 Profile Ni-MH
BatteryElectrochemical
Processes |
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|
ELECTROCHEMICAL
PROCESSES |
|
 Ni-MH
BATTERY |
|
 Specification
tables |
1.Cell
Fundamentals |
| Overview |
The
GREENCELL nickel-metal hydride cell chemistry is a hybrid of the
proven positive
electrode chemistry
of the sealed nickel-cadmium cell with the energy storage
features of metal alloys developed
for advanced hydrogen energy storage concepts.
This heritage in a positive-limited
cell design results in batteries providing enhanced
capacities while retaining the well-characterized
electrical and physical design
features of the sealed nickel-cadmium cell design.
|
| Features |
|
| Comparison
of Ni-MH and Ni-Cd Cells |
|
| Major
applications |
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| Structural
designs |
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| Electrochemical
processes |
|
| Discharge
characteristics |
2.Principle of Electrochemistry |
| Charge
characteristics |
The
electrochemistry of the GREENCELL nickel-metal hydride cell is generally
represented
by the
following charge and discharge reactions: |
| Charging
methods |
|
| Cycle
life characteristics |
Charge |
| Storage
characteristics |
At
the negative electrode, in the presence of the alloy and with an
electrical potential applied,
the water in the electrolyte is decomposed into hydrogen atoms,
which are absorbed
into the alloy, and hydroxyl ions as indicated below: |
| Safety
characteristics |
|
| Designing
for Ni-MH cells |
|
| Battey
pack designs |
Alloy
+ H2O + e=Alloy (H) + OH- |
| Battery
pack configurations designation system |
At
the positive electrode, the charge reaction is based on the oxidation
of nickelhydroxide
just as it is in the nickel-cadmium couple: |
| Precautions
for using Ni-MH batteries |
|
| Battery
selection |
Ni(OH)2
+ OH-=NiOOH + H2O + e |
| Discharge | |
At
the negative electrode, the hydrogen is desorbed and combines with
a hydroxyl ion
to form
water while also contributing an electron to the circuit: |
|
Alloy
(H) + OH- =Alloy + H2O + e |
|
At the positive electrode, nickel oxyhydroxide is reduced to its
lower valence state, nickel
hydroxide: |
|
NiOOH
+ H2O + e=Ni(OH)2 +
OH- |
|
 |
|
Fig.6
Schematic Discharge of GREENCELL Ni-MH battery |
|
Overcharge: |
|
Sealed
nickel-metal hydride cells have a negative electrode with an excess
of activematerial.
on charge, the positive electrode will thus first become charged
and will evolve
oxygen
before the negative electrode has reached a fully charged state:At
the positive electrode: |
|
OH-
=1/4O2 + 1/2H2O + e |
|
The
oxygen passes through the porous separator and is reduced at the
negative electrode
according
to the following reaction: |
|
1/4O2 + 1/2H2O + e =OH-
|
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This
is called the recombination reaction and makes the sealed nickel-metal
hydride cell
feasible. |
|
| Principle of Sealed cell: | |
When
the battery is being charged, the positive electrode becomes fully
charged first due
to its
small capacity. After this, overcharging occurs the reaction shown
in reaction
Formula-1 occurs
which produces oxygen gas. |
|
OH- =1/2H2O
+ 1/4O2 + e --------Formula-1 |
|
However,
at this point the negative electrode is not yet fully charged, and
therefore, intheory,
hydrogen gas does not form. The oxygen gas formed at the positive
electrode passes
though the separator, is diffuses into the negative electrode, and
cause the formation
o water by oxidizing the hydrogen in the hydrogen absorbing alloy
which is being
charged
(Reaction Formula-2) |
|
4MH + O2= 4M + 2H2O
--------Formula-2 |
|
The
water formed in reaction formula -2 is consumed by the normal charging
reaction (Reaction
Formula -3) |
|
Charging:
M + H2O + e = MH + OH-------Formula-3 |
|
Apart
from the oxygen consuming reaction of reaction formula ¢Ú, oxygen
gas is also
consumed by the electrochemical reaction (Reaction Formula -4) |
|
O2 + 2H2O + 4e ¡ú 4OH-------Formula-4 |
|
In
this way, the oxygen gas formed at the positive electrode is consumed
at the negative
electrode,
making it possible to seal the Ni-MH battery. |
|
| Cell components | |
GREENCELL
Nickel-metal hydride cells, with the exception of the negative
electrode,
use the
same general types of components as the sealed nickel-cadmium cell.
|
|
| Negative Electrode | |
The
basic concept of the nickel-metal hydride cell negative electrode
emanated from research
on the storage of hydrogen for use as an alternative energy source
in the 1970s.
Certain
metallic alloys were observed to form hydrides that could capture
(and release) hydrogen in volumes up to nearly a thousand times their own volume. By careful selection of the alloy constituents and proportions, the thermodynamics could be balanced to permit the absorption and release process to proceed at room temperatures and pressures. The general result is shown schematically in Figure 7 where the much smaller hydrogen atom is shown absorbed into the interstices of a bimetallic alloy crystal structure. |
|
 |
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Figure
7.Schematic of Metal-Alloy Crystal Structure Within Nickel-Metal Hydride Negative Electrode |
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Two
general classes of metallic alloys have been identified as possessing
characteristics
desirable for battery cell use. These are rare earth/nickel alloys
generally
based around
LaNi5 (the so-called AB5
class of alloys) and alloys consisting primarily of titanium and zirconium (designated as AB2 alloys). In both cases, some fraction of the base metals is often replaced with other metallic elements. The AB5 formulation appears to offer the best set of features for commercial nickel-metal hydride cell applications.The metal hydride electrode has a theoretical capacity approximately 40 percent higher than the cadmium electrode in a nickel-cadmium couple. As a result, nickel-metal hydride cells provide energy densities that are 20-40 percent higher than the equivalent nickel-cadmium cell. |
|
Positive
Electrode |
|
The
nickel-metal hydride positive electrode design draws heavily on
experience withnickel-cadmium
electrodes. Electrodes that are economical and rugged exhibiting
excellent
high-rate
performance, long cycle life, and good capacity include pasted and
sintered-type positive
electrodes.The
balance between the positive and negative electrodes is adjusted
so that the cell is
always positive-limited as illustrated in Figure 8. This means that
the negative electrode
possesses
a greater capacity than the positive. The positive will reach full
capacity
first as the
cell is charged. It then will generate oxygen gas that diffuses
to the
negative electrode where
it is recombined. This oxygen cycle is a highly efficient way
of handling moderate overcharge
currents. |
|
 |
|
Figure8.Relative
Electrode Balances for Nickel-Metal Hydride Cell During Discharge/Charge/Overcharge. |
|
| Electrolyte | |
The
electrolyte used in the nickel-metal hydride cell is alkaline, a
dilute solution of potassium
hydroxide containing other minor constituents to enhance cell performance. |
|
| Separator | |
The
baseline material for the separator, which provides electrical isolation
between the
electrodes
while still allowing efficient ionic diffusion between them, is
a nylon blend
similar to
that currently used in many nickel-cadmium cells. |
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