, Zn, Mn and Co）; Mm = mischmetal containing Ce, La, Nd and Pr） alloys were hydrided by an electrochemical process to determine the alloys with the most potential for electrochemical hydrogen storage. The alloys were hydrided in a 6 mol/L KOH solution at 80 °C for 480 min and at 100 A/m2. To assess the electrochemical hydriding performance of alloys, maximum hydrogen concentrations, hydrogen penetration depths and total mass of absorbed hydrogen in the alloys were measured by glow discharge spectrometry. In addition, the structures and phase compositions of the alloys both before and after hydriding were studied by optical and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. It was determined that the highest total amount of hydrogen was absorbed by the Mg-25Ni-12Mm and Mg-26Ni （mass fraction, %） alloys. The maximum hydrogen concentrations in the Mg-25Ni-12Mm and Mg-26Ni alloys were 1.0% and 1.6%, respectively. The main hydriding product was the binary MgH2 hydride, and the ternary Mg2NiH4 hydride was also detected in the Mg-25Ni-12Mm alloy. The electrochemical hydriding parameters achieved are discussed in relation to the structures of alloys, alloying elements and hydriding mechanisms.

Nanocrystalline Mg and Mg alloy powders by hydriding-dehydriding processing

The process of mechanically assisted hydriding and subsequent thermal dehydriding was proposed to produce nanocrystalline Mg and Mg alloy powders using pure Mg and Mg-5.5%Zn-0.6%Zr(mass fraction)(ZK60 Mg) alloy as the starting materal.The hydriding was achieved by room-temperature reaction milling in hydrogen.The dehydriding was carried out by vacuum annealing of the as-milled powders.The microstructure and morphology of both the as-milled and subsequently dehydrided powders were characterized by X-ray diffraction analysis(XRD) ,transmission electron microscopy(TEM) ,and scanning electron microscopy(SEM) ,respectively.The results show that,by reaction milling in hydrogen,both Mg and ZK60 Mg alloy can be fully hydrided to form nanocrystalline MgH2 with an average grain size of 10 nm.After subsequent thermal dehydriding at 300℃,the MgH2 can be turned into Mg again,and the newly formed Mg grains are nanocrystallines,with an average grain size of 25 nm.

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.

Influence of chloride salts on hydrogen generation via hydrolysis of MgH_2 prepared by hydriding combustion synthesis and mechanical milling

The effects of chloride salts(NaCl,MgCl2and NH4Cl)on the hydrolysis kinetics of MgH2prepared by hydridingcombustion synthesis and mechanical milling(HCS+MM)were discussed.X-ray diffraction(XRD)analyses show that high-purityMgH2was successfully prepared by HCS.Hydrolysis performance test results indicate that the chloride salt added during the millingprocess is favorable to the initial reaction rate and hydrogen generation yield within60min.A MgH2?10%NH4Cl composite exhibitsthe best performance with the hydrogen generation yield of1311mL/g and a conversion rate of85.69%in60min at roomtemperature.It is suggested that the chloride salts not only play as grinding aids in the milling process,but also create fresh surface ofreactive materials,favoring the hydrolysis reaction.

Electrochemical hydriding and thermal dehydriding properties of nanostructured hydrogen storage MgNi26 alloy

The MgNi26 alloy was prepared by three different methods of gravity casting (GC), mechanical alloying (MA) and rapid solidification (RS). All samples were electrochemically hydrided in a 6 mol/L KOH solution at 80 °C for 240 min. The structures and phase compositions of the alloys were studied using optical microscopy and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. A temperature-programmed desorption technique was used to measure the absorbed hydrogen and study the dehydriding process. The content of hydrogen absorbed by the MgNi26-MA (approximately 1.3%, mass fraction) was 30 times higher than that of the MgNi26-GC. The MgNi26-RS sample absorbed only 0.1% of hydrogen. The lowest temperature for hydrogen evolution was exhibited by the MgNi26-MA. Compared with pure commercial MgH2, the decomposition temperature was reduced by more than 200 °C. The favourable phase and structural composition of the MgNi26-MA sample were the reasons for the best hydriding and dehydriding properties.

Hydriding and microstructure nanocrystallization of ZK60 Mg alloy by reaction milling in hydrogen

The hydriding of as-cast Mg-5.5%Zn-0.6%Zr(ZK60 Mg)(mass fraction)alloy was achieved by room-temperature reaction milling in hydrogen,with the mechanical energy serving as the driving force for the process.The hydriding progress during milling was examined by hydrogen absorption measurement,and the microstructure change was characterized by X-ray diffraction analysis(XRD),scanning electron microscopy(SEM),and transmission electron microscopy(TEM),respectively.The results show that,by room-temperature reaction milling in hydrogen,the as-cast ZK60 Mg alloy can be fully hydrided to form a nanocrystalline MgH_(2) single-phase microstructure.In particular,the average grain size of the MgH_(2) phase obtained by room-temperature reaction milling in hydrogen for 16.2 h is about 8-10 nm,and the average particle size of the as-milled hydrided powders is 2-3μm.

Hydriding and dehydriding characteristics of nanocrystalline and amorphous Mg_(20-x)La_xNi_(10)(x=0-6) alloys prepared by melt-spinning

In order to improve the hydrogenation and dehydrogenation performances of the Mg2Ni-type alloys, Mg was partially substituted by La in the alloy, and melt spinning technology was used for the preparation of the Mg20-xLaxNi10 （x=0, 2, 4, 6） hydrogen storage alloys. The structures of the alloys were studied by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and high-resolution transmission electron microscopy （HRTEM）. It was found that no amorphous phase formed in the as-spun La-free alloy, but the as-spun alloys containing La held a major amorphous phase. When La content x≤2, the major phase in the as-cast alloys was Mg2Ni phase, but with further increase of La content, the major phase of the as-cast alloys changed into LaNi5＋LaMg3 phase. Thermal stability of the as-spun alloys was studied by differential scanning calorimetry （DSC）, showing that spinning rate was a negligible factor on the crystallization temperature of the amorphous phase. The hydrogen absorption and desorption kinetics of the as-cast and as-spun alloys were measured using an automatically controlled Sieverts apparatus, confirming that the hydrogen absorption and desorption capacities and kinetics of the as-cast alloys clearly increased with rising La content. For La content x=2, the as-spun alloy displayed optimal hydrogen desorption kinetics at 200 ℃.

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 INFLUENCE OF INDIUM ON Pd-In ALLOYS

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The Cracking Induced by Oxidation-Hydriding in Welding Joints of Zircaloy-4 Plates

The welding joints of Zircaloy 4 plates obtained by diffusion welding at 800°C under pressure in vacuum were cracked during autoclave tests at 400°C superheated steam after exposure longer than 150 days. The section of specimens was examined by optical microscopy and the composition at the tips of cracking was analyzed by electron microprobe. The result shows that the combination of oxidation and hydriding induced cracking is responsible for this failure of the welding joints.

Investigation of Structure and Hydriding Properties of La0.6 Nd0.4 Ni4.s Mn0.2 Cux Alloys

The structure and hydriding performance of La0.6Nd0.4Ni4.8Mn0.2Cux (x = 0 ～ 0.4) alloys were investigated in order to develop suitable materials for metal hydride air conditioner. The effect of Cu addition on the crystal structure,equilibrium pressure, hydrogen capacity and hysteresis as well as hydrogen absorption/desorption kinetics were systematically studied by using the measurement of P-C isotherms, X-ray diffraction and scanning electron microscopy. As the amount of Cu increases, the plateau pressure increases and hydrogen absorption/desorption kinetics is improved, but the effective hydrogen storage capacity decreases. It is shown that variations in the basal plane parameter a can be used as an indication for the plateau pressure changes. With the increase of parameter a, the plateau pressure decreases. For La0.6 Nd0.4Ni4.8Mn0.2Cux(x = 0 ～ 0.4) alloys there is a relationship between the effective hydrogen storage capacities and the ratios of their unit cell parameters c and a. The effective hydrogen storage capacity decreases with increase of a/c.

Deflagration characteristics of freely propagating flames in magnesium hydride dust clouds

The flame propagation processes of MgH_(2)dust clouds with four different particle sizes were recorded by a high-speed camera.The dynamic flame temperature distributions of MgH_(2)dust clouds were reconstructed by the two-color pyrometer technique,and the chemical composition of solid combustion residues were analyzed.The experimental results showed that the average flame propagation velocities of 23μm,40μm,60μm and 103μm MgH_(2)dust clouds in the stable propagation stage were 3.7 m/s,2.8 m/s,2.1 m/s and 0.9 m/s,respectively.The dust clouds with smaller particle sizes had faster flame propagation velocity and stronger oscillation intensity,and their flame temperature distributions were more even and the temperature gradients were smaller.The flame structures of MgH_(2)dust clouds were significantly affected by the particle sinking velocity,and the combustion processes were accompanied by micro-explosion of particles.The falling velocities of 23μm and 40μm MgH_(2)particles were 2.24 cm/s and 6.71 cm/s,respectively.While the falling velocities of 60μm and 103μm MgH_(2)particles were as high as 15.07 cm/s and 44.42 cm/s,respectively,leading to a more rapid downward development and irregular shape of the flame.Furthermore,the dehydrogenation reaction had a significant effect on the combustion performance of MgH_(2)dust.The combustion of H_(2)enhanced the ignition and combustion characteristics of MgH_(2)dust,resulting in a much higher explosion power than the pure Mg dust.The micro-structure characteristics and combustion residues composition analysis of MgH_(2)dust indicated that the combustion control mechanism of MgH_(2)dust flame was mainly the heterogeneous reaction,which was affected by the dehydrogenation reaction.

Recent progress in thermodynamic and kinetics modification of magnesium hydride hydrogen storage materials

Hydrogen energy has emerged as a pivotal solution to address the global energy crisis and pave the way for a cleaner,low-carbon,secure,and efficient modern energy system.A key imperative in the utilization of hydrogen energy lies in the development of high-performance hydrogen storage materials.Magnesium-based hydrogen storage materials exhibit remarkable advantages,including high hydrogen storage density,cost-effectiveness,and abundant magnesium resources,making them highly promising for the hydrogen energy sector.Nonetheless,practical applications of magnesium hydride for hydrogen storage face significant challenges,primarily due to their slow kinetics and stable thermodynamic properties.Herein,we briefly summarize the thermodynamic and kinetic properties of MgH2,encompassing strategies such as alloying,nanoscaling,catalyst doping,and composite system construction to enhance its hydrogen storage performance.Notably,nanoscaling and catalyst doping have emerged as more effective modification strategies.The discussion focuses on the thermodynamic changes induced by nanoscaling and the kinetic enhancements resulting from catalyst doping.Particular emphasis lies in the synergistic improvement strategy of incorporating nanocatalysts with confinement materials,and we revisit typical works on the multi-strategy optimization of MgH2.In conclusion,we conduct an analysis of outstanding challenges and issues,followed by presenting future research and development prospects for MgH2 as hydrogen storage materials.

Improving hydrogen storage thermodynamics and kinetics of Ce-Mg-Ni-based alloy by mechanical milling with TiF_(3)

Mg-based hydrides are too stable and the kinetics of hydrogen absorption and desorption is not satisfactory.An efficient way to improve these shortcomings is to employ reactive ball milling to synthesize the nanocomposite materials of Mg and additives.In this experiment,TiF_(3)was selected as an additive,and the mechanical milling method was employed to prepare the experimental alloys.The alloys used in this experiment were the as-cast Ce_(5)Mg_(85)Ni_(10),as-milled Ce_(5)Mg_(85)Ni_(10)and Ce_(5)Mg_(85)Ni_(10)+3 wt.%TiF3.The phase transformation,structural evolution,isothermal and non-isothermal hydrogenation and dehydrogenation performances of the alloys were inspected by XRD,SEM,TEM,Sievert apparatus,DSC and TGA.It revealed that nanocrystalline appeared in the as-milled samples.Compared with the as-cast alloy,ball milling made the particle dimension and grain size decrease dramatically and the defect density increase significantly.The addition of TiF_(3)made the surface of ball milling alloy particles markedly coarser and more irregular.Ball milling and adding TiF_(3)distinctly improved the activation and kinetics of the alloys.Moreover,ball milling along with TiF_(3)can decrease the onset dehydrogenation temperature of Mg-based hydrides and slightly ameliorate their thermodynamics.

Inclusion of CoTiO_(3) to ameliorate the re/dehydrogenation properties of the Mg–Na–Al system

For the first time,the MgH_(2)–NaAlH_(4)(ratio 4:1)destabilized system with CoTiO_(3) addition has been explored.The CoTiO_(3)-doped MgH_(2)–NaAlH_(4) sample begins to dehydrogenate at 130℃,which is declined by 40℃ compared to the undoped MgH_(2)–NaAlH_(4).Moreover,the de/rehydrogenation kinetics characteristics of the CoTiO_(3)-doped MgH_(2)–NaAlH_(4) were greatly ameliorated.With the inclusion of CoTiO_(3),the MgH_(2)–NaAlH_(4) composite absorbed 5.2 wt.%H_(2),higher than undoped MgH_(2)–NaAlH_(4).In the context of dehydrogenation,the CoTiO_(3)-doped MgH_(2)–NaAlH_(4) sample desorbed 2.6 wt.%H_(2),almost doubled compared to the amount of hydrogen desorbed from the undoped MgH_(2)–NaAlH_(4) sample.The activation energy obtained by the Kissinger analysis for MgH_(2) decomposition was significantly lower by 35.9 kJ/mol than the undoped MgH_(2)–NaAlH_(4) sample.The reaction mechanism demonstrated that new phases of MgCo and AlTi_(3) were generated in situ during the heating process and are likely to play a substantial catalytic function and be useful in ameliorating the de/rehydrogenation properties of the destabilized MgH_(2)–NaAlH_(4) system with the inclusion of CoTiO_(3).