Electrochemical Hydrogen Storage Kinetics Research Articles

Effect of Co content on electrochemical hydrogen kinetics properties of single-phase BCC-type MgAlTiCoxNi high entropy alloys used as a negative electrode in basic and acidic electrolyte

In this report, the effect of Co content on the electrochemical hydrogen storage reaction kinetic of MgAlTiCoxNi (x = 1, 1.5 and 2) High Entropy Alloys (HEAs) as negative electrodes under basic (6 M KOH) and acidic (1 M H2SO4) electrolyte was studied. Empirical thermodynamic parameters were used to predict the formation of HEAs. The alloys were obtained after 8 h of high-energy ball-milling. The structural analysis showed the formation of a BCC structure and a reduction of the unit lattice with increasing Co content. At high temperatures, the structure changes from BCC-type to B2-type. The morphology and surface area of the MgAlTiCoNi, MgAlTiCo1.5Ni, and MgAlTiCo2Ni alloy powders are very similar. Electrochemical measurements show that alloys exposed to acidic solutions as negative electrodes exhibit higher discharge capacity and better electrochemical hydrogen storage kinetics properties. The increase in Co content in the alloys in both basic and acidic solutions reduced the discharge capacity. Furthermore, a high Co content in the electrodes exposed to an acidic electrolyte reduces the anticorrosion properties of the alloy. The MgAlTiCoNi HEA electrode achieved the best electrochemical performance with a discharge capacity of 465 mAh/g in an acidic electrolyte after a short galvanic charge (60 min). The concentration of Co in the HEAs revealed a direct relationship with the hydrogen diffusion rate, while the lattice parameter is associated with the discharge capacity.

Electrochemical Hydrogen Storage Kinetics and Microstructure of the Rapidly Quenched Alloy Mg2Ni0.75Zn0.25

The Mg2Ni0.75Zn0.25 alloys are prepared from Mg2Ni as master alloy and pure Zinc ingot by remelting process in this work. The microstructures, electrochemical properties and desorption kinetics of the as-cast and melt-spun alloys are investigated. The phase compositions and microstructures of the alloys are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD). The results show that the segregation atoms rich in Zn can inhibit the growth of Mg2Ni grains, especially the nanocrystalline grains formed in Mg2Ni0.75Zn0.25 alloy after melt-spinning process. The rapidly quenched Mg2Ni0.75Zn0.25 alloy exhibits the largest discharge performance (102.1 mAh g−1) and dehydrogenation activation energy (14.68 kJ mol−1). The improvement of the kinetic properties is related to the hydrogen diffusion rate (D) in the alloys and the charge transfer reaction on the surface. The rapidly quenched Mg2Ni0.75Zn0.25 alloy has the largest D, 6.187 × 10−11 cm2 s−1. The doping of Zn aggravates the corrosion of alloy electrodes, which destroys the oxide layer on the alloy surface, provides a channel for the diffusion of hydrogen atoms.

Highly ameliorated gaseous and electrochemical hydrogen storage kinetics of nanocrystalline and amorphous CeMg12-type alloys by mechanical milling

An attempt that effectively advances the quantity of hydrogen storage for CeMg12-type alloys was adopting a method of partial substitution of Mg in alloy by Ni. It is also employing mechanical ball-milling technology to prepare the nanocrystalline and amorphous CeMg11Ni + x wt.% Ni (x = 100, 200) alloys. And these two factors, Ni content after replacement as well as ball grinding time, affecting the structures and hydrogen storage dynamics belonging experimental alloys were studied systematically. Thereinto, characterization analysis on structures was using XRD, SEM and HRTEM. Then, the test of gaseous hydrogen storage performance was carried out through Sievert device as well as differential scanning calorimeter (DSC) that connected with H2 detector. Besides, Arrhenius and Kissinger equations were served as a convenient method for calculating the hydrogen desorption activation energy of alloys. Finally, it can adopt the automatic galvanostatic system to measure the electrochemical hydrogen storage performance. It is shown that, for experimental alloys, the more Ni content shows beneficial on the formation of glass, meanwhile, it also strikingly ameliorates both gaseous and electrochemical hydrogen storage dynamics properties. In addition, milling time varying brings a very large impact upon hydrogen storage properties of alloys. When the ball-grinding time is constantly changing, the gaseous hydrogen absorption capacity, gaseous hydriding rate as well as high-rate discharge ability (HRD) of alloys are able to appear the maximum values respectively. However, following the prolongation of milling time, both electrochemical discharge capacity and gaseous dehydrogenating rate raise all the time. The increase in Ni content and the prolongation of milling time can both enhance the gaseous hydrogen storage dynamics of alloys, inferring that their essence all can be attributed to the reduction of the activation energy of hydrogen desorption.

An Investigation on Hydrogen Storage Kinetics of the Nanocrystalline and Amorphous LaMg12-type Alloys Synthesized by Mechanical Milling

Nanocrystalline and amorphous LaMg12-type LaMg11Ni + x wt% Ni (x = 100, 200) alloys were synthesized by mechanical milling. Effects of Ni content and milling time on the gaseous and electrochemical hydrogen storage kinetics of as-milled alloys were investigated systematically. The electrochemical hydrogen storage properties of the as-milled alloys were tested by an automatic galvanostatic system. And the gaseous hydrogen storage properties were investigated by Sievert apparatus and a differential scanning calorimeter (DSC) connected with a H2 detector. Hydrogen desorption activation energy of alloy hydrides was estimated by using Arrhenius and Kissinger methods. It is found that the increase of Ni content significantly improves the gaseous and electrochemical hydrogen storage kinetic performances of as-milled alloys. Furthermore, as ball milling time changes, the maximum of both high rate discharge ability (HRD) and the gaseous hydriding rate of as-milled alloys can be obtained. But the hydrogen desorption kinetics of alloys always increases with the extending of milling time. Moreover, the improved gaseous hydrogen storage kinetics of alloys are ascribed to a decrease in the hydrogen desorption activation energy caused by increasing Ni content and milling time.

Improved hydrogen storage kinetics of nanocrystalline and amorphous Pr-Mg-Ni-based PrMg12-type alloys synthesized by mechanical milling

In this paper, the nanocrystalline and amorphous PrMg11Ni + x wt.% Ni (x = 100, 200) alloys were synthesized by mechanical milling. The gaseous and electrochemical hydrogen storage performances were studied in detail. The results reveal that increasing Ni content facilitates the glass forming of the alloys, and it significantly improves the gaseous and electrochemical hydrogen storage kinetics performance. Furthermore, milling time varying significantly affects the hydrogen storage properties of the alloys. The hydrogen capacity of the alloys first increases and then decreases with milling time prolonged. The hydriding rate and high-rate discharge ability (HRD) of the as-milled alloys have maximum values with milling time varying. But dehydriding rate always increases with milling time prolonged. The improved gaseous hydrogen storage kinetics of alloys are convinced to be ascribed to a reduction in hydrogen desorption activation energy caused by increasing Ni content and prolonging milling time.

Hydrogen Storage Characteristics of Nanocrystalline and Amorphous Nd-Mg-Ni-Based NdMg12-Type Alloys Synthesized via Mechanical Milling

In this study, Mg was partially substituted by Ni with the intent of improving the hydrogen storage kinetics performance of NdMg12-type alloy. Mechanical milling technology was adopted to fabricate the nanocrystalline and amorphous NdMg11Ni + x wt pct Ni (x = 100, 200) alloys. The effects of Ni content and milling duration on the microstructures and hydrogen storage kinetics of as-milled alloys have been systematically investigated. The structures were characterized by XRD and HRTEM. The electrochemical hydrogen storage properties were tested by an automatic galvanostatic system. Moreover, the gaseous hydrogen storage properties were investigated by Sievert apparatus and a differential scanning calorimeter connected with a H2 detector. Hydrogen desorption activation energy of alloy hydrides was estimated by using Arrhenius and Kissinger methods. The results reveal that the increase of Ni content dramatically ameliorates the gaseous and electrochemical hydrogen storage kinetics performance of the as-milled alloys. Furthermore, high rate discharge ability (HRD) reach the maximum value with the variation of milling time. The maximum HRDs of the NdMg11Ni + x wt pct Ni (x = 100, 200) alloys are 80.24 and 85.17 pct. The improved gaseous hydrogen storage kinetics of alloys via increasing Ni content and milling time can be attributed to a decrease in the hydrogen desorption activation energy.

Hydrogen storage kinetics of nanocrystalline and amorphous NdMg12-type alloy–Ni composites synthesized by mechanical milling

Abstract Nanocrystalline and amorphous NdMg11Ni + x wt.% Ni (x = 100, 200) composites were synthesized by mechanical milling, and their gaseous and electrochemical hydrogen storage kinetic performances were systematically investigated. Hydrogen absorption and desorption properties were investigated by means of a Sievert apparatus and a differential scanning calorimeter connected with an H2 detector. Electrochemical hydrogen storage kinetics of the as-milled alloys were tested by an automatic galvanostatic system. Results show that increasing Ni content significantly improves gaseous and electrochemical hydrogen storage kinetics. The improved gaseous hydrogen storage kinetics of the alloys are ascribed to the decrease in hydrogen desorption activation energy caused by increasing Ni content and milling time.

Hydrogen Storage Kinetics of Nanocrystalline and Amorphous LaMg12-Type Alloy–Ni Composites Synthesized by Mechanical Milling

The nanocrystalline and amorphous LaMg11Ni + x wt% Ni (x = 100, 200) composites were synthesized by the mechanical milling, and their gaseous and electrochemical hydrogen storage kinetics performance were systematically investigated. The results indicate that the as-milled composites exhibit excellent hydrogen storage kinetic performances, and increasing Ni content significantly facilitates the improvement of the hydrogen storage kinetics properties of the composites. The gaseous and electrochemical hydrogen storage kinetics of the composites reaches a maximum value with the variation of milling time. Increasing Ni content and milling time both make the hydrogen desorption activation energy lower, which are responsible for the enhancement in the hydrogen storage kinetics properties of the composites. The diffusion coefficient of hydrogen atom and activation enthalpy of charge transfer on the surface of the as-milled composites were also calculated, which are considered to be the dominated factors for the electrochemical high rate discharge ability.

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.

Hydrogen storage kinetics of as-cast and spun (Mg24Ni10Cu2)100–x Nd x (x = 0–20) alloys

Abstract (Mg24Ni10Cu2)100–x Nd x (x = 0, 5, 10, 15, and 20) alloys with nanocrystalline and amorphous structures were prepared by melt-spinning. X-ray diffraction and high-resolution transmission electron microscopy 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. Melt spinning significantly improves the gaseous and electrochemical hydrogen storage kinetics of the alloys. Furthermore, melt spinning 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 gives rise to the high-rate dischargeability of the alloy electrode, first mounting up and then going down with the increase in spinning rate.

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.

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).

An investigation on electrochemical and gaseous hydrogen storage performances of as-cast La1−xPrxMgNi3.6Co0.4 (x = 0–0.4) alloys

The as-cast RE–Mg–Ni-based AB2-type La1−xPrxMgNi3.6Co0.4 (x = 0–0.4) alloys were prepared by vacuum induction furnace with a high purity helium gas as the protective atmosphere. The phase composition and microstructure of the as-cast alloys were characterized by XRD, SEM equipped with EDS. The results indicate that the as-cast alloys consist of two phases of LaMgNi4 and LaNi5. The measurements of the electrochemical properties show that the discharge capacity of the alloys slightly decreases with Pr content rising. As the Pr content grows from 0 to 0.4, the maximum discharge capacity decreases from 347.0 to 310.4 mAh/g. However, the cycle stability and the high-rate dischargeability of the alloy obviously augment with the Pr content increasing. Furthermore, the measurements of the electrochemical hydrogen storage kinetics reveal that the limiting current density (IL) first increases then decreases whereas the exchange current density I0 of the alloys first decreases then increases with the rising amount of Pr substitution, which indicates that the electrochemical dynamic of the alloy electrode are jointly dominated by the charge-transfer resistance and diffusion ability of hydrogen atoms. The measuring of the gaseous hydrogen storage reveals two pressure plateaus appear on each pressure–concentration–isotherm (P–C–T) curve of the as-cast alloys, which correspond to the LaMgNi4 and LaNi5 phases. Furthermore, we note that the pressure plateau of the P–C–T curve visibly rises with Pr content increasing.

Influence of Melt Spinning on the Electrochemical Hydrogen Storage Kinetics of RE-Mg-Ni-Based A2B7-Type Alloys

Abstract In order to improve the electrochemical characteristics of the La-Mg-Ni system A2B7-type electrode alloys, the partial substitution of M (M=Pr, Zr) for La has been performed. The melt spinning technology was used to prepare the La0.65M0.1Mg0.25Ni3.2Co0.2Al0.1 (M=Pr, Zr) electrode alloys. The influences of the melt spinning and substithting La with M (M=Pr, Zr) on the structures and the electrochemical hydrogen storage kinetics of the alloys were investigated. The results obtained by XRD and TEM reveal that the as-cast and spun alloys hold a multiphase structure, containing two main phases (La, Mg)2Ni7 and LaNi5 as well as a residual phase LaNi2. The as-spun (M=Pr) alloy displays an entire nanocrystalline structure, while an amorphous-like structure is detected in the as-spun (M=Zr) alloy, implying that the substitution of Zr for La facilitates the amorphous formation. The electrochemical measurement indicates that the high rate discharge ability (HRD) of the alloys first mounts up and then falls with rising of spinning rate. Furthermore, the electrochemical impedance spectrum (EIS), the Tafel polarization curves and the potential-step measurements all indicate that the electrochemical kinetics first increase and then decreases with growing of spinning rate.

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.

Preparation and Hydrogen Storage Kinetics of Nanocrystalline and Amorphous Mg<sub>2</sub>Ni-Type Alloys

In order to obtain a nanocrystalline and amorphous structure in the Mg2Ni-type alloy, the melt spinning technology has been used to prepare the Mg20Ni8M2 (M = Co, Cu) hydrogen storage alloys. The microstructures of the alloys were characterized by XRD, SEM and HRTEM. The effects of the melt spinning on the gaseous and electrochemical hydrogen storage kinetics of the alloys were investigated. The results indicate that the as-spun (M = Co) alloys display a nanocrystalline and amorphous structure as spinning rate grows to 20 m/s, while the as-spun (M = Cu) alloys hold an entire nanocrystalline structure even if a limited spinning rate is applied, suggesting that the substitution of Co for Ni facilitates the glass formation in the Mg2Ni-type alloy. The melt spinning remarkably ameliorates the gaseous hydriding and dehydriding kinetics of the alloys. As the spinning rate is raised from 0 (As-cast was defined as the spinning rate of 0 m/s) to 30 m/s, the hydrogen absorption saturation ratio ( ), a ratio of the hydrogen absorption capacity in 5 min to the saturated hydrogen absorption capacity, are enhanced from 80.43% to 94.38% for the (M = Co) alloy and from 56.72% to 92.74% for the (M = Cu) alloy. The hydrogen desorption ratio ( ), a ratio of the hydrogen desorption capacity in 20 min to the saturated hydrogen absorption capacity of the alloy, are increased from 24.52% to 51.67% for the (M = Co) alloy and from14.89% to 40.37% for the (M = Cu) alloy. Furthermore, the high rate discharge ability (HRD) and the hydrogen diffusion coefficient (D) of the alloys notably mount up with the growing of the spinning rate.

Impacts of Melt Spinning and Element Replacement on the Electrochemical Performances of the RE–Mg–Ni-Based A<sub>2</sub>B<sub>7</sub>-Type Electrode Alloys

The RE–Mg–Ni-based A2B7-type La0.75−xPrxMg0.25Ni3.2Co0.2Al0.1(x = 0, 0.1, 0.2, 0.3, 0.4) electrode alloys fabricated by melt spinning technology. The impacts of the melt spinning and the replacement of La by Pr on the microstructures and electrochemical performances of the alloys were systematically investigated. The results indicate that the as-cast and spun alloys hold a compound phase structure, containing (La, Mg)2Ni7and LaNi5phases as well as a residual phase LaNi2. A notable grain refinement of the alloys without altering the phase structures of the alloys obtained by melt spinning. The discharge capacity of the alloy (x = 0.2) tend to first augments and then falls with the growing spinning rate. And the as-spun (10 m/s) alloy gains the maximum discharge capacity as Pr content augmenting in the alloys. Furthermore, the measurements of the electrochemical hydrogen storage kinetics reveal that the high rate discharge ability (HRD), the hydrogen diffusion coefficient (D) and the limiting current density (ILSubscript text) of the alloys first increase then decrease with the rising of the spinning rate and the amount of Pr substitution.

Impacts of Melt Spinning and Mn Replacement on Electrochemical Hydrogen Storage Performances of Nanocrystalline and Amorphous Mg<sub>2</sub>Ni-Type Alloys

The Mg2Ni-type alloys with a nanocrystalline and amorphous structure have been confirmed possessing superior electrochemical hydrogen storage kinetics. The melt-spinning technique is used to preparing the nanocrystalline and amorphous Mg2Ni-type alloys with the nominal compositions of Mg20Ni10-xMnx (x = 0, 1, 2, 3, 4). The impacts of the melt spinning and the replacement of Ni by Mn on the structures and the electrochemical performances of the alloys are investigated systematically. The analysis of the structures by XRD and HRTEM reveals that the replacement of Ni by Mn facilitates the glass formation in the Mg2Ni-type alloy, and the amorphization degree of the as-spun alloys increases with the growing of the spinning rate. Furthermore, the replacement renders the formation of secondary phases MnNi and Mg instead of altering the Mg2Ni major phase in the alloys. The measurement of the electrochemical characteristics by an automatic galvanostatic system indicates that the discharge capacity and cycle stability of the alloys dramatically grow with the rising of the spinning rate and the amount of Mn replacement, with which the high rate discharge ability (HRD) of the alloys first augments and then falls.

Investigation on Gaseous and Electrochemical Hydrogen Storage Kinetics of As-Quenched Nanocrystalline Mg2Ni-type Alloys

Abstract In order to improve the gaseous and electrochemical hydrogen storage kinetics of the Mg 2 Ni-type alloys, Ni in the alloy was partially substituted by element Cu. Rapid quenching technology was used for the preparation of the Mg 2 Ni 1- x Cu x ( x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys. The structures of the as-cast and quenched alloys were characterized by XRD, SEM and TEM. The gaseous hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage kinetics of the as-quenched alloys was tested by an automatic galvanostatic system. The results show that all the as-quenched alloys hold an entire nanocrystalline structure and are free of amorphous phase. The substitution of Cu, instead of changing the major phase Mg 2 Ni, leads to a visible refinement of the grains of the as-cast alloys. Furthermore, both the rapid quenching treatment and Cu substitution significantly improve the gaseous and electrochemical hydrogen storage kinetics of the alloys. As the quenching rate grows from 0 (as-cast is defined as quenching rate of 0 m/s) to 30 m/s, the hydrogen absorption saturation ratio in 5 min, for the Mg 2 Ni 0.7 Cu 0.3 alloy, increases from 57.2% to 92.8%, the hydrogen desorption ratio in 20 min from 21.6% to 49.6%, the high rate discharge ability (HRD) from 40.6% to 73.1%, the hydrogen diffusion coefficient ( D ) from 1.02×10 −11 to 4.08×10 −11 cm 2 /s and the limiting current density ( I L ) from 113.0 to 715.3 mA/g.

Influence of Substituting La with Pr on Electrochemical Characteristics of RE–Mg–Ni-Based A<sub>2</sub>B<sub>7</sub>-Type Alloys Prepared by Melt Spinning

The RE–Mg–Ni-based A2B7-type La0.75−xPrxMg0.25Ni3.2 Co0.2Al0.1 (x = 0, 0.1, 0.2, 0.3, 0.4) electrode alloys were fabricated by casting and melt spinning. The microstructures and electrochemical characteristics of the as-cast and spun alloys were investigated in detail. The results indicate that the as-cast and spun alloys have a multiphase structure, consisting of two main phases (La, Mg)2Ni7 and LaNi5 as well as a residual phase LaNi2. The substitution of Pr for La results in a notable grain refinement of the as-cast alloys without altering the phase structure of the alloys. The discharge capacity of the alloys first rises and then falls with the variation of the Pr content. As Pr content grows from 0 to 0.4, the discharge capacity increases from 389.4 (x = 0) to 392.4 (x = 0.1) and then drops to 383.7 mAh/g (x = 0.4) for the as-cast alloy. And it mounts up from 393.5 (x = 0) to 397.9 (x = 0.1) and then declines to 382.5 mAh/g for the as spun (5 m/s) alloys. Furthermore, the measurements of the electrochemical hydrogen storage kinetics reveal that the high rate discharge ability (HRD), the limiting current density (IL) and the hydrogen diffusion coefficient (D) of the alloys first increases then decreases with the rising amount of Pr substitution.