Page 254 - Battery Reference Book

19/20 Nickel batteries
Table 19.5 Nickel-hydrogen cell electrochemistry
Operation mode Electrochemistr?,

Normal
Charge
Nickel electrode Ni(OH)2 + OH- W NiOOH + H2O + e-
Discharge
Charge
aH2
Hydrogen H20 + e- e + OH-
Discharge
Charge
Net reaction Ni(0H)z ___\ NiOOH + 4H2
Discharge
Overcharge
Nickel electrode 20H- --f $02 + H2O + 2e-
Hydrogen electrode 2 HzO + 2e- .+ H2 + 20H-
Net reaction H2 + a02 .+ H2O + HEAT
Overdischarge (cell reversal)
Nickel electrode H20 + e- .+ $Hz +OH-
Hydrogen electrode OH- + bH2 .+ H2O + e-

&;ressure vessel Gas screen wick. Hence, electrolyte which leaves the stack dur-
ing cycling will be wicked back. The nickel electrode
Wall wick (not shown) consists of a sintered nickel powder plaque containing
a nickel screen substrate, which is electrochemically
Hydrogen impregnated with nickel hydroxide active material by
electrode the Pickett process. The gas screens are polypropylene.
Separator The electrolyte is a 31% aqueous solution of potassium
Nickel hydroxide.
stack electrode
Hydrogen gas 19.3 Nickel-zinc secondary batteries
These batteries are not yet available in a sealed form.
Figure 19.22 Air Force/Hughes individual pressure vessel This type of battery comprises a positive nickel elec-
nickel-hydrogen cells
trode of the sintered or pasted type similar to that used
in the nickel-cadmium battery.
configuration electrodes of different types directly face The zinc electrode is usually prepared from pow-
each other. The stack is packaged in a cylindrical dered zinc oxide, often mixed with a Teflon binder,
pressure vessel, with hemispherical end caps. The and pressed on to an expanded silver grid. The zinc
vessel is made of Inconel 718. It is lined with electrode is made somewhat oversized compared to
a zirconium oxide wall wick. The components are the nickel electrode and is contoured to minimize zinc
shaped in a ‘pineapple’ slice pattern and the electrodes anode shape changes during cycling of the cell. Typical
separator materials are fibrous sausage skin or cello-
are connected in parallel. phane and Perlon wrapping.
If a high bubble pressure separator, such as asbestos, Potassium hydroxide solutions between 30 and 40%
is used the oxygen generated at the nickel electrode by weight are the usual electrolytes for the cell. Cell
on charge is directed to the hydrogen electrode of constructions, prismatic and flat, are similar to those
the next unit cell, where it recombines chemically used for nickel-cadmium cells.
with hydrogen to form water. If a low bubble pres- The nickel-zinc battery has had a checkered history
sure separator, such as ZircarTM (ceramic fabric), is since its inception in 1887. This battery development
used, the oxygen can pass through the separator and has been plagued by poor cyclic life and is now being
recombine on the hydrogen electrode of the same unit surpassed in performance by new battery systems.
cell. This reaction is quite exothermic and care must The basic cell reaction is:
be taken to limit damage to the hydrogen electrode 2NiOOH + Zn + H20 + Ni(OH)2 + Zn(0H)z (19.21)
surface due to the heat of recombination. The separa-
tors extend beyond the electrodes to contact the wall 6-7 M potassium hydroxide is used as electrolyte.