Highly Improved Electrochemical Performances of the Nanocrystalline and Amorphous Mg_2Ni-type Alloys by Substituting Ni with M (M=Cu,Co,Mn)

The element Ni in the Mg2Ni alloy is partially substituted by M（M = Cu, Co, Mn） in order to ameliorate the electrochemical hydrogen storage performances of Mg2Ni-type electrode alloys. The nanocrystalline and amorphous Mg20Ni10-xMx（M = None, Cu, Co, Mn; x = 0-4） alloys were prepared by melt spinning. The effects of the M（M = Cu, Co, Mn） content on the structures and electrochemical hydrogen storage characteristics of the as-cast and spun alloys were comparatively studied. The analyses by XRD, SEM and HRTEM reveal that all the as-cast alloys have a major phase of Mg2Ni but the M（M = Co, Mn） substitution brings on the formation of some secondary phases, MgCo2 and Mg for the（M = Co） alloy, and Mn Ni and Mg for the（M = Mn） alloy. Besides, the as-spun（M = None, Cu） alloys display an entirely nanocrystalline structure, whereas the as-spun（M = Co, Mn） alloys hold a nanocrystalline/amorphous structure, suggesting that the substitution of M（M = Co, Mn） for Ni facilitates the glass formation in the Mg2Ni-type alloys. The electrochemical measurements indicate that the variation of M（M = Cu, Co, Mn） content engenders an obvious effect on the electrochemical performances of the as-cast and spun alloys. To be specific, the cyclic stabilities of the alloys augment monotonously with increasing M（M = Cu, Co, Mn） content, and the capacity retaining rate（S20） is in an order of（M = Cu） 〉（M = Co） 〉（M = Mn） 〉（M = None） for x≤1 but changes to（M = Co） 〉（M = Mn） 〉（M = Cu） 〉（M = None） for x≥2. The discharge capacities of the as-cast and spun alloys always grow with the rising of M（M = Co, Mn） content but first mount up and then go down with increasing M（M = Cu） content. Whatever the M content is, the discharge capacities are in sequence：（M = Co） 〉（M = Mn） 〉（M = Cu） 〉（M = None）. The high rate discharge abilities（HRDs） of all the alloys grow clearly with rising M（M = Cu, Co） content except for（M = Mn） alloy, whose HRD has a maximum value with varying M（M = Mn） content. Furthermore, for the as-cast alloys, the HRD is in order of（M = Co） 〉（M = Mn） 〉（M = Cu） 〉（M = None）, while for the as-spun（20 m·s^-1） alloys, it changes from（M = Co） 〉（M = Mn） 〉（M = Cu） 〉（M = None） for x = 1 to（M = Cu） 〉（M = Co） 〉（M = None） 〉（M = Mn） for x = 4.

An investigation on electrochemical hydrogen storage performances of Mg-Y-Ni alloys prepared by mechanical milling

Abstract： The nanocrystalline and amorphous Mg2Ni-type electrode alloys with a composition of Mg20-xYxNi10 （x=0, 1, 2, 3 and 4） were fabricated by mechanical milling. Effects of Y content on the structures and electrochemical hydrogen storage performances of the alloys were investigated in detail. The inspections of X-ray diffraction （XRD）, transmission electron microscopy （TEM） and scanning electron microscopy （SEM） revealed that the substitution of Y for Mg brought on an obvious change in the phase composition of the alloys. The substitution of Y for Mg resulted in the formation of secondary YMgNi4 phases without altering the major phase Mg2Ni when Y content x≤1. But with the further increase of Y content, the major phase of the alloys changed into YMgNi4 phase. In addition, such substitution facilitated the glass forming of the Mg2Ni-type alloy. The discharge capacities of the as-milled alloys had the maximum values with Y content varying, but Y content with which the alloy yielded thc biggest discharge capacity was changeable with milling time varying. The substitution of Y for Mg had an insignificant effect on the activation ability of the alloys, but it dramatically improved the cycle stability of the as-milled alloys. The effect of Y content on the electrochemical kinetics of the alloys was related to milling time. When milling time was 10 h, the high rate discharge ability （HRD）, diffusion coefficient of hydrogen atom （D） and charge transfer rate all had the maximum value with Y content increasing, but they always decreased in the same condition when milling time increased to 70 h.

An investigation of hydrogen storage kinetics of melt-spun nanocrystalline and amorphous Mg_2Ni-type alloys

The nanocrystalline and amorphous Mg2Ni-type Mg2- xLaxNi （x=0, 0.2） hydrogen storage alloys were synthesized by melt-spinning technique. The as-spun alloy ribbons were obtained. The microstructures of the as-spun ribbons were characterized by X-ray diffraction （XRD）, high resolution transmission electronic microscopy （HRTEM） and electron diffraction （ED）. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus, and their electrochemical kinetics were tested by an automatic galvanostatic system. The electrochemical impedance spectrums （EIS） were plotted by an electrochemical workstation （PARSTAT 2273）. The hydrogen diffusion coefficients in the alloys were calculated by virtue of potential-step method. The obtained results showed that no amorphous phase was detected in the as-spun La-free alloy, but the as-spun alloys substituted by La held a major amorphous phase, con- firming that the substitution of La for Mg markedly intensified the glass forming ability of the Mg2Ni-type alloy. The substitution of La for Mg notably improved the electrochemical hydrogen storage kinetics of the Mg2Ni-type alloy. Furthermore, the hydrogen storage kinetics of the experimental alloys was evidently ameliorated with the spinning rate growing.

Structure and electrochemical hydrogen storage behaviors of Mg-Ce-Ni-Al-based alloys prepared by mechanical milling

The influences of milling time and Ce content on the electrochemical property and micro structure of asmilled Mg1-xCexNi0.9Al0.1(x=0,0.02,0.04,0.06,0.08)+50 wt%Ni alloys were investigated systematically.The as-milled alloys have an outstanding activation property.The cycle stability conspicuously grows up with milling time and Ce proportion increasing.The capacity retention rate at 100 th cycle of x=0.02 alloy augments from 47% to 63% when prolonging milling time from 5 to 30 h and it grows from55% to 82% for the 30 h milled alloy with Ce content growing from 0 to 0.08.The discharge capacity of x=0.02 alloy grows up invariably with milling time prolonging,while that of the 30 h milled alloys has the maximal value of 578.4 mAh/g with Ce content increasing.Moreover,the electrochemical kinetic properties of alloys significantly improve with milling duration extending,while they have the maximal values with Ce proportion varying.

Structure and Electrochemical Hydrogen Storage Properties of as-Milled Mg-Ce-Ni-Al-Based Alloys

At room temperature,crystalline Mg-based alloys,including Mg2 Ni,MgNi,REMg12 and La2 Mg17,have been proved with weak electrochemical hydrogen storage performances.For improving their electrochemical property,the Mg is partially substituted by Ce in Mg-Ni-based alloys and the surface modification treatment is performed by mechanical coating Ni.Mechanical milling is utilized to synthesize the amorphous and nanocrystalline Mg1-xCexNi0.9Al0.1(x=0,0.02,0.04,0.06,0.08)+50 wt%Ni hydrogen storage alloys.The effects made by Ce substitution and mechanical milling on the electrochemical hydrogen storage property and structure have been analyzed.It shows that the as-milled alloys electrochemically absorb and desorb hydrogen well at room temperature.The as-milled alloys,without any activation,can reach their maximal discharge capacities during first cycling.The maximal value of the 30-h-milled alloy depending on Ce content is 578.4 mAh/g,while that of the x=0.08 alloy always grows when prolonging milling duration.The maximal discharge capacity augments from337.4 to 521.2 mAh/g when milling duration grows from 5 to 30 h.The cycle stability grows with increasing Ce content and milling duration.Concretely,the S100 value augments from 55 to 82%for the alloy milled for 30 h with Ce content rising from 0 to 0.08 and from 66 to 82%when milling the x=0.08 alloy mechanically from 5 to 30 h.The alloys’electrochemical dynamics parameters were measured as well which have maximum values depending on Ce content and keep growing up with milling duration extending.

Properties of Mechanically Milled Nanocrystalline and Amorphous Mg–Y–Ni Electrode Alloys for Ni–MH Batteries

Nanocrystalline and amorphous Mg2Ni-type Mg20-xYxNi10（x = 0,1,2,3 and 4） electrode alloys were fabricated using mechanical milling.The effects of the Y content and milling time on the microstructures and electrochemical performances of the alloys were investigated in detail.X-ray diffraction and transmission electron microscopy analyses revealed that the substitution of Y for Mg yields an obvious change in the phase composition and micro morphology of the alloys.When the Y content x ≤ 1,the substitution of Y for Mg does not change the major phase Mg2 Ni,but with a further increase in the Y content,the major phase of the alloys transforms into the YMg Ni4 YMg3 phase.A nanocrystalline and amorphous structure can be obtained by mechanical milling,and the amorphisation degree of the alloy visibly increases with increased milling time.Electrochemical measurements indicate that the discharge capacity of the alloys first increases and then decreases with increasing Y content and milling time.The substitution of Y for Mg dramatically ameliorates the cycle stability of the as-milled alloys,and the mechanical milling more or less impairs the cycle stability of the alloys.Furthermore,the high rate discharge ability,electrochemical impedance spectrum,Tafel polarisation curves and potential step measurements indicate that the electrochemical kinetic properties of the as-milled alloys first increase and then decrease with increasing Y content and milling time.

Structure and electrochemical characteristics of Mg–Ti–Ni-based electrode alloys synthesized by mechanical milling

The vacuum induction melting was adopted to fabricating Mg_(50−x)Ti_(x)Ni_(45)Al_(3)Co_(2)(x=0,1,2,3,4 at.%)composites protected by the high-purity helium atmosphere.Subsequently,the surface modification treatment of the as-cast alloys was carried out by mechanically coating nickel.The amorphous and nanocrystalline Mg_(50−x)Ti_(x)Ni_(45)Al_(3)Co_(2)(x=0–4)+50 wt.%Ni hydrogen storing alloys as the negative materials in batteries were prepared through ball milling,and the influences of milling time and Ti dosage on the structure and electrochemical hydrogen storing behaviors of the corresponding samples were studied in detail.The electrochemical testing reveals that the as-milled alloys have excellent performances and can finish the electrochemical hydrogenation and dehydrogenation at indoor temperature.In the first cycle without activation,the ball milling alloy obtains the maximum value of discharge capacity.Discharge capacity and cyclic steadiness of the composites conspicuously grow as Ti content and milling duration increase.Concretely,the capacity retaining rate at 100th cycle and the discharge capacity of 30 h milling samples augment from 53%to 78%and from 435.2 to 567.2 mAh/g with changing Ti content from 0 to 4.The same performances of the alloy(x=4)are enhanced from 61%to 78%and from 379.9 to 567.2 mAh/g,respectively,with extending milling duration.Moreover,high rate discharge ability,potential-step measurements,potentiodynamic polarization curves and electrochemical impedance spectrum manifest that the electrochemical kinetics properties can achieve significant amelioration as Ti content varies and milling duration is extended.