Effect of Heat-Treatment Process on Properties of Rare Earth Mg-Based System Hydrogen Storage Alloys with AB_3-Type

The effect of heat-treatment process on the properties of Mm0.8Mg0.2(NiCoAlMn)3.5 hydrogen storage alloy was discussed . The electrochemical properties such as cycling stability, activation property, and the plateau voltage of the alloy which was heat-treated in various temperatures and times had different changes during the cycle process, the optimum heat-treatment conditions of this alloy were determined by this work.

Study on Nanocrystalline Rare Earth Mg-Based System Hydrogen Storage Alloys with AB_3-Type

A sort of rare earth Mg-based system hydrogen storage alloys with AB3-type was prepared by double-roller rapid quenching method. The alloys were nanocrystalline multi-phase structures composed of LaNi3 phase and LaNi5 phase by X-ray diffraction and scanning electron microscopy analyses, and the suitable absorption/desorption plateau was revealed by the measurement of P-C-I curve. Electrochemical studies indicate that the alloys exhibit good electrochemical properties such as high capacity and stable cycle life, and the discharge capacity is 369 mAh·g-1 at 0.2 C (72 mA·g-1). after 460 cycles, the capacity decay was only 19.4% at 2 C (720 mA·g-1).

Phase Structure and Electrochemical Properties of Rare Earth Based Hydrogen Storage Alloys

The rare earth based hydrogen storage alloys MmxM1 1 - x ( Ni3.55 Co0.75 Mn0.4 A10.3 ) ( x = 0 ～ 0.5 ) were investigated in this work.Adjusted Ml: Mm ratio to change the content of La,Ce,Pr and Nd in the alloys and then to change the phase structure, the influences of phase structure on the electrochemical properties were analyzed.The results indicate that the main phase of all alloys is LaNi5 with CaCu5 type structure and the crystal lattices constants of LaNi5 are changed with increasing x value, i.e, decreased a-axis, increased c-axis and axis ratio and nonlinear decreased crystal volume.The crystal volume of the alloy with x = 0.3 is larger than others.There is second phase A1LaNi4 in alloys when x≥0.3, which decrease the discharge capacity, but increase the cycling stability and high rate discharge ability.Compared comprehensively, the alloy with x = 0.3 shows the higher discharge capacity and the better cycling stability.

Study on Particle Size and Electrochemical Properties of Rare Earth Based Hydrogen Storage Alloys

The rare earth based hydrogen storage alloys Ml_ 0.7Mm_ 0.3(Ni_ 3.55Co_ 0.75Mn_ 0.4Al_ 0.3) were chosen as objects of investigation in this paper. The effects of particle size on electrochemical properties of the alloy were investigated. The results indicate that the alloy with particle size of 100 and 150 mesh shows good activation behavior and high discharge capacity (the first discharge capacity and the maximum discharge capacity), but poor cycling stability, low capacity retention and high discharge capacity rate. The Ml_ 0.7Mm_ 0.3(Ni_ 3.55Co_ 0.75Mn_ 0.4Al_ 0.3) alloy with particle size of 150 mesh shows excellent electrochemical properties.

Study on Phase Structure and PCT Characteristics of Rare Earth Based Hydrogen Storage Alloys

The rare earth based hydrogen storage alloys Mm_xMl_ 1-xNi_ 3.55Co_ 0.75Mn_ 0.4Al_ 0.3(x=0～0.50) were investigated in this work. The influences of phase structure on the PCT characteristics were analyzed by means of electrochemical measurements. The results indicate that there is a strict relationship between crystal volume and PCT characteristics.

Effects of Strip Casting and Annealing on Electrochemical Properties of LPCNi_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3) Hydrogen Storage Alloys

The effect of thickness (1 similar to 10 mm) of the ingots on the electrochemical properties of as-cast and annealed strip cast LPCNi3.55Co0.75Mn0.4Al0.3 hydrogen storage alloys was investigated. It is found that the 0.2 C discharge capacity of as-cast LPCNi3.55Co0.75Mn0.4Al0.3 alloy increases with the increase of the thickness of the ingots. As-east alloy with the thickness of 10 mm shows better activation property, higher 1C discharge capacity and better cyclic stability than others. It is mainly contributed to its larger unit cell volume and less internal stress. Annealed LPCNi3.55Co0.75Mn0.4Al0.3 alloy with the thickness of 3 mm shows much better comprehensive electrochemical properties than as-east one; The cyclic. stability of the alloy with the thickness of 6 mm and the activation properties of the alloys with the thickness of 3 similar to 6 mm are improved after annealing. It is mainly owing to the great release of internal stress and the decrease of the segregation of Mn in the alloys.

Microstructure and electrochemical properties of LaNi_(4-x)FeMn_x (x=0-0.8) hydrogen storage alloys

In order to reduce the cost of LaNi5 based hydrogen storage alloys, effect of substitution of Mn for Ni on structural and electrochemical properties of LaNi4-xFeMnx (x=0-0.8) hydrogen storage alloys was studied systematically. X-ray diffraction (XRD) and scanning electron microscope (SEM) showed that LaNi5 and La2Ni7 phases were invariably present in all alloy samples, and when x >= 0.4, (Fe, Ni) phase was observed. Electrochemical studies revealed that the discharge capacity reached a maximum value of 306.4 mAh/g when x=0.2 and the cycling stability decreased with the increase of x. With the increase of Mn content, hydrogen diffusion coefficient decreased, whereas high rate discharge-ability (HRD) and exchange current density first increased slowly when x <= 0.2 and then decreased markedly when x=0.8, indicating that electrochemical reaction on the surface of alloy electrodes had strong influence on kinetic property.

Effect of ball milling on hydrogen storage of Mg_3La alloy

Hydrogen storage and microstructure of ball milled Mg3La alloy were investigated by X-ray diffraction and pressure-composition-isotherm measurement. The ball milled Mg3La alloy could absorb hydrogen up to 4wt.% at 300 ℃ for the first time, along with a decomposing course. Following tests showed that the average reversible hydrogen storage capacity was 2.7wt.%. The enthalpy and entropy of dehydrogenation reaction of the decomposed ball milled Mg3La and hydrogen were calculated. XRD patterns indicated the existence of MgH2 and LaH3 in the decomposed hydride and the formation of Mg when hydrogen was desorbed. After the first hydrogenation, all the latter hydrogenation/dehydrogenation reactions could be taken place between Mg and MgH2. The ball milled Mg3La alloy exhibited better hydriding kinetics than that of the as-cast Mg3La alloy at room temperature. The kinetic curve could be well fitted by Avrami-Erofeev equation.

Electrochemical Properties of Strip Casting LPCNi_(3.55) Co_(0.75) Mn_(0.4) Al_(0.3) Hydrogen Storage Alloys

LPCNi 3.55 Co 0.75 Mn 0.4 Al 0.3 hydrogen storage alloy was investigated, and the effects of thickness of its strip casting ingots(as cast) on the electrochemical performances were discussed. It was found that the 0.2 C discharge capacity increased with the increase of the thickness (from 1 mm to 10 mm) of the ingots, mainly due to the enlargement of the unit cell volume; Among the thickness of the ingots in our study, 10 mm sample showed a better activation property; LPCNi 3.55 Co 0.75 Mn 0.4 Al 0.3 alloy with 10mm showed higher comprehensive properties than those with other thickness under 1C rate.

Effects of Mm(NiCoAlMn)_5 hydrogen storage alloy coated with Ni-Co-P alloy by electroless plating on electrochemical properties of hydride electrodes

The effect of chemical plating with Ni Co P alloy on the properties of MH electrodes is investigated. The results show that the efficiency of storage alloy and the activation of MH electrode have been improved by introducing 1.74% cobalt in the Ni Co P alloy coating. The initial discharge capacity is 208 mAh/g. The maximum discharge capacity gets to 298.5 mAh/g. At the same time the cycle life of MH electrodes is improved. The discharge capacity of MH electrodes coated with Ni Co P is 88% of the maximum discharge capacity after 300 cycles. Whereas the discharge capacity of bare alloy electrodes retains 62% of the maximum capacity after 300 cycles. An increment of discharge capacity is mainly due to the superposition of the oxidation current of Co as well as improved efficiency of microcurrent collection. The effect of Ni Co P alloy coating by electroless plating on the kinetic properties of hydride electrode has been systematically investigated by electrochemical techniques. The results indicate that the kinetic properties of MH electrodes, including exchange current density, limiting current density, have been improved markedly. This improvement of kinetic properties leads to the decrease of the overpotential of anodic and cathodic polarization.

Electronic Structure of Hydrogen Storage Compounds, LaNi_5 and Its Micro-Hydrogenated Compounds

The electronic structures of LaNi5 hydrogen storage alloys and its micro-hydrogenated compounds with two hydrogen atoms in the center of two octahedral interstices and two tetrahedral interstices, were investigated by the first principles discrete variational method （DVM）. The results of density of states and the difference of charge distribution clearly show that the s electrons of H mainly interact with the s electrons of hydride-non-forming element Ni, despite there being a larger affinity of La for hydrogen than that of Ni in pure metal-hydrogen system. From the cohesive energy of systems, we also found two systems have almost same stability with occupation of H atoms.

Microstructure and Hydrogen Absorption Pro-perties of Mg/MmNi_(5-x) (CoAlMn)_x Composite Prepared by Mechanical Alloying

The microstructure ofthe Mg/MmNi5-x (CoAlMn )x composite hydrogen storage material preparedby the method of mechanical alloyingwas characterized by X-ray diffraction, SEM and particle size distribution analysis. By measuring PCTcurves, the hydrogen absorption properties of the composite was evaluated.The results show that nanocrystallinecomposite structure can be obtainedunder adequate ball milling condition. The reactive activation and hydrogen absorption capacity are improved compared with the sole MmNi5-x(CoAlMn)x alloy. The effect ofmagnesium on the microstructure andhydrogen absorption properties of thecomposite were also evaluated.

Electrochemical properties of cobalt-free La_(0.80)Mg_(0.20)Ni_(2.85)Al_(0.11)M_(0.53) (M=Ni,Si,Cr,Cu,Fe) alloys

In order to investigate the effect of different B-site additions on phase structure and electrochemical properties of cobalt-free La-Mg-Ni based alloys, La0.80Mg0.20Ni2.85Al0.11M0.53 （M=Ni, Si, Cr, Cu, Fe） hydrogen storage alloys were prepared and studied systematically. X-ray powder diffraction showed that the alloys consisted mainly of LaNi3 phase and LaNi5 phase except that Cr addition caused a minor Cr phase. Electrochemical testing indicated that alloys with additional Ni, Cr, Cu or Fe were activated within only 1-2 cycles, while that with Si addition needed 4 cycles. Adding Si, Cu and Fe increased cycling stability of La-Mg-Ni based alloys. However, maximum discharge capacity decreased from 362 mAh/g to 215 mAh/g in the order of Ni〉Fe〉Cu〉Cr〉Si. In addition, electrochemical kinetics of alloy electrodes was also researched by measuring high rate discharge ability （HRD）, hydrogen diffusion coefficient （D） and limiting current density （IL）.

化学计量比对无钴AB5型贮氢合金相结构及电化学性能的影响

用冷坩埚磁悬浮熔炼结合热处理制备了La（Ni0.87Mn0.07Al0.06）；（x=4．7-5．3）贮氢合金，采用XRD、夹片式模拟开口电池测试体系研究了合金的相结构及电化学性能。XRD分析表明，X为5．0～5．3的合金为单-CaCu5型六方结构，X为4．7-4．9的合金含有少量Ce2Ni，型和BCr型结构的杂相；随着x的增大，晶胞体积先基本保持不变，然后从89．65A3（x=4．9）减小到89．20A3（x=5．31；晶胞c／a轴比先基本保持不变，然后从0．7982（x=5．0）明显增加到0．8000（x=5．3）。电化学性能测试表明，该系列合金活化性能优异：最大放电容量Cmax从308．4mAh·g^-1（x=4．7）增加到338．5mAh·g^-1（x=5．0），然后降低到325．1mAh·g^-1（x=5．3）；循环稳定性（S100×C300）先基本保持不变，然后明显提高，与晶胞c／a轴比的变化有良好的对应关系。

稀土元素改性钒基固溶体储氢合金研究进展

钒基固溶体储氢合金具有体心立方(BCC)结构,储氢质量分数在3.8%以上,充放电容量为1052 mA·h/g,优于AB2和AB5等系列合金,并且在常温常压下表现出较高的氢溶解度和扩散系数,因此在氢储运系统以及氢能源供应等领域具有广阔的应用前景,但钒基固溶体合金存在着活化难度大、放氢条件苛刻、循环寿命短以及对氧敏感易氧化等问题。研究表明,稀土对多种固态储氢材料均有很好的改性作用,将稀土元素通过元素替代或掺杂的方式加入到钒基固溶体合金中,有助于生成高活性的稀土或稀土氧化物第二相,可明显改善材料的吸放氢热力学、循环稳定性以及抗毒化性质,同时可减少材料内的氧含量,提高材料的活化特性。电化学性能方面,稀土元素的添加能显著提升合金电极的循环稳定性、耐腐蚀能力以及高倍率放电性能。因此,稀土元素取代是实现钒基固溶体储氢材料实际应用的一项行之有效的方法。本文报道了近30年稀土改性钒基固溶体储氢合金的研究现状,重点总结了稀土元素的作用机制,并对今后重点研究方向进行了展望。

Effect of annealing treatment on the anti-pulverization and anti-corrosion properties of La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5) hydrogen storage alloy

The Lao.67Mg0.33Ni2.5Co0.5 hydrogen storage alloy was prepared by the vacuum intermediate frequency induction furnace followed by annealing treatment. The pulverization degree of both the as-cast and annealed alloy powders after gaseous hydriding and dehydriding cycle was investigated and the discovery was that annealing treatment could hardly ameliorate their anti-pulverization ability. The element content of La, Mg, Ni and Co existing in electrolyte before and after the electrochemical cycles by using ICP-AES technology was also analyzed and it showed that a large amount of La and Mg were dissolved in the electrolyte, but the amount of dissolution for La and Mg significantly declined when the alloy was annealed. The XRD analysis revealed that all the al- loys consisted of two main phases AB3 and AB2 and a residual phase AB5 while annealing treatment made the AB2 phase decrease slightly. Furthermore, the anti-corrosion abilities of various elements in different phases of the as-cast and annealed alloy samples were studied by analyzing the element （La, Mg, Ni, Co） change with the corrosion time in phases AB3 and AB2 by means of EDS. It turned out that the element of La was mainly corroded out from the phase AB2 while not easily from the phase AB3. However, the element of Mg was both easily corroded out from the phases AB2 and AB3, but the corrosion was more obvious in the phase AB3. Therefore, annealing improved the anti-corrosion performances of La and Mg in the phase AB2.

Influence of magnesium content on structure and electrochemical properties of La_(1-x)Mg_xNi_(1.75)Co_(2.05) hydrogen storage alloys

La（1-x）MgxNi（1.75）Co（2.05）（x=0.07, 0.08, 0.10, 0.13, 0.15） alloys were prepared by high-frequency inductive method, and then their structure and electrochemical properties were investigated systematically. The XRD analysis revealed that the alloys consisted of LaNi5 phase and La4MgNi（19）（Ce5Co（19） ＋ Pr5Co（19）） phase, and the introduction of Mg could promote the formation of La4MgNi（19） phase. The observation of microstructure showed that all the alloys processed dendritic structure, which was refined with the increase of x value. The electrochemical measurements showed that all the alloys could be activated within 2 cycles, and with increasing x, the maximum discharge capacity obviously increased from 254.00 m Ah/g（x=0.07） to 351.51 mAh/g（x=0.15）, but the cycling stability（S（80）） decreased somehow from 78.4% to 73.9%. Meanwhile, the appropriate addition of Mg could improve the high-rate discharge capacity（HRD） of the alloy electrodes, which was mainly controlled by the electrochemical reaction rate on the surface of the alloys.

Phase structure and hydrogen storage properties of REMg8.35Ni2.18Al0.21 （RE=La, Ce, Pr, and Nd） hydrogen storage alloys

REMg8.35Ni2.18Al0.21 （RE=La, Ce, Pr, and Nd） alloys were prepared by induction melting and following annealing. X-ray diffraction （XRD） and scanning electron microscopy （SEM） results showed that the alloys were composed of Mg2Ni, （La, Pr, Nd）MgzNi, （La, Ce）2MgxT, （Ce, Pr, Nd）Mg12 and Ce2Ni7 phases. The above phases were disproportioned into Mg2NiH4, MgH2 and REHx （x=2.5 1 or 3） phases in hydriding. CEH2.51 phase transformed into CEH2.29 phase in dehydriding, whereas LaH3, PrH3 and NdH3 phases re- mained unchanged. The PrMg8.41Ni2.14Al0.20 alloy had the fastest hydriding kinetics and the highest dehydriding plateau pressure while the CeMg8.35Ni2.18Al0.21 alloy presented the best hydriding/dehydriding reversibility. The onset hydrogen desorption tempera- ture of the CeMg8.35Ni2.18Al0.21 hydride decreased remarkably owing to the phase transformation between the Cell2.51 and the CEH2.29.

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.

Investigations on hydrogen storage properties of LaMg_(8.52)Ni_(2.23)M_(0.15) (M=Ni, Cu, Cr) alloys

LaMg8.52Ni2.23M0.15 （M=Ni, Cu, Cr） alloys were prepared by induction melting. X-ray diffraction showed that all the three alloys had a multiphase structure, consisting of La2Mg17, LaMg2Ni and Mg2Ni phases. Energy dispersive X-ray spectrometer results revealed that most of Cu and Cr distributed in MgzNi phase. La2Mg17 and LaMg2Ni phases decomposed into MgHz, Mg2NiH4 and LaH3 phases during the hydrogenation process. Hydriding/dehydriding measurements indicated that the reversible hydrogen storage capacities of Mg2Ni phase in LaMgs.52Ni2.23M0.15 （M=Cu, Cr） alloys increased to 1.05 wt.% and 0.97 wt.% from 0.79 wt.% of Mg2Ni phase in LaMgs.52Ni2.38 alloy at 523 K. Partial substitution of Cu and Cr for Ni decreased the onset dehydrogenation temperature of the alloy hydrides and the temperature lowered by 18.20 and 5.50 K, respectively. The improvement in the dehydrogenation property of the alloys was attributed to that Cu and Cr decreased the stability of Mg2NiH4 phase.