Yukio Fukumoto

Effect of the stoichiometric ratio on electrochemical properties of hydrogen storage alloys for nickel-metal hydride batteries

Effect of stoichiometric ratio on the electrochemical properties of negative electrodes was investigated for alloys with composition Mm(Ni3.6Mn0.4Al0.3Co0.7)x (Mm = misch metal, 0.88 ⩽ x ⩽ 1.12). The discharge capacity at a current density of 0.2 A g−1 increased with an increase in unit cell volume of the alloys in the range of x = 0.96−1.12. The discharge efficiency increased with increasing x value, and was about 85.2% even at a high current density of ca 2 A g−1 in the case of x = 1.12. The improved high-rate dischargeability was explained on the basis of high diffusibility of hydrogen in the bulk of alloys as the result of the relatively low stability of the hydride, in addition to the high electrocatalytic activity for the hydrogen electrode reactions of the negative electrode.

Electrochemical characterization of surface-modified negative electrodes consisting of hydrogen storage alloys

Abstract Negative electrodes consisting of MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 alloy powder filled in a porous nickel substrate were modified by using different electroless plating baths or alkaline solutions containing hypophosphite as a reducing agent. The deposited metals function as a microcurrent collector and then increase the discharge capacity of the negative electrodes. In contrast, modification with a reducing agent gives rise to high electrocatalytic activity of the negative electrodes which leads to a remarkable reduction in overvoltage in charging and discharging even at the first cycle. It was found that such simple surface modifications improved the performance of negative electrodes under appropriate conditions with respect to the amount of electroless deposit or the concentration of reducing agent.

Electrochemical characterization of hydrogen storage alloys modified with metal oxides

Abstract Negative electrodes were made from hydrogen storage alloys (MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 ) modified by mixing them with metal oxide powders. The discharge capacity of cathodes was greatly increased by the modification with metal oxides with high electric conductivity, for example RuO 2 and Co 3 O 4 . An exchange current density ( J 0 ) for the hydrogen electrode reaction was also increased remarkably by modification with metal oxides. High-rate dischargeability was found to increase asymptotically with an increment of J 0 . An impedance analysis indicated that the modification of the alloy powder with the metal oxides decreased the charge-transfer resistance. The mode of action of metal oxides was discussed on the basis of the electrocatalytic activity of the modified surface.

Surface modification of hydrogen storage alloy electrode with an alkaline solution containing potassium borohydride as a reducing agent

Negative electrodes consisting of MmNi3.6Mn0.4Al0.3Co0.7 alloy in porous nickel substrate were modified by immersing them in an alkaline solution of potassium borohydride (KBH4). Such a simple modification results in rapid activation and increases the saturation capacity and high-rate dischargeability of the negative electrodes. Electrochemical analysis indicates that the overvoltages in charging and discharging processes were drastically decreased by the reducing treatment. Electrons liberated from borohydride ions and intermediary hydrogen play an important role for improving the characteristics of negative electrodes. The evidence for hydride formation during the surface modification was detected by X-ray diffraction measurements as well as by an electrochemical method.

Compositional and structural characteristics of MgNi alloy prepared by mechanical alloying for use as negative electrodes in nickel-metal hydride batteries

From the elemental analysis by EPMA, it was found that the bulk of MgNi alloy particles prepared by mechanical alloying (MA) were uniform composition with MgNi = 1 and a nickel-rich surface, while in the bulk of the alloy particles prepared by induction melting (IM) the MgNi ratio was not uniform. Moreover, it was confirmed by transmission electron microscopy (TEM) and electron diffractometry that the MgNi alloys prepared by MA and IM had amorphous and crystalline structures, respectively. This was believed to be responsible for the difference of the discharge capacities between two kinds of MgNi alloys.

Electrodeposition of NiP alloys from Ni-citrate bath

Abstract In the electrodeposition of NiP alloy from a Ni-citrate bath, changes in P content and current efficiency of NiP deposits with plating variables such as H2PHO3 concentration, current density, bath pH and temperature were investigated. P content of the deposits was found to be constant around 25 at.% in a wide range of plating conditions. It was concluded that NiP alloy electrodeposition is controlled by Ni deposition rate and by the strong interaction between P and Ni, and that NiP alloy is formed through a direct deposition mechanism.

Effect of stoichiometric ratio on discharge efficiency of hydrogen storage alloy electrodes

Abstract Discharge efficiencies were evaluated for nonstoichiometric and stoichiometric Mm(B5), (B5 = Ni3.6Mn0.4Al0.3Co0.7, x = 0.88, 0.96, 1.00, 1.12) alloys. The discharge efficiency decreased with an increase in activation energy for hydrogen diffusion in the alloys. X-ray diffraction data showed the appearance of Ce2Ni7-type second phase, which formed a very stable hydride, for the Mm(B5)0.88 alloy and Ni3Al-type one, which had high catalytic activity, for the Mm(B5)1.12 alloy, respectively. The Mm(B5), alloys with larger x values or higher electrocatalytic activity showed higher discharge efficiency. It was therefore considered that not only hydrogen diffusion in the alloys but also charge transfer at the alloy/solution interface are responsible for the improvement of the discharge efficiency.

Surface modification of MgNi alloy with graphite by ball-milling for use in nickel-metal hydride batteries

Both discharge capacity and charge–discharge cycle life of MgNi negative electrodes prepared by mechanical alloying (MA) are greatly improved by surface modification with graphite using a planetary mill.

Kinetics of the Hydrogen Evolution Reaction at Various Zr‐Based Loves‐Phase Hydrogen‐Storage Alloy Electrodes

The kinetics of the hydrogen evolution reaction at various Zr-based laves-phase hydrogen-storage alloy electrodes was investigated by measuring an overpotential-decay curve immediately after the interruption of an applied constant current. The measured overpotential was divided into two partial overpotentials corresponding to the elementary reactions, i.e., the Volmer (discharge) and Tafel (combination) reactions. The ratio of exchange current density for the Volmer (discharge) reaction to that for the Tafel (combination) reaction increased with increasing unit-cell volume of the hydrogen-storage alloys used, suggesting that the Tafel (combination) reaction is effectively suppressed.

Kinetics of the hydrogen evolution reaction in stoichiometric and non-stoichiometric hydrogen storage alloys for nickel-hydrogen batteries

Abstract The kinetics of the hydrogen evolution reaction in stoichiometric and non-stoichiometric hydrogen storage alloys with compositions of Mm(Ni 3.6 Mn 0.4 Al 0.3 Co 0.7 ) x (Mm = misch metal, 0.88 ⩽ x ⩽ 1.12) were investigated by measuring the overvoltage decay immediately after the interruption of an applied cathodic current. The total overvoltage η for hydrogen evolution was divided into two components (η 1 and η 2 ) corresponding to the elementary reactions. The mechanism of the hydrogen evolution reaction was analyzed on the basis of the dependence of η 1 and η 2 on current density. The effect of the stoichiometric ratio on the performance of the alloys as a negative electrode material in nickel-hydrogen batteries was discussed from the standpoint of relative velocity of the elementary reactions.