Negative electrodes for electric cells

The invention relates to a negative electrode for an alkaline electrolyte electric cell. The main electrode material is nickel lanthanide and is characterized by the fact that it also includes a mercury compound. It is applicable to secondary electric cells, in particular of the nickel- or silver-hydrogen type. Cells embodying such negative electrodes exhibit improved capacity irrespective of temperature and electrolyte concentration conditions as compared with like cells in which the mercury compound is not used with the nickel lanthanide.

RELATED APPLICATIONS 
No related applications of applicant are copending. 
BACKGROUND AND BRIEF SUMMARY OF INVENTION 
The present invention relates to electrodes whose main material is 
lanthanum and nickel alloys, which can be called nickel lanthanide, and to 
their use. It is known that nickel lanthanides whose composition is close 
to the formula La Ni.sub.5 fix hydrogen, forming hydrides which can then 
give up their hydrogen by a reversible reaction. 
It has been proposed to use these nickel lanthanides in alkaline secondary 
cells of the nickel or silver-hydrogen type for example, in order to lower 
the hydrogen pressure during operation. It has been observed that 
electrodes which are cathodically polarized and hence charged, to several 
times their theoretical capacity (counting six or seven atoms of hydrogen 
for the formula La Ni.sub.5), yielded a capacity of about 92% of the 
theoretical capacity at 0.degree. C. and in concentrated electrolyte (12M 
potassium hydroxide); but it has been observed that this efficiency fell 
to 83% at ambient temperature and became only 50 to 60% in 5M potassium 
hydroxide. 
Preferred embodiments of the present invention are intended to improve the 
capacity of such electrodes whatever the temperature and electrolyte 
concentration conditions. 
The present invention provides an electrode whose main material is nickel 
lanthanide for use in an alkaline electrolyte, to which material a mercury 
compound is added. 
Preferably, the compound is mercury oxide HgO and the weight of HgO added 
to the lanthanide is equal aproximately to 2% of the weight of the 
lanthanide. 
The invention will be better understood by referring to the following 
detailed descriptive example of the electrode embodying the invention and 
its use.

DETAILED DESCRIPTION 
An electrode was made from a powder contained from an alloy having a 
composition close to La Ni.sub.5 by subjecting the alloy to several 
successive hydrogenation cycles. The resultant powder was then mixed at a 
temperature of approximately 60.degree. C. with 2% by weight of 
polytetrafluoroethylene. A first series of perforated sheet metal pockets 
was filled with this material to be used as comparison electrodes of the 
prior art (about 1g of La Ni.sub.5 per squ. cm. of pocket). 
Mercury oxide HgO was then added in a proportion of 2% of the weight of La 
Ni.sub.5 to what remained of the above described 
polytetrafluoroethylene-bound mass of La Ni.sub.5, and a second series of 
like-dimensioned perforated sheet metal pockets was filled in the same way 
as for the first series. This second series of HgO containing La Ni.sub.5 
filled pockets comprise electrodes embodying the present invention. 
The two series of electrodes were then tested respectively in cells where 
they were opposed respectively to a nickel counter-electrode immersed in 
aqueous solutions of potassium hydroxide. Two potassium hydroxide 
concentrations were used for respective different cells of each series, 
namely 5M and 12M. The potential of the pockets was referenced in relation 
to an Hg/HgO reference electrode. 
All these first and second series electrodes were successively charged 
(i.e., negatively polarized in relation to the nickel counter-electrode) 
for 15 hrs. at C/5 A (C being the theoretical capacity of an electrode in 
Ah on the basis of 300 Ah/Kg), and then discharged (i.e., positively 
polarized in relation to the nickel counter-electrode) at C/5 A. 
The results obtained are gathered in the table herebelow, giving the ratio 
of the discharged capacity to the theoretical capacity of 300 Ah/Kg. 
______________________________________ 
Efficiency 
% 
Electrolyte Second series 
Concentration 
Temperature 
First series 
(Embodying 
KOH C. (Prior Art) 
the invention) 
______________________________________ 
5M 21.degree. C 
53.4% 79.1% 
12M 21.degree. C 
82.5% 92.3% 
5M 0.degree. C 
91.3% 100.7% 
______________________________________ 
The 12M potassium hydroxide concentration is too high for yielding results 
at 0.degree. C. that are of any value, the solution then being near to 
saturation. 
These tabular results clearly indicate the material advantages obtained 
with the electrodes embodying the invention. Indeed, it is seen that at 
ambient temperature, e.g., 21.degree. C., the efficiency of the electrodes 
according to the invention used with a diluted alkaline electrolyte 5M is 
practically as good or even better than the efficiency of the electrodes 
(first series) of the prior art in a concentrated electrolyte also at 
21.degree. C. 
In all the conditions tested the efficiency of the electrodes embodying the 
invention was materially higher than that of the electrodes of the prior 
art under the same conditions of temperature and electrolyte 
concentration. 
It must be understood that the invention is not limited to the embodiment 
which has just been described. Variations within the scope of the appended 
claims are possible and are contemplated. There is no intention of 
limitation to the exact disclosure herein presented.