稀有金属(英文版) 2019,38(10),954-964

Structure and electrochemical performances of Mg_20-xY_xNi₁₀(x=0-4) alloys prepared by mechanical milling

Yang-Huan Zhang Xi-Ping Song Pei-Long Zhang Yong-Guo Zhu Hui-Ping Ren Bao-Wei Li

Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources,Inner Mongolia University of Science and Technology

State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing

Beijing Whole Win Materials Sci.& Tech.Co.,Ltd.

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

收稿日期：2 April 2015

基金：financially supported by the National Natural Science Foundations of China (Nos.51161015 and 51371094);the State Key Laboratory of Advanced Metals and Materials (No.2011-ZD06);

Structure and electrochemical performances of Mg_20-xY_xNi₁₀(x=0-4) alloys prepared by mechanical milling

Yang-Huan Zhang Xi-Ping Song Pei-Long Zhang Yong-Guo Zhu Hui-Ping Ren Bao-Wei Li

Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources,Inner Mongolia University of Science and Technology

State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing

Beijing Whole Win Materials Sci.& Tech.Co.,Ltd.

Abstract：

Mg₂Ni-type Mg_20-xY_xNi₁₀(x=0,1,2,3 and4) electrode alloys were fabricated by vacuum induction melting.Subsequently,the as-cast alloys were mechanically milled on a planetary-type ball mill.The effects of milling time and Y content on the microstructures and electrochemical performances of the alloys were investigated in detail.The results show that nanocrystalline and amorphous structure can be successfully obtained through mechanical milling.The substitution of Y for Mg facilitates the glass forming of the Mg₂Ni-type alloy and significantly enhances the electrochemical characteristics of the alloy electrodes.Moreover,the discharge capacity of Y-free alloy monotonously grows with the milling time prolonging,while that of the Y-substituted alloys has the maximum values in the same case.The milling time of obtaining the greatest discharge capacity markedly decreases with Y content increasing.The electrochemical kinetics of the alloys,including high rate discharge ability(HRD),diffusion coefficient(D),limiting current density(I_L) and charge transfer rate,monotonously increase with milling time extending.

Keyword：

Mg₂Ni-type alloy; Y substitution for Mg; Milling duration; Electrochemical performances;

Received： 2 April 2015

1 Introduction

In the coming decades,human will face two serious challenges,namely the decreasing world supply of fossil fuels and the increasing rate of global warming and climate change.An attractive strategy for limiting the use of fossil fuels is to develop cleaner and renewable energy sources.Among available alternative energy sources,hydrogen energy is regarded as the promising candidate since it is quite abundant and inexhaustible on earth [ 1] .To achieve the application goal of hydrogen energy,some key technology issues,including hydrogen production,separation(purification) and storage,need to be solved.Among these challenges,how to separate and store hydrogen efficiently and safely has been regarded as the critical problems [ 2] .The present work deals with the hydrogen storage.As to the conventional methods of hydrogen storage,metal hydride systems are considered to be more accurate,efficient and safer [ 3] .Solid-state hydrogen storage medium,particularly Mg and Mg-based alloy,have been extensively investigated due to their high hydrogen capacity,safety and reversibility.Unfortunately,high thermodynamic stability,sluggish kinetics and very poor electrochemical cycle stability are still major obstacles for practical applications of the alloys either as hydrogen storage materials for on-board use or as negative electrode materials in Ni-MH batteries [ 4] .In recent years,various approaches,particularly mechanical alloying [ 5, 6] ,surface modification [ 7] ,alloying with other elements [ 8] and adding catalyst [ 9] ,have been adopted to overcome these drawbacks,and there has been a remarkable improvement in the desorption kinetics [ 10] .

The specific capacity and hydriding/dehydriding kinetics of hydride electrode materials depend on their chemical composition and crystalline structure.Especially,nanocrystalline or amorphous Mg-based hydrogen storage alloys show a higher hydrogen absorption capacity at low temperatures and ambient pressure,and better kinetics of hydriding and dehydriding compared with their bulk counterparts [ 11] .As reported by Hima Kumar et al. [ 12] ,changing the structure of Mg2Ni alloy from polycrystalline to nanocrystalline enables its hydrogen absorbing/desorbing temperature to decrease by 100℃,namely from300 to 200℃.Mechanical milling,although having some insurmountable disadvantages in processing by powder metallurgy such as the surface contamination and the timeconsuming,is considered to be a quite appropriate technique for producing amorphous and nanocrystalline Mg2Ni alloys with different compositions [ 13] .Moreover,it is well known that the high hydride formation enthalpy of Mg2Ni is the reason for its low discharge capacity and that the sharp degradation of its discharge capacity during the charge-discharge cycling is caused by forming and thickening Mg(OH)2 surface layer [ 3, 14] .It is convinced that alloying and microstructure modification are the effective approaches to improving the hydriding properties.Particularly,the partial substitution of rare earth elements (La,Ce,Pr,Nd and Y) for Mg makes the stability of the hydride decrease and facilitates hydrogen desorption [ 15] .Also,such substitution could be very effective in protecting the materials from corrosion,hence improving the cycle performance [ 16] .Aono et al. [ 17] also synthesized the MgNi₄Y intermetallic compound via mechanical milling and casting methods from YNi₂ and MgNi₂ powders.They also studied hydriding properties of this compound.So far,the studies on electrochemical properties of Y-added Mg2Ni-type alloys are seldom.

Our previous works reported that a series of Mg2Ni-type alloys were fabricated by melt spinning,and their structures and electrochemical performances were investigated [ 18, 19] .In the present work,the Mg₂Ni-type Mg_20-xY_xNi₁₀(x=0-4) alloys were prepared by mechanical milling,and the effects of milling duration and Y content of x=0-4 on the structures and electrochemical performances of the alloys were investigated in detail.

2 Experimental

Experimental alloys with the chemical composition of Mg_20-xY_xNi₁₀ (x=0-4) were prepared by a vacuum induction furnace in a helium atmosphere at a pressure of0.04 MPa to prevent Mg from volatilizing.The molten alloy was poured into a copper mold cooled by water;thus,a cast ingot was obtained.The chemical composition analysis shows that the volatilization amount of magnesium was<6%.A part of the as-cast alloys was mechanically crushed into power with a particle size of about 50μm.Then,the prepared power was mechanically milled in a planetary-type mill in an argon atmosphere to prevent the powders from being oxidized during ball milling.The samples were handled in a glove box under Ar atmosphere.Cr-Ni stainless steel balls and the powders with a weight ratio of 35:1 were put into Cr-Ni stainless steel vials together.The milling speed was 135 r·min^-1 and the durations of 10,30,50 and 70 h were used.For convenience of description,the alloys were denoted with Y contents of x=0-4 as Y₀,Y₁,Y₂,Y₃ and Y_4.

The phase structures of the as-cast and milled alloys were determined by means of X-ray diffractometer (XRD,D/max/2400).The diffraction with experimental parameters of 160 mA,40 kV and 10 (°)·min^-1 was performed with Cu Kαradiation filtered by graphite.A Philips scanning electron microscopy (SEM,QUANTA 400) linked with an energy dispersive spectrometer (EDS) was used for observing morphologies and analyzing chemical composition of the as-cast alloys.The powder samples of the asmilled alloys were examined using transmission electron microscopy (TEM,JEM-21 00F,operated at 200 kV),and their crystalline states were ascertained by electron diffraction (ED).

A mixture of the alloy powder and carbonyl nickel powder in a weight ratio of 1:4 was cold-pressed under a pressure of 35 MPa into round electrode pellets with a diameter of 15 mm whose total weight was 1 g.The electrochemical performances were measured at 30℃by a tri-electrode open cell consisting of a working electrode(the metal hydride electrode),a sintered Ni(OH)2/NiOOH counter electrode as well as a Hg/HgO reference electrode,which were immersed in 6 mol·L^-1 KOH electrolyte.The voltage between the negative electrode and the reference one was defined as the discharge voltage.In every cycle,the alloy electrode was first charged with a constant current density,and after resting for 15 min it was discharged at the same current density to cutoff voltage of-500 mV.

To determine the electrochemical kinetics of the alloy electrodes,the electrochemical impedance spectra (EIS)and the Tafel polarization curves of the alloys were measured at 30℃using an electrochemical workstation(PARSTAT 2273).Prior to measurement,several electrochemical charging and discharging cycles were carried out to activate the materials.The fresh electrodes were fully charged and then rested for 2 h up to the stabilization of the open-circuit potential.The EIS of the alloy electrodes were measured at 50%depth of discharge (DOD),frequency range of 10 kHz-5 mHz,amplitude of signal potentiostatic or galvanostatic measurements being 5 mV,and the number of points per decade of frequencies of 60.The Tafel polarization curves were measured in the potential range of-1.2-+1.0 V (vs.open-circuit voltage) with a scan rate of5 mV·s^-1.For the potentiostatic discharge,the test electrodes in the fully charged state were discharged at 500 mV potential steps for 5000 s on electrochemical workstation(PARSTAT 2273),using electrochemistry corrosion software (CorrWare).

3 Results and discussion

3.1 Microstructure characteristics

The phase components and structure characteristics of ascast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys were subjected to XRD detections,just as demonstrated in Fig.1.Evidently,mechanical milling gives rise to diffraction peaks of the alloys merging and broadening,which can be ascribed to the changes of crystal grain size and a heterogeneous strain.Meanwhile,it is also noted that,for the same milling time,the diffraction peaks of the Y₁ alloy exhibit much lower intensity and larger width than those of the Y₀ alloy,indicating that the substitution of Y for Mg facilitates the crystalline structure transforming to a nanocrystalline or amorphous structure.Furthermore,the diffraction peaks,microstructural characteristics and electrochemical properties of Y₂,Y₃ and Y₄ alloys are basically the same as those of Y₁ alloy,so it just gives the experimental data of Y₁ alloy as the representative of Y-substituted alloys.In addition,it can be found that the substitution of Y for Mg results in the formation of secondary phases YMgNi₄ and YMg3 without altering the major phase of the Mg₂Ni for Y content of x=1.The morphologies of the as-cast and milled Y₁ alloy powers are observed by SEM,as illustrated in Fig.2.It is clearly found that the mechanical milling makes the size of the powder particles decrease dramatically.And some ultrafine particles take place cold welding together,aggregating into larger particles.

The SEM images and EDS spectra of the as-cast Mg_20-xY_xNi₁₀ (x=0-4) alloys are presented in Fig.3,from which it is found that the substitution of Y for Mg results in an obvious change in the morphologies of the alloys.Evidently,the as-cast Y₀ alloy exhibits a typical dendritic structure,but for the Y₁ alloy,it almost vanishes and some secondary phases emerge,which are ascertained to be YMgNi₄ and YMg₃ by EDS spectra.

The structure characteristics of the as-milled Mg_20-xY_xNi₁₀ (x=0-4) alloys were inspected by TEM,just as shown in Fig.4.It can be seen that,although dispersed strongly in alcohol,the alloy particles agglomerate clearly.The average size of the alloy particles was measured by the linear intercept method,to be in the range of20-30 nm.With the aim of displaying the effects ofmilling duration and Y content on the microstructure,the locally amplified micrographs of the as-milled alloys are also exhibited in Fig.4.It is evident that the mechanical milling makes the crystalline alloy strongly disordered and nanostructured;meanwhile,some crystal defects such as subgrains and grain boundaries can be seen clearly.This also is the major reason for the merging and broadening of the XRD peaks described in Fig.1.After carefully checking,it is found that with milling time prolonging from 10 to 70 h,the grain size of the Y₀ alloy markedly decreases and the disordered degree of its microstructure obvious increases,meaning that a large amount of internal energy is stored,leading to non-stabilization of the lattice,yielding fine grain sizes.As stated by Niua and North wood et al. [ 20] ,the stored internal strain grows with longer milling time.The longer the milling time is,the more the defects are introduced.Notably,it is also find that the Y₀alloy,even if milled for 70 h,exhibits a nearly entire nanocrystalline structure,while Y₁ alloy milled for 70 h have an obvious nanocrystalline and amorphous one,meaning that the substitution of Y for Mg facilitates the glass forming of the Mg₂Ni alloy,which is supported by analysis of Debye-Scherrer rings.A similar conclusion was stated by Teresiak et al. [ 15] .

Fig.1 XRD patterns of as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys:a Y₀ alloy and b Y₁ alloy

Fig.2 SEM images of Y₁ alloy a before milling and b after milling for 70 h

Fig.3 SEM images of a Y₀ alloy and b Y₁ alloy together with typical EDS spectra of c Mg₂Ni phase,d YMgNi₄ phase and e Mg₃Y phase

Fig.4 TEM images,enlarged images and ED patterns of as-milled Mg_20-xY_xNi₁₀ (x=0-4) alloys:a Y₀ alloy milled for 10 h,b Y₀ alloy milled for 70 h,c Y₁ alloy milled for 10 h,and d Y₁ alloy milled for 70 h

3.2 Electrochemical hydrogen storage performances

3.2.1 Activation capability and discharge capacity

Easy to be activated is indispensable for the alloy electrode applied in Ni-MH battery.The activation capability was evaluated by the number of charging-discharging cycles required for attaining the greatest discharge capacity through repeated charge-discharge process at a constant current density.The fewer the number of charging-discharging cycle is,the better the activation performance will be.The evolutions of the discharge capacities of the as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys with cycle number are described in Fig.5,from which it can be seen that all the alloys attain their maximum discharge capacities at the first charging-discharging cycle,indicating an excellent activation capability.Moreover,it is found that the milling time has an obvious effect on the discharge capacity of the alloys.Figure 6 depicts the relationship of the discharge capacities of the alloys with milling time.It is very evident that the discharge capacities of the alloys have a different change with milling time increasing.The discharge capacity of the Y₀ alloy monotonously grows with milling time prolonging.Namely it increases from 42.2 to 269.5 mAh·g-1 when milling time rises from 0 (the as-cast is defined as milling time of 0 h) to 70 h.Differing from the Y₀ alloy,the discharge capacities of the Y-substituted alloys yield the maximum values with milling time varying,namely 418.5,379.6,352.8 and 341.5 mAh·g-1 corresponding to Y_l,Y₂,Y₃ and Y₄ alloys,respectively.Apparently,the greatest discharge capacities of the Y-substituted alloys clearly decline with Y content increasing.Notably,the milling time of yielding the biggest discharge capacity obviously shortens with Y content increasing.

Here,some elucidations can be given about the variation in the discharge capacity of the alloys with milling time.The monotonously enhanced discharge capacity of the Y₀alloy by prolonging milling time is definitely ascribed to the nanocrystalline structure created by mechanical milling.The structure analysis results (Fig.4) show that the formation of nanocrystalline grains creates numerous grain boundaries in the as-milled alloy,which is no doubt to promote the hydrogen storage properties due to the fact that hydrogen concentration in the grain boundary is much higher than that in the grain interior region and amorphous region [ 21, 22] .Furthermore,internal strain increases with crystal size decreasing,which accelerates hydrogen absorbing/desorbing process [ 20] .That is to say,the amount of hydrogen absorption in the as-milled Mg2Ni increases with internal strain increasing.Also,it has been reported that nanocrystalline Mg₂Ni alloys can electrochemically absorb and desorb a large amount of hydrogen at room temperature [ 23] .

Fig.5 Evolution of discharge capacity of as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys with cycle number:a Y₀ alloy and b Y₁ alloy

Fig.6 Evolution of discharge capacity of as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys with milling time

Presently,it is discussed the reasons why the discharge capacities of the Y-substituted alloys first go up and then go down with milling time varying.The increased discharge capacity of the alloys by milling is most likely attributed to the formation of the nanocrystalline structure.The positive contribution of nanocrystalline structure to the discharge capacity has been mentioned previously,and here is no longer.The decreased capacities of the Y-substituted alloys by milling means that milling for a longer time gives rise to a harmful action on the discharge capacity,for which two aspects are considered to be responsible.Firstly,the substitution of Y for Mg facilitates to the glass forming,and the amount of amorphous structure increases with milling time growing.Evidently,an amorphous structure is disadvantageous to capacity as it is known that the nanocrystalline microstructures can accommodate larger amounts of hydrogen than amorphous ones.Secondly,the mechanical milling inevitably brings on the plastic deformation of intermetallics,which produces different types of crystal defects.As a result,internal strain will be produced by storing sufficient energy.And the internal strain increases with milling time prolonging.Also there is a view that the lattice strain results in a decrease in the number of hydrogen occupation sites,hence decreasing the discharge capacity [ 24] .However,Niua and North wood [ 20] believed that the internal strain introduced by the defects accelerates the hydrogen absorbing/desorbing process;hence,the amount of absorbed hydrogen in ball-milled Mg₂Ni increases with internal strain cumulating.Apparently,there is still no uniform view about whether the lattice strain is beneficial or harmful for the capacity of the alloy.The authors consider that the action of the lattice strain on the discharge capacity is most probably dependent on the compositions of the alloys.

3.2.2 Electrochemical cycle stability

The cyclic stability of an alloy electrode,which is one of the major performance indicators that evaluate whether or not a kind of alloy can be applied as a negative electrode material,was characterized by the capacity retaining rate(S₂₀),which is defined as:

where C_max is the maximum discharge capacity and C₂₀ is the discharge capacity of the 20th charge-discharge cycle at a current density of 40 mA·g^-1.If an alloy electrode still has a high capacity retention rate after repeated chargedischarge cycles,it can be said that it possesses superior electrochemical cyclic stability and it is more appropriate to be the negative electrode materials for Ni/MH batteries.

The variations in S₂₀ values of the as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys with milling time are described in Fig.7,from which it can be seen that the S₂₀values clearly decline with milling time prolonging.To be specific,increasing the milling time from 0 to 70 h makes S₂₀ value decrease from 43.6%to 33.9%for Y₀ alloy and from 89.8%to 74.7%for Y₄ alloy.Notably,for the same milling time,theS₂₀ values increase sharply with Y content increasing,indicating that the substitution of Y for Mg exerts a positive impact on the cycle stability of Mg₂Ni alloy.It is well known that the sharp degradation of the discharge capacity of Mg₂Ni alloy during the charge-discharge cycling is caused by forming and thickening Mg(OH)₂ surface layer,which hinders the hydrogen atoms from diffusing in or out,in alkaline solution [ 25] .On the other hand,the hydrogen storage material sustains an inevitable volume change during the charge/discharge process,resulting in alloy cracking and pulverizing,which,in turn,makes the surface of the material apt to be oxidized.Moreover,the capacity degradation of the alloy is strongly related to the formation of lattice strain containing lattice defects during the milling [ 26] .The formation of the nanocrystalline structure and production of the lattice strain resulted from milling are the main reasons of the mechanical milling impairing cycle stability because the intergranular corrosion is unavoidable [ 27] .

Fig.7 Evolution of S₂₀ values of as-cast and milled Mg_20-xY_xNi₁₀(x=0-4) alloys with milling time

The experimental results also provide evidence that the major cause of the capacity degradation of the alloy is corrosion,as shown in Fig.8.It can be noted that the sizes of the alloy particles have no obvious change after the electrochemical cycle,indicating that the pulverization of alloy particles scarcely takes place in the process of the electrochemical cycle.A rough and porous layer can clearly be seen on the surface of the alloy particles after electrochemical cycling,which is determined to be magnesium hydroxide from XRD analysis,as presented in Fig.8c.Hence,it can be concluded that it is the rough and flocculous layer that leads to the capacity deterioration of the alloy.The positive contribution of Y substitution for Mg on the cycle stability is surmised to be ascribed to following several aspects.Firstly,the rare earth elements(La,Nd,Sm and Y) can be dissolved in the Mg₂Ni alloy,forming solid solution (EDS spectra in Fig.3) which enables the cell volume of the Mg₂Ni alloy to enlarge obviously [ 28] ,decreasing the expansion-to-contraction ratios in the process of hydrogen absorption/desorption,thereby enhancing the anti-pulverization capability.Secondly,the partial substitution of Mg with rare earth elements (La,Ce,Pr,Nd and Y) can improve the corrosion resistance of the alloy in alkaline and increase the cycle life of the alloy [ 29, 30] .

Fig.8 SEM images of Y₀ alloy a before cycling and b after cycling together with c typical XRD pattern of as-milled (50 h) Y₀ alloys before and after electrochemical cycle

Fig.9 Evolution of HRDs of as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys with current density:a Y₀ alloy and b Y₁ alloy

3.2.3 Electrochemical kinetics

Keeping a high discharge capacity even during the process of charge-discharge cycles with a big current density is necessary for the practical application of alloy electrode in Ni-MH battery,especially power battery.It is well known that a decrease in the discharge capacity of an alloy electrode with current density increasing is unavoidable.Usually,the electrochemical kinetics of an alloy electrode is symbolized by its high rate discharge ability (HRD),defined as:

where C_i and C₄₀ are the maximum discharge capacities of the alloy electrode charged-discharged at current densities of i and 40 mA·g^-1,respectively.The variation in the HRDs of the as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4)alloys with discharge current density are provided in Fig.9.It is evident that the milling engenders a positive contribution to the HRD of the alloys.On the basis of the data in Fig.9 at a current density of 120 mA·g^-1,the relationships between the HRDs and the milling time can be established,as shown in Fig.9.It is evident that the HRDs of the alloys always grow with milling time elongating.To be specific,the HRD is raised from 53.8%to82.2%for the Y₀ alloy and from 68.7%to 82.4%for the Y₁ one by prolonging milling time from 0 to 70 h.It has come to light that the HRD of an alloy electrode is basically determined by the charge transfer rate on the alloy electrode surface and the hydrogen diffusion capability in the alloy bulk [ 31] .Hence,it is very necessary to investigate the effects of the milling time on the diffusion ability of hydrogen atoms and the charge transfer rate to reveal the mechanism of the electrochemical kinetics of the alloy impacted by milling time varying.

The charge transfer capability can be qualitatively evaluated by its EIS based on Kuriyama's model [ 32] .The EIS results of the as-cast and milled Mg_20-xY_xNi₁₀(x=0-4) alloys are depicted in Fig.10,in which Z_real and-Z_imag are the real and imaginary parts of the complex plane,respectively.From Fig.10 it is found that the EIS has two distorted capacitive loops at the high and middle frequency regions separately as well as a line at the low frequency region,which very well expresses the electrochemical process of the alloy electrode.Among them,the smaller semicircle in the high frequency region reflects the contact resistance between the alloy powder and the conductive material,and the larger one in the middle frequency region corresponds to the charge transfer resistance on the alloy surface,while the straight line in low frequency represents the atomic hydrogen diffusion in the alloy.In such cases,the charge transfer capability can be estimated easily;namely,the larger the radius of the semicircle in the middle frequency region is,the higher the charge transfer resistance of the alloy electrode is.Apparently,the radii of the large semicircle of the alloys in the middle frequency region clearly shrink with milling time increasing,meaning that prolonging milling time is beneficial to enhancing the charge transfer ability.

With respect to the hydrogen diffusion ability,it is evaluated by the hydrogen diffusion coefficient,which can be derived by means of the semilogarithmic curves of anodic current versus working duration of an alloy electrode,as shown in Fig.11.Based on White's model [ 33] ,the diffusion coefficient of hydrogen atoms in the bulk alloy could be calculated easily through the slope of the linear region of the corresponding plots according to the following formulae:

Fig.10 EIS results of as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys:a Y₀ alloy and b Y₁ alloy

Fig.11 Semilogarithmic curves of anodic current versus time responses of as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys:a Y₀ alloy and b Y₁ alloy

where i is the diffusion current density (A·g^-1),F is the Faraday constant (96,485 C mol^-3),D is the hydrogen diffusion coefficient (cm²·s^-1),C₀ is the initial hydrogen concentration in the bulk of the alloy (mol·cm^-3),C_s is the hydrogen concentration on the surface of the alloy particles(mol·cm^-3),a is the alloy particle radius (cm),d is the density of the hydrogen storage alloy (g·cm^-3) and t is the discharge time (s),respectively.The evolutions of the D values of the alloys derived by Eq.(4) with milling time are also shown in Fig.11.Figure 11 demonstrates that the D values of the alloys always grow with milling time extending,which is very similar to the variation tendencies of the HRDs of the alloys with milling time,indicating that the diffusion ability of hydrogen atoms is a very important factor for the electrochemical kinetics of the alloys.

Limiting current density (I_L),another important electrochemical kinetic parameter of an alloy electrode,to be sure of relating to the diffusion rate of hydrogen in alloy electrode [ 34] ,can be obtained by measuring the Tafel polarization curve,as demonstrated in Fig.12.It can be noted that,in all cases,each anodic polarization curve has a clear inflection point,namely a critical value existing;here,it is termed as I_L.It is regarded as an oxidation reaction taking place on the surface of the alloy electrode and the oxidation layer hindering hydrogen atoms from further penetrating [ 31] .Therefore,theI_L can be viewed as a critical current density for passivating.In consideration of the data in Fig.12,the relationships between theI_L values of the alloys and the milling time can be established,just as shown in Fig.12.It is evident thatI_L values of the alloys clearly increase with milling time elongating.Here,some elucidations about why ball milling improves the electrochemical kinetics of the alloys are provided.It was reported that the hydrogen absorbing and desorbing rates of Mg-based alloys were greatly enhanced by refining grain [ 35] .Crystalline alloy,when mechanically milled,becomes at least partially disordered,yielding the ultrafine grain sizes.Meanwhile,some defects such as dislocations,stacking faults and grain boundaries are introduced.By refining the microstructure,a lot of new crystallites and grain boundaries are created which can act as fast diffusion paths for hydrogen absorption.Moreover,the introduction of defects,disordering and internal strain facilitates to increase hydriding/dehydriding rates because the internal strain introduced by the defects accelerates the hydrogen absorbing/desorbing process [ 20] .The enhanced charge transfer rate by milling is basically ascribed to the increased surface area,formation of micro/nanostructures and creation of defects on the surface.As demonstrated by Jafarian et al. [ 36] ,increasing the surface area of the electrode can markedly ameliorate the electrocatalytic activity for hydrogen adsorption of the electrode.Hydrogen attaches more strongly to the surface of a nanocrystalline alloy than that of poly crystalline one,accelerating the reaction of the dissociation of hydrogen and giving rise to an improvement of the electrocatalytic activity of the electrode [ 31] .

Fig.12 Tafel polarization curves (overpotential vs current density and insets being limiting current density vs milling time) of as-cast and milled Mg_20-xY_xNi₁₀ (x=0-4) alloys:a Y₀ alloy and b Y₁ alloy

4 Conclusion

Nanocrystalline and amorphous Mg2Ni-type Mg_20-xY_xNi₁₀(x=0-4) alloys were fabricated by mechanical milling,and the effects of milling time on the structures and electrochemical performances of the alloys were investigated.The results reveal that the substitution of Y for Mg facilitates the glass forming in the as-milled alloy.Furthermore,such substitution brings on the formation of the secondary phases YMgNi₄ and YMg₃ without altering the major phase Mg₂Ni.The electrochemical measurement indicates that all the experimental alloys exhibit excellent activation capability.The effect of the milling time on the discharge capacity of the alloys is associated with the compositions of the alloy.The discharge capacity of the Y-free alloy always grows with milling time prolonging,while those of the Y-substituted alloys yield the maximum values in the same condition.Moreover,the milling impairs the electrochemical cycle stability of the alloys more or less.Furthermore,prolonging milling time is beneficial to improving the electrochemical kinetics of the alloys.The electrochemical kinetics,including HRD,D,I_L and charge transfer rate,monotonously increase with milling time extending.

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