The nickel/metal hydride battery operates in concentrated KOH (potassium hydroxide) electrolyte. The electrode reactions in a nickel/metal hydride battery are as follows:
Cathode (+): NiOOH + H2O + e- Ni(OH)2 + OH- (1)
Anode (-): (1/x) MHx + OH- (1/x) M + H2O + e- (2)
Overall: (1/x) MHx + NiOOH (1/x) M + Ni(OH)2 (3)
The KOH electrolyte can only transport the OH- ions and, to balance the charge transport, electrons must circulate through the external load. The nickel oxy-hydroxide electrode (equation 1) has been extensively researched and characterized, and its application has been widely demonstrated for both terrestrial and aerospace applications. Most of the current research in Ni/Metal Hydride batteries has involved improving the performance of the metal hydride anode. Specifically, this requires the development of a hydride electrode with the following characteristics: (1) long cycle life, (2) high capacity, (3) high rate of charge and discharge at a constant voltage, and (4) retention capacity.
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