In recent decades, nickel metal hydride rechargeable batteries have been widely used in automobiles and relatively large portable electronic devices. The positive electrode is nickel hydroxide, and the negative electrode is an intermetallic compound. The most common metal has the general form AB5, where A is a mixture of rare earth elements, lanthanum, cerium, neodymium, praseodymium and B is nickel, cobalt, manganese, and aluminum.
The electrochemistry is as follows. During discharging, at the positive electrode, NiOOH is reduced,
NiOOH + H2O + e – -^ Ni(OH)2 + OH-. (12.22)
At the negative electrode, metal hyride is oxidized,
MH + OH – -^ M + H2O + e-. (12.23)
During charging, at the positive electrode, Ni(OH)2 is oxidized,
Ni(OH)2 + OH – -^ NiOOH+H2O + e-. (12.24)
At the negative electrode, metal is reduced,
M + H2O + e – —> MH + OH-. (12.25)
When overcharged at low rates, the oxygen produced at the positive electrode passes through the separator and recombines at the surface of the negative. Hydrogen evolution is suppressed and the charging energy is converted to heat. This process allows NiMH cells to remain sealed in normal operation and to be maintenance free.
NiMH batteries have been applied extensively in electric automobiles, such as the General Motors EV1, Honda EV Plus, Ford Ranger EV, and Vectrix Scooter. Hybrid vehicles such as the Toyota Prius, Honda Insight, Ford Escape Hybrid, Chevrolet Malibu Hybrid, and Honda Civic Hybrid also use them. NiMH technology is used extensively in rechargeable batteries for consumer electronics.