简介概要

Electrochemical hydrogen storage behaviors of as-cast and spun RE-Mg-Ni-Co-Al-based AB₂-type alloys applied to Ni-MH battery

来源期刊：Rare Metals2020年第2期

论文作者：Yang-Huan Zhang Gang Huang Ze-Ming Yuan Shi-Hai Guo Yan Qi Dong-Liang Zhao

文章页码：181 - 192

摘要：Preparation of La-Mg-Ni-Co-Al-based AB_2-type alloys La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1（x=0,0.05,0.10,0.15,0.20） was performed using melt spinning technology.The influences of spun rate and Y content on structures and electrochemical hydrogen storage characteristics were studied.The base phase LaMgNi₄ and the lesser phase LaNis were detected by X-ray diffraction（XRD） and scanning electron microscope（SEM）.The variations of spinning rate and Y content cause an obvious change in phase content,but without altering phase composition,namely,with spinning rate and Y content growing,LaMgNi4 phase content augments while LaNi₅ content declines.Furthermore,melt spinning and the replacing La by Y refine the grains dramatically.The electrochemical tests show a favorable activation capability of the two kinds of alloys,and the maximum discharge capacities are achieved during the first cycle.Discharge capacity firstly increases and subsequently decreases with spinning rate rising,while cycle stability is ameliorated and discharge capacity decreases with Y addition increasing.It is found that the amelioration of cycle stability is due to the enhancement of anti-pulverization,anti-corrosion and antioxidation abilities by both replacement of La with Y and melt spinning.Moreover,with the increase of Y addition and/or spinning rate,the electrochemical kinetics that contain charge transfer rate,limiting current density（I_L）,hydrogen diffusion coefficient（D） and the high rate discharge ability（HRD） firstly augment and then reduce.

稀有金属(英文版) 2020,39(02),181-192

Electrochemical hydrogen storage behaviors of as-cast and spun RE-Mg-Ni-Co-Al-based AB₂-type alloys applied to Ni-MH battery

Yang-Huan Zhang Gang Huang Ze-Ming Yuan Shi-Hai Guo Yan Qi Dong-Liang Zhao

Key Laboratory of Integrated Exploitation of Baiyun Obo MultiMetal Resources,Inner Mongolia University of Science and Technology

Department of Functional Material Research,Central Iron and Steel Research Institute

作者简介：*Yang-Huan Zhang,e-mail:zhangyh59@sina.com;

收稿日期：30 July 2017

基金：financially supported by the National Natural Science Foundations of China (Nos.51761032, 51871125 and 51471054);

Electrochemical hydrogen storage behaviors of as-cast and spun RE-Mg-Ni-Co-Al-based AB₂-type alloys applied to Ni-MH battery

Yang-Huan Zhang Gang Huang Ze-Ming Yuan Shi-Hai Guo Yan Qi Dong-Liang Zhao

Key Laboratory of Integrated Exploitation of Baiyun Obo MultiMetal Resources,Inner Mongolia University of Science and Technology

Department of Functional Material Research,Central Iron and Steel Research Institute

Abstract：

Preparation of La-Mg-Ni-Co-Al-based AB_2-type alloys La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1(x=0,0.05,0.10,0.15,0.20) was performed using melt spinning technology.The influences of spun rate and Y content on structures and electrochemical hydrogen storage characteristics were studied.The base phase LaMgNi₄ and the lesser phase LaNis were detected by X-ray diffraction(XRD) and scanning electron microscope(SEM).The variations of spinning rate and Y content cause an obvious change in phase content,but without altering phase composition,namely,with spinning rate and Y content growing,LaMgNi4 phase content augments while LaNi₅ content declines.Furthermore,melt spinning and the replacing La by Y refine the grains dramatically.The electrochemical tests show a favorable activation capability of the two kinds of alloys,and the maximum discharge capacities are achieved during the first cycle.Discharge capacity firstly increases and subsequently decreases with spinning rate rising,while cycle stability is ameliorated and discharge capacity decreases with Y addition increasing.It is found that the amelioration of cycle stability is due to the enhancement of anti-pulverization,anti-corrosion and antioxidation abilities by both replacement of La with Y and melt spinning.Moreover,with the increase of Y addition and/or spinning rate,the electrochemical kinetics that contain charge transfer rate,limiting current density(I_L),hydrogen diffusion coefficient(D) and the high rate discharge ability(HRD) firstly augment and then reduce.

Keyword：

Electrochemical characteristic; AB₂-type alloy; Y substitution; Melt spinning;

Received： 30 July 2017

1 Introduction

Over-consumption of limited fossil fuels not only gives rise to a series of environmental problems,but also speeds up its exhaustion.The severe environmental pollution and the high rate of global warming compel an initiative change from fossil fuels to renewable energy sources.Recently,an investigation shows that 20%energy on our earth is depleted by transportation [ 1] and approximately 23%CO₂emission released by vehicle exhaust through burning of fossil fuels [ 2] .A strategy has been presented for reducing energy consumption and CO₂ emission.According to an authoritative report,the main cause for the appearance of fog and haze in Beijing is vehicle exhaust.As is well known,the end of the Stone Age is not due to the exhaustion of stones.As the saying goes,the end of the fossil fuel era does not need to wait until it dries up.That is to say,human beings cannot be constrained to give up the car for the depletion of fossil fuels;in contrast,mankind is highly possible to forwardly abandon fossil fuels because of a substantial growth of automobiles.Actually,as early as 2009,some related documents about developing new energy vehicles have been published by the authoritative departments of China.Nowadays,the hybrid electric vehicle (HEV) has been applied in our lives.There is a part of HEV that contains nickel-metal hydride battery,which in turn,ushers in the best development situation.A part of hydrogen storage materials are deemed to be the most promising candidates,especially AB₂-type alloys and the rare earth-based AB₅-type alloys which have been applied to business;however,none of them is satisfactory as a result of their inherent weaknesses,such as activation difficulty for AB₂-type alloys and relatively low specific capacity for AB₅-type alloys.For the negative electrode of Ni/MH battery,La-Mg-Ni-based A₂B₇ and AB₂ alloys were seen as one of the most suitable materials in consideration of the comprehensive electrochemical performances and lower production cost [ 3] ,as put forward by Kohno et al. [ 4] and Kadir et al. [ 5] .The maximum discharge capacity of LaMgNi₄ was～400 mAh·g^-1 by synthesizing the alloy with method of mechanical milling,as proposed by Wang et al. [ 6] .Guenee et al. [ 7] detected a cubic MgCu₄Sn (AuBe₅-type) structure by studying the structures of LaMgNi₄ and NdMgNi₄.An enormous number of researches have been performed to reach objectives of business application.Nevertheless,bad cycle stability hiders the development of these alloys.Nowadays,it has not found a suitable alloy that could be applied as the negative electrode material.

Alloying and microstructural transformation have been deemed to be the impactful way to enhance the hydrogen storage properties of RE-Mg-Ni-based alloys [ 8, 9] .Particularly,electrochemical cycle stability of complex alloys can be enhanced by partially replacing La with Co,Mn and lanthanon,Ni with Al and Cu [ 10, 11, 12] .Besides,prior studies show that melt spinning can markedly enhance cycle stabilities of La-Mg-Ni system alloys and enormously refine the structure [ 13, 14] .Thus,it is expected to prepare a high-discharge capacity alloy with favorable cycling stability through exploring an optimized combination of Y substitution ratio with the suitable melt spinning technique.

It has been corroborated in some documents that appending lanthanons Y and Ce enormously increases electrochemical properties,including cycle stability and anti-corrosion of electrode materials [ 15] .In this paper,the La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys were dealt with the technology of melt spinning and a partial substitution for La by Ce and Y.Whereafter,the influences of spinning rate and the degree of Y substitution on microstructure and electrochemical characteristic were studied in specifics.

2 Experimental

The experimental alloys are La_0.8-xCe_0.2 Y_xMgN i_3.4-Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20).In order to simplify expression,the alloys in different Y contents are written in a simple Y₀,Y_0.05,Y_0.10,Y_0.15 and Y_0.20,respectively.Vacuum induction furnace filled with0.04 MPa helium was used to synthesize the alloy ingots.A part of the as-cast alloys were re-melted and then spun by melt-spinning with a rotating roller.Because spinning rate cannot be accurately determined,it can be seen as the outer edge speed of the copper roller.The spinning rates of 2,5,10 and 12 m·s^-1 were set to investigate the effect of different spinning rates on properties.

X-ray diffraction (XRD,D/max/2400) using Cu Kα1radiation filtered by graphite at 40 kV,160 mA and10 (°)·min^-1 served as analysis tool for testing components of as-cast and melt-spun alloys and the phase structure.Scanning electron microscope (SEM,Quanta 400) was used to detect the morphologies of two kinds of alloys.

The carbonyl nickel powder and alloy powder were mixed according to the ratio of 1:4 and then weighed to1 g.The weighed powder was made into an electrode with15 mm in diameter under a pressure of 35 MPa.The trielectrode contains a working electrode (the sample cathode electrode),a sintered Ni(OH)2/NiOOH positive electrode and a Hg/HgO reference electrode.It was applied in measuring electrochemical capability at 303 K.In each cycle,the electrode was charged at a constant current density,discharged at the same current density and ended when cutoff voltage was up to-500 mV after resting for15 min.

Electrochemical workstation (PARSTAT 2273) was adopted to survey the potentiodynamic polarization and the electrochemical impedance spectra (EIS) at 303 K.The sample was charged at first and then kept for some hours until the open circuit voltage became stable.EIS were tested under 50%depth of discharge (DOD),frequency of10 kHz-5 mHz,amplitude of 5 mV and the number of points per decade of frequencies being 60.Electrochemistry corrosion software (CorrWare) was utilized to test the potentiostatic discharge at 500 mV and steps for 5000 s.The Tafel polarization curves were acquired at scan rate of5 mV·s^-1 in the potential range from-1.2 to+1.0 V(between the alloy electrode and Hg/HgO).

3 Results and discussion

3.1 Microstructural characterization

XRD patterns of La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys prepared by casting and melt spinning are shown in Fig.1,which reveals that there are multiphase structures inside the two kinds of alloys.LaMgNi₄ is the major phase corresponding to the SnMgCu₄ (AuBe₅)-type structure that belongs to (216) space group,and LaNi₅ is the less phase.The melt spinning and replacing La with Y obviously bring about changes in the phase abundances without changing phase component.Meanwhile,it is found that diffraction peak intensities of the LaNi₅ phase significantly reduce when the spinning velocity and Y content increase,suggesting that replacing La with Y and melt spinning can reduce LaNi₅phase.According to XRD,the lattice constant of LaMgNi₄and LaNi₅ phases were worked out by Jade 6.0 software,and all data are shown in Tables 1 and 2.Lattice constants and crystal sizes of LaMgNi₄ and LaNi₅ phases obviously increase when spinning speed grows,as shown in Table 1,which are probably related to crystal lattice distortion and lattice stress caused by melt spinning.Meanwhile,melt spinning brings about the increase in LaMgNi₄ phase and the decrease in LaNi₅ phase amount.The lattice constants and crystal sizes of LaMgNi₄ and LaNi₅ obviously reduce with the increase of Y content,as shown in Table 2,which is because La is bigger than Y in grain size.Also,with Y content rising,the amount of LaMgNi₄ phase grows and that of LaNi₅ phase decreases.

Fig.1 XRD patterns of as-cast and spun La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys:a Y_0.20 alloy and b as-spun(12 m·s^-1)

The illustrations of SEM and EDS of as-cast La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20)alloys are shown in Fig.2.Clearly,the as-cast alloy presents typical dendritic structures.The morphologies of the as-cast alloy display the peritectic reaction by two different color regions.According to XRD detection,LaNi₅ phase is the bright region and LaMgNi₄ phase is the gray region,and they are distinguished by EDS patterns.Moreover,it is also found that replacing La with Y clearly facilitates the refinement of alloy grains.

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Table 1 Lattice constants and abundances of LaMgNi₄ and LaNi₅ phases in as-spun Y_0.2 alloy

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Table 2 Lattice constants and abundances of LaMgNi₄ and LaNi₅ phases in as-spun (12 m·s^-1) alloys

Fig.2 SEM images of as-cast a Y₀ alloy,b Y_0.10 alloy and c Y_0.20 alloy;typical EDS spectra of d LaMgNi₄ and e LaNi₅ phases of as-cast Y_0.10alloy

The morphologies of La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1(x=0,0.05,0.10,0.15,0.20) treated with different spinning speeds and Y substitution were detected with SEM,as presented in Fig.3.Using line-intercept technique,the average grain sizes of LaMgNi₄ are determined to be 2.37,2.24,1.87,and 1.08μm in Y_0.2 alloy spun at 2,5,10 and12 m·s^-1,respectively.Apparently,grain sizes of the melt spinning alloys obviously decrease when spinning rate and Y content increase.By comparing Figs.2 and 3,a particularly obvious characteristic can be found that the melt spinning dramatically promotes the refinement of alloy grains.Moreover,Fig.3 shows that the as-spun alloys include two phases,LaMgNi₄ as main phase and LaNi₅ as secondary phase,and the amount of LaNi₅ phase decreases as spinning rate decreases,which agrees well with XRD results.

Fig.3 SEM images of as-spun La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys:Y_0.20 alloy spun at a 2 m·s^-1,b 5 m·s^-1,c 10 m·s^-1 and d 12 m·s^-1;e Y₀,f Y_0.05,g Y_0.10 and h Y_0.20 alloys spun at 12 m·s^-1

3.2 Electrochemical characteristics

3.2.1 Discharge potential characteristic,discharge capacity and activation capability

As Fig.4 shows,at a current density of 60 mA·g-1,the discharge potential curves of La_0.8-xCe_0.2Y_xMgNi_3.4-Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) are acquired.The potential plateau of the discharge curves is regarded as the output power of battery,and the values of output power are determined by the discharge potential characteristics.With time going on,the discharge potential plateau becomes more horizontal and the discharge potential characteristics become more favorable.Experimental alloys show good potential characteristics by replacing La with Y and melt spinning,and it shows that discharge plateau is lengthened and discharge potential is heightened.Furthermore,it has been confirmed that variations of Y content and spinning rate engender a different result on the discharge capacity.Figure 4a,b shows the relationships between spinning rate/Y content and discharge capacity.The tendency that rises first and then falls with the augment of rotation speed is shown in Fig.4a for discharge capacity of Y_0.20 alloy.When spinning speed is up to 10 m·s^-1,Y_0.20 alloy obtains the maximum discharge capacity of 363.4 mAh·g^-1.Moreover,it can be easily observed that the discharge capacity clearly reduces with the growth of Y substitution.As Y content of as-spun (12 m·s^-1) alloys increases from 0to 0.20,the discharge capacity reduces from 377.8 to357.5 mAh·g^-1.

Figure 5 presents the discharge capacities of the alloys which were dealt with casting and melt spinning at current density of 60 mA·g^-1.Usually,after charging-discharging cycles of many times,the greatest discharge content is reached,which can be characterized by activation capacity.The activation performance becomes more and more favorable with the abatement of charging-discharging cycles needed.In Fig.5,the maximum discharge capacity can be reached after once hydrogen charging,suggesting that the sample has a prominent activation capability.The variations of spinning rate and Y content make no difference to the activation capability.

Fig.4 Discharge potential curves of as-cast and spun La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys:a Y_0.20 alloy and b as-spun (12 m·s^-1)

Fig.5 Evolution of discharge capacity of as-cast and spun La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys with cycle number:a Y_0.20 alloy and b as-spun (12 m·s^-1)

The electrochemical behaviors of the alloys can be obviously affected by variations of spinning rate and Y content according to above results,which is thought to be related to the structures altered by melt spinning and replacement of La by Y.The potential characteristic of an alloy has a high correlation with internal resistance of a battery.Moreover,the battery resistance contains polarization resistance and ohmic internal resistance,and they are all basically affected by the diffusion of hydrogen atoms of electrode [ 16] .The beneficial effect of melt spinning and replacing La with Y on the potential characteristics are probably caused by the dramatic refinement of grains.As we all know,hydrogen atoms can diffuse fast in the grain boundaries [ 17] .The discharge capacity is affected by a variety of factors,for example,the crystal structure,the diameter of grains,the surface configuration,etc.With the change in spinning speed,discharge capacity achieves a maximum value which indicates that melt spinning brings about both positive and passive functions on discharge capacity.Positive action is a decrease in grain size caused by melt spinning since a large number of hydrogen atoms can be assembled on the grain boundaries [ 18] .Because discharge capacity of LaMgNi₄ phase is higher than that of LaNi₅ phase,the reduction of LaNi₅amount in the procedure of melt spinning is also very beneficial to enhance the electric capacity.A negative effect on discharge capacity caused by melt spinning is the generation of lattice stress.It is confirmed that the lattice stress is disadvantage to the discharge capacity [ 19] .Figure 4b indicates that replacing La with Y engenders an obviously negative action on the discharge capacity,which is most likely in connection with the decrease in cell cubage after substitution.The activation energy is dominated by the change in intrinsic energy before and after the absorption of hydrogen.The activation performance will be poor with the internal energy and the surface energy increasing [ 19] .In addition,surface energy is derived from an oxidation layer and the tension caused by atoms entering the tetrahedron or octahedron gap.The superior activation properties of the electrode alloys fabricated by casting and melt spinning are principally associated with their multiphase micro structures on a basis of a fact that phase boundary is regarded as buffer area to mitigate strain energy due to the lattice distortion after hydrogen absorption.Melt spinning and replacement of La with Y can refine grains.It is helpful to enhance the activation ability of an electrode.

3.2.2 Cycle stability

The capacity retaining rate (S_n) in general can be utilized to characterize cycle stability,which shows the ability to resist capacity reduction during the electrochemical cycle process.The formula of S_n is S_n=C_n/C_max×100%,where C_max characterizes the maximum electricity content of discharge and C_n is electricity content of discharge after nth cycle at a current density of 300 mA·g^-1.Figure 6depicts the different S_n values in different cycles of La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys,these values clearly exhibit degradation process of discharge capacity.It is evident that degradation rate of discharge capacity remarkably slackens as Y substitution and spinning rate increase,indicating that replacing La with Y and melt spinning have an extremely active effect on cycle stability of the samples.To show how spinning rate and Y addition influence the cycle stability directly,the relationship between S₁₅₀ (n=150)values and spinning rate or Y addition are constructed from the data in Fig.6a,b.The S₁₅₀ values considerably grow with spinning rate and Y content increasing.With spinning speed enhancing from 0 (as-cast is defined as the spinning rate of 0 m·s^-1) to 12 m·s^-1,the S₁₅₀ value of Y_0.20 alloy increases from 71.3%to 92.4%;similarly,S₁₅₀ value grows to 92.4%from 76.5%with Y addition increasing from 0 to0.20 for as-spun (12 m·s^-1) alloy.

As is known to all,the fundamental causes resulting in capacity degradation are pulverization and oxidation [ 20, 21] .Hydrogen atoms entering the lattice interstitials can lead to the internal stress which enlarges the unit cell inevitably,which is considered to be a factor leading to the pulverization.To research the capacity degradation mechanism of electrode,the morphological changes in negative electrode before and after charge-discharge cycle were observed by SEM,as shown in Fig.7.Many flaws can be found on the surface after 300 charge-discharge cycles,whereas Y₀ alloy particle has an extremely glossy surface without any crack before cycling,illustrating that the pulverization occurs with the electrochemical cycles.It has been found that the surface of Y₀ alloy powders is overlapped by the rough and gossypine layer after cycling(Fig.7b),which is discerned to be La(OH)3 and Mg(OH)2by XRD,as Fig.7d shows.There is more or less hydroxide coating to come into being on the surface of Y_0.15 specimen particle,implying that anti-corrosion and anti-oxidation abilities are enormously enhanced by the replacement of La with Y.According to above explanation,it can receive a conclusion that an active effect of replacing La with Y and melt spinning on keeping stabilization in cycles is ascribed to the grain refinement on the basis of a generally acknowledge fact that the toughness and strength of an alloy increase as grain size decreases.That is to say,strength and toughness are better when grain size is smaller,implying the higher the anti-pulverization capability will be.Besides,the improvement of oxidation replacing La with Y is extremely conducive to further improve electrochemical cycle stability.

Fig.6 Evolution of capacity retaining rates (S_n) of as-cast and spun La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys with cycle number:a Y_0.20 alloy and b as-spun (12 m·s^-1)

Fig.7 SEM images of a as-spun (12 m·s^-1 Y₀ alloy before cycling),b as-spun (5 m·s^-1) Y₀ and c as-spun (5 m·s^-1) Y_0.20 alloys after cycling;d XRD pattern of as-spun (12 m·s^-1) Y₀ alloy after cycling

3.2.3 Electrochemical kinetics

Electrochemical activity in general can be characterized by high rate discharge (HRD) performance.The formula of HRD is HRD=C_i/C₆₀×100%,where C_i and C₆₀ are the maximum discharge capacities at the current densities of i and 60 mA·g^-1,respectively.Figure 8 shows the variations of HRDs with current density for the alloys m the state of melt-spun and as-cast.With current density growing,HRDs of the alloys conspicuously lessen.Taking i=300 mA·g^-1 as a reference for comparison,the relations are ascertained between Y addition/spinning rate and HRDs,as seen in Fig.8a,b.With spinning rate and Y content growing,HRDs increase at first and then decrease,and the maximum HRD for as-spun (12 m·s^-1) Y_0.10electrode is 93.8%and 94.2%for Y_0.20 alloy,respectively,which is almost the same with the HRD (92.6%-94.6%) of tombarthite-based AB₅-type alloy [ 22] .

Fig.8 Evolution of HRD of as-cast and spun La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys with current density:a Y_0.20alloy and b as-spun (12 m·s^-1)

Actually,HRD of electrode is mainly affected by two aspects:one is hydrogen diffusion ability inside bulk and the other is charge transfer rate on exterior [ 23] .To investigate kinetic mechanism,the hydrogen diffusion ability and charge transfer rate influenced by spinning rate and Y content were studied.According to the Kuriyama's model [ 24] ,the charge transfer rate was ascertained by electrochemical impedance spectrum (EIS).An emblematical example of EIS curves in spinning (2 m·s^-1) is displayed in Fig.9,whose abscissa is the real part of impedance (Z_real) and the ordinate is the imaginary part of impedance (-Z_imag).It can be found that both of high and middle-frequency regions demonstrate two twisted capacitor rings and the low-frequency region is a line in each EIS curve.The electrochemical process can be correctly reflected by the patterns of different frequency regions.The smaller semicircle in high-frequency area indicates contact resistance between detection electrode and nickel strap which is conductive material;the larger one in middlefrequency region presents the charge transfer resistance(R_ct) on electrode surface,and beeline in low-frequency area indicates Warburg impendence.R_ct value was derived using a matching program Z-View via the equivalent circuit (inset Fig.9),and it principally rests with both the reaction surface and the proportion of the reaction proposed by Kuriyama et al. [ 24] .In the equivalent circuit in Fig.9,R_el stands for electrolyte resistance,R_cp and C_cp are the contact resistance and capacitance between the current collector and the electrode plate,the contact resistance and capacitance between the alloy particles are represented by R_pp and C_pp,R_ct and C_ct represent the charge transfer reaction resistance and capacitance on alloy particle surface,and R_w stands for the Warburg impedance.With apparent activation enthalpy (Δ_rH^*),electrochemical reaction on electrode surface can be confirmed,which comes down to the following [ 25] :

Fig.9 EIS results of as-spun (2 m·s^-1) La_0.8-xCe_0.2Y_xMgNi_3.4-Co_0.4Al_0.1(x=0,0.05,0.10,0.15,0.20) alloys and equivalent circuit

where T stands for temperature of the sample,R_ct presents the charge transfer resistance of the charged or discharged alloy electrodes,R and C₀ are constants of the gas and the surface area dependent,respectively.Taking design conditions of Eq.(1) into consideration,EIS curves of La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys were measured at temperatures of 303,318 and333 K,and EIS curves of Y_0.10 and Y_0.20 prepared by melt spinning (12 m·s^-1) are shown in Fig.10.The data are listed in Fig.10,and Kuriyama diagrams of ln(T/R_ct) versus 1/T were drawn inserted in Fig.10a,b by using logarithmic transform of Eq.(1).Δ_rH^*values were figured up readily based on the slopes of the Kuriyama diagrams.Hence,the relationships among A_rH^*v alue,spinning rate and Y content are established,as illustrated in Fig.11.There exist optimal spinning rate and Y content for achieving the maximum value of transfer rate.It can be seen thatΔ_r^H*values firstly decline and then rise with spinning rate and Y content growing.It suggests that the melt spinning and replacing La with Y have two aspects on the role for the charge transfer rate.Melt spinning causes an active effect probably bearing on crystallographic characteristics,and crystallographic and electronic structure mainly rests with charge transfer rate on surface proposed by Kleperis et al. [ 26] .Moreover,the ultra-refined grains after melt spinning are deemed to produce an active impact,because refinement grains intensively prevent the alloy from pulverization during electrochemical cycle [ 27] and reduce effective superficial area and charge transfer rate on the interface.Concerning replacing La with Y,an active impact is regarded as the enhancement of corrosion resistance such as preventing La and Mg from further oxidation and forming a nickel-rich layer on the surface [ 28] ,which can enhance surfactivity.The negative impact is to bound up with grain refinement.

Diffusion coefficient of hydrogen atoms was deduced from semilogarithmic curves illustrated in Fig.12.On the basis of linear slope region from corresponding plots,hydrogen coefficient in the bulk was easily conjectured to fit the following equations [ 29] :

Fig.10 EIS results of as-spun (12 m·s^-1) a Y_0.10 and b Y_0.20 alloys at various temperatures

Fig.11 Evolutions of activation enthalpy (Δ_rH^*) values of La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys with Y content and spinning rate:a Y_0.20 alloy and b as-spun (12 m·s^-1)

where i is diffusion current density (A·g^-1),F represents the Faraday constant (C·mol^-1),D presents hydrogen diffusion coefficient (cm²·s^-1) in the alloy bulk,C₀ and C_spresent original hydrogen thickness on particles surface and inside the bulk (mol·cm^-3),inpidually,a presents radius of particle (cm),d presents density of the alloy(g·cm^-3) and t presents discharge time (s).Figure 12a,b shows the D values by calculating Eq.(3),indicating that D values firstly grow and then lessen when spinning rate and Y content grow.Another kinetic parameter of hydrogen diffusion ability inferred from the potentiodynamic polarization curve is the limiting current density (I_L).Figure 13 shows an outstanding inflection point in each anodic polarization curve.The limiting current density (I_L)appears at inflection point,which determines the occurrence of oxidation reaction on the electrode surface.Hence,oxide layer can prevent the hydrogen from diffusing into alloys [ 11] .So,the limiting current density (I_L) in general is deemed for a critical current density for passivation.Relationships between spinning rate/Y content and I_L values were built,as shown in Fig.13a,b.Obviously,the values of I_L increase at beginning and then decrease when the spinning speed and Y addition increase.Comparing Figs.12 and 13,it can found that D and I_L values are similarly affected by spinning rate and Y content,indicating that diffusion capability of hydrogen atoms can be truly reflected via the two parameters.Besides,it also can be seen that the transformation trends of D and I_L values with spinning rate and Y content are same with HRD,implying that the diffusion capability is a crucial diathesis which affects electrochemical characteristics of an electrode.Grain refinement by melt spinning and replacement of La by Y has a greatly active impact on the increase in hydrogen atom diffusion capacity,which has been discussed above [ 17] .An inactive effect of melt spinning on hydrogen diffusion is extremely likely related to lattice distortion,which hinders hydrogen diffusion by increasing diffusion activation energy of hydrogen atoms [ 30] .The negative effect of replacement of La by Y on hydrogen diffusion is put down to cell volume diminishing on the basis of a fact that diffusion activation energy of hydrogen atom will be augmented when cell volume and lattice constants decrease,consequently impeding hydrogen diffusion [ 30] .

Fig.12 Semilogarithmic curves of anodic current versus time responses of as-cast and spun La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys:a Y_0.20 alloy and b as-spun (m·s^-1)

Fig.13 Potentiodynamic polarization curves of as-cast and spun La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys:a Y_0.20alloy and b as-spun (12 m·s^-1)

4 Conclusion

The structure characterization of La_0.8-xCe_0.2Y_xMgNi_3.4-Co_0.4Al_0.1 (x=0,0.05,0.10,0.15,0.20) alloys prepared by casting and spinning shows that Y substitution and melt spinning cause an ascent in LaMgNi₄ phase content while descent in LaNi₅ phase content,but phase compositions are stabilized.Melt spinning and replacing La with Y make the grains evidently refined.Besides,the melt spinning can lead to the enlargement of cell volumes and lattice constant in sizes,whereas replacing La with Y generates a completely opposite result.Preeminent activation capability of samples is represented by means of electrochemical tests.The discharge potential characteristics can be obviously enhanced by the melt spinning and replacing La with Y.As spinning speed increases,the discharge capacity increases to a maximum value at first and then decreases,but it remarkably decreases as Y content increases.Moreover,the spinning rate and replacing La with Y dramatically improve the electrochemical cycle stability,which can be explained by the improvement in anti-pulverization and anti-corrosion and anti-oxidation abilities produced by melt spinning and replacement of La by Y.The electrochemical kinetics,including HRD,D,I_L and charge transfer rate,of both as-cast and spun alloys promoted at beginning and later reduced as spinning rate and Y content grow,for which structure change by spinning and replacing La with Y are mainly responsible.Activation enthalpy (Δ_rH^*) and hydrogen diffusion coefficient (D) of samples are deemed as primary factors influencing HRD abilities.