Guo Fang Zhang

Structures and electrochemical hydrogen storage performance of Si added A2B7-type alloy electrodes

Abstract In order to ameliorate the electrochemical hydrogen storage performance of La–Mg–Ni system A 2 B 7 -type electrode alloys, a small amount of Si was added. The La 0.8 Mg 0.2 Ni 3.3 Co 0.2 Si x ( x =0–0.2) electrode alloys were prepared by casting and annealing. The effects of adding Si on the structure and electrochemical hydrogen storage characteristics of the alloys were investigated systematically. The results indicate that the as-cast and annealed alloys hold multiple structures, involving two major phases of (La, Mg) 2 Ni 7 with a Ce 2 Ni 7 -type hexagonal structure and LaNi 5 with a CaCu 5 -type hexagonal structure as well as one residual phase LaNi 3 . The addition of Si results in a decrease in (La, Mg) 2 Ni 7 phase and an increase in LaNi 5 phase without changing the phase structure of the alloys. What is more, it brings on an obvious effect on electrochemical hydrogen storage characteristics of the alloys. The discharge capacities of the as-cast and annealed alloys decline with the increase of Si content, but their cycle stabilities clearly grow under the same condition. Furthermore, the measurements of the high rate discharge ability, the limiting current density, hydrogen diffusion coefficient as well as electrochemical impedance spectra all indicate that the electrochemical kinetic properties of the electrode alloys first increase and then decrease with the rising of Si content.

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

Abstract The nanocrystalline and amorphous Mg 2 Ni-type alloys with a chemical composition of (Mg 24 Ni 10 Cu 2 ) 100− x Nd x ( 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 Mg 2 Ni-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 ( S 20 ) 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.

Effects of stoichiometric ratio La/Mg on structures and electrochemical performances of as-cast and annealed La–Mg–Ni-based A2B7-type electrode alloys

Abstract In order to investigate the influences of the stoichiometric ratio of La/Mg (increasing La and decreasing Mg on the same mole ratio) on the structure and electrochemical performances of the La–Mg–Ni-based A 2 B 7 -type electrode alloy, the as-cast and the annealed ternary La 0.8+ x Mg 0.2− x Ni 3.5 ( x =0−0.05) electrode alloys were prepared. The characterization of electrode alloys by X-ray diffraction (XRD) and scanning electron microscopy (SEM) shows that all the as-cast and the annealed alloys hold two major phases of (La,Mg) 2 Ni 7 and LaNi 5 as well as a residual phase of LaNi 3 . Moreover, the increase of La/Mg ratio brings on a decline of (La,Mg) 2 Ni 7 phase and a rise of LaNi 5 and LaNi 3 phases. The variation of La/Mg ratio gives rise to an evident change of the electrochemical performances of the alloys. The discharge capacities of the as-cast and the annealed alloys evidently decrease with growing the La/Mg ratio, while the cycle stabilities of the alloys visibly augment under the same condition. Furthermore, the high rate discharge ability (HRD), the electrochemical impedance spectrum (EIS), the Tafel polarization curves, and the potential step measurements all indicate that the electrochemical kinetic properties of the alloy electrodes increase with the La/Mg ratio rising.

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−xNdx (xxa0=xa00, 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 0xa0m·s−1) to 40xa0m·s−1, the discharge capacity increases from 86.4 to 452.8 mAh·g−1, the S20 (the capacity maintain rate at 20th cycle) value increases from 53.2xa0% to 89.7xa0%, the hydrogen absorption saturation ratio (R5a, a ratio of the hydrogen absorption quantity in 5xa0min to the saturated hydrogen absorption capacity) increases from 36.9xa0% to 91.5xa0%, and the hydrogen desorption ratio (

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

R_{ 1 0}^{d}

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

R10d, a ratio of the hydrogen desorption quantity in 10xa0min to the saturated hydrogen absorption capacity) increases from 16.4xa0% to 47.7xa0% for the (xxa0=xa010) alloy, respectively.

Hydrogen storage properties of mechanically milled La2Mg17-x wt.%Ni (x=0, 50, 100, 150 and 200) composites

Abstract Nanosized Fe3+ and Eu3+ codoped CeO2 solid solutions were synthesized via hydrothermal method. The crystalline structure of Ce1−x(Fe0.5Eu0.5)xO2−δ (x=0.00–0.30) solid solutions was carried out by the X–ray diffraction technique, and the spectrum features were identified by UV–Vis and Raman spectroscopy, respectively. It was observed that the cell parameters were first increased then decreased by increasing the doped ions content. The phase separation was detected when the dopant concentration reached to x=0.30. UV–Vis spectrum showed that the width of the band gap gradually reduced by increasing the doped content, and the solid solubility was determined to be x=0.20. The Raman technique displayed that the peak position of F2g mode gradually shifted to lower frequencies from 465 cm−1 for x=0.00 to 440 cm−1 for x=0.20. The catalytic effects of Ce1−x(Fe0.5Eu0.5)xO2−δ solid solutions on the electrochemistry properties of Mg2Ni/Ni were measured by mixing them together via ball milling technique. The electrochemical properties of the Mg2Ni/Ni–Ce1–x(Fe0.5Eu0.5)xO2−δ composites showed that the maximum discharge capability Cmax and the cycle stability were improved obviously. Meanwhile, the EIS characteristic also indicated that the doped solid solutions could enhance the rate of charge transfer on the surface of alloy. The catalytic effect of the solid solutions was speculated to rely on both the concentration of oxygen vacancies and the cell volumes of the solid solutions.

Electrochemical hydrogen storage behaviors of the nanocrystalline and amorphous Nd-Cu-added Mg 2 Ni-type alloy electrodes applied to Ni-MH battery

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.