The lead-acid cell

The lead-acid cell can be represented schematically as having a negative electrode of porous lead (lead sponge) and a positive electrode of lead dioxide, Pb02, both immersed in an aqueous solution of sulphuric acid  

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Table 1 gives the calculated open circuit voltages of the lead—acid cell at 25°C at the sulfuric acid molalities shown. The corrected activities of sulfuric acid from vapor pressure data (20) are also given. 

Table 1. Thermodynamic Values for the Lead-acid Cell at 25°C...

Table 2. Temperature Coefficients of the Lead—Acid Cell, Ej mV...

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. 

The separator must be stmcturaHy sound to withstand the rigors of battery manufacturing, and chemically inert to the lead—acid cell environment. Numerous materials have been used for separators ranging from wood, paper, and mbber to glass and plastic. The majority of separators used are either nonwoven—bound glass or microporous plastic such as PVC or polyethylene. 

J. Burbank, A.C. Simon, E. Willihnganz, The lead acid cell, in Advances in Electrochemistry and Electrochemical Engineering, Vol. 8, John Wiley, New York, 1971, p. 170. 

By contrast, secondary batteries may be reused after regenerating their original redox chemicals. This is achieved by passing a current through the battery in the opposite direction to that during normal battery usage. The most common examples of secondary batteries are the lead-acid cell (there is one inside most cars) and nickel-cadmium batteries (commonly called NiCad batteries). 

The lead-acid cell was invented by Plante in 1859, and has remained more-or-less unchanged since Faurd updated it in 1881. The lead-acid cell is the world s most popular choice of secondary battery, meaning it is rechargeable. It delivers an emf of about 2.0 V. Six lead-acid batteries in series produce an emf of 12 V. 

Fig. 5.2 Approximate open circuit voltage and electrolyte density as a function of percentage service capacity for the lead-acid cell...

Fig. 5.12 (a) Discharge curves for a typical lead-acid cell at various rales, (b) Charging curve for the lead-acid cell ai C/10... 

The most well-known storage cell is the lead-acid cell, which was invented by GASTON PLANTE in 1859 and is still the most widely used device of its type. The cell is represented by... 

The Edison cell uses an iron anode, nickel oxide eathode, and KOH electrolyte. This cell is extremely rugged and is still used in certain industrial apphcations, but it was never able to displace the lead-acid cell as Edison had hoped.. 

Figure 7. OCV and electrolyte density as a function of the percentage of discharge capacity for the lead-acid cell [1] (by permission of Arnold C.A. Vincent, B. Scrosati, Modern Batteries. An Introduction to Electrochemical Power Sources, 2nd edition, Edward Arnold, London, 1997).

Unlike the Daniell and Leclanche cells, the lead-acid cell is rechargeable. So, when the battery runs down, you do not need to replace it. Instead, an electric current is applied in a direction opposite to that discussed above. As a result of the input of energy, the reactions are reversed. The cell is eventually restored to its charged state. During recharge, the cell functions as an electrolytic cell, which you will learn about in the next section. 

The discharge reactions of the lead-acid cell are as follows ... 

The voltage efficiency of the lead-acid cell is about 80 %. This near reversibility is a consequence of the rapidity of the chemical reactions in the cell. As we have seen, the ability to supply large currents at potentials near the open-circuit potential means that the chemical reactions at the electrodes are fast as the charge is drained away by the current, the potential should drop, but the chemical reaction occurs rapidly enough to rebuild the potential. 

If we compare the quantity of charge obtained from the lead-acid cell to the quantity that must be passed in to charge the cell, we obtain values of 90 to 95 %, or even higher in special circumstances. This means that very little of the charging current is dissipated in side reactions (such as electrolysis of water). Overall, the lead storage cell is an extra-... 

For the standard Gibbs energy change in the lead-acid cell we have (for a two-electron change) ... 

The lead—acid cell utilises the reactions of lead oxidation (Pb Pb + 2e ) and of lead... 

So the electromotive force for the lead—acid cell can be expressed by ... 

The lead—acid cell operates with H2SO4 solutions of concentrations up to 1.28 g cm (at 25 °C). At higher acid concentrations, the cycle life of the lead—acid cell is shortened. 

Thus, the plates in the lead—acid cell pulsate during cycling, expanding in thickness during discharge (because of the increased volume of the solid phases in the active masses, which reduces the pore volume at the same time), and then during re-charge, the plate thickness decreases and the active mass pore volume increases at that. 

Brief Summary of the Lead Compounds Involved in the Manufacture and Operation of the Lead—Acid Cell... 

The potential/pH diagram in Fig. 2.1 shows that the lead—acid cell is thermodynamically unstable on open circuit. Processes of self-discharge proceed on the two electrodes as a result of which water decomposes to H2 and O2, and discharge reactions commence at the positive and negative plates as follows ... 

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