Lead-acid battery electrodes

FIGURE 3.1 Lead-acid battery electrode stmctures (a) flat and tubular plates (b) pasted flat electrode, in which the two grids on the left are made of carbon and lead, respectively. After the grid is pasted and cured, the electrode is formed as shown at right, ([a] From http //www.check thatcar.com/carfaq2.asp. [b] From A. Kirchev et al., J. Power Sources, 196(20), 8773-8788,2011.)... 

To calculate the open circuit voltage of the lead—acid battery, an accurate value for the standard cell potential, which is consistent with the activity coefficients of sulfuric acid, must also be known. The standard cell potential for the double sulfate reaction is 2.048 V at 25 °C. This value is calculated from the standard electrode potentials for the (Pt)H2 H2S04(yw) PbS04 Pb02(Pt) electrode 1.690 V (14), for the Pb(Hg) PbS04 H2S04(yw) H2(Pt) electrode 0.3526 V (19), and for the Pb Pb2+ Pb(Hg) 0.0057 V (21). 

The mercurous sulfate [7783-36-OJ, Hg2S04, mercury reference electrode, (Pt)H2 H2S04(y ) Hg2S04(Hg), is used to accurately measure the half-ceU potentials of the lead—acid battery. The standard potential of the mercury reference electrode is 0.6125 V (14). The potentials of the lead dioxide, lead sulfate, and mercurous sulfate, mercury electrodes versus a hydrogen electrode have been measured (24,25). These data may be used to calculate accurate half-ceU potentials for the lead dioxide, lead sulfate positive electrode from temperatures of 0 to 55°C and acid concentrations of from 0.1 to Sm. 

Some battery designs have a one-way valve for pressure rehef and operate on an oxygen cycle. In these systems the oxygen gas formed at the positive electrode is transported to the negative electrode where it reacts to reform water. Hydrogen evolution at the negative electrode is normally suppressed by this reaction. The extent to which this process occurs in these valve regulated lead —acid batteries is called the recombination-efficiency. These processes are reviewed in the Hterature (50—52). 

A lead-acid battery consists of electrolytic cells, each containing an anode of porous lead, a cathode of primarily lead peroxide (PbO,), and electrodes of metallic lead. The anode and cathode are separated by nonsulfuric acid and water. 

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. 

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.

Although the rate of these reactions is slow, according to its thermodynamic situation the lead dioxide electrode is not stable. Since a similar situation applies to the negative electrode, the lead-acid battery system as a whole is unstable and a certain rate of water decomposition cannot be avoided. 

In the lead-acid battery, the reactions at both electrodes include the dissolved state, which means that the reacting species are dissolved in the course of the reaction. The new chemical compounds formed during the reaction are precipitated again as solid matter. This explains the completely different appearance of the material in the charged and discharged states. 

The lead electrode used as anode in the well-known lead-acid battery is a rather... 

The very first functioning lead-acid battery was presented by Gaston Plante in 1860 spirally would lead sheets served as electrodes, separated by a layer of felt — the first separator of a lead-acid battery [12], This assembly in a cylindrical vessel in 10% sulfuric acid had only a low capacity, which prompted Plante to undertake a variety of experiments resulting in many improvements that are still connected with... 

The electrolyte was a solution of ammonium chloride that bathed the electrodes. Like Plante s electrochemistry of the lead-acid battery, Leclanche s electrochemistry survives until now in the form of zinc-carbon dry cells and the use of gelled electrolyte.12 In their original wet form, the Leclanche electrochemistry was neither portable nor practicable to the extent that several modifications were needed to make it practicable. This was achieved by an innovation made by J. A. Thiebaut in 1881, who through encapsulating both zinc cathode and electrolyte in a sealed cup avoided the leakage of the liquid electrolyte. Modern plastics, however, have made Leclanche s chemistry not only usable but also invaluable in some applications. For example, Polaroid s Polar Pulse disposable batteries used in instant film packs use Leclanche chemistry, albeit in a plastic sandwich instead of soup bowls.1... 

In 1899, the nickel-cadmium battery, the first alkaline battery, was invented by a Swedish scientist named Waldmar Jungner. The special feature of this battery was its potential to be recharged. In construction, nickel and cadmium electrodes in a potassium hydroxide solution, it was the first battery to use an alkaline electrolyte. This battery was commercialized in Sweden in 1910 and reached the Unites States in 1946. The first models were robust and had significantly better energy density than lead-acid batteries, but nevertheless, their wide use was limited because of the high costs. 

Micka K., Rousar I. Theory of Porous Electrodes. XII. The Negative Plate of the Lead-Acid Battery. Electrochim. Acta. 1974 19 499-502. 

Worked Example 7.9 Consider the electrode potentials for metallic lead within a lead-acid battery. The lead has three common redox states, Pb4+, Pb2+ and Pb°, so there are three possible equilibria to consider ... 

As the name suggests, the materials used in a lead-acid battery include lead and an acid. Figure 11.19 shows that the electrodes in each cell are constructed using lead grids. One electrode consists of powdered lead packed into one grid. The other electrode consists of powdered lead(IV) oxide packed into the other grid. The electrolyte solution is fairly concentrated sulfuric acid, at about 4.5 mol/L. 

Each cell of a lead-acid battery is a single compartment, with no porous barrier or salt bridge. Fibreglass or wooden sheets are placed between the electrodes to prevent them from touching. 

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