The nickel-oxide electrode common to the nickel hydrogen and nickel cadmium cell is by design the limiting or capacity determining electrode on both charge and discharge. The useable discharge capacity from this electrode, and since it is the limiting electrode, the useable discharge capacity of the cell as well, can and is optimized by rate of charge, charge temperature and additives to electrode and electrolyte. Recent tests with nickel hydrogen cells and tests performed almost 25 years ago with nickel cadmium cells indicate an improvement of capacity as a result of using increased electrolyte concentration

Ritterman, P. F.

English

NASA Technical Reports Server

FACTORS AFFECTING NICKEL-OXIDE ELECTRODE 
CAPACITY IN NICKEL-HYDROGEN CELLS 
Paul F. Ritterman 
COMSAT 
The nickel-oxide electrode common to the nickel hydrogen and 
nickel cadmium cell is by design the limiting or capacity 
determining electrode on both charge and discharge. The 
useable discharge capacity from this electrode. and since it is 
the limiting electrode, the useable discharge capacity of the 
cell as well can and has been optimized by rate of charge, 
charge temperature and additives to electrode and electrolyte. 
Recent tests at COMSAT Labs with nickel hydrogen cells (Ref. 1) 
and tests performed almost 25 years ago with nickel cadmium 
cells (Ref. 2) indicate an improvement of capacity as a result 
of using increased electrolyte concentration. 
The capacity attainable from the nickel oxide electrode and 
hence from the nickel-hydrogen (and nickel cadmium) cells 
depend on how effectively and extensively the electrode can be 
charged. Full charge or the conversion of all active material 
to a higher valence state is extremely difficult to achieve 
under spacecraft flight conditions where there are rate, time, 
and temperature constraints on charge. The difficulty in 
charging the nickel-oxide electrode is due to the occurance of 
a competing parasitic reaction involving the evolution of 
oxygen at approximately the same potential as that of the 
charging reaction of the nickel-oxide electrode. The two 
reactions and their open circuit potentials as well as the 
relative effect of current density and the temperature on 
potentials are shown in Figure 1. 
At reversible (extremely low current densities) conditions a 
nickel hydrogen cell would never be charged because 81mv more 
voltage is required to charge the nickel-oxide electrode than 
to evolve oxygen in alkaline media. Because of the relative 
effect of temperature and charge current density on the 
potentials of these two reactions. it has been possible to 
combine these parameters so that the charge reaction 
predominates over the parasitic evolution of oxygen (Figure 1). 
Within the practical limits of cell charging; between 1 and 10 
ma/cm.2; the voltage-current density curves favor the 
charging of the nickel electrode (Figure 2). The 
https://ntrs.nasa.gov/search.jsp?R=19840025628 2020-03-22T10:17:53+00:00Z
temperature-voltage coefficient for the oxygen evolution and 
that of nickel oxide charging reaction are both negative. 
however, that of the oxygen evolution is more negative (Figure 
2). Therefore, a decrease in temperature would result in 
relative voltages favoring the changing.of the nickel 
electrode. The effect of decreasing temperature and increasing 
charging rate to improve the efficiency of nickel hydrogen and 
nickel cadmium cells is well known and has been empirically 
applied by users of these alkaline systems for many years. 
The thermodynamic effect of va~ying the KOH concentratiun is 
common to both the nickel electrode charging and the oxygen 
evolution reaction, nevertheless, a significant increase in 
available capacity has been noted as a result in increasing 
electrolyte concentration under constant condition of 
overcharge. (Ref. 1) 
Figure 3 shows capacities obtained with nickel hydrogen cells 
at 00 and 2806. The cells are identical except for their 
electrolyte concentrations. The capacities obtained at both 
temperatures are almost in linear proportion to electrolyte 
concentration. 
The effect sf increased electrolyte concentrations seems to be 
the same on the relative potentials of the nickel-oxide vs and 
parasitic oxygen evolution as that observed for decreasing 
temperature and increasing charge current. Thus there is an 
additional parameter which can be varied to create the most 
favorable conditions for nickel electrode cell charge 
acceptance, 
Figure 4 shows plots of nickel hydrogen cell pressure vs. 
charge input for the cells whose capacities are shown in 
Figure 3. When the positive electrodes in a nickel hydrogen 
cell evolve oxygen, hydsogen and oxygen react causing a 
decrease in the slope of pressure vs. Ah input. When all of 
the charging current is used for oxygen evolution, all hyd~ogen 
evolved at the negative electrode is consumed and the c e l l  
pressure remains constant as charge continues, 
As shown in Figure 4, the cells with the highest electrolytes 
concentration accept the most charge prior to 'pressure plateau 
indicating the greatest charge acceptance prior to predominant 
oxygen gassing. 
The effect of electrolyte concentration on cell and electrode 
capacities may also have frequently been demonstrated by the 
capacity d%ffer@nces reported for flooded electrode tests and 
for the same electrodes as part of electrolyte starved 
cells. Flooded capacities have generally exceeded cell 
capacities by about 1% to 20%. It is widely accepted that 
nickel oxide ehectrodes absorbs KOM from the electrolyte during 
charge and at the same time yields water to the electrolyte. 
The equations shown in Figure 5 are only one representation of 
what is believed to occur during the charge of a nickel oxide 
electrode (Ref. 3). COMSAT Lab has developed data for nickel 
hydrogen cells which indicate a PO% decrease in electrolyte 
concentration when a cell is taken from complete discharge to 
full charge (Ref. 4). 
The extent of decrease in electrolyte concentration as a result 
of charge is a function of ampere hours charged per unit volume 
of electrolyte. In a flooded condition the ampere hours of 
charge per unit volume of electrolyte is considerably saaller 
than that of a cell, therefore the decrease in KOH 
concentration will also be considerably less for electrodes 
charged in flooded electrolyte conditions than in case of the 
same electrodes contained in an electrolyte starved cell. 
Therefore, the electrodes in a flooded condition will accept 
more charge prior to gassing oxygen than the same electrode 
charged in a limited electrolyte volume cell as a result of the 
greater decrease in electrolyte concentration in the case of a 
cell. 
The charge acceptance of a cell can be affected by both 
electrolyte concentration and volume. Electrolyte can 
therefore be considered a variable for improving charge 
efficiency and utilization in a NI-NZ cell and adds another 
dimension to charge management of these cells and batteries. 
By appropriate increase of electrolyte concentration and/or 
volume, a lower charge current and/or higher temperature, can 
be used resulting in smaller batteries and smaller charge 
arrays. This will impact weight and cost of a spacecraft. 
With respect to the effect of electrolyte concentration on 
life, data exists for geosynchronous orbit application, which 
indicates no adverse effects on battery life in the range of 
electrolyte concentrations investigated. 
References: (1) M. Earl - COMSAT Labosatories Technical Note 
PSD-83-029. 
(2) L. Belove and I. M. Schulman - Proc. Ann 
Power Sources Conference 13, 82 1959. 
(3) Bourgault and Conway - Can. J. Chem 38, 1557 
(1960). 
(4) J. F .  Stockeb - COMSAT Laboratories Technical 
Memo PSD-83-002. 


8 .rl 2 k C, 8 8 a, C, 3, d k0 C, 0 a, d a, !i h C, .rl & 0 t-l 1 5 1 .: t-l d 2 m $ $2 

THE CONCENTRATION OF KOH DECREASES AS THE STATE OF CHARGE OF A 
CELL INCREASES; AS A RESULT OF KOH ABSORBED BY THE CHARGED 
POSITIVE, AND H20 RELEASED FROM THE DISCHARGED POSlTlVE ACTIVE 
MATERIAL. 
2 Ni(OH)2 1.28 H20 + 0.14 KOH + 2 OH- -- 2 NiOOH.0.14 KOH + 
4.56 H20 + 2f- 
Figure 5. KOH concantration vs. s t a t e  of charge. 
A. Thaller, NASA/Lewis Research Center: I take i t  Paul that  the benefits 
are  related to  using higher concentrations of KOH rather than using 
nickel hydrogen? 
Q. Ritterman, Comsat: Beg your pardon? 
Q. Thaller, NASA/Lewis Research Center: The benefits were they related 
to  using higher concentrations of KOH or benefits of using nickel 
hydrogen? 
Q. R i  tterman, Comsat: I don't  know why you are  comparing concentration 
with system you can combine these. You can use these in both nickel 
hydrogen and nickel cadmium b u t  I am talking primarily about nickel 
hydrogen because you have greater flexibi 1 i t y  as f a r  as current 
density and temperature goes. You are  not limited by cadmium in 
nickel hydrogen. 
Q. Green, RCA-Astro: Paul, would the l i f e  expectancy of the nickel 
hydrogen battery increase with the KOH concentration that  you've 
described? 
A. Ritterman, Comsat: Yes. Okay. There have been ce l l s  that  have r u n  
for  accelerated l i f e  t e s t  of 30 seasons a t  TRW with an end of dis- 
charge KOH concentration in excess of 38%. So within the range tha t  
I 've  talked about there has been reported no other problems in 
relat ion t o  l i f e  expectancy. 
Q, Green, RCA-Astro: No problem? 
A. Ri tterman, Comsat: No problem. 
Q. Milden, Aerospace Corporation: The choice of the 31 or 32% 
electrolytes  was over the years an optimum or a trade off between 
voltage character is t ics  and everything e lse ,  how would you suspect 
or expect the voltage on charge and discharge to  vary with the 
increased concentration and also the effects  of swelling and a l l  
other kinds of good s tuff  l ike  that  with a varied concentration. 
In other words, i s  t h i s  a l l  good or  i s  there a trade-off? 
A. Ritterman, Comsat: So f a r  i t  looks l ike  a l l  good. I guess some 
more s ~ e c i f i c  t e s t  would have to  be done to  determine the e f fec t  
on sweil ing. B u t  I think 1 i f e  data shows that  you can sustain 
certainly a t  the end of discharge with a concentration as high as 
40%. 
Q. Unidentifiedt: What I'm saying Paul, when the current was higher 
the difference between the end of discharge voltages was higher? 
Q. Ritterman, Comsat: Yeah. Was i t  a1 1 the way through on the discharge? 
A. Unident i f ied:  It was more pronounced toward the end. At  the very 
beginning they were a 1 i t t l e  c loser  bu t  they spread ou t  as the 
discharge went on. 
Q. Un ident i f ied:  For the same amp output? 
A. Ritterman, Comsat: I can ' t  answer that .  
COMMENT 
Unident i f ied:  This i s  a comment. There are two po in ts  coming ou t  o f  
t h i s  session. One po in t  i s  when you are t a l k i n g  about n icke l  cadmium 
o r  n icke l  hydrogen there are s i g i f i c a n t  changes i n  the e l e c t r o l y t e  
concentrat ion between charge and discharge. There are a lso 
s i g n i f i c a n t  d i f ferences i n  the va r i a t i on  i n  the KOH concentrat ion o f  
one t o  the other because the react ion t ha t ' s  tak ing place dur ing t h a t  
t ime of discharge. The other po in t  i s  t h a t  there are  d i f f e r e n t  n icke l  
hydrogen systems i n  operation using d i f f e r e n t  l eve l s  o f  KOH 
concentration. And one other t h i ng  t ha t  Paul has pointed ou t  very 
n i c e l y  i n  h i s  paper i s  t ha t  there i s  a s i g n i f i c a n t  impact on c e l l s  
performance as a resu l  t o f  KOH concentration. 
Ritterman, Comsat: Yes I guess I d i d n ' t  make my po in t  c lea r  about 
$ icke l  cadmium and n icke l  hydrogen. I n  n icke l  hydrogen you r e a l l y  
have no net  change i n  hydroxide due t o  compound formulat ion but  you 
do have a change i n  absorption o f  water and KOH concentration. But 
1 d i d n ' t  want t o  l i m i t  t h i s  t o  n icke l  hydrogen I th ink  i t ' s  
appl icable t o  n icke l  cadmium ce l l s .  
Hendee, Telesat Canada: Bas ica l l y  the comment you made was the one 
1 was qoinq t o  make. You have t o  be carefu l  about t r ans la t i ng  t h i s  
i n t o  n i cke l  cadmium as wel l .  One o f  the reasons f o r  the 31 t o  32% 
was the nylon separator was it not? The degradation o f  the nylon 
separator was shown t o  be the l e a s t  around a 31 o r  32% KOH 
concentration. I f  you can change t h a t  you have a new b a l l  game. 
Q. Rogers, Hughes A i r c r a f t  Corporation: Paul , I 'm wondering the t e s t  
data t h a t  you presented i s  t ha t  what we go t  a f t e r  bu i l d i ng  the c e l l s  
a t  Hughes o r  i s  add i t iona l  data which i s  the same bas i ca l l y  as what 
I remember we got? 
A. R i  tterman, Comsat: The i n i t i a l  data about the e f f e c t i v e  concentrat ion 
was found sometime p r i o r  t o  the Hughes tes t ing .  
A. R i  tterman, Comsat: Yes these c e l l s  are  48 amp per hour ce l l s .  


Factors Affecting Nickel-oxide Electrode Capacity in Nickel-hydrogen Cells

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