Abstract

The La_0.7Mg_0.3Ni_2.875Co_0.525Mn_0.1 alloy was synthesized by melting with intermetallic alloy La₂Mg₁₇. The composites contained NiB with different weight ratios (4%, 6%, 8%). The XRD analyses showed that all these alloys mainly consist of the (La,Mg)Ni₃ phase with the rhombohedral PuNi₃-type structure and the LaNi₅ phase with the hexagonal CaCu₅-type structure, the lattice parameters and cell volumes of the (La,Mg)Ni₃ phase increased with the increasing NiB weight ratio, while the LaNi₅ phase kept almost the same value. The electrochemical measurements showed that the maximum discharge capacity of the composites decreased, and the cycling life improved distinctly. At the discharge current density 1500 mA/g, the HRD (high rate discharge ability) increased from 57.0% to 71.3% (6%). Moreover, according to the electrochemical impedance spectroscopy (EIS) and the potentiodynamic polarization, the charge-transfer reaction resistance decreased and the anticorrosion performance improved in the alkali solution.

Abstract

This paper reviews the development of hydrogen storage alloys prepared by an effective method of mechanical alloying and milling. It emphasizes alloys based on Mg or that contain Mg due to their low cost, low weight and high hydrogen storage capacity. Hydrogen absorption/desorption and electrochemical measurements are briefly discussed. The electrochemical properties of the alloys that contain Mg are covered in detail, emphasizing the effects of changes in alloy composition. The system of Ti–Ni-based alloys is also introduced. At present, composite hydrogen storage alloys may be the most effective materials for practical application in new nickel/metal hydride secondary batteries. The steps of hydrogen absorption/desorption such as charge-transfer and hydrogen diffusion for evaluating the electrochemical properties of hydrogen storage alloys are discussed. The relationship between alloy composition and electrochemical properties is noted and evaluated.

Abstract

Mg₂Ni-based hydride was prepared by hydriding combustion synthesis (HCS), and subsequently modified with various metal elements (Ti, Co, Cr and Y) by mechanical milling. The effects of the modifications on electrochemical properties of the hydride were investigated. Both of the maximum discharge capacity and the high rate dischargeability (HRD) are increased by the modification of Co, while the cycling stability is improved for the hydride modified with Ti, Cr and Y. The Tafel polarization shows that the addition of the metals has a positive effect on improving the anti-corrosion ability. The electrochemical kinetics was also characterized by linear polarization, electrochemical impedance spectroscopy and potentiostatic discharge. The results show that the hydrogen diffusion coefficient and the electrochemical reaction resistance are increased by the modifications, but the exchange current density decreases.

Abstract

Microstructure and some dynamic performances of Ti_0.17Zr_0.08V_0.34RE_0.01Cr_0.1Ni_0.3 (, Dy) hydrogen storage electrode alloys have been investigated using XRD, FESEM–EDS, ICP-MS and EIS measurements. The alloy is composed of V-based solid solution phase with a dendritic shape and a continuous C14 Laves phase with a network shape surrounding the dendrite. Pressure–composition isotherm curves indicate that the alloy with Dy addition has a lower equilibrium hydrogen pressure and a wider plateau region. The alloy electrode with Dy addition has higher discharge capacity, while the alloy electrode with Ce addition has better activation and higher cycle stability. The alloy electrode with Ce addition has better electrochemical activity with higher exchange current density , lower charge transfer resistance and lower apparent activation energy . The capacity degradation behavior for the alloy electrode is attributed to two main factors: one is the dissolutions of V and Zr element to KOH solution, and another is the larger charge transfer resistance which increases with increasing cycle number. Hydrogen diffusion coefficient is a rate-limiting factor at higher discharge current, while exchange current density is the rate-limiting factor at lower discharge current, both hydrogen diffusion coefficient and exchange current density are responsible for the poor high-rate dischargeability of the alloy in this case.

Abstract

In the present work, new type hydrogen storage composites electrodes Ti_0.9Zr_0.2Mn_1.5Cr_0.3V_0.3–x wt% La_0.7Mg_0.25Zr_0.05Ni_2.975 were successively prepared by ball milling method. The structure and electrochemical properties of the composites were investigated by means of XRD, SEM, EDS and electrochemical measurements. It was found that the bulk of the composites still retained the hexagonal C14 Laves structure after short-term ball milling and that the maximum capacity of the composite electrodes was significantly improved to 292.4 and 314.0 mA h/g for x=5 and 10, respectively, from maximum 48.6 mA h/g of Ti_0.9Zr_0.2Mn_1.5Cr_0.3V_0.3 alloy electrode. Electrochemical impedance spectra and cyclic voltammograms (CV) measurements revealed that the charge-transfer resistance was reduced with increasing amount of La–Mg-based alloy. The increasing x amount also lifted the values of hydrogen diffusion coefficient D of the composite electrodes obtained by anodic polarization (AP) measurements. These indicated that the La–Mg-based alloy as a surface modifier not only increased the discharge capacity but also improved the charge–discharge kinetics of composite electrode greatly.

Abstract

Hydrogen storage alloy composites were prepared by ball milling method. Their structures, morphologies and the hydrogen storage characteristics were intensively studied in the present work. It was found that the bulk of composites maintained the hexagonal C14 Laves phase structure after ball milling with additional La–Mg-based alloy for two hours. Scanning electron microscopy (SEM) observations revealed that the average size of Ti_0.9Zr_0.2Mn_1.5Cr_0.3V_0.3 and La_0.7Mg_0.25Zr_0.05(Ni_0.85Co_0.15)_3.5 particles were reduced to several hundred nanometers after ball milling process. Energy dispersive X-ray spectrometer (EDS) patterns of the composites showed that the La_0.7Mg_0.25Zr_0.05(Ni_0.85Co_0.15)_3.5 phase was uniformly distributed on the surface of Ti_0.9Zr_0.2Mn_1.5Cr_0.3V_0.3. The hydrogen absorption–desorption characteristics of the composites were improved with increasing dispersivity of the La_0.7Mg_0.25Zr_0.05(Ni_0.85Co_0.15)_3.5 alloy particles. Pressure–composition–temperature (PCT) measurements revealed that the pressure plateau and the hysteresis decreased with increasing x. Among these composites, the sample of x=10 exhibited more significant hydrogen storage performances. Its hydrogen absorption and desorption plateau pressure were 0.547 and 0.302 MPa, respectively, which were much lower than those of the Ti_0.9Zr_0.2Mn_1.5Cr_0.3V_0.3 alloy. Moreover, it took only 200 and 2500 s to reach the constant pressure for the hydrogen absorption and desorption reaction. These improvements can be attributed to the preferable dispersivity of La–Mg-based alloy particles and also the formation of micro-crack after ball milling. Both factors synergetically facilitate the diffusion of hydrogen atoms in the composites and accelerate the dissociation of hydrogen molecules and recombination of hydrogen atoms on the surface of composites during the hydrogen absorption–desorption process.

Abstract

The structure and electrochemical characteristics of melted composite Ti_0.10Zr_0.15V_0.35Cr_0.10Ni_0.30 + x% LaNi₅ (x = 0, 1, 5 and 10) hydrogen storage alloys have been investigated systematically. XRD shows that the matrix phase structure of V-based solid solution phase with a BCC structure and C14 Laves phase with hexagonal structure is not changed after adding LaNi₅ alloy. However, the amount of the secondary phase increases with increasing LaNi₅ content. Field emission scanning electron microscopy–energy dispersive spectroscopy (FESEM–EDS) shows that the C14 Laves phase contains more Zr and the white lard phase has a composition close to (Zr, Ti)(V, Cr, Ni, La)₂. The electrochemical measurements show that the hysteresis effect decreases dramatically with increasing x. The activation performance, the low temperature dischargeability, high rate dischargeability and cyclic stability of composite alloy electrodes increase greatly with increasing x. The maximum discharge capacity first increases as x increases from 0 to 5 and then decreases when x increases further from 5 to 10. The improvement of the electrochemical characteristics caused by adding LaNi₅ seems to be related to formation of the secondary phase.

Abstract

Composite LaNi₅+x wt.% Ti_0.10Zr_0.16V_0.34Cr_0.10Ni_0.30 (x=0, 1, 5, 10) alloys were prepared by two-step re-melting. X-ray diffractometer (XRD), scanning electron microscopy (FESEM), energy dispersive spectrometry (EDS), inductively coupled plasma (ICP) and electrochemical impedance spectroscopy (EIS) analyses showed that the matrix phase of LaNi₅ alloy with CaCu₅ structure remained unchanged after additive alloy was added, the amount of the second phase increased with increasing x. The synergetic effect within constituent alloys appeared during the composite process. The electrochemical characteristics of the composite alloy electrodes were greatly improved, and the optimum composition was x=5, at which the low temperature dischargeability at 233 K was 87.37 %, and the maximum discharge capacity and the high rate dischargeability at discharge current density of 1800 mA/g were 326.1 mAh/g and 71.98 % at 303 K, respectively. The HRD was controlled by both the charge-transfer reaction of hydrogen on the electrode/electrolyte interface and hydrogen diffusion coefficient in the bulk of the alloys at discharge current density of 1800 mA/g.

Abstract

The phase structures and hydrogen storage properties of the Ca_3-xLa_xMg₂Ni₁₃ alloys were investigated. It was found that the La substitution is unfavorable for the formation of the Ca₃Mg₂Ni₁₃-type phase. The La-substituted alloys consist of multiple phases. Increasing La content to x = 2.25 leads to a disappearance of Ca₃Mg₂Ni₁₃-type phase. Among these alloys, the Ca_1.5La_1.5Mg₂Ni₁₃ alloy has highest equilibrium pressures of hydrogen absorption–desorption and a highest hydrogen desorption capacity of 1.34 wt.% at 318 K. The discharge capacity decreases for La-substituted alloys. However, the cycling capacity retention rate (S₃₀) increases from 13.7 to 67.6% when x increases from 0 to 3.

Abstract

The microstructure and electrochemical hydrogen storage characteristics of La_0.67Mg_0.33−xCa_xNi_2.75Co_0.25 (x = 0, 0.05, 0.10 and 0.15) alloys are investigated. The results show that all alloys mainly consist of (La, Mg)Ni₃ and LaNi₅ phases, besides a small amount of (La, Mg)₂Ni₇ phase. The cycle stability (S₈₀) after 80 charge/discharge cycles of all alloy electrodes first increases from 60.1% (x = 0) to 64.2% (x = 0.05), then decreases to 45.9% (x = 0.15). The high rate dischargeability of all alloy electrodes first increases from 52.6% (x = 0) to 61.4% (x = 0.10), then decreases to 57.2% (x = 0.15). Moreover, the charge-transfer resistance (R_ct) first decreases from 168.2 mΩ (x = 0) to 125.7 mΩ (x = 0.10), then increases to 136.6 mΩ (x = 0.15). All the results indicate that the substitution of Mg with a certain amount of Ca can improve the overall electrochemical characteristics.

Research highlights

? The microstructure of La_0.67Mg_0.33Ni_2.75Co_0.25 alloy is almost unchanged by substituting Mg with Ca. ? The cycle life of La_0.67Mg_0.33Ni_2.75Co_0.25 alloy is improved by substituting Mg with Ca (x = 0.05). ? The electrochemical kinetics of La_0.67Mg_0.33Ni_2.75Co_0.25 alloy is improved by substituting Mg with Ca (x = 0.10).