The overall reversible chemical reaction for a Nickel metal hydride cell is
MH + NiOOH = M – Ni(OH)2
The process is reversed during charging. The active material in the positive electrode is nickel hydroxide which often has a conductive network composed of cobalt oxides and a current collector which commonly is a nickel foam skeleton, nickel fiber matrix or sintered filamentary nickel fibers.
NiOOH + H20 + e = Ni(OH)2 + OH-
The active ingredient in the negative electrode is an alloy capable of storing hydrogen with complex microstructures such as AB5 (LaCePrNdNiCoMnAl) or AB2 (VTiZrNiCrCoMnAlSn) where AB represents the ratio of the type A materials (LaCePrNd) or TiZr ) to that of the B type materials (VNiCrCoMnAlSn). The reaction at the negative electrode where the complex metallic microstructures are represented by M.
The metal hydroxide is oxidized to the metal alloy M.
MH + NiOOH = M + H20 + e
Nickel metal hydride batteries are currently the top contended for the battery of choice for EV and HEV applications due to its operating characteristics. These cells will operate at temperatures ranging from -30 to 65 deg C, have a specific energy of 63-75 Wh/kg, an energy density of 220 Wh/L and a power density of 850 W/L. The exhibit a cycle lifetime of 600 to 1200 cycles, safely produce voltages greater than 320 volts with flexible cell sizes from .3 to 250 Ah. The cost ranges from $220 to $400 per kWh.
