|
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. |
