Yunfeng Zhu

An investigation on the structural and electrochemical properties of La0.7Mg0.3(Ni0.85Co0.15)x (x=3.15–3.80) hydrogen storage electrode alloys

Abstract In this paper, the structural and electrochemical properties of the La 0.7 Mg 0.3 (Ni 0.85 Co 0.15 ) x ( x =3.15, 3.30, 3.50, 3.65, 3.80) hydrogen storage electrode alloys have been studied systematically. From the XRD analyses, each alloy of this series is composed of the LaNi 3 phase and the LaNi 5 phase, and the phase abundance of each phase varies with the degree of non-stoichiometry x and determines the hydrogen absorption capacity of the alloy. The electrochemical studies show that as x increases the maximum discharge capacity first increases from 365.7 mAh/g ( x =3.15) to 398.4 mAh/g ( x =3.50) and then decreases to 328.5 mAh/g ( x =3.80). Moreover, as x increases from 3.15 to 3.80, the high rate dischargeability (HRD), the exchange current density ( I 0 ), the limiting current density ( I L ) and the diffusion coefficient ( D ), of the alloy electrodes all increase first and then decrease.

The effect of Mn substitution for Ni on the structural and electrochemical properties of La0.7Mg0.3Ni2.55−xCo0.45Mnx hydrogen storage electrode alloys

Abstract The structures, hydrogen storage property and electrochemical properties of the La 0.7 Mg 0.3 Ni 2.55−x Co 0.45 Mn x (x=0.0,0.1,0.2,0.3,0.4,0.5) electrode alloys has been studied systematically. It can be found that, by X-ray powder diffraction, the alloys are all consisted of the (La,Mg)Ni3 phase and the LaNi5 phase, and the lattice parameters and cell volumes of both the (La,Mg)Ni3 phase and the LaNi5 phase increase with increasing Mn content in alloys. The P–C isotherms curves indicate that the hydrogen storage capacity first increases and then decreases with increasing x, and the equilibrium pressure decreases. The electrochemical measurements show that the maximum discharge capacity increases from 342.6 (x=0.0) to 368.9 mA h / g (x=0.3) and then decreases to 333.5 mA h / g (x=0.5) . For the discharge current density of 1000 mA / g , the high rate dischargeability (HRD) of the alloy electrodes increases from 55.8% (x=0.0) to 72.3% (x=0.4) and then decreases to 70.0% (x=0.5). Moreover, according to the electrochemical impedance spectroscopy, linear polarization and anodic polarization measurements, the exchange current density I0 and the limiting current density IL of the alloy electrodes also all increase first and then decrease with increasing Mn content in alloys.

Investigation of the Structural and Electrochemical Properties of Superstoichiometric Ti-Zr-V-Mn-Cr-Ni Hydrogen Storage Alloys

In this paper, the structural and electrochemical properties of the superstoichiometric (Ti 0.8 Zr 0.2 )(V 0.533 Mn 0.107 Cr 0.16 Ni 0.2 ) x (x = 2, 3, 4, 5, 6) hydrogen storage alloys have been studied systematically. It is found by X-ray diffraction and energy dispersive spectra analysis that all these alloys mainly consist of two phases, a C 14 Laves phase with hexagonal structure and a V-based solid solution phase with body-centered cubic structure. The lattice parameters and thus the cell volumes of the two phases all decrease when x is increased. The electrochemical measurements indicate that the maximum discharge capacity, the discharge equilibrium potential, the high rate dischargeability, the cyclic stability, the exchange current density I 0 , and the limiting current density I L of the alloys all increase with increasing x from 2 to 5. When x reaches 6, the discharge equilibrium potential, the high rate dischargeability, and the cyclic stability are still increasing proportionately, while the maximum discharge capacity, the exchange current density I 0 , and the limiting current density 1 L all decrease. Furthermore, the alloy electrodes are activated with more difficulty for the alloys with higher stoichiometry x. Consequently, we believe that the superstoichiometry is an effective way to improve the overall electrochemical properties of the Ti-based Laves-phase hydrogen storage alloys used for the negative electrodes of the Ni-MH secondary batteries.

Hydrogen storage and electrochemical properties of the La0.7Mg0.3Ni3.825−xCo0.675Mnx hydrogen storage electrode alloys

Abstract In this paper, the structure, hydrogen storage and electrochemical properties of the La0.7Mg0.3Ni3.825−xCo0.675Mnx (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys have been investigated systematically. It is found that, by X-ray diffraction and Rietveld analysis, all alloys consist mainly of two phases: an (La,Mg)Ni3 phase with the rhombohedral PuNi3-type structure and an LaNi5 phase with the hexagonal CaCu5-type structure. Moreover, with increasing x, the abundance of the (La,Mg)Ni3 phase decreases, but the abundance of the LaNi5 phase increases progressively, which indicates that the Mn element is beneficial for the formation of LaNi5 phase in the alloy. The pressure–composition (P–C) isotherm curves reveal that the equilibrium pressure decreases and the hydrogen storage capacity increases first and then decreases with increasing x. Electrochemical studies show that the maximum discharge capacity of the alloy electrodes increases from 225.2 to 328.8 mAh/g and then decreases to 292.2 mAh/g with increasing x from 0.0 to 0.5. Meanwhile, the high rate dischargeabilities of the alloy electrodes are also improved with an optimum Mn content in the alloy (x=0.4). In addition, the exchange current density I0, the limiting current density IL and the hydrogen diffusion coefficient D of the alloy electrodes all increase first and then decrease with increasing x.

The structural and electrochemical properties of La0.7Mg0.3(Ni0.85Co0.15)x(x=3.0–5.0) hydrogen storage alloys

Abstract In this paper, the effect of the compositions on the structural and electrochemical characteristics of the La0.7Mg0.3(Ni0.85Co0.15)x (x=3.0,3.5,4.0,5.0) hydrogen storage alloys have been investigated systematically. The results of the XRD Rietveld analyses show that the structures of the alloys change obviously with increasing x from 3.0 to 5.0. The main phase of the alloys with x=3.0–3.5 is (La,Mg)Ni3 phase (PuNi3-type structure), but the main phase of the alloys with x=4.0–5.0 is LaNi5 phase (CaCu5-type structure). Moreover, the phase abundance, lattice parameters and cell volumes of the (La,Mg)Ni3 phase and the LaNi5 phase change with increasing x. The electrochemical studies show that the maximum discharge capacity increases from 355.4 mAh/g (x=3.0) to 395.6 mAh/g (x=3.5) and then decreases to 226.8 mAh/g (x=5.0). As the discharge current density is 1250 mA/g , the high rate dischargeability (HRD) increases from 67% (x=3.0) to 81.3% (x=3.5) and then decreases to 74.9% (x=5.0). Furthermore, the exchange current density (I0), the limiting current density (IL) and the diffusion coefficient (D), of the alloy electrodes all first increase and then decrease with increasing x from 3.0 to 5.0, which is consistent with the variation of the HRD.

A study on the effect of annealing treatment on the electrochemical properties of La0.67Mg0.33Ni2.5Co0.5 alloy electrodes

Abstract In this paper, the influence of heat treatment on the electrochemical properties of La 0.67 Mg 0.33 Ni 2.5 Co 0.5 alloy electrodes has been studied. The results show that, after heat treatment, the electrochemical properties including the discharge capacity, width of discharge plateau and cyclic stability have been improved markedly. The discharge capacity of the alloy annealed at 1123 K is the highest and the cycle life of the alloy annealed at 1223 K is the longest, respectively. In addition, the exchange current density I 0 and the limiting current density I L of the alloy electrode both increase when annealed at 1123 K and then decrease when the annealing temperature increases to 1223 K .

The effect of Zr substitution for Ti on the microstructures and electrochemical properties of electrode alloys Ti1-xZrxV1.6Mn0.32Cr0.48Ni0.6

Abstract The effect of Zr substitution for Ti on the microstructures and electrochemical properties of the electrode alloys Ti 1− x Zr x V 1.6 Mn 0.32 Cr 0.48 Ni 0.6 ( x =0.2,0.3,0.4,0.5) has been studied. It is found by X-ray powder diffraction and energy dispersive X-ray spectroscope analyses that all the alloys consist of a C14 Laves main phase with hexagonal structure and a V-based solid solution secondary phase with b.c.c. structure. A small amount of TiNi-based third phase with b.c.c. structure has been found precipitated in the C14 Laves main phase in addition. With the increase in the amount of Zr substitution, the lattice parameters of the main phase and secondary phase are found increased and decreased, respectively. The electrochemical PCT curves indicate that the maximum hydrogen absorption capacity [H/M] max decreases with increasing Zr substitution, which may be attributed to the decrease in the content of V-based solid solution. The maximum discharge capacity and high rate dischargeability of the alloy electrodes both decrease, while the cyclic durability increases with the increasing amount of Zr substitution.

A study on improving the cycling stability of (Ti0.8Zr0.2)(V0.533Mn0.107Cr0.16Ni0.2)4 hydrogen storage electrode alloy by means of annealing treatment: II. Effects on the electrochemical properties

Abstract In this part of our study, the effects of annealing treatment on the electrochemical properties of the (Ti0.8Zr0.2)(V0.533Mn0.107Cr0.16Ni0.2)4 hydrogen storage electrode alloy are reported. The annealing treatment effectively improves the discharge capacity, the high rate dischargeability and the cycle life of the alloy. The association of the improvement in electrochemical properties with the changes in alloy structures after annealing treatment is discussed. The variations of the exchange current density I0 and the limiting current density IL by annealing treatment correlates well with the variation of the high rate dischargeability. However, the annealing treatment at the temperature studied does not affect the activation property of the alloy very much as all electrodes made of the alloys, either as casted or annealed, can be activated in a small number of charge–discharge cycles.

Electrochemical studies on the Ti–Zr–V–Mn–Cr–Ni hydrogen storage electrode alloys

Abstract In this paper, the electrochemical properties of the ( Ti 0.8 Zr 0.2 )( V 0.533 Mn 0.107 Cr 0.16 Ni 0.2 ) x (x=2,3,4,5,6) hydrogen storage electrode alloys have been studied. X-ray powder diffraction analysis showed that all alloys mainly consisted of a C14 Laves phase with hexagonal structure and a V-based solid solution phase with BCC structure. The lattice parameters and the unit cell volumes of the two phases decrease when x is increased. The electrochemical performances of the alloy electrodes, including the cycle life, the linear polarization, the anodic polarization, the Tafel polarization and the electrochemical impedance spectra, have been investigated systematically. After considering the global effect of the non-stoichiometry of B-side elements on the performances of the alloy electrodes, the optimum composition was found to lie in x =5. Consequently, we believe that the non-stoichiometry is an effective way to improve the overall electrochemical properties of the Ti-based Laves phase hydrogen storage alloys used for the negative electrodes of Ni–MH secondary batteries.

Structural and Electrochemical Properties of the La0.7Mg0.3Ni2.975 − x Co0.525Mn x Hydrogen Storage Electrode Alloys

The effect of partial substitution of Mn for Ni on the structural and electrochemical properties of the La 0.7 Mg 0.3 Ni 2975-x Co 0.525 Mn x (x = 0.0, 0,1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys has been investigated systematically. The results of X-ray powder diffraction and Rietveld analyses showed that all alloys consisted of the (La, Mg)Ni 3 phase and the LaNi 5 phase, and the content of the (La, Mg)Ni 3 phase first remained unchanged (∼77 wt %) and then decreased, but the content of the LaNi 5 phase increased progressively with increasing x. Meanwhile, the lattice parameters and cell volumes of the ( La, Mg)Ni 3 phase and the LaNi 5 phase all increased with increasing Mn content. The pressure composition isotherms showed that the hydrogen storage capacity first remained almost unchanged and then decreased with increasing x from 0.0 to 0.5, and the equilibrium pressure decreased from 0.51 atm to 0.06 atm. The electrochemical measurements indicated that the maximum discharge capacity first remains unchanged (∼400 mAh/g) with increasing x from 0.0 to 0.2 and then decreased when x increased further. Moreover, the high rate discharge-ability, the exchange current density I 0 , the limiting current density I L , and the hydrogen diffusion coefficient D of the alloy electrodes all increased first and then decreased with increasing x, which indicates that the kinetics of hydriding/dehydriding of the La 0.7 Mg 0.3 Ni 2.975-x Co 0.525 Mn x (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) hydrogen storage alloys increased first up to x = 0.1 and then decreased with further increasing x.