Hydrogen storage behaviors and microstructure of MF3 (M=Ti, Fe)-doped magnesium hydride

MgH2+10%MF3(M=Ti,Fe)(mass fraction) composites were prepared by ball-milling in hydrogen atmosphere,and their hydrogen storage behaviors and microstructure were investigated systematically.The results show that the hydriding and dehydriding kinetics of MgH2 are markedly improved by doping TiF3 and FeF3 fluorides.At 573 K,the two composites can absorb 5.67%-6.07%(mass fraction) hydrogen within 5 min under an initial hydrogen pressure of 3.5 MPa,and desorb 5.34%-6.02% hydrogen within 6 min.Furthermore,the composites can absorb hydrogen rapidly in moderate temperature range of 313-473 K.In comparison,TiF3-doped sample has a better hydriding-dehydriding kinetics than FeF3-doped sample.The microstructure analysis shows that some active particles including MgF2,TiH2 and Fe could be formed in the hydriding-dehydriding processes of the MF3-doped composites.From the Kissinger's plot,the apparent activation energies for the hydrogen desorption of the composites are estimated to be 74.1 kJ/mol for TiF3-doped composite and 77.6 kJ/mol for FeF3-doped composite,indicating MgH2 is significantly activated due to the catalytic effect of the doping of MF3.     

Development of hydrogen-storage alloys by mechanical alloying of Mg with Fe and Co

The hydriding and dehydriding kinetics of Mg are reviewed. In order to improve the reaction kinetics of Mg with hydrogen, mechanically-alloyed Mg-10 wt.%Fe and Mg-10 wt.%Co mixtures are prepared and their hydrogen-storage properties are investigated. The activation of Mg-10 wt.%Fe is easier than that of Mg-10 wt.%Co. However, The hydriding rates (at 569–589 K, 7–11 bar H₂) and dehydriding rates (at 589 K, 1.0 bar H₂) of the mechanically-alloyed Mg-10wt.%Co are higher than those of the mechanically-alloyed Mg-10 wt.%Fe after activation. The H_a value of Mg-10 wt.%Co after 60 min is 3.08 wt.% at 589 K under 11 bar H₂ and its H_d value after 60 min is 1.48 wt.% at 589 K under 1.0 bar H₂. Mg-10 wt.%Co has a smaller particle size than has Mg-10 wt.%Fe after hydriding and dehydriding cycling. The mechanical alloying of Mg with Fe and Co and the hydriding-dehydriding cycling increased the hydriding and dehydriding rates by facilitating nucleation (by creating defects on the surface of the Mg particle and by the additive) and by shortening the diffusion distances (by reducing the Mg particle sizes).     

Hydrogen storage performance of Mg-based composites prepared by spark plasma sintering

The hydrogen storage capacities, hydrogen absorption mechanism and hydride stability of Mg-based composites prepared by spark plasma sintering (SPS) were investigated in this paper. The results showed that the composites had a double-phase microstructure of Mg phase and V-based solid solution, with nanocrystalline magnesium existing at their sintering interface. With the addition of the V-based solid solution in 20% volume fraction, the composite exhibited a maximum reversible hydrogen storage capacity of 4.2 wt.% at 573 K, compared with that of pure Mg of almost zero. DSC results indicated that the hydride decomposition temperature of MgH₂ decreased sharply from 708 K in pure Mg to 636 K and to 591 K as the volume of V-based solid solution increased from 20 vol.% to 50 vol.%. With the addition of V-based solid solution, the hydrogen absorption kinetics of pure Mg was greatly improved at 573 K, and its hydrogen absorption mechanism changed from surface reaction control to diffusion control in the composite. Based on these experimental results, a model was put forward to describe the hydrogen absorption/desorption mechanism in these composites.     

Hydrogen storage properties of MgH2/SWNT composite prepared by ball milling

A systematic investigation was performed on the hydrogen storage properties of mechano-chemically prepared MgH₂/single-walled carbon nanotube (SWNT) composites. It is found that the hydrogen absorption capacity and hydriding kinetics of the composites were dependent on the addition amount of SWNTs as well as milling time. A 5 wt.% addition of SWNTs is optimum to facilitate the hydrogen absorption and desorption of MgH₂. The composite MgH₂/5 wt.% SWNTs milled for 10 h can absorb 6.7 wt.% hydrogen within about 2 min at 573 K, and desorb 6 wt.% hydrogen in about 5 min at 623 K. Prolonging the milling time over 10 h leads to a serious degradation on hydrogen storage property of the MgH₂/SWNT composite, and property/structure investigations suggest that the property degradation comes from the structure destruction of the SWNTs.     

Improvement of the hydrogen-storage kinetics of Mg by reactive mechanical grinding with 10 wt.% Fe2O3 and 5 wt.% Ni

The addition of Fe₂O₃ to Mg is believed to be able to increase the hydriding rate of Mg, and the addition of Ni is thought to be able to increase the hydriding and dehydriding rates of Mg. A sample Mg-(10wt.%Fe₂O₃, 5 wt.%Ni) was prepared by mechanical grinding under H₂ (reactive mechanical grinding). The as-milled sample absorbed 4.61 wt.% of hydrogen at 593 K under 12 bar H₂ for 60 min. Its activation was accomplished after two hydriding-dehydriding cycles. The activated sample absorbed 4.59 wt.% of hydrogen at 593 K under 12 bar H₂ for 60 min, and desorbed 3.83 wt.% hydrogen at 593 K under 1.0 bar H₂ for 60 min. The activated Mg-(10wt.%Fe₂O₃, 5 wt.%Ni) had a slightly higher hydriding rate at the beginning of the hydriding reaction but a much higher dehydriding rate compared with the activated Mg-10 wt.%Fe₂O₃. prepared via spray conversion. After hydriding-dehydriding cycling, Fe₂O₃ was reduced, Mg₂Ni was formed by the reaction of Mg with Ni, and a small fraction of Mg was oxidized.     

Preparation and hydrogen sorption properties of Mg-Cu-Y-H systems

Mg-xwt.%CuY (x=15, 20, 25) composites were successfully prepared by reactive mechanical alloying (RMA).X-ray diffraction (XRD) measurement shows that main phases of the as milled composites are MgH2 and Mg2Cu, and they converted into Mg and MgCu2 after dehydrogenation, respectively.Pressure-Composition-Isotherm (PCI) test shows that the composites exhibit double pressure plateau at each isothermal desorption process.The hydrogen absorption and desorption kinetics of the composites become worse with increasing x content, indicating that Mg-Cu phase has a negative effect on the hydrogen sorption properties of the composites.It is supposed that the good hydrogen sorption properties of the composites attribute to the catalyst effect of yttrium hydride distributed in Mg substrate and the particles size reduction and crystal defects formed by RMA.     

铟对MgH2脱氢性能的影响

将MgH2和In的混合粉末在行星式球磨机上进行球磨,制备了一种Mg H2-In的复合物。利用XRD分析了复合物的相组成以及吸放氢过程中的相转变;用气相色谱仪和差示扫描量热仪测定了复合物的脱氢性能和相转变温度,并用基于Sievert原理的全自动气体吸附仪测定了复合物的吸放氢热力学和动力学性能。结果表明,脱氢过程中Mg3In和Mg(In)固溶体的形成使Mg H2的脱氢反应焓和激活能显著降低,从而降低了Mg H2的脱氢温度,并显著改善了动力学性能。     

INFLUENCE OF TEMPERATURE AND PRESSURE ON THE KINETICS OF Mg-6mol%LaNi PREPARED BY HYDRIDING COMBUSTION SYNTHESIS

A new model to study the hydriding/dehydriding （H/D） kinetic mechanism has been applied in the two-phase （α-β） region of the Mg-6mol%LaNi composite at temperature and pressure ranging from 523 to 623K and 0.256 to 0.992MPa H2. respectively. The coincidence of the theoretical calculation with the experimental data indicates that the rate-limiting step is hydrogen diffusion in the β phase for hydriding process and the diffusion of hydrogen in the α solid solution for hydrogen desorption with activation energies 89500 and 87900J/mol H2 for HID processes, respectively, which were much smaller than those of MgH2 and can be attributed to the La and Ni additions.     

Mg-Ni-Ti19Cr50V22Mn9的结构及氢化动力学研究

采用机械合金化方法，将质量百分数为85Mg-5Ni-10Ti19Cr5oV22Mn9的复合材料在氢气保护气氛下球磨8h制备出复合储氢材料。用体积法测量了它在不同条件下的储氢性能，利用X射线衍射、显微镜技术和激光粒度分布仪考察了球磨时间对材料结构的影响，分析了氢化动力学与结构的相互关系。研究了材料在523K-573K的氢化反应动力学机理。结果表明，该复合材料在573K，20min内的吸放氢量分别为6．7％和6．6％。氢扩散为其限制性环节，吸放氢活化能分别为63kJ／mol和69kJ／mol。     

机械合金化La-Mg-Ni系三元储氢合金的性能

采用机械合金化制备了La-Mg-Ni系三元储氢材料，并对其热力学、动力学进行了研究，该材料具有很好的活性和较高的储氢量，在553K时储氢量达到5．23％(质量百分数)。在3．0MPa氢气压力和423K～573K之间的条件下，可以在1min之内完成饱和吸氢量的90％以上。采用XRD衍射、SEM对材料的物相和形貌进行分析和研究。实验证明：物相组成为La2Mg17，Mg2Ni，LaH2和单质La，颗粒的最大粒径为4μm。混合粉末材料的非晶化和体系中催化物质的存在使其氢化动力学性能得以明显改善。     

Hydrogenation reaction of Mg-Based alloys fabricated by rapid solidification

Mg-23.5wt%Ni-xwt%Cu (x=2.5, 5 and 7.5) alloys for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The alloys were ground by a planetary ball mill for 2 h in order to obtain a fine powder. The Mg-23.5Ni-5Cu alloy had crystalline Mg and Mg₂Ni phases. Mg-23.5Ni-5Cu had an effective hydrogen capacity of near 5 wt%. The activated Mg-23.5Ni-5Cu alloy absorbed 4.50 and 4.84 wt%H at 573K under 12 bar H₂ for 10 and 60 min, respectively, and desorbed 3. 21 and 4.81 wt%H at 573K under 1.0 bar H₂ for 10 and 30 min, respectively. The activated Mg-23.5Ni-5Cu alloy showed a quite high hydriding rate like Mg-10Fe₂O₃, and higher dehydriding rates than the activated Mg-xFe₂O_3?yNi. This likely resulted because the melting before melt spinning process has led to the homogeneous distribution of Ni and Cu in the melted Mg, and the Mg-23.5Ni-5Cu alloy has a larger amount of the Mg₂Ni phase than the Mg-xFe₂O_3?yNi alloy.     

Hydrogen storage properties of a Ni,Fe and Ti-added Mg-based alloy

Mg-5wt%Ni-2.5wt%Fe-2.5wt%Ti (referred to as Mg-5Ni-2.5Fe-2.5Ti) hydrogen storage material was prepared by reactive mechanical grinding, after which the hydrogen absorption and desorption kinetics were investigated using a Sievert-type volumetric apparatus. A nanocrystalline Mg-5Ni-2.5Fe-2.5Ti sample was prepared by reactive mechanical grinding and hydriding-dehydriding cycling. Analysis by the Williamson-Hall method from an XRD pattern of this sample after 10 hydriding-dehydriding cycles showed that the crystallite size of Mg was 37.0 nm and that its strain was 0.0407%. The activation of Mg-5Ni-2.5Fe-2.5Ti was completed after three hydriding-dehydriding cycles. The prepared Mg-5Ni-2.5Fe-2.5Ti sample had an effective hydrogen-storage capacity near 5 wt% H. The activated Mg-5Ni-2.5Fe-2.5Ti sample absorbed 4.37 and 4.90 wt% H for 5 and 60 min, respectively, at 593K under 12 bar H₂, and desorbed 1.69, 3.81, and 4.85 wt% H for 5, 10 and 60 min, respectively, at 593K under 1.0 bar H₂.     

Dehydriding reaction kinetic mechanism of MgH_2-Nb_2O_5 by Chou model

Chou model was used to investigate the dehydriding reaction kinetic mechanism of MgH_2-Nb_2O_5 hydrogen storage materials at 573 K.A new conception,characteristic absorption/desorption time(t_c)was introduced to characterize the reaction rate The fitting results show that for the hydrogen desorbing mechanism,the surface penetration is the rate-controlling step.The mechanism remains the same even when the original panicle size of Nb_2O_5 is before ball milling(BM) or when the BM time changes And t_c indic...     

Hydriding/dehydriding behavior of MgH<Subscript>x</Subscript>-iron oxides composites

Hydrogen has considerable potential as a renewable substitute for fossil fuels due to its high gravimetric energy density and environment friendliness. In particular, metal hydrides were attracted much interest given that its hydrogen capacity exceeds. One approach that can improve the kinetics is the addition of iron oxide. In this study, the hydrogen absorption/desorption properties of Mg were improved. The effect of the iron oxide concentration on the kinetics of the Mg hydrogen absorption reaction was investigated. MgH_x-iron oxide composites were synthesized by hydrogen-induced mechanical alloying. The synthesized powder was characterized by XRD, SEM, and simultaneous TG/DSC analysis. The hydriding behaviors were evaluated, using an automatic Sievert’s-type PCT apparatus. The absorption and desorption kinetics of Mg catalyzed with 5 and 10 wt.% Fe₂O₃/Fe₃O₄ were determined at 423, 473, 523, 573, and 623K, respectively. The results of hydrogenation properties on MgH_x-Iron oxide composites were measured to be about 1.0∼4.7 wt.% under 1 MPa H₂ atmosphere.     

Improvement in hydrogen sorption properties of Mg by reactive mechanical grinding with Cr2O3, Al2O3 and CeO2

We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H₂ (reactive mechanical grinding) with oxides Cr₂O₃, Al₂O₃ and CeO₂. The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH₂ during reactive mechanical grinding. The Mg+10wt.%Cr₂O₃ powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. The Mg+10wt.%Cr₂O₃ powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. Mg+10wt.%Cr₂O₃ absorbs 5.87wt.% H at 573 K, 11 bar H₂ during 60 min at the first cycle. The Mg+10wt.%Cr₂O₃ powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. It desorbs 4.44 wt.% H at 573 K, 0.5 bar H₂ during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding–dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr₂O₃ in the Mg+10wt.%Cr₂O₃ powder is reduced after hydriding–dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr₂O₃ after cycling.     

Cyclic hydrogen storage properties of Mg milled with nickel nano-powders and NiO

The cyclic hydrogen storage properties of nanocrystalline composite of Mg–3Ni–2NiO (wt.%) produced by mechanical milling with nickel nano-powders under the hydrogen pressure have been investigated. The results show that the composite has excellent hydrogen storage properties. Its maximum hydrogen capacity is about 6.4 wt.%. The hydrogen absorption time is about 54 s in which 6.1 wt.% of hydrogen has been absorbed at 200 °C. The hydrogen desorption time is about 310 s in which 6.2 wt.% of hydrogen has been discharged in the temperature range from 290 °C to 320 °C under the hydrogen pressure of 0.1 MPa. The composite also has a better cyclic hydrogen storage behavior. The hydrogen capacity is decreased less than 0.1 wt.% after 60 times of cycling. However, the hydrogen absorption time and desorption time are increased in proportion with the cyclic numbers. The reason for these may be that the MgO, formed during the cycling process, impedes an intimate contact of the catalyst with the MgH₂.     

Characteristics of plasma electrolytic oxide coatings on Mg-Al-Zn alloy prepared by powder metallurgy

Mg-6 wt.%Al-1 wt.%Zn alloy powders were produced by gas atomization, and subsequently compacted and sintered under various conditions of temperature, time, and pressure. The bulk Mg-6 wt.%Al-1 wt.%Zn alloy was coated by the plasma electrolytic oxidation (PEO) method. The optimum condition of compaction and sintering for PEO coatings was established based on the investigation of microstructure, microhardness, and corrosion properties of coatings which were compared to those of cast Mg-6 wt.%Al alloy. The coatings on Mg-6 wt.%Al and Mg-6 wt.%Al-1 wt.%Zn alloys consisted of MgO, MgAl₂O₄, and Mg₂SiO₄. The Mg-6 wt.%Al-1 wt.%Zn alloy compacted at room temperature for 10 min and sintered at 893 K for 3 h showed the most porous and nonuniform coating layer because the coatings had grown through grain boundaries that resulted from poor bonding between powder particles in the substrate. However, the coated Mg-6 wt.%Al-1 wt.%Zn alloy hot-compacted at 593 K for 10 min had the thickest coating layer and the highest microhardness. In addition, it demonstrated the best corrosion resistance as verified by polarization curves in 3.5% NaCl solution.     

Improved hydrogenation properties of Mg-x(Ti0.9 Zr0.2 Mn1.5 Cr0.3) composites

Mg-x(Ti_0.9 Zr_0.2 Mn_1.5 Cr_0.3)(x=20%, 30%, 40%) (mass fraction) composite powders were prepared by reactive ball milling with hydrogen and their hydrogen storage properties and microstructure were investigated by XRD, SEM and pressure-composition-temperature measurement. The results show that the composites have 3.83%-5.07% hydrogen capacity at 553 K and good hydrogenation kinetics, even at room temperature. Among them, the milled Mg-30%(Ti_0.9Zr_0.2Mn_1.5Cr_0.3) composite has the highest hydrogenation kinetics as it can quickly absorb 2.1% hydrogen at 373 K, 3.5% in 2 000 s at 473 K, even 3.26% in 60 s at 553 K under 3 MPa hydrogen pressure. The improved hydrogenation properties come from the catalytic effect of Ti_0.9 Zr_0.2 Mn_1.5 Cr_0.3 particles dispersed uniformly on the surface of Mg particles.     

定向凝固多孔Mg-Zn合金的制备及气孔结构

采用Bridgman型定向凝固法制备出藕状多孔Mg-Zn合金。研究了不同锌含量和氢气压力对气孔形貌的影响。通过理论计算对Mg-1Zn(质量分数,%,下同)合金的孔隙率进行了预测。结果表明,锌元素的加入会对孔结构产生重大影响。随着Zn含量从0%增加到2%,平均孔径增加。随着氢气压力从0.1 MPa增加到0.6 MPa,Mg-1Zn合金的孔隙率明显降低。基于氢气在多组分熔融金属中溶解度的计算模型,Mg-1Zn合金铸锭凝固高度为20 mm的不同氢压孔隙率的计算结果与实验的结果比较吻合。通过组织观察表明,随着Zn含量的增加,凝固组织由柱状晶转变为等轴晶。此外,研究了在不同凝固阶段孔的形成过程,可为在生物医用材料中应用的定向凝固多孔Mg-Zn合金的制备提供理论依据。     

Ce基催化剂对2LiBH4/MgH2放氢性能的影响

采用球磨方法制备了2LiBH4/MgH2复合储氢材料体系,用XRD、FTIR和储氢性能测试手段等对复合体系结构和储氢性能进行表征,研究了不同Ce基催化剂对复合体系放氢性能的影响,分析了催化剂的催化机理。结果表明:2LiBH4/MgH2复合物加热过程为明显的两步放氢,第1步主要发生MgH2的分解放氢;第2步为第1步生成的Mg与LiBH4发生放氢反应;添加Ce和CeF3都能提高2LiBH4/MgH2体系的放氢性能。Ce主要改善体系第2步放氢特性,CeF3对体系两步放氢反应均产生显著效果。添加5mol%CeF3使2LiBH4/MgH2体系起始放氢温度降低约100℃,体系最大放氢量达到10.6%(质量分数,下同);F-取代部分H-形成LiBH1-xFx,改善了LiBH4的分解特性,从而显著改善了2LiBH4/MgH2体系的放氢性能。