Lead charge-discharge reactions

PIa.tes, Plates are the part of the cell that ultimately become the battery electrodes. The plates consist of an electrically conductive grid pasted with a lead oxide—lead sulfate paste which is the precursor to the electrode active materials which participate in the electrochemical charge—discharge reactions. 

In the lead-acid battery, sulfuric acid has to be considered as an additional component of the charge-discharge reactions. Its equilibrium constant influences the solubility of Pb2+ and so the potential of the positive and negative electrodes. Furthermore, basic sulfates exist as intermediate products in the pH range where Fig. 1 shows only PbO (cf. corresponding Pour-baix diagrams in Ref. [5], p. 37, or in Ref. [11] the latter is cited in Ref. [8]). Table 2 shows the various compounds. 

The charge-discharge reaction of the negative electrode corresponds to curve A in Fig. 1, but the Pb2+ ion activity is now determined by the solubility of lead sulfate (PbS04). Thus Eq. (12) has to be modified into... 

Figure 2. Reactions that occur in lead-acid batteries versus electrode potential (thermodynamic situation). Their equilibrium potentials are inserted as boxed numbers. Equilibrium potentials of the charge-discharge reactions (Pb/PbS04 and PhS04/Pb02) are represented by hatched columns, to indicate their dependence on acid concentration. The inserted equilibrium potentials (-0.32 and +l. 75 V) of the charge discharge reactions correspond to an acid density of 1.23 gem 3.

The active material comprises the substances that constitute the charge-discharge reaction. In the positive electrode of lead-acid batteries, the active material in the charged state is lead dioxide (PbOj), which is converted into lead sulfate (PbS04) when the electrode is discharged. The active material is the most essential part of a battery, and battery technology has to aim at optimum constitution and performance for the expected application. This does not only concern the chemical composition but also the physical structure and its stability. Specialized methods have been developed to fulfill these requirements, and the primary products as well as the manufacturing process are usually specified by the individual battery manufacturer. 

Figure 6.2 Reactions that occur in lead-add batteries versus electrode potential (thermodynamic situation). Their equilibrium potentials are inserted as boxed numbers. Equilibrium potentials of the charge-discharge reactions (Pb/PbS04 and...

Alkaline gel electrolytes based on potassium-salt poly (acrylic acid) (PAAK), typically, PAAK-KOH-H2O was used in metal hydride reactions to enhance the charge-discharge reactions and capacity retention [62]. A silica-based gel electrolyte was used in lead dioxide-positive electrode reaction to give superior discharge capacity and greatly reduced the rate of electrode corrosion [63]. The Na2S04-PAAK gel electrolyte was employed in the middle to separate acid and alkaline chambers with an AEM and a CEM. Detailed information on preparation of gel electrolytes and setup is reported in Ref. [56]. 

An excellent review covers the charge and discharge processes in detail (30) and ongoing research on lead—acid batteries may be found in two symposia proceedings (32,33). Detailed studies of the kinetics and mechanisms of lead —acid battery reactions are pubUshed continually (34). Although many questions concerning the exact nature of the reactions remain unanswered, the experimental data on the lead—acid cell are more complete than for most other electrochemical systems. 

An excellent review covers die charge and discharge processes in detail and ongoing research on lead—acid batteries may be found in two symposia proceedings. Detailed studies of the kinetics and mechanisms of lead-acid battery reactions arc published eondnually. 

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