Yongquan Lei

Electrochemical Behaviour of Some Mechanically Alloyed Mg — Ni-Based Amorphous Hydrogen Storage Alloys*

The electrochemical behaviours of some amorphous Mg5oNi5o-;t-),MxN), (M, N = Co, Al and Si) hydrogen storage alloys prepared by mechanical alloying (MA) were investigated. It is found that the MA amorphous alloys are easier to activate electrochemically than crystalline alloys. All MA amorphous alloys reach their respective maximum capacities at the first charge/discharge cycle. At a current density of 20 mA/g the MA Mg5oNi5o has a maximum discharging capacity around 500 mAh/g, which is ten times higher than that of the crystalline alloy. The partial substitution of Co, Si and Al for Ni in Mg5oNi50 alloy decreases its discharging capacity each to a different extent. However, the durability of the amorphous Mg5oNi5o-:t-,,M.tN), alloys is rather poor, and their capacity degradation rates amount to 10

A Study of the Structural and Electrochemical Properties of La0.7Mg0.3 ( Ni0.85Co0.15 ) x ( x = 2.5 ­ 5.0 ) Hydrogen Storage Alloys

In this paper, the structural and electrochemical properties of La 0.7 Mg 0.3 (Ni 0.85 Co0.15) x (x = 2.5,3.0,3.5,4.0,4.5,5.0) hydrogen storage alloy electrodes were studied systematically. X-ray diffraction Rietveld analyses show that all these alloys consist of a (La, Mg)Ni 3 phase with the PuNi 3 -type rhombohedral structure and a LaNi 5 phase with the CaCu 5 -type hexagonal structure. The (La, Mg)Ni 3 phase abundance first increases to a high percentage (∼75%) and then decreases with increasing x. In contrast, the LaNi 5 -phase abundance first remains low and almost unchanged and then increases to a high percentage (∼70%) with increasing x. The pressure-composition isotherms shows that for each isotherm the plateau region widens and the plateau pressure is maintained almost unchanged when x increases from 2.5 to 3.5. However, as x increases further, the plateau region is shortened and becomes flatter, and the plateau pressure increases with x. Electrochemical studies indicate that all important electrochemical properties, including the maximum discharge capacity, the high rate dischargeability, the exchange current density, and the limiting current density of the alloy electrodes increase as x increases from 2.5 to 3.5 and then decrease when x increases further from 3.5 to 4.5.

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.

A study of the degradation of the electrochemical capacity of amorphous Mg50Ni50 alloy

Abstract The degradation of the electrochemical capacity of amorphous Mg 50 Ni 50 alloy during charge-discharge cycling is investigated. The surface morphology and structure of the alloy before and after electrochemical cycling is examined by scanning electron microscopy and X-ray diffraction, respectively. The composition profiles of magnesium, nickel and oxygen are determined by X-ray photoelectron spectroscopy. These results suggest that the capacity deterioration can be attributed to the oxidation of magnesium and nickel in KOH solution during the discharge process, and the reduction of Ni(OH) 2 to nickel during the charge process.

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.

THE RELATION BETWEEN THE DISCHARGE CAPACITY AND CYCLING NUMBER OF MECHANICALLY ALLOYED MGXNI100-X AMORPHOUS ELECTRODE ALLOYS

Abstract The relation between the discharge capacity and cycling number of mechanically alloyed Mg x Ni 100− x amorphous electrode alloys was proposed on the basis of the fact that the degradation of discharge capacity was mainly caused by the oxidation of magnesium and nickel. The experimental data fitted the deduced equation well. Partial substitution of Co, Si or Al for Ni may improve the cycling life of amorphous Mg x Ni 100− x alloys just as predicted by the deduced equation; however, their initial discharge capacities were dramatically reduced.

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.

Electrochemical Properties of the La0.7Mg0.3Ni2.65 − x Mn0.1Co0.75Al x ( x = 0 ­ 0.5 ) Hydrogen Storage Alloy Electrodes

In our endeavor to improve the cyclic stability of La-Mg-Ni-Co type alloys, La 0 . 7 Mg 0 . 3 Ni 2 . 6 5 - x Mn 0 . 1 Co 0 . 7 5 Al x (x = 0-0.5) hydrogen storage alloys were prepared and the structure and electrochemical properties of these alloys were investigated systematically. X-ray diffraction and Rietveld analyses revealed that the major phases are the (La,Mg)Ni 3 phase and the LaNi 5 phase. Electrochemical studies indicate that as Ni is progressively substituted by Al, the cyclic stability of alloy electrodes is noticeably improved due to the formation of a dense oxide film on the alloy surface. The capacity retention of the alloy electrodes for 100 cycles increases from 32.0% (x = 0) to 73.8% (x = 0.3). The maximum discharge capacity decreases with increasing Al content, and the high rate dischargeability, electrochemical impedance spectra, linear polarization, Tafel polarization, and potential-step studies all indicate that the electrochemical kinetics of the alloy electrodes is deteriorated. We ascribe this phenomenon to the increase of the charge-transfer resistance of the alloy electrodes and reduction of the diffusion rate of hydrogen from interior of the bulk to the surface due to the presence of the aforesaid Al-containing oxide film. The optimal content of Al in La 0 . 7 Mg 0 . 3 Ni 2 . 6 5 - x Mn 0 . 1 Co 0 . 7 5 Al x alloys in this study is in the range from 0.2 to 0.3.

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.

The study on the electrochemical performance of mechanically alloyed Mg–Ti–Ni-based ternary and quaternary hydrogen storage electrode alloys

In this paper, ternary and quaternary magnesium-based hydrogen storage electrodes prepared by means of mechanical alloying (MA) were investigated. Through X-ray diffractometer (XRD) and scanning electron microscope (SEM) investigation, the phase structure transformation and morphology of Mg35Ti15Ni50 during MA process were examined. The effects of Ti content on discharge capacity, the cycling life and high rate dischargeability of the ternary Mg–Ti–Ni electrode alloys were studied in the discharge tests. It was found that as the Ti content increased the cycling stability of Mg50−xTixNi50 alloys improved but the discharge capacity and high rate discharge-ability both decreased. The MA quaternary alloys had significantly longer cycling life but lower discharge capacity than the ternary alloy Mg35Ti15Ni50. Surface coating of alloy particles with Ni was also tested for improving the cycling stability of the ternary alloys and found beneficial for cycling stability and bad for discharge capacity