Electrochemical oxidation of zinc, thermal decomposition of silver oxide, and solubility determinations of silver oxides in potassium hydroxide

Arcand, G. M.

English

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THIRD QUARTERLY REPORT 
THE REACTIONS PERTAINING TO ZINC-SILVER AND CADMIUM-SILVER BATTERIES 
J PL 95 1 887 
This work was performed for the Jet Propulsion Laboratory, California Institute of 
Technology, sponsored by the National Aeronautics and Space Administration under 
Contract NAS7-100. 
G. Myron Arcand 
Department of Chemistry 
Idaho State University 
Pocatel lo, Idaho 
- - .  
GPO PRICE $ 
December 15, 1967 CFSTI PRICE(S) $ 
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ABSTRACT. 
Preliminary experiments indicate that electrochemical oxidation of zinc in 
10 - VF KOH produces less than 10% Zn(OH)2 and more than 90% ZnO. The product 
i s  blue-gray while i n  the KOH but changes to white upon being washed with water. 
The rate of conversionofAg0 to A920 rapidly increases with increasing temp- 
erature. Fifty percent conversion occurs in 24 hours at 121°, 11 hours at 131°, 4 
hours at  141°, 2 hours at 151°, and 1 hour at 160'. 
on the method of preparation of the original material. 
The rate of conversion depends 
The solubility concentration of  Ag(l) in 10 - VF KOH saturated with ZnO i s  
about 4 x 
stirred system containing solid Ago and somewhere between 25 and 100 hours i n  an 
VF. This concentration is  reached in  less than a half-hour in a - 
unstirred 
less than 
provided 
system containing a A g o  electrode. The concentration of Ag(l> drops to 
1 x 
no solid A g o  is  present. 
- VF in 24 hours when a zinc plate i s  placed m the sotution, , 
1 
The objectives of the contract are three-fold: 
(1) 
(2) 
(3) 
The characterization of cadmium and zinc anodic reaction products. 
The study of the thermal decomposition of silver oxide. 
The study of the deposition of silver on zinc anodes. 
This report i s  divided into three sections corresponding to the objectives outlined 
above. 
A. THE PRODUCT OF THE ELECTROCHEMICAL OXIDATION OF ZINC 
Expe r imen tal 
The construction of the two-compartment cel l  was described in the previous 
report (1 ). The working-electrode compartment contained 2 hl of electrolyte solution 
which was 10 - VF in KOH saturated with ZnO and having a tritium activity of 
8.20 x 106 dpm/ml. The auxiliary-electrode compartment and the ''salt bridge" con- 
tained 10 - VF KOH. The auxiliary electrode was platinum wire. 
A constant, 30-ma current was passed through the system until the zinc was 
completely oxidized. 
dissolved in 1 ml of 5 - VF HNO3 and the solution diluted to 5.00 ml. 50-h.portions 
of the solution were pipetted into 15 ml of scintillating fluid and counted with a 
Nuclear-Chicago Model 703A liquid scintillation counting system. 
The cell was then dismantled and the zinc precipitate was 
A rectangular Lucite cell was constructed to hold a 20 mm x 20 mm zinc 
electrode and two A g o  electrodes. 
0.25 mm thick manufactured by Electronic Space Products, Inc., was wrapped in one 
layer of polypropylene and three layers of sausage casing. 
The zinc electrode, a sheet of 6N Grade metal 
The two Ago electrodes 
2 were each about 2 cm and were wrapped in five layers of sausage casing. 
plate was placed between the A@ plates and the electrodes were separated by Lucite 
spacers 1 mm thick. 
The zinc 
The cel l  was activated with 4 ml of the tritiated solutiondescribed 
i 
2 
above. The cel I was operated at 50 ma for 4 hours and was then dismantled and the 
oxidized product recovered and treated as described dbove. 
Results and Discussion 
Several experiments were made w i th  the two-compartment cell, but only one 
has been evaluated. The results of the experiment indicate that only about 1% of 
the product was Zn(OH)2. However, the data are suspect because some untritiated 
KOH solution probably drained into the working compartment in spite of the closed 
stopcock in the bridge. Since the calculation of the relative amount of Zn(OH)2 
formed depends on knowledge of the total H in the system, any unknown addition of 
electrolyte solution would cause an error. However, even if the volume of electrolyte 
were increased by 50% with inactive solution, the calculated amount o f  Zn(OH)2 
would be only 2%. 
The remaining runs made with the two-compartment cell w i l l  not be analyzed 
because of the uncertainty in electrolyte volume. The system is  being modified to 
minimize the drainage problem. 
Analysis of the results obtained from the run with the Lucite, single-compartment 
cell i s  not complete. Data available so far indicate an even lower activity in this 
precipitate than was found in the two-compartment system. 
that the Zn(OH)2 content in the anodic product i s  very smal I. 
In any event, i t  appears 
The physical appearance of the product i s  interesting. In both cells, the product 
was contaminated with a grayish or blue-gray material which turned white on washing 
with water. The product from the Lucite cell was entirely blue-gray with no indica- 
tion of the white which i s  characteristic of zinc oxide. Where the electrode was 
exposed to air, a white residue, possibly KOH or K2CO3, was formed. It isdif f icult  
to imagine that this material i s  finely divided zinc metal because there was too much 
of i t  present. Furthermore, no gas was evolved when i t  was washed as would be ex- 
pected i f  the metal were oxidized during that process. Still, the prospect of blue ZnO 
! 
3 
i s  not appealing. 
ProDosed Work 
Tracer experiments with zinc anodes w i l l  be continued and similar experiments 
with cadmium anodes w i l l  be initiated. An attempt w i l l  be made to identify the blue 
product described above. 
B. THERMAL DECOMPOSITION OF A g o  
ExDe r i  men ta I 
Constant-temperature thermograms of electrochemical ly-prepared A g o  (3) 
were continued as described i n  the previous report (1). Similar experiments at 
1600 were performed on commercial Ago supplied by M. Ames Chemical Works, 
Inc., Glen Falls, N. Y. The Ames material was used both as powder and in pellet 
form. Pellets were made in a Parr calorimetric press using a 1/4-inch diameter die. 
Increasing-temperature thermograms were run on both materials, the temperature 
being followed with the thermocouple which i s  mounted inside the hangdown tube near 
the sample. Ten-degree temperature intervals were recorded on the chart by means 
of an event marker. The heating rate was about 5q/per minute in al l  cases. 
Resu Its and Discuss ion 
Constant- tempera ture thermograms of e I ec trochem ical l y*prepared A g o  are 
The curves suggest that the rate of decomposition is  approxi- shown in Figure 1. 
mately doubled for each 100 increase in temperature. 
at a l l  temperatures investigated. 
The loss in 24 hours i s  serious 
Figure 2 shows the thermograms of electrochemically-prepared and of Ames 
Ago at 1600. Obviously, the Ames material decomposes much more rapidly than 
that prepared in  this laboratory. The reason for the difference is  not yet known. 
Possibly the Ames A g o  consists of smaller particles. That the state of aggregation 
4 
i s  relatively unimportant i s  indicated by the similarity of  the two Ames curves, one 
for.powder and one for a pellet. Ames is  being queried to learn the method of manu- 
facture of this material. 
Differences in behavior are seen also in the increasing-temperature thermograms 
shown in Figure 3. The Ames Ago shows sharper breaks and a "plateau" at a smaller 
relative loss than does the electrochemical ly-prepared compound. There is, again, 
l i t t le difference between the two Ames samples. The slight shift toward higher temp- 
eratures seen for the pellets i s  probably causedibya slower mechanical loss of oxygen from 
the compacted mass. This could account for the difference at  the constant 1600 for 
the Ames samples. 
The results shown here for the 1200 case compare favorably with those presented 
by Nagy, Moroz, and Casey (4). They report that pellets decompose about twice as 
fast as powder, but they formed their pellets at 70,000 psi. These authors suggest 
that the basic crystal size i s  the important factor i n  determining the rate of decompo- 
sition. This could account for the difference in  behavior between the electrochemically- 
prepared and Ames samples. 
Preliminary calculations indicate that the reaction mechanism i s  quite complex 
and may involve consecutive and/or concurrent reactions. An analysis by the method 
of Chatterjee (5) indicates an order of 0.7 for the init ial reaction, i.e. , near the 
beginning of the first rapid change in weight. 
yields 33 KCal/mole for the activation energy of this reaction and indicates that 
The method of  Coats and Redfern (6) 
another reaction becomes important around 1800. 
Proposed Work 
Further work w i l l  be done on chemically-prepared Ago and on A g o  taken from 
electrodes formed in KOH at 1600. If serious differences are observed, other temp- 
eratures w i l l  be used. Equipment i s  on hand to begin controlled-atmosphere 
experiments and these should start soon. Particle-size distributions wi l  I be determined 
5 
as soon as sedimentation equipment becomes available. 
C. SOLUTION OF A g o  AND A920 IN KOH 
Experimental 
The procedure for solubility determinations was described in the previous 
report (1). Solutions saturated w i th  both ZnO and A g o  but containing none of 
the solid were allowed to react with sheet zinc, the solution being analyzed for 
Ag(l) periodically. 
Results and Discussion 
The results of another determination of the solubility of A g o  in 10 VF KOH -
saturated with ZnO are shown in Table 1. The init ial  concentrations are com- 
parable, but the slow decrease noted previously (1) does not appear. In fact, the 
concentration of Ag(l) seems to remain constant over the eight days of  the experi- 
ment. It i s  probable that these results are more reliable than those found earlier 
because considerably greater care was exercised. I t  i s  evident from the data 
akcumulated thus far that the maximum concentration of Ag(l) in 10 VF KOH 
saturated with ZnO is 4 b o u t  4 x  VF and i s  reached within one-half hour 
of mixing the solid A g o  with the electrolyte solution. 
- 
Figure 4 of the last report (1) showed the change of concentration of Ag(l) 
wi th time WhenaAgOelectrode was placed in an unstirred 10 VF KOH solution 
saturated with ZnO. The concentration rose to about 4 x 
- 
VF in less than 
100 hours and stayed essentially constant for the next 60 hours. The experiment 
was continued for another 200 hours; no change in cencentration was observed. 
The concentration i s  comparable to those found in the experiments using powdered 
A@* 
The results of measurements made in the presence of zinc plates have not been 
satisfactory. Concentrations in the control solutions have been about 4 x VF, -
6 
I 
as expected, but the determinations in the systems containing solid zinc were not 
dependable. Rough )estimates indicate that the concentration of Ag( I )  drops to some- 
what less than 1 x 
the first hour or so. 
Proposed Work 
- VF within 24 hours with the most rapid change occurring in  
This work wi l l  be continued with a new method of analysis. 
The polarographic procedure used to analyze silver did not prove satisfactory 
for the systems involving reaction with solid zinc. A method using ''OrnAg w i l l  be 
tried for subsequent experiments. The rate of reaction of silver oxide with zinc in 
KOH solutions w i l l  be studied and the effects of  temperature and KOH concentra- 
tion w i l l  be investigated. 
behavior of the zinc electrode w i l l  be considered. 
The effect of silver deposition on the electrochemical 
7 
Table 1 
Solubility of Ago .in 10 VF KOH Saturated with ZnO -
Time (hours) 
0.25 3.7 
1 48 3.8 
1 74 3.7 
192 3.6 
8 
- 
I 
0 0 0 0 0 
cc 
0 
9 
I I I I I 
0 - N c) 
10 
n 
U 
0.1 
0.2 
0.3 
0.4 
0.5 
0.6 
0.7 
0.8 
0.9 
1 .o I 
1 00 1 40 180 220 260 300 340 380 420 
Time (hours) 
Figure 3. Thermograms of A g o  at AT Rate of 5'/min. 
11 
RE FE RE N C  ES 
(1) G. M. Arcand, "The Reactions Pertaining to Zinc-Silver and Cadmium- 
Silver Batteries'', September 15, 1 9 6 7 ~  Second Quarterly Report on. 
Contract JPL 951887. 
G. M. Arcand, "The Reactions Pertaining to Zinc-Silver and Cadmium- 
Silver Batteries", April 1, 1967, Final Report on Contract JPL 951458. 
W. L. Jolly, "Synthetic Inorganic Chemistry", Prentice-Hall, Inc., 
Englewood Cliffs, N. J., 1960, pp. 148-9. 
G. D. Nagy, W. J. Moroz, and E. J. Casey, Proc. Ann. Power Sources 
Conf., - 19, 80 (1965). 
P. K. Chatterjee, J. Polymer Sci. - 3, 4253(1965). 
A. W. Coats and J. P. Redfern, Nature - 201~. 68(1964). 
(2) 
(3) 
(4) 
(5) 
(6) 


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