Structures and hydrogen storage properties of RE-Mg-Ni-Mn-based AB2-type alloys prepared by casting and melt spinning
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 La1-xCexMgNi3.5Mn0.5(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
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)
Previous research results indicated that melt spinning technology can dramatically refine the microstructure of A2B7-type alloys and significantly improve their cycle stability
2 Experimental
The chemical compositions of the alloys were La1-xCexMgNi3.5Mn0.5 (x=0,0.1,0.2,0.3,0.4).For convenience,they were denoted with Ce content as Ce0,Ce0.1,Ce0.2,Ce0.3 and Ce0.4 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 La1_xCexMgNi3.5Mn0.5 (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
Fig.1 XRD patterns of a as-cast and b as-spun (15 m·s-1) La1-xCexMgNi3.5Mn0.5 (x=0-0.4) alloys
Table 1 Lattice constants and abundances of LaMgNi4 and LaNi5 phases
Fig.2 SEM images together with typical EDS spectra of as-cast La1-xCexMgNi3.5Mn0.5 (x=0-0.4) alloys:a Ce0,b Ce0.2,c Ce0.4,and d EDS spectra of d LaNi5 phase and e LaMgNi4 phase for Ce0.2 alloy
SEM images and EDS results of as-cast La1-xCexMgNi3.5Mn0.5 (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 La1-xCexMgN13.5Mn0.5 (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 Ce0.2and Ce0.4 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 La1-xCexMg Ni3.5Mn0.5 (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 La1-xCexMgNi3.5Mn0.5(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) La1-xCexMgNi3.5Mn0.5 (x=0-0.4) alloys:a Ce0,b Ce0.2 and c Ce0.4 alloy
Fig.4 Evolution of discharge capacity of a as-cast and b as-spun (15 m·s-1) Lar-xCexMgNi3.5Mn0.5 (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
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
Fig.5 Evolution of maximum discharge capacity of as-cast and as-spun La1-xCexMgNi3.5Mn0.5 (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 (Sn) is defined as:
where Cn is the discharge capacity of the nth chargedischarge cycle and Cmax is the maximum discharge capacity.Figure 7 describes the variations of Sn values of as-cast and as-spun La1-xCexMgNi3.5Mn0.5 (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.
Fig.6 Discharge potential curves of a as-cast and b as-spun (15 m·s-1) La1-xCexMgNi3.5Mn0.5 (x=0-0.4) alloys
Fig.7 Evolution of capacity retaining rates (Sn) of a as-cast and b as-spun (15 m·s-1) La1-xCexMgNi3.5Mn0.5 (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) Ce0.2 alloy before and after electrochemical cycle:SEM images of Ce0.2 a before and b after cycling and c XRD pattern of Ce0.2 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 Ci 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 La1-xCexMgNi3.5Mn0.5 (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 AB5-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) La1-xCexMgNi3.5Mn0.5 (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
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 (IL),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
Fig.10 Semilogarithmic curves of anodic current versus time responses of a as-cast and b as-spun (15 m·s-1) La1-xCexMgNi3.5Mn0.5(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) La1-xCexMgNi3.5Mn0.5 (x=0-0.4) alloys.Inset beingIL values 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
Fig.12 EIS curves of as-cast La1-xCexMgNi3.5Mn0.5 (x=0-0.4)alloys and equivalent circuit (inset)
where Rct 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 La1-xCexMg Ni3.5Mn0.5 (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) Ce0.2 alloy are presented in Fig.13 as a representative.Based on the data in Fig.13,the Kuriyama graphs of ln(T/Rct) 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,ΔrH*values can be easily derived.Thus,the relationship betweenΔrH*values of as-cast and as-spun alloys and Ce content is established,as demonstrated in Fig.14.It is clear thatΔrH*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
Fig.13 EIS curves of a as-cast and b as-spun (15 m·s-1) Ce0.2 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 (ΔrH*) values of as-cast and as-spun (15 m·s-1) La1-xCexMgNi3.5Mn0.5 (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 (IL) 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 (ΔrH*) of the alloys are deemed to be the main controlling factors of HRD of the as-cast and as-spun alloys.
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