简介概要

Structures and hydrogen storage properties of RE-Mg-Ni-Mn-based AB₂-type alloys prepared by casting and melt spinning

来源期刊：Rare Metals2019年第11期

论文作者：Yang-Huan Zhang Zhong-Hui Hou Ying Cai Feng Hu Yan Qi Dong-Liang Zhao

文章页码：1086 - 1096

摘要：To ameliorate the electrochemical hydrogen storage properties of RE-Mg-Ni-Mn-based AB₂-type electrode alloys,La element was partially substituted by Ce,and La_1-xCe_xMgNi_3.5Mn_0.5（x=0,0.1,0.2,0.3,0.4）alloys were fabricated by casting and melt spinning.The effects of Ce content on structures and electrochemical hydrogen storage properties of prepared alloys were studied in detail.Results show that the experimental alloys consist of LaMgNi₄ and LaNi₅ phases.The variation of Ce content,instead of changing phase composition,results in an obvious phase abundance change in the alloys,namely the amount of LaMgNi₄ and LaNi₅ phases,respectively,increases and decreases with Ce content growing.Moreover,the partial substitution of Ce for La leads to that the lattice keeps constant,cell volumes clearly decreases and the alloy grains are markedly refined.The electrochemical measurements reveal that the as-cast and as-spun alloys obtain the maximum discharge capacities at the first cycling without any activation needed.With Ce content increasing,the discharge capacity of as-cast alloys visibly decreases.By contrast,the as-spun alloys have the maximum discharge capacity value.The substitution of Ce for La dramatically promotes the cycle stability.Moreover,the electrochemical kinetic performances of as-cast and asspun alloys first increase and then decrease with Ce content increasing.

稀有金属(英文版) 2019,38(11),1086-1096

Structures and hydrogen storage properties of RE-Mg-Ni-Mn-based AB₂-type alloys prepared by casting and melt spinning

Yang-Huan Zhang Zhong-Hui Hou Ying Cai Feng Hu 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：2 November 2015

基金：financially supported by the National Natural Science Foundation of China(Nos.51161015,51371094 and 51471054);the Natural Science Foundation of Inner Mongolia,China(No.2015MS0558);

Structures and hydrogen storage properties of RE-Mg-Ni-Mn-based AB₂-type alloys prepared by casting and melt spinning

Yang-Huan Zhang Zhong-Hui Hou Ying Cai Feng Hu 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：

To ameliorate the electrochemical hydrogen storage properties of RE-Mg-Ni-Mn-based AB₂-type electrode alloys,La element was partially substituted by Ce,and La_1-xCe_xMgNi_3.5Mn_0.5(x=0,0.1,0.2,0.3,0.4)alloys were fabricated by casting and melt spinning.The effects of Ce content on structures and electrochemical hydrogen storage properties of prepared alloys were studied in detail.Results show that the experimental alloys consist of LaMgNi₄ and LaNi₅ phases.The variation of Ce content,instead of changing phase composition,results in an obvious phase abundance change in the alloys,namely the amount of LaMgNi₄ and LaNi₅ phases,respectively,increases and decreases with Ce content growing.Moreover,the partial substitution of Ce for La leads to that the lattice keeps constant,cell volumes clearly decreases and the alloy grains are markedly refined.The electrochemical measurements reveal that the as-cast and as-spun alloys obtain the maximum discharge capacities at the first cycling without any activation needed.With Ce content increasing,the discharge capacity of as-cast alloys visibly decreases.By contrast,the as-spun alloys have the maximum discharge capacity value.The substitution of Ce for La dramatically promotes the cycle stability.Moreover,the electrochemical kinetic performances of as-cast and asspun alloys first increase and then decrease with Ce content increasing.

Keyword：

AB₂-type alloy; Ce substitution for La; Melt spinning; Electrochemical properties;

Received： 2 November 2015

1 Introduction

The excessive consumption of limited fossil fuels greatly accelerates its depletion and simultaneously causes a series of environmental problems.In particular,an amazing growth in the rate of global warming and serious air pollution have forced us to make a change from fossil fuels to cleaner and renewable energy sources.Some investigations and statistical results showed that a quarter of the world total energy was consumed by transport [ 1] and globally about 23%CO₂ emission derives from automobile exhaust fumes through the combustion of fossil fuels [ 2] .

Recently,an authoritative survey report from the Ministry of Environmental Protection of China declared that automobile exhaust is the main culprit giving rise to the severe haze in Beijing.Hence,a widespread application of electric vehicle(EV) and the hybrid electric vehicle (HEV) is considered to be an attractive strategy to reduce both energy consumption and CO₂ emissions.As a matter fact,in 2009,the Ministry of Industry and Information Technology of China enacted"the regulation of access of new energy automobile production enterprises and products"in which the HEV with nickel-metal hydride battery as auxiliary power has been classified as a mature product for nationwide selling,which,in turn,brings up a golden age for the development of the nickel-metal hydride(Ni-MH) battery.S ome hydrogen storage materials are deemed to be candidates as the negative electrode for Ni-MH batteries.For example,LaNi₅-type hydrogen storage alloys have been commercialized,but none of them is satisfactory because of their relatively low specific capacity.

Considering comprehensive electrochemical properties,La-Mg-Ni-based negative electrode alloys have been viewed as the most promising candidates for Ni/MH batteries on account of their higher discharge capacities(380-410 mA·h·g-1) [ 3, 4] .Wang et al. [ 5] utilized mechanical milling to synthesize a LaMgNi₄ alloy and found that it had a maximum discharge capacity of about400 mA·h·g-1.Guenee et al. [ 6] studied the structures of LaMgNi₄ and NdMgNi₄ alloys and found that these alloys have a cubic MgCu₄Sn (AuBe₅-type) structure.Although extensive research has recently been performed to achieve the goal of applications [ 7, 8] ,the practical application of these alloys is still a long way to go.Numerous literatures reported that alloying with other elements and microstructure modification can effectively improve the hydrogen storage properties of La-Mg-Ni-based alloys [ 9, 10, 11] .Particularly,the electrochemical cycle stability of the alloys can be markedly improved by the partial substitution of RE (RE=Ce,Pr,Nd and Sm) for La and Co,Mn,Cu and Al for Ni [ 12, 13] .Moreover,it was identified that the electrochemical performances of the alloys depend on their structures [ 14] .Melt spinning technique is a useful method to obtain an ultrafine grain,even nanocrystalline structure.It was ascertained that alloys produced by melt spinning exhibit excellent hydriding characteristics.Also,the microstructure created by melt spinning displays a much higher stability during the hydrogen absorbing and desorbing cycles.

Previous research results indicated that melt spinning technology can dramatically refine the microstructure of A₂B₇-type alloys and significantly improve their cycle stability [ 15, 16] .Hence,it is expected that the partial substitution of Ce for La and melt spinning technique may improve the electrochemical properties of the alloys.In this paper,the La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys were prepared by casting and melt spinning.The structures and electrochemical hydrogen storage characteristics were examined in detail.

2 Experimental

The chemical compositions of the alloys were La_1-xCe_xMgNi_3.5Mn_0.5 (x=0,0.1,0.2,0.3,0.4).For convenience,they were denoted with Ce content as Ce₀,Ce_0.1,Ce_0.2,Ce_0.3 and Ce_0.4 alloys.The as-cast alloys were prepared by vacuum induction melting with the protection of helium atmosphere with a pressure of 0.04 MPa.Then,the as-cast alloys were spun with a water-cooled rotating copper roller.Because it is extremely difficult to measure the cooling rate during melt spinning,the spinning rate was expressed by linear velocity of the copper roller.In the present experiment,the spinning rates were 2,6,10 and15 m·s^-1.

The phase compositions and structures of as-cast and asspun alloys were carried out by means of X-ray diffractometer (XRD,D/max/2400) with scan rate of 4 (°)·min^-1.Before XRD testing,the alloy ingots were mechanically ground into powders with below 50μm in size.The micromorphologies of the alloys were examined by scanning electron microscopy (SEM,QUANTA 400).

The electrochemical performances were carried out by using a tri-electrode open cell at 303 K.Metal hydride electrodes were prepared by pressing the alloy powders and carbonyl nickel powders in a weight ratio of 1:4.The total weight was about 1 g,and the diameter was 15 mm.A sintered Ni(OH)2/NiOOH counter electrode,a Hg/HgO reference electrode and the prepared metal hydride electrode were immersed in 6 mol·L^-1 KOH electrolyte.The charge/discharge cycles were carried out with a LAND(CT2001A) battery test instrument.In every cycle,the experimental cells were charged at the current density followed by a rest for 10 min and then were discharged at the same current density to cutoff voltage of-0.5 V.

The electrochemical kinetic properties of the alloy electrodes,including electrochemical impedance spectra (EIS),Tafel polarization curves and hydrogen diffusion coefficients,were measured using an electrochemical workstation (PAR-STAT 2273).After the alloy electrodes were completely charged and discharged for three cycles,the EIS was conducted at 50%depth of discharge (DOD) in the frequency range from 10 kHz to 5 mHz with an alternative current (AC)amplitude perturbation of 5 mV at 303,313 and 323 K.Then,the Tafel polarization curves were measured in the potential range from-1.2 to+1.0 V (vs.open circuit potentials) with a scan rate of 5 mV·s^-1 at 303 K.The hydrogen diffusion coefficients were also determined by potential step method at303 K.After being fully charged,the alloy electrodes were discharged with potential step of 600 mV for 5000 s.

3 Results and discussion

3.1 Structural characteristics

The phase compositions and structural characteristics of ascast and as-spun La_1＿xCe_xMgNi_3.5Mn_0.5 (x=0-0.4)alloys are subjected to XRD detections,as shown in Fig.1.It is clear that the diffraction patterns are similar and exhibit sharp peaks,indicating that these alloys have excellent crystallinities [ 17] .Clearly,the as-cast and asspun alloys have multiphase structures.The main phase is LaMgN_i4 corresponding to SnMgCu₄ (AuBe₅)-type structure with F 43m (216) space group,and the secondary phase is LaNi₅ corresponding to CaCu₅-type structure with P6/mmm (191) space group.The substitution of Ce for La and melt spinning,instead of altering the phase composition,result in an obvious change in phase abundances of the alloys.In particular,it can be seen that the diffraction peaks of LaNi₅ phase in as-spun Ce_0.4 alloy nearly disappear,indicating that Ce partial substitution and melt spinning decrease the amount of LaNi₅ phase.The lattice parameters of LaMgNi₄ and LaNi₅ phases of as-cast and as-spun (15 m·s^-1) alloys are also listed in Table 1.Obviously,the substitution of Ce for La reduces the lattice constants and cell volumes of LaMgNi₄ and LaNi₅ phases.This can be ascribed to that the atom radius of Ce is smaller than that of La.Meanwhile,it is also noted that increasing Ce substitution amount and spinning rate brings on an increase in LaMgNi₄ phase amount and a decrease in LaNi₅ phase amount.

Fig.1 XRD patterns of a as-cast and b as-spun (15 m·s^-1) La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys

下载原图

Table 1 Lattice constants and abundances of LaMgNi₄ and LaNi₅ phases

Fig.2 SEM images together with typical EDS spectra of as-cast La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys:a Ce₀,b Ce_0.2,c Ce_0.4,and d EDS spectra of d LaNi₅ phase and e LaMgNi₄ phase for Ce_0.2 alloy

SEM images and EDS results of as-cast La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys are presented in Fig.2.It is found that the morphologies of as-cast alloys display typical dendritic structures.The substitution of Ce for La gives rise to an evident refinement of the alloy grains.There are two regions with different colors,bright and gray regions.The analysis of EDS identifies that the gray region is LaMgNi₄ phase and the bright region is LaNi₅ phase,which is consistent with XRD detection.

Figure 3 demonstrates the morphologies of as-spun La_1-xCe_xMgN1_3.5Mn_0.5 (x=0-0.4) alloys.Figures 2 and 3show that melt spinning results in a dramatic decrease in grain size.In addition,it is also observed that the grain size of as-spun alloys notably decreases with the increase in Ce content.A careful observation will find that LaNi₅ phase which is shown in Fig.3 a almost vanishes in as-spun Ce_0.2and Ce_0.4 alloys,indicating that the partial substitution of Ce for La alters the phase abundances of the alloys.

3.2 Electrochemical performances

3.2.1 Activation capability,discharge capacity and potential characteristic

Generally speaking,the activation capability of an alloy can be characterized by the number of charging-discharging cycles required for attaining the maximum discharge capacity.Figure 4 describes the variations of the discharge capacities of as-cast and as-spun La_1-xCexMg Ni_3.5Mn_0.5 (x=0-0.4) alloys with cycle number at the charge and discharge current density of 60 mA·g^-1.It can be seen that all the alloys can reach their maximum discharge capacities at the first cycle,exhibiting an excellent activation performance.Both Ce substitution for La and melt spinning have no effect on activation capability of the alloys.

Figure 5 presents the relationship between discharge capacity of as-cast and as-spun alloys and Ce content.It is evident that the discharge capacity of as-cast alloys always declines with Ce content increasing.To be specific,increasing Ce content from 0 to 0.4 results in the decrease in discharge capacity of as-cast alloy from 308.5 to272.3 mA·h·g^-1.Differing from as-cast alloy,as-spun alloys have the maximum discharge capacity values with the variation of Ce content,which are 335.6,341.8,358.9and 352.6 mA·h·g^-1 for the spinning rates of 2,6,10 and15 m·s^-1,respectively.Figure 6 depicts discharge potential curves of as-cast and as-spun La_1-xCe_xMgNi_3.5Mn_0.5(x=0-0.4) alloys at a current density of 60 mA·g^-1.The discharge potential characteristic,a very important performance of an alloy electrode,can be characterized by the potential plateau of the discharge curves of the alloy.It is noted that the experimental alloys exhibit good potential characteristics.The substitution of Ce for La and melt spinning markedly improve discharge potential characteristics of the alloys,enhancing discharge potential and lengthening discharge plateau.

Fig.3 SEM images of as-spun (15 m·s^-1) La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys:a Ce₀,b Ce_0.2 and c Ce_0.4 alloy

Fig.4 Evolution of discharge capacity of a as-cast and b as-spun (15 m·s^-1) La_r-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys with cycle number (n)

The above results indicate that Ce substitution and melt spinning generate an obvious effect on electrochemical performances of the alloys,which is considered to be associated with the structural change by Ce substitution for La and melt spinning.In terms of activation capability,it directly depends on the change of internal energy.The larger the added internal energy is,the poorer the activation performance of the alloy will be.The internal energy change before and after absorbing hydrogen mainly involves lattice strain energy resulting from hydrogen atom entering the interstitial sites [ 18] .The excellent activation capability of as-cast and as-spun La_1-xCe_xMgNi_3.5Mn_0.5(x=0-0.4) alloys is principally attributed to their refined grains owing to the fact that the grain boundary can be viewed as buffer zone to relieve the lattice distortion and strain energy formed in the process of hydrogen absorption.

As well known,the discharge capacity is affected by crystal structure,phase composition and structure,grain size,composition uniformity and surface state,etc.The decreased discharge capacity of as-cast alloys as a result of Ce substitution for La is most likely associated with the reduction in cell volume due to the fact that the discharge capacity of an alloy is proportional to cell volume.However,the discharge capacities of as-spun alloys have the maximum values with Ce content increasing.This may be attributed to the grain refinement produced by Ce substitution because the grain boundary exhibits the distribution of the maximum hydrogen concentrations [ 19] .As to the potential characteristics of an alloy,it is thought to be closely relevant to internal resistance of a battery,including ohmic internal resistance and polarization resistance,which are basically dominated by the diffusion of hydrogen atoms in the alloy.The positive contribution of Ce substitution and melt spinning to the potential characteristic is most probably ascribed to the refinement of the alloy grains due to the fact that grain boundaries provide the fast diffusion paths for hydrogen atoms [ 20] .

Fig.5 Evolution of maximum discharge capacity of as-cast and as-spun La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys with Ce content

3.2.2 Electrochemical cycle stability

Electrochemical cyclic stability is also a key factor that evaluate whether a kind of electrode alloy can be applied in Ni-MH battery.The cyclic stability of the experimental alloy electrodes can be evaluated by capacity retaining rate.Here,the capacity retaining rate (S_n) is defined as:

where C_n is the discharge capacity of the nth chargedischarge cycle and C_max is the maximum discharge capacity.Figure 7 describes the variations of S_n values of as-cast and as-spun La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4)alloys with cycle number at a current density of300 mA·g^-1.It can be clearly seen that the degradation of discharge capacity significantly declines with Ce content growing,indicating that the substitution of Ce for La makes a positive contribution to the cycle stability of the alloys.The relationship between S₁₅₀ (n=150) values of as-cast and as-spun alloys and Ce content is also illustrated in Fig.7.It is evident that S₁₅₀ values conspicuously increase with Ce content growing.To be specific,increasing Ce content from 0 to 0.4 enables S₁₅₀ value to rise from 40.2%to 59.2%for as-cast alloy and from63.2%to 80.2%for as-spun (15 m·s^-1) alloy,respectively.For the same Ce content,as-spun alloy exhibits a much higher S₁₅₀ value than as-cast one,indicating that melt spinning facilitates the amelioration of the cycle stability of the alloy.

It has been documented that the primary causes leading to the capacity degradation of the alloys are the pulverization and oxidation.During the charge-discharge process,the alloy suffers an inevitable volume expansion and contraction,which aggravates the cracking and pulverizing of the alloy particles.The fresh alloy surface contacting with electrolyte will be oxidized,forming La(OH)2 and Mg(OH)₂ surface layer,which is also evidenced by experimental result,as presented in Fig.8.It is observed that many cracks appear on the alloy particle surface after electrochemical cycle.Meanwhile,it can be also seen that the alloy particles after cycling are covered by a rough and gossypin layer,which is identified to be La(OH)₂ and Mg(OH)₂ by XRD,as shown in Fig.8c.The positive contribution of Ce substitution for La to the cycle stability of as-cast and as-spun alloys is ascribed to two aspects.Firstly,the grain refinement caused by Ce substitution La facilitates to increase the strength and toughness,so as to enhance the anti-pulverization capability.Secondly,thus substitution can significantly improve the anti-corrosion and antioxidation ability of the alloys.A similar result was reported by Teresiak et al. [ 14] .

Fig.6 Discharge potential curves of a as-cast and b as-spun (15 m·s^-1) La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys

Fig.7 Evolution of capacity retaining rates (S_n) of a as-cast and b as-spun (15 m·s^-1) La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys.Inset being S₁₅₀ values as function of Ce content x

Fig.8 SEM images together with typical XRD pattern of as-spun (15 m·s^-1) Ce_0.2 alloy before and after electrochemical cycle:SEM images of Ce_0.2 a before and b after cycling and c XRD pattern of Ce_0.2 after cycling

3.2.3 Electrochemical kinetics

Another important factor for hydrogen storage electrode alloy is the electrochemical kinetics characterized by the high-rate discharge (HRD),which can be calculated by following equation:

where C_i and C₆₀ are the discharge capacities of the alloy electrode charged-discharged at the current densities of i and 60 mA·g-1.Figure 9 shows the variations of HRDs of as-cast and as-spun La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4)alloys with current density.It is evident that HRDs of the alloys clearly decline with current density rising.To facilitate comparison,current density of 300 mA·g^-1(i=300 mA·g^-1) was taken as a criterion to establish the relationship between HRDs of as-cast and as-spun alloys and Ce content,as shown in Fig.9a,b.It is found that HRDs of the alloys first increase and then decline with Ce content increasing,and the maximum HRD is 90.5%for as-cast alloy and 92.6%for as-spun alloy,which are almost equal to the electrochemical kinetics of rare earth-based AB_5-type alloy electrode.Moreover,it is also observed that the as-spun alloy shows a higher HRD than the as-cast alloy,indicating that melt spinning facilitates to improve the electrochemical kinetic properties of the alloys.

Fig.9 Evolution of HRD of a as-cast and b as-spun (15 m·s^-1) La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys with current density.Inset being C₃₀₀/C₆₀ values variation with Ce content x

It is convinced that HRD property of an alloy electrode mainly depends on charge transfer rate on the alloy electrode surface and the hydrogen diffusion ability in alloy bulk [ 21] .To investigate the mechanism of electrochemical kinetics of the experimental alloys,the effects of Ce content on the hydrogen diffusion ability and charge transfer rate were studied.The hydrogen diffusion coefficient can be derived by measuring the semilogarithmic curves of anodic current versus working duration of an alloy electrode,as illustrated in Fig.10.According to the model of Zheng et al. [ 22] ,the diffusion coefficient of the hydrogen atoms in the bulk of the composite could be estimated through the slope of linear region of the corresponding plots by the following formula:

where D is the hydrogen diffusion coefficient (cm²·s^-1),a is the alloy particle radius (cm),i is the diffusion current density (A·g-1) and t is the discharge time (s).D values of the alloys can be estimated by Eq.(3) and are also shown in Fig.10a,b.Clearly,D values of the alloys first increase and then decrease with Ce content growing.Another important parameter associating with diffusion ability of hydrogen atoms is limiting current density (I_L),which can be obtained by measuring the potentiodynamic polarization curve of an alloy electrode,as presented in Fig.11.It is noted that,in all cases,the current density tends to reach a limiting value in activation region due to the diffusionlimited kinetics of hydrogen absorption/desorption [ 21] .Thus,I_L value indicates the diffusion rate of hydrogen atoms from the bulk to the surface of the electrode.Based on the data in Fig.11,the relationships betweenI_L values and milling time can be constructed and are shown in Fig.11.It is found by comparing Figs.10 and 11 that the variation tendencies of D and I_L values with Ce content are very similar,suggesting that these two parameters factually reflect the diffusion ability of hydrogen atoms.Obviously,D and I_L values of the alloys increase first and then decrease with Ce content increasing,which is consistent well with the change of HRD.Therefore,it can be concluded that the hydrogen diffusion rate is a very important factor for the electrochemical kinetics of the alloy.The grain refined by Ce substitution for La evidently facilitates the enhancement of diffusion ability of hydrogen atoms,which has been mentioned previously.The negative impact on hydrogen diffusion caused by Ce substitution is ascribed to the decrease in cell volume due to the fact that a reduction in lattice constants and cell volume increases the diffusion activation energy of hydrogen atoms,consequently impairing hydrogen diffusion [ 23] .

Fig.10 Semilogarithmic curves of anodic current versus time responses of a as-cast and b as-spun (15 m·s^-1) La_1-xCe_xMgNi_3.5Mn_0.5(x=0-0.4) alloys.Inset being D values change with Ce content x

Fig.11 Potentiodynamic polarization curves of a as-cast and b as-spun (15 m·s^-1) La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys.Inset beingI_Lvalues variation with Ce content x

Regarding the charge transfer rate on the alloy electrode surface,it can be determined from its EIS curve in terms of Kuriyama's model [ 24] .As a typical representative,EIS curves of as-cast La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4)alloys are presented in Fig.12,where Z_real is the real part of impedance and Z_imag is the imaginary part of impedance.It can be clearly seen that there are two distorted capacitive loops at high-and middle-frequency regions separately as well as a line at low-frequency region in each EIS curve.As reported by Kuriyama et al. [ 24] ,these EIS curves can be analyzed using an equivalent circuit,as shown in Fig.12.In this equivalent circuit,R_el is ascribed to the electrolyte resistance between the metal hydride electrode and the reference electrode;R_cp and C_cp represent the semicircle in the high-frequency region,which is ascribed to the contact resistance between the alloy particles and the current collector;R_pp and C_pp represent the contact resistance and capacitance between the alloy particles;^thesemicircles in low-frequency region,modeled by R_ct and C_ct,represent the charge transfer reaction resistance and the double-layer capacitance;R_w is the Warburg impedance.With the aid of the equivalent circuit shown in Fig.12,the R_ct values can be obtained with the fitting program Z-view.Kuriyama et al. [ 24] considered that R_ct value depends on both the reactivity of the alloy surface and reaction area.And the electrochemical reactivity of the alloy surface can be evaluated with apparent activation enthalpy (Δ_rH^*),which can be evaluated by the following equation:

Fig.12 EIS curves of as-cast La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4)alloys and equivalent circuit (inset)

where R_ct is the charge transfer resistance for the metal hydride electrodes,R is the gas constant (8.314 J·mol^-1·K^-1),T is the temperature and C₀ is a constant in which the surface area is included.In consideration of the calculation conditions of Eq.(4),EIS curves of as-cast and as-spun La_1-xCe_xMg Ni_3.5Mn_0.5 (x=0-0.4) alloys were measured at different temperatures (303,318 and 333 K),and EIS curves of ascast and as-spun (15 m·s^-1) Ce_0.2 alloy are presented in Fig.13 as a representative.Based on the data in Fig.13,the Kuriyama graphs of ln(T/R_ct) versus 1/T can be plotted by using logarithmic transform of Eq.(4),as shown in Fig.13a,b,respectively.

From the slopes of Kuriyama's plots,Δ_rH^*values can be easily derived.Thus,the relationship betweenΔ_rH^*values of as-cast and as-spun alloys and Ce content is established,as demonstrated in Fig.14.It is clear thatΔ_rH^*values of the alloys first decrease and then increase with Ce content growing,implying that as-cast and as-spun alloys have an optimal Ce content with which the alloy possesses the highest charge transfer rate.It indicates that increasing Ce content plays a positive or negative role in the charge transfer rate of the alloys.The positive contribution of Ce substitution for La to charge transfer is considered to be associated with the improved corrosion resistance which further suppresses oxidation of La and Mg and induces a Ni-enriched layer on alloy surface [ 25] ,enhancing the electrocatalytic activity of the surface of the alloy electrode.The disadvantage of Ce substitution for La on charge transfer is most likely attributed to the grain refinement because the refined grains can strongly prohibit the cracking and pulverizing of the alloy during chargedischarge cycles [ 26, 27] ,reducing available new surfaces of the alloy particles and decreasing charge transfer rate at the alloy-electrolyte interface.

Fig.13 EIS curves of a as-cast and b as-spun (15 m·s^-1) Ce_0.2 alloys at various temperatures (R²,correlation coefficient).Inset being Kuriyama's graphs of ln(T/Rct) versus 1/T

Fig.14 Evolution of activation enthalpy (Δ_rH^*) values of as-cast and as-spun (15 m·s^-1) La_1-xCe_xMgNi_3.5Mn_0.5 (x=0-0.4) alloys

4 Conclusion

The substitution of Ce for La gives rise to an increase in the amount of LaMgNi₄ phase and a decrease in that of LaNi₅phase without altering phase compositions.And this partial substitution results that the grains of the as-cast and asspun alloys obviously are refined.Furthermore,the lattice parameters and cell volumes of the as-cast and as-spun alloys visibly decrease with Ce content growing.The electrochemical measurements show that the as-cast and as-spun alloys exhibit excellent activation capability.The discharge capacity of the as-cast alloys visibly declines with Ce content increasing.By contrast,the as-spun alloy has the maximum value with the increase in Ce content.The partial substitution of Ce for La markedly improves the electrochemical cycle stability of the alloys.The electrochemical kinetics of the as-cast and as-spun alloys,including HRD,diffusion coefficient (D),limiting current density (I_L) and charge transfer rate,first increase and then decrease with Ce content growing,for which the structure modified by Ce substitution is principally responsible.The hydrogen diffusion coefficient (D) and the activation enthalpy (Δ_rH^*) of the alloys are deemed to be the main controlling factors of HRD of the as-cast and as-spun alloys.