Negative plates charge reactions

The drying of negative plates is not possible without precautions, because of the tendency to spontaneous oxidation. This oxidation reaction is much ac-celerated by water, and the active material of a moist negative electrode is spon-taneously converted into lead oxide when exposed to air. When, on the other hand, the charged plate is dry, a thin layer of oxide covers the surface of the active material, and prevents... 

Two other reactions must be taken into account during the charging of a VRLA cell. These are, the evolution of hydrogen at the negative plate ... 

Resolution of the above problems is made more difficult by the sensitivity of negative plates to various secondary reactions, such as those caused by impurities and additives. These may affect the self-discharge rate, as well as the charge-acceptance and the efficiency at which the charge input is subsequently used. The transient nature of these effects increases the difficulty of understanding their influence. Several studies are now underway to increase this understanding and... 

During the initial stages of the discharge of a fully charged negative plate, eleetron transfer can take place at any location because the entire plate is conduetive. Accordingly, the discharge process (both dissolution and deposition steps) oeeurs both on the surfaces and in the interior of the plate. Nevertheless, the reaction in the... 

Fig. 17.4. Discharge and charge reactions at the negative plate of a lead-acid cell.

As pointed out in the preceding reviews of separator function and separator materials (Chapters 6 and 7, respectively) and of charging strategies (Chapter 9), the separator plays a crucial role in VRLA cells. Provision of a separator with the appropriate microstructure can constrain the rate of oxygen arrival at the negative plate and thus restrict the suppression of the plate potential caused by the oxygen-recombination reaction. 

It is worthwhile mentioning that charging the lead storage battery would not be possible if it were not f or a high hydrogen overvoltage on the negative plate, which permits the reaction... 

Parallel mechanism of the charge reactions on negative plates containing carbons or graphites [36]... 

The cycle life within one cycle-set depends strongly on the nature and properties of the carbon or graphite additives used. These materials differ in particle size, structure and affinity to lead and to the expander. Of special importance is the interface between carbon and lead particles, and its area as it determines the resistance that electrons have to overcome when transferred between these two phases and thus affects the potential and the rate of the electrochemical reactions at the carbon/solution interface. Only a limited number of carbon and graphite materials have optimum structural characteristics and may improve substantially the cycle life performance of the cells. It is of crucial importance to identify the most effective carbon (graphite) additives, i.e. with most beneficial effect on the parallel mechanism of charge of the negative plates. 

The reactions that proceed in the negative plates of wet-charged batteries on storage can be represented by the following equations ... 

The second reaction that accelerates the self-discharge processes of the negative plates in wet-charged batteries is the oxidation of lead by the oxygen evolved by the self-discharge processes at the positive plates. This oxidation of the lead active mass can be represented by the following equation ... 

During charge of a VRLA battery, the only reaction that proceeds at the negative plates, up to 70% SOC of the battery, is the charge reaction (Fig. 14.8) ... 

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