Chiaki Iwakura

Some factors affecting the cycle lives of LaNi5-based alloy electrodes of hydrogen batteries

The electrode characteristics, the hydrogen equilibrium properties, the crystallographic and mechanical properties of simple ternary alloys of the type LaNi5−xMx(M ≡ Ni, Mn, Cu, Cr, AlandCo) were examined. The effect of substituting elements to improve the cycle life increased in the order: M ≡ Mn, Ni, Cu, Cr, AlandCo. The lower the capacity, the smaller the volume expansion ratio, the slower the pulverizing rate and the lower the Vickers hardness were, the longer the cycle life became. The alloy LaNi2.5Co2.5, which showed the highest durability as a negative electrode, satisfied all these requirements.

The influence of small amounts of added elements on various anode performance characteristics for LaNi2.5Co2.5-based alloys

The addition of small amounts of elements such as silicon, aluminium and titanium to LaNi2.5Co2.5 greatly influenced anode performance characteristics such as usable temperature range, capacity and its decay rate during repeated cycles, rate capability, low temperature dischargeability and self-discharge rate. The capacity decay was suppressed by the addition of silicon, but the rate capability decreased and the self-discharge rate increased. The alloy containing titanium exhibited a much longer cycle life, but much lower storage capacity and worse low temperature dischargeability. The addition of aluminium was very useful for improving the usable temperature range, cycle life and charge retention, and it did not cause too great a decrease in capacity and/or increase in overpotentials.

Effects of substitution with foreign metals on the crystallographic, thermodynamic and electrochemical properties of AB5-type hydrogen storage alloys

Effects of substitution with foreign metals on the crystallographic, thermodynamic and electrochemical properties of AB5-type hydrogen storage alloys were investigated. The unit cell volume of MmNi5 − xAlx (Mm = mischmetal, 0.2 ⩽ x ⩽ 1.0) alloys and the values of enthalpy change (−ΔH) in hydride formation increased monotonously with an increase in x-value, but the discharge capacity and high-rate dischargeability had a limiting value. For the MmNi4.2Al0.4M0.4 (M = Cr, Mn, Fe, Co) alloys, both the unit cell volume and the −ΔH value increased with increasing metallic bond radius of the foreign metal. Especially, manganese improved the discharge capacity and the high-rate dischargeability.

Electroless deposition of Ni–B, Co–B and Ni–Co–B alloys using dimethylamineborane as a reducing agent

Fundamental aspects of electroless Ni–B, Co–B and Ni–Co–B alloys have been systematically examined. The composition, crystal structure and deposition rate of the alloys were determined as a function of the concentration of reducing agent (dimethylamineborane) and complexing agents (tartrate, citrate, malonate and succinic acid), bath pH and Ni2+/Co2+ ratio. Changes in the deposition rate and metallurgical features of the alloys induced by the change in plating parameters are discussed, based on electrochemical polarization data and the formation enthalpy of the nickel and cobalt borides.

Surface modification of metal hydride negative electrodes and their charge/discharge performance

Negative electrodes consisting of a multicomponent alloy (MmNi3.6Mn0.4Al0.3Co0.7) in a porous nickel substrate were modified by using different kinds of electroless plating baths or alkaline solutions containing hypophosphite as a reducing agent. Electrochemical properties of the negative electrodes such as discharge capacity, electrocatalytic activity for the hydrogen electrode reaction (HER), and high-rate dischargeability were examined in a 6 M KOH solution. It was found that such simple surface modifications as described above improved the performance of negative electrodes.

Self‐Discharge Mechanism of Nickel‐Hydrogen Batteries Using Metal Hydride Anodes

The self-discharge mechanism of nickel-hydrogen batteries using metal hydride anodes is investigated and discussed by dividing the capacity loss during the storage in open-circuit conditions into two parts, i.e., reversible and irreversible ones. The reversible capacity loss is attributed to the desorption of hydrogen from the metal hydride anode and the irreversible capacity loss to the deterioration of the hydrogen-absorbing alloy. Microencapsulation of the alloy with a thin copper layer is found to improve the self-discharge characteristics.

Electrochemical characteristics of a homogeneous amorphous alloy prepared by ball-milling Mg2Ni with Ni

Ball-milling Mg2Ni with metallic Ni (70 wt.% vs. Mg2Ni) lead to the formation of a homogeneous amorphous alloy, which exhibited a maximum discharge capacity of ca. 870 mAh g (Mg2Ni)−1 at 30°C. Its electrochemical and microstructural characteristics indicated that the homogeneous amorphous structure of the alloy was an important factor for such an improvement in charge–discharge characteristics.

Hydriding and electrochemical characteristics of a homogeneous amorphous Mg2Ni-Ni composite

Abstract Hydriding and electrochemical characteristics of a homogeneous amorphous Mg 2 Ni-Ni composite prepared by ball-milling of Mg 2 Ni alloy with Ni (70 wt.% vs. Mg 2 Ni) were investigated. It was found that the Mg 2 Ni-Ni composite absorbed hydrogen to the amount of 4.0 wt.% vs. Mg 2 Ni [2.4 wt.% vs. (Mg 2 Ni+70 wt.% Ni)] at a high rate under a hydrogen pressure of 3 MPa at 30°C. Furthermore, this alloy exhibited an extremely high discharge capacity of ∼1082 mAh g(Mg 2 Ni) −1 [636 mAh g(Mg 2 Ni+70 wt.% Ni) −1 ] at 30°C, which exceeded the theoretically calculated value of 999 mAh g(Mg 2 Ni) −1 on the basis of Mg 2 NiH 4 for crystalline Mg 2 Ni.

Electrochemical characterization of MmNi4.0−xMn0.75Al0.25Cox electrodes as a function of cobalt content

Abstract Hydrogen storage alloys based on misch metal (Mm) are commercially used as negative electrode materials for the nickel–hydride battery. In general, cobalt is contained in the alloys to guarantee the long cycle life of the negative electrode. Consequently, cobalt is an essential element in the hydrogen storage alloy. In order to elucidate the function of cobalt in the charge and discharge processes, the electrochemical feature of the negative electrode as well as the diffusion behavior of hydrogen in the alloy were systematically examined for the MmNi 4.0− x Mn 0.75 Al 0.25 Co x (0≤ x ≤0.6) alloys as a function of the cobalt content. The results of electrochemical measurements are interpreted from the viewpoint of the changes in unit cell volume of the alloys that strongly influences the stability of the hydride.

Electrochemical characterization of various metal foils as a current collector of positive electrode for rechargeable lithium batteries

Abstract Electrochemical characterization of various metal foils as a current collector of the positive electrode for rechargeable lithium batteries was carried out by cyclic voltammetry, potentiostatic electrolysis and a.c. impedance measurement. Products of the potentiostatic electrolysis were determined by atomic absorption spectroscopy and gas chromatography/mass spectrometry. As a result, it was found that an aluminum foil seems to be the most suitable material as a current collector of the positive electrode, whose electrochemical stability in 1 M LiClO4/ethylene carbonate—diethyl carbonate and 1 M LiPF6/ethylene carbonate—diethyl carbonate solutions increases with increasing aluminum purity.