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March 16, 2019

Nickel Metal Hydride (NiMH) Chemistry FAQ

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Nickel/Metal Hydride (NiMH) Cells

Anode: Rare-earth or nickel alloys with many metals

Cathode: Nickel oxyhydroxide

Electrolyte: Potassium hydroxide

Applications: Cellular phones, camcorders, emergency backup lighting, power tools, laptops, portable, electric vehicles

This sealed cell is a hybrid of the NiCd and NiH2 cells. Previously, this battery was not available for commercial use because, although hydrogen has wonderful anodic qualities, it requires cell pressurization. Fortunately, in the late 1960s scientists discovered that some metal alloys (hydrides such as LiNi5 or ZrNi2) could store hydrogen atoms, which then could participate in reversible 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:

Location Reactions Voltage
Anode MH + OH- —> M + H2O + e- 0.83
Cathode NiOOH + H2O + e- —> Ni(OH)2 + OH- 0.52
Overall NiOOH + MH —> Ni(OH)2 + M 1.35

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

A very interesting fact about these alloys is that some metals absorb heat when absorbiong hydrogen, and some give off heat when absorbing hydrogen. Both of these are bad for a battery, since we would like the hydregen to move easily in and out without any energy transfer. The successful alloys are all mixtures of exothermic and endothermic metals to achieve this.

Hydrogen Storage Metals Comparison:

Material Density H2 Storage Capacity
LaNi5 8.3 0.11 g/cc
FeTi 6.2 0.11
Mg2Ni 4.1 0.15
Mg 1.74 0.13
MgNi Eutectic 2.54 0.16
liquid H2 0.07 0.07

Please notice 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, of which LaNi5 is a representative. 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 Molar KOH or potassium hydroxide

The NiMH cell does cost more and has half the service life of the NiCd cell, but it also has 30% more capacity, increased power density (theoretically 50% more, practically 25% more). The memory effect, which was at one time thought to be absent from NiMH cells, is present if the cells are treated just right. To avoid the memory effect fully discharge once every 30 or so cycles. There is no clear winner between the two. The better battery depends on what characteristics are more crucial for a specific application.

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