Mg2Ni-type Alloy Research Articles

Hydrogen storage behavior of nanocrystalline and amorphous Mg-Ni-Cu-La alloys.

Alloying and structural modification are two effective ways to enhance the hydrogen storage kinetics and decrease the thermal stability of Mg and Mg-based alloys. In order to enhance the characteristics of Mg2Ni-type alloys, Cu and La were added to an Mg2Ni-type alloy, and the sample alloys (Mg24Ni10Cu2)100−xLax (x = 0, 5, 10, 15, 20) were prepared by melt spinning. The influences of La content and spinning rate on the gaseous and electrochemical hydrogen storage properties of the sample alloys were explored in detail. The structural identification carried out by XRD and TEM indicates that the main phase of the alloys is Mg2Ni and the addition of La results in the formation of the secondary phases LaMg3 and La2Mg17. The as-spun alloys have amorphous and nanocrystalline structures, and the addition of La promotes glass formation. The electrochemical properties examined by an automatic galvanostatic system show that the samples possess a good activation capability and achieve their maximal discharge capacities within three cycles. The discharge potential characteristics were vastly ameliorated by melt spinning and La addition. The discharge capacities of the samples achieve their maximal values as the La content changes, and the discharge capacities always increase with increasing spinning rate. The addition of La leads to a decline in hydrogen absorption capacity, but it can effectively enhance the rate of hydrogen absorption. The addition of La and melt spinning significantly increase the hydrogen desorption rate due to the reduced activation energy.

Highly improved electrochemical performances of the nanocrystalline and amorphous Mg2Ni-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-x M x (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 MnNi 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.

Effect of melt spinning on gaseous hydrogen storage characteristics of nanocrystalline and amorphous Nd-added Mg2Ni-type alloys

Nanocrystalline and amorphous Mg-Nd-Ni-Cu quaternary alloys with a composition of (Mg24Ni10Cu2)100-xNdx (x=0, 5, 10, 15, 20) were prepared by melt spinning technology and their structures as well as gaseous hydrogen storage characteristics were investigated. The XRD, TEM and SEM linked with EDS detections reveal that the as-spun Nd-free alloy holds an entire nanocrystalline structure but a nanocrystalline and amorphous structure for the as-spun Nd-added alloy, implying that the addition of Nd facilitates the glass forming in the Mg2Ni-type alloy. Furthermore, the degree of amorphization of the as-spun Nd-added alloy and thermal stability of the amorphous structure clearly increase with the spinning rate rising. The melt spinning ameliorates the hydriding and dehydriding kinetics of the alloys dramatically. Specially, the rising of the spinning rate from 0 (the as-cast was defined as the spinning rate of 0 m/s) to 40 m/s brings on the hydrogen absorption saturation ratio (R5a) (a ratio of the hydrogen absorption quantity in 5 min to the saturated hydrogen absorption capacity) increasing from 36.9% to 91.5% and the hydrogen desorption ratio (R10d) (a ratio of the hydrogen desorption quantity in 10 min to the saturated hydrogen absorption capacity) rising from 16.4% to 47.7% for the (x=10) alloy, respectively.

Structure and electrochemical performances of Mg20−xYxNi10 (x = 0–4) alloys prepared by mechanical milling

Mg2Ni-type Mg20−xYxNi10 (x = 0, 1, 2, 3 and 4) 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 (IL) and charge transfer rate, monotonously increase with milling time extending.

Improved hydrogen storage kinetics of nanocrystalline and amorphous Mg-Nd-Ni-Cu-based Mg2Ni-type alloys by adding Nd

In order to improve the gaseous and electrochemical hydrogen storage kinetics of the Mg2Nitype alloy, the elements Cu and Nd were added in the alloy. The nanocrystalline and amorphous Mg2Ni-type alloys with the composition of (Mg24Ni10Cu2)100-xNdx (x = 0, 5, 10, 15, 20) were prepared by melt spinning technology. The effects of Nd content on the structures and hydrogen storage kinetics of the alloys were investigated. The characterization by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) reveals that all the as-cast alloys hold multiphase structures, containing Mg2Ni-type major phase as well as some secondary phases Mg6Ni, Nd5Mg41, and NdNi, whose amounts clearly grow with increasing Nd content. Furthermore, the as-spun Nd-free alloy displays an entire nanocrystalline structure, whereas the as-spun Nd-added alloys hold a mixed structure of nanocrystalline and amorphous structure and the amorphization degree of the alloys visibly increases with the rising of the Nd content, suggesting that the addition of Nd facilitates the glass forming in the Mg2Ni-type alloy. The measurement of the hydrogen storage kinetics indicates that the addition of Nd significantly improves the gaseous and electrochemical hydrogen storage kinetics of the alloys. The addition of Nd enhances the diffusion ability of hydrogen atoms in the alloy, but it impairs the charge-transfer reaction on the surface of the alloy electrode, which makes the high rate discharge ability (HRD) of the alloy electrode first mount up and then go down with the growing of Nd content.

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

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 the 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.

Electrochemical hydrogen-storage performance of Mg20−x Y x Ni10 (x = 0–4) alloys prepared by mechanical milling

Mg in Mg2Ni-type alloy was partially substituted with Y to improve the electrochemical performance of the alloy. The as-cast Mg2Ni-type Mg20−x Y x Ni10 (x = 0, 1, 2, 3, 4) electrode alloys were prepared by vacuum induction melting under the protection of high-purity helium atmosphere and subsequent mechanical milling for different time points. Effects of Y substitution with Mg on the microstructures and electrochemical performance of the as-cast and milled alloys were investigated in detail by X-ray diffraction, transmission electron microscopy, scanning electron microscopy coupled with energy-dispersive spectrometry, and automatic galvanostatic system. Results revealed that the substitution of Y with Mg evidently changed the phase composition of alloys. At a Y content of x ≤ 1, the major phase in alloys was Mg2Ni, but the phase changed to YMgNi4 + YMg3 with further addition of Y content. Electrochemical measurement showed that variations in the discharge capacity of alloys with Y content were closely related to the milling time. At ≤ 10-h milling time, the discharge capacity of alloy invariably increased with the increasing Y content; at > 10-h milling time, the discharge capacity initially increased, and then decreased with the increasing Y content. The substitution of Y with Mg dramatically ameliorated the cycle stability of the as-cast and milled alloys. Furthermore, high rate of discharge ability, electrochemical impedance spectrum, Tafel polarization curves, and potential-step measurements indicated that the electrochemical kinetic properties of the as-cast and milled alloys initially increased and then decreased with the increasing Y content.

Electrochemical hydrogen storage behaviors of the nanocrystalline and amorphous Nd-Cu-added Mg2Ni-type alloy electrodes applied to Ni-MH battery

In order to ameliorate the electrochemical characteristics of Mg2Ni-type alloy, the elements Cu and Nd are added in the alloy. The nanocrystalline and amorphous Mg2Ni-type alloys with the composition of (Mg24Ni10Cu2)100–xNdx (x = 0, 5, 10, 15, 20) are prepared by melt spinning technology. The effects of Nd content and spinning rate on structures and electrochemical performances of the alloys are investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) reveal that the as-spun Nd-free alloy displays an entirely nanocrystalline structure, whereas the as-spun Nd15 alloy, differing from Nd0 alloy, exhibits nanocrystals embedded in an amorphous matrix. This suggests that the addition of Nd facilitates the amorphous formation ability of the alloys. The electrochemical measurement indicates that both adding Nd and melt spinning significantly improve the electrochemical performances of the alloys, involving the discharge potential characteristics, discharge capacity, cycle stability, and the high rate dischargeability (HRD). The Nd addition and melt spinning enhance the diffusion ability of hydrogen atoms in the alloy, but both of them impair the charge-transfer ability on the surface of the alloy electrode, which gives rise to the HRD values first mounting up and then going down with the growing of the Nd content and the spinning rate.

Effect of mechanical grinding on the electrochemical hydrogen storage properties of Mg–Ni–Y alloy

The nanocrystalline and amorphous Mg2Ni-type electrode alloys with a composition of Mg20 − xYxNi10 (x = 1 and 4) were fabricated by mechanical milling, and the effects of milling time on the structures and electrochemical hydrogen storage performances of the alloys were investigated in detail. Test results of X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) reveal that a nanocrystalline structure can be obtained through mechanical milling, and the amorphization degree of the alloy visibly increases with milling time prolonging. When Y content is x = 1, the substitution of Y for Mg results in the formation of secondary YMgNi4 + YMg3 phases without altering the major phase Mg2Ni, but when Y content is x = 4, the major phase of the alloys changes into YMgNi4 + YMg3 phases. The capacity retaining rate of 20th cycle is reduced from 48.3 to 37.7 % for the x = 1 alloy and from 89.8 to 74.7 % for the x = 4 alloy by increasing milling time from 0 (as-cast is defined as milling time of 0 h) to 70 h. The x = 1 alloy milled for 30 h obtains the maximum discharge capacity of 418.5 mA h g−1, and for x = 4 alloy milled for 10 h, it is 341.5 mA h g−1. The effect of milling time on the electrochemical kinetics of the alloys is related to Y content. When Y content is x = 1, the high rate discharge ability (HRD), diffusion coefficient of hydrogen atom (D), and charge transfer rate all increase with milling time prolonging. However, for x = 4 alloy, the electrochemical kinetics is getting steadily worse with ball milling time increasing.

Hydrogen storage kinetics of nanocrystalline and amorphous Cu—Nd-added Mg2Ni-type alloys

The (Mg24Ni10Cu2)100—xNdx (x=0, 5, 10, 15, 20) alloys with nanocrystalline and amorphous structures were prepared by melt spinning technology. The structures of the as-cast and spun alloys were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The effects of Nd content and spinning rate on the structures and hydrogen storage kinetics of the alloys were investigated. The results show that the as-spun Nd-free alloy displays an entire nanocrystalline structure, whereas the as-spun Nd-added alloys hold nanocrystalline and amorphous structures, suggesting that the addition of Nd facilitates the glass forming of the alloys. Both the Nd-addition and the melt spinning significantly improve the gaseous and electrochemical hydrogen storage kinetics of the alloys. The addition of Nd and melt spinning enhance the diffusion ability of hydrogen atoms in the alloy, but both of them impair the charge-transfer reaction on the surface of the alloy electrode, which makes the high rate discharge ability (HRD) of the alloy electrode first mount up and then go down with the growing Nd content and spinning rate.

Electrochemical Performance of Nanocrystalline and Amorphous Mg–Nd–Ni–Cu-Based Mg2Ni-type Alloy Electrodes Used in Ni-MH Batteries

Nanocrystalline and amorphous Mg2Ni-type (Mg24Ni10Cu2)100–x Nd x (x = 0, 5, 10, 15, 20) alloys were prepared by melt-spinning technology. The structures of as-cast and spun alloys were characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Electrochemical performance of the alloy electrodes was measured using an automatic galvanostatic system. The electrochemical impedance spectra and Tafel polarisation curves of the alloy electrodes were plotted using an electrochemical work station. The hydrogen diffusion coefficients were calculated using the potential step method. Results indicate that all the as-cast alloys present a multiphase structure with Mg2Ni type as the major phase with Mg6Ni, Nd5Mg41 and NdNi as secondary phases. The secondary phases increased with the increasing Nd content. The as-spun Nd-free alloy exhibited nanocrystalline structure, whereas the as-spun Nd-doped alloys exhibited nanocrystalline and amorphous structures. These results suggest that adding Nd facilitates glass formation of Mg2Ni-type alloys. Melt spinning and Nd addition improved alloy electrochemical performance, which includes discharge potential characteristics, discharge capacity, electrochemical cycle stability and high-rate discharge ability.

Gaseous and electrochemical hydrogen storage behaviors of nanocrystalline and amorphous Nd-added Mg2Ni-type alloys

Melt spinning technology was used to prepare the Mg2Ni-type (Mg24Ni10Cu2)100−x Nd x (x = 0, 5, 10, 15, 20) alloys in order to obtain a nanocrystalline and amorphous structure. The effects of the spinning rate on the structures and gaseous and electrochemical hydrogen storage behaviors of the alloys were investigated. The analysis of X-ray diffraction (XRD), transmission electron microscope (TEM), and scanning electron microscope (SEM) linked with energy-dispersive spectroscopy (EDS) reveals that all the as-cast alloys hold a multiphase structure, involving the main phase Mg2Ni and some secondary phases such as Mg6Ni, Nd5Mg41, and NdNi. The as-spun Nd-free alloy displays an entire nanocrystalline structure, whereas the as-spun Nd-added alloys hold a nanocrystalline and amorphous structure, and the amorphization degree visibly increases with the spinning rate increasing. The melt spinning ameliorates the hydrogen storage performances of the alloys dramatically. When the spinning rate rises from 0 (the as-cast was defined as the spinning rate of 0 m·s−1) to 40 m·s−1, the discharge capacity increases from 86.4 to 452.8 mAh·g−1, the S 20 (the capacity maintain rate at 20th cycle) value increases from 53.2 % to 89.7 %, the hydrogen absorption saturation ratio (R 5 a , a ratio of the hydrogen absorption quantity in 5 min to the saturated hydrogen absorption capacity) increases from 36.9 % to 91.5 %, and the hydrogen desorption ratio (\(R_{ 1 0}^{d}\), a ratio of the hydrogen desorption quantity in 10 min to the saturated hydrogen absorption capacity) increases from 16.4 % to 47.7 % for the (x = 10) alloy, respectively.

Influences of substituting Ni with M (M=Cu, Co, Mn) on gaseous and electrochemical hydrogen storage kinetics of Mg20Ni10 alloys

In this work, a comprehensive comparison regarding the impacts of M (M=Cu, Co, Mn) substitution for Ni on the structures and the hydrogen storage kinetics of the nanocrystalline and amorphous Mg20Ni10−xMx (M=Cu, Co, Mn; x=0–4) alloys prepared by melt spinning has been carried out. The analysis of XRD and TEM reveals that the as-spun (M=None, Cu) alloys display an entire nanocrystalline structure, whereas the as-spun (M=Co, Mn) alloys hold a mixed structure of nanocrystalline and amorphous structure when M content x=4, indicating that the substitution of M (M=Co, Mn) for Ni facilitates the glass formation in the Mg2Ni-type alloy. Besides, all the as-spun alloys have a major phase of Mg2Ni but M (M=Co, Mn) substitution brings on the formation of some secondary phases, MgCo2 and Mg phases for M=Co as well as MnNi and Mg phases for M=Mn. Based upon the measurements of the automatic Sieverts apparatus and the automatic galvanostatic system, the impacts engendered by M (M=Cu, Co, Mn) substitution on the gaseous and electrochemical hydrogen storage kinetics of the alloys appear to be evident. The gaseous hydriding kinetics of the alloys first rises and then declines with the growing of M (M=Cu, Co, Mn) content. Particularly, the M (M= Mn) substitution results in a sharp drop in the hydriding kinetics when x=4. The M (M=Cu, Co, Mn) substitution ameliorates the dehydriding kinetics dramatically in the order (M=Co)>(M=Mn)>(M=Cu). The electrochemical kinetics of the alloys visibly grows with M content rising for (M=Cu, Co), while it first increases and then declines for (M=Mn).

Highly Improved Gaseous Hydrogen Storage Characteristics of the Nanocrystalline and Amorphous Nd–Cu-added Mg2Ni-type Alloys by Melt Spinning

The nanocrystalline and amorphous Mg–Nd–Ni–Cu quaternary alloys with a composition of (Mg 24 Ni 10 Cu 2 ) 100− x Nd x ( x = 0–20) were prepared by melt spinning. The X-ray diffraction and transmission electron microscopy inspections reveal that, by varying the spinning rate and the Nd content, different microstructures could be obtained by melt spinning. Particularly, the as-spun Nd-free alloy holds an entire nanocrystalline structure but the as-spun Nd-added alloy has a nanocrystalline and amorphous structure, which implies that the addition of Nd facilitates the glass forming in the Mg 2 Ni-type alloy. Also, the degree of the amorphization in the as-spun Nd-added alloys clearly increases with increasing the spinning rate and the Nd content. The H-storage capacity and the hydrogenation kinetics of amorphous, partially and completely nanocrystalline alloys were investigated and it was found that they are dependent on the microstructure and the phase composition of the alloys. Specially, enhancing the spinning rate from 0 (the as-cast was defined as the spinning rate of 0 m/s) to 40 m/s makes the hydrogen absorption saturation ratio ( R 5 a ) (a ratio of the hydrogen absorption quantity in 5 min to the saturated hydrogen absorption capacity) increase from 35.2% to 90.3% and the hydrogen desorption ratio ( R 10 d ) (a ratio of the hydrogen desorption quantity in 10 min to the saturated hydrogen absorption capacity) rise from 12.7% to 44.9% for the ( x = 5) alloy. And the growing of the Nd content from 0 to 20 gives rise to the R 5 a and R 10 d values rising from 85.7% to 94.5% and from 36.7% to 54.8% for the as-spun (30 m/s) alloys, respectively.

Hydrogen storage characteristics of the nanocrystalline and amorphous Mg–Nd–Ni–Cu-based alloys prepared by melt spinning

Nanocrystalline and amorphous Mg–Nd–Ni–Cu-based (Mg24Ni10Cu2)100−xNdx (x = 0–20) alloys were prepared by melt spinning and their structures as well as hydrogen storage characteristics were investigated. The analysis of XRD, TEM and SEM linked with EDS reveal that all the as-cast alloys hold a multiphase structure, containing Mg2Ni-type major phase as well as some secondary phases Mg6Ni, Nd5Mg41 and NdNi, whose amounts clearly grow with Nd content rising. Furthermore, the as-spun Nd-free alloy displays an entire nanocrystalline structure whereas the as-spun Nd-added alloys have a mixed structure of nanocrystalline and amorphous, moreover, the amorphization degree of the alloys visibly increases with Nd content rising, implying that the addition of Nd facilitates the glass forming in the Mg2Ni-type alloy. The addition of Nd results in a slight decrease in the hydrogen absorption capacity of the as-cast and spun alloys, but it significantly enhances their hydrogen storage kinetics and hydriding/dehydriding cycle stability of the alloy. In order to reveal the capacity degradation mechanism of the as-spun alloy, the structure evolution of the nanocrystalline and amorphous alloys during the hydriding–dehydriding cycles was investigated. It is found that the root causes of leading to the capacity degradation of the nanocrystalline and amorphous alloys are nanocrystalline coarsening, crystal defect decreasing and amorphous phase crystallizing.

Effect of Nd content on electrochemical performances of nanocrystalline and amorphous (Mg24Ni10Cu2)100−xNdx (x=0−20) alloys prepared by melt spinning

The nanocrystalline and amorphous Mg2Ni-type alloys with a chemical composition of (Mg24Ni10Cu2)100−xNdx (x=0, 5, 10, 15, 20) were fabricated by melt spinning technology. The effects of spinning rate on the structure and electrochemical hydrogen storage performance of the alloys were investigated. The as-spun Nd-free alloy displays an entire nanocrystalline structure, whereas the as-spun Nd-added alloys hold a nanocrystalline and amorphous structure, suggesting that the addition of Nd facilitates the glass forming of the Mg2Ni-type alloys. Increasing the spinning rate from 0 to 40 m/s gives rise to the discharge capacity growing from 42.5 to 100.6 mA·h/g for the x=0 alloy and from 86.4 to 452.8 mA·h/g for the x=10 alloy. And the cycle stability (S20) rises from 40.2% to 41.1% for the x=0 alloy and from 53.2% to 89.7% for the x=10 alloy, respectively.

Electrochemical hydrogen storage performances of the nanocrystalline and amorphous (Mg24Ni10Cu2)100–xNdx (x=0–20) alloys applied to Ni-MH battery

Melt spinning technology was used to prepare the Mg2Ni-type (Mg24Ni10Cu2)100–xNdx (x=0, 5, 10, 15, 20) alloys in order to obtain a nanocrystalline and amorphous structure. The effects of Nd content and spinning rate on the structures and electrochemical hydrogen storage performances of the alloys were investigated. The structure characterizations of X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) linked with energy dispersive spectroscopy (EDS) revealed that the as-spun Nd-free alloy displayed an entire nanocrystalline structure, whereas the as-spun Nd-added alloys held a nanocrystalline and amorphous structure and the degree of amorphization visibly increased with the rising of Nd content and spinning rate, suggesting that the addition of Nd facilitated the glass forming of the Mg2Ni-type alloy. The electrochemical measurements indicated that the addition of Nd and melt spinning improved the electrochemical hydrogen storage performances of the alloys significantly. The discharge capacities of the as-cast and spun alloys exhibited maximum values when Nd content was x=10, which were 86.4, 200.5, 266.3, 402.5 and 452.8 mAh/g corresponding to the spinning rate of 0 (As-cast was defined as the spinning rate of 0 m/s), 10, 20, 30 and 40 m/s, respectively. The cycle stability (S20, the capacity maintain rate at 20th cycle) of the as-cast alloy always rose with the increasing of Nd content, and those of the as-spun alloys exhibited the maximum values for Nd content x=10, which were 77.9%, 83.4% 89.2% and 89.7%, corresponding to the spinning rate of 10, 20, 30 and 40 m/s, respectively.

Electrochemical and kinetic study of as-cast and as-quench Mg2Ni-type hydrogen storage alloys

Abstract

Highly improved electrochemical hydrogen storage performances of the Nd–Cu–added Mg2Ni-type alloys by melt spinning

Abstract In order to improve the electrochemical hydrogen storage performances of the Mg 2 Ni-type alloy, Nd and Cu were added jointly. The (Mg 24 Ni 10 Cu 2 ) 100− x Nd x ( x = 0, 5, 10, 15, 20) alloys with nanocrystalline and amorphous structure were fabricated by melt-spinning technique. The effects of spinning rate and Nd content on the structures and electrochemical hydrogen storage performances of the alloys were investigated. The structure characterizations of XRD, TEM and SEM linked with EDS reveal that the as-cast ( x = 15) alloy holds a multiphase structure, containing Mg 2 Ni major phase as well as some secondary phases Mg 6 Ni, Nd 5 Mg 41 and NdNi. The as-spun Nd-free alloy displays an entire nanocrystalline structure, whereas the as-spun Nd-added alloys hold a nanocrystalline and amorphous structure, meanwhile, the degree of amorphization visibly increases with both spinning rate and Nd content rising, suggesting that the addition of Nd facilitates the glass forming of the Mg 2 Ni-type alloys. Furthermore, the addition of Nd and melt spinning dramatically improve the electrochemical hydrogen storage performances of the alloys. The discharge capacity and cycle stability of the alloys conspicuously augment with spinning rate growing. When the spinning rate rises from 0 (The as-cast was defined as the spinning rate of 0 m/s) to 40 m/s, the discharge capacity of the ( x = 10) alloy is enhanced from 86.4 to 452.8 mA h/g, and capacity maintain rate ( S 20 ) (the capacity maintain rate at 20th cycle) from 53.2% to 89.7%, respectively.

Gaseous and electrochemical hydrogen storage kinetics of as-quenched nanocrystalline and amorphous Mg2Ni-type alloys

The nanocrystalline and amorphous Mg2Ni-type Mg2Ni1−xCox (x = 0, 0.1, 0.2, 0.3, 0.4) alloys were synthesized by melt quenching technology. The structures of the as-cast and quenched alloys were characterized by XRD, SEM and HRTEM. The gaseous hydrogen storage kinetics of the alloys was measured using an automatically controlled Sieverts apparatus. The alloy electrodes were charged and discharged with a constant current density in order to investigate the electrochemical hydrogen storage kinetics of the alloys. The results demonstrate that the substitution of Co for Ni results in the formation of secondary phases MgCo2 and Mg instead of altering the major phase Mg2Ni. No amorphous phase is detected in the as-quenched Cofree alloy, however, a certain amount of amorphous phase is clearly found in the as-quenched alloys substituted by Co. Furthermore, both the rapid quenching and the Co substitution significantly improve the gaseous and electrochemical hydrogen storage kinetics of the alloys, for which the notable increase of the hydrogen diffusion coefficient (D) along with the limiting current density (IL) and the obvious decline of the electrochemical impedance generated by both the Co substitution and the rapid quenching are basically responsible.