Cells overcharge reactions

The overcharge reactions for the cell are the same as for nickel—cadmium and nickel—hydrogen cells. The oxygen generated on the nickel electrode at the end of charge and overcharge finds its way to the anode and reacts to form water in the Ni—H2 case and Cd(OH)2 in the Ni—Cd case. 

The cell reactions are shown in Fig. 8.21, from which it can be seen that overdischarge and overcharge reactions are possible. Use is made of the overdischarge reaction to allow the assembly of the cell without the addition of elemental sodium. To the products of the discharge reaction - nickel and sodium chloride - is added a small quantity of aluminium which on the first charge generates enough excess sodium to prime the sodium electrode ... 

Fig. 9.15. Representation of primary charge-discharge processes and overcharge reactions in VRLA cell. Vertical arrow just to the left of j-axis denotes potential for oxygen reduction to water.

Figures 10.1 and 10.2 compare the 12-V overcharge reaction divided into four regions, and provide a diagram of the factors affecting the increase in the cell temperature during a portion of the overcharge tests. Regions III and IV should be...

During cell overcharge, a part of the irreversible energy 7a F and - t7c 1 rn y be used to overcome the activation energy of anode and cathode degradation, respectively, and of electrolyte anodic oxidation and cathodic reduction reactions, which account for cell selfdischarge and aging [11,21-24],... 

Overcharge reaction. The charge capacity of the ZEBRA cell is determined by the quantity of salt (NaCl) available in the cathode. In case a cell is fully charged and the charge voltage continues to be applied to the cell for whatever reasons, the liquid salt NaAlCU supplies a sodium reserve following the reversible reaction... 

Barrier by the chemistry. In case of severe mechanical damage of the battery the brittle ceramic breaks, whereas the cell case made out of steel is deformed and most likely remains closed. In any case the liquid electrolyte reacts with the liquid sodium to form salt and aluminum equal to the overcharge reaction described above. These reaction products form a layer covering the NiCl2 cathode and thus passivate it. This reaction reduces the thermal load by about 1 /3 compared to the total electrochemically stored energy. 

The result in a sealed cell is that electricity is converted into heat without any net chemical change in the cell. The overcharge reaction is exothermic, particularly the chemical recombination reaction in the sealed cell. 

This overcharge reaction consumes water and thereby decreases the level of electrolyte in the cell. The water loss can be limited by controlling the amount of overcharge so as to maximize the interval between needed water replenishments. 

When these half-reactions are summed, there is no net reaction. Thus the material balance of the cell is not altered by overcharge. At open circuit, equation 19 at the negative electrode is the sum of a two-step process, represented by equation 15 and... 

These equations are based on the thermodynamically stable species. Further research is needed to clarify the actual intermediate formed during overcharge. In reahty, the oxygen cycle can not be fully balanced because of other side reactions, that include gtid corrosion, formation of residual lead oxides in the positive electrode, and oxidation of organic materials in the cell. As a result, some gases, primarily hydrogen and carbon dioxide (53), are vented. 

Oxygen evolution occurs on nickel oxide electrodes throughout charge, on overcharge, and on standby. It is the anodic process in the self-discharge reaction of the positive electrode in nickel-cadmium cells. Early work in the field has been reviewed [9], No significant new work has been reported in recent years. 

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