Nickel Metal Hydride (NiMH) Cells

Anode : Rare-earth or nickel alloys with many metals

Cathode : Nickeloxyhydroxide

Electrolyte : Potassium hydroxide

This cell is a hybrid of the NiCd and NiH2 cells. Hydrogen has good anodic qualities but required cell pressurization until the 1960s. The discovery that some metal alloys (hydrides such as LiNi5 or ZrNi2) could store hydrogen atoms solved this problem. These metals could participate irreversible chemical reactions. In modern NiMH batteries, the anode consists of many metals alloys, including V, Ti, Zr, Ni, Cr, Co, and Fe.

Except for the anode, the NiMH cell very closely resembles the NiCd cell in construction. Even the voltage is virtually identical, at 1.2 volts, making the cells interchangeable in many applications. Here are the cell reactions:

Anodes used in these cells are complex alloys containing many metals, such as an alloy of V, Ti, Zr, Ni, Cr, Co, Fe. The chemistry of these alloys and the reason for superior performance are not clearly understood. The compositions are determined by empirical testing methods.

Some metals give off heat while others absorb heat when absorbing hydrogen. Both of these are undesirable, since hydrogen needs to move easily in and out without any energy transfer. Good alloys are all mixtures of exothermic and endothermic metals to reduce any heat transfer.

Hydrogen Storage Metal Chart :

Note that the density of hydrogen stored in a metal hydride is higher than that of pure liquid hydrogen. Commercial NiMH batteries are mostly of the rare earth-nickel type, such as LaNi5. These alloys can store six hydrogen atoms per unit cell such as LaNi5H6. Even misch metal nickel alloys are used to save the cost of separation.

The electrolyte of commercial NiMH batteries is typically 6 M KOH. The NiMH cell cost more and has half the service life of the NiCd cell. It also has 30% more capacity and increased power density (theoretically50% more, practically 25% more). To reduce any so called memory effect, fully discharge once every 30 or so cycles. NiCD and NiMH rechargeable batteries each has its pros and cons depending on the use.