Charging/Discharging Stability of a Metal Hydride Battery Electrode

Abstract

The metal hydride (MH) alloy powder for the negative electrode of the Ni/MH battery was first pulverized and oxidized by electrochemically charging and discharging for a number of cycles. The plate of the negative electrode of an experimental cell in this study was made from a mixture of a multicomponent AB 5 -based alloy powder, nickel powder, and polytetrafluoroethylene (PTFE). The characteristics of the negative electrode, including discharge capacity, exchange current density, and hydrogen diffusivity, were studied by means of the electrochemical experiments and analysis in an experimental cell. The exchange current density of a Mm 0.95 Ti 0.05 Ni 3.85 Co 0.45 Mn 0.35 Al 0.35 alloy electrode increases with increasing number of charge/discharge cycles and then remains almost constant after 20 cycles. A microcracking activation, resulting from an increase in reaction surface area and an improvement in the electrode surface activation, increases the hydrogen exchange current densities. Measurement of hydrogen diffusivities for Mm 0.95 Ti 0.05 Ni 3.85 Co 0.45 Mn 0.35 Al 0.35 alloy powder shows that the ratio of D/a 2 (D ϭ hydrogen diffusivity; a ϭ sphere radius) increases with increasing number of cycles but remains constant after 20 cycles.

Effect of substitution of Mm for La on the electrochemical properties of the LaNi3.55Mn0.4Al0.3Co0.75 compound

“…(8), the value ofD a 2 can be evaluated from the slope of the plot of log(i) versus t. Taking into account that the hydrogen diffusion coefficientD is determined by the cyclic voltammetry described above, the radius of the particles participating in the electrochemical reaction, assumed as spherical, can be calculated from the value of the slopeD a 2 . the values of the mean diffusion coefficient of hydrogen calculated by the cyclic voltammetry and given in Table 2, the radius of the particles participating in the electrochemical reaction in the cases of the MmNi 3.55 Mn 0.4 Al 0.3 Co 0.75 and the LaNi 3.55 Mn 0.4 Al 0.3 Co 0.75 alloys are, respectively, 12 and 31 m. Before the cycling, the particle size was about 63 m. So, we assume that the decrease of the particle size is due to the microcracking of the electrode powder alloys, during the hydrogen absorption and desorption processes [6].…”

“…cycles, decreases to about 180 mAh/g after 50 cycles and then remains constant. The capacity decrease is mainly due to the surface layer oxidation [6] and the corrosion phenomena. The lanthanum-containing alloy has a higher capacity but it has a lesser stability than the Mm-containing alloy.…”

“…If the particles of the hydride alloy have a spherical form [6,7], the diffusion phenomena can be given by Eq. (5) as follows:…”

“…Different techniques have been proposed for the evaluation of the kinetic process in the hydrogen storage alloys. These have included the cyclic voltammetry [3,10,14], the chronoamperometry 0925 [6,7,11,[22][23][24] and the electrochemical impedance spectroscopy [2,10,12,[15][16][17][18][19][20][21]24] methods. For various AB 5 -type alloys, the variation of the kinetic parameters is due to the differences in the alloys composition, the alloying process and also the hydrogen concentration in the alloys [24].…”

Electrochemical properties of the MmNi3.55Mn0.4Al0.3Co0.4Fe0.35 compound

“…After that, the discharge capacity decreases linearly to about 200 mAh g −1 after 70 cycles. This decrease in discharge capacity is due to the deterioration of the negative electrode material with increasing the number of charge-discharge cycles [9,10]. In fact, during the cycling, rare earth elements such as La segregate to the grain boundaries, where they were subject to corrosion in the alkaline electrolyte.…”