Analysis of the Activation Mechanism of Hydrogen Storage Alloy Negative Electrodes Containing Palladium Ion-Adsorbed Carbon Powder

Shinya MORISHITA*, Katsuyoshi FUJITA, Kazuhiko ITOH, Shin-ichi TOWATA and Katsushi ABE

Toyota Central Research and Development Laboratories, Inc.; 41-1 Yokomichi, Nagakute, Aichi 480-1192 Japan
Toyoda Automatic Loom Works, Ltd.; 2-1 Toyoda-cho, Kariya-shi 448-8671 Japan

In the case of Ni-MH batteries, the initial activation of the hydrogen storage alloy negative electrodes was promoted by adding palladium ion-adsorbed carbon powder (Pd2+/C). From electrochemical impedance analysis of the negative electrodes, and TEM observation and XRD measurement of Pd2+/C after cathodic polarization, the mechanism of this phenomenon was presumed to be as follows:
1)  Palladium ion is reduced to fine palladium particles during the first charge.
2)  The fine palladium particles act as active sites for charge/discharge reactions.
3)  Pulverization of the alloy particles occurs due to volume expansion by the hydrogen absorption into the alloy, and new active sites are formed on the fresh alloy surface.
4)  Therefore, the promotion of the initial activation of the negative electrode is observed by addition of Pd2+/C.

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