Polarization lead-acid batteries

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... 

PbO(OH)2 and H2Pb02, on their part, stay in equilibrium with ions in the solution. The access of ions from the solution to the bulk of the particles and the relatively quick response of PAM crystallinity imply that the ions processes that take place at the interfaces particle solution and crystal zone gel zone are almost reversible and relatively fast. These phenomena allow the electrochemical reactions in PAM to proceed in the bulk of the particles and agglomerates during discharge of the positive plates and thus increase the latter s capacity. This causes the current density and hence the polarization of the electrochemical reaction to decrease. As known, discharge of the positive lead—acid battery plates proceeds at low polarizations. 

The driving force for an electrochemical reaction to proceed is polarization of the electrode, i.e. the potential difference between the equilibrium potential of the reaction and the electrode potential. The rate of the electrochemical reaction depends on the hindrances that have to be overcome by the reacting particles for the reaction to proceed. The hydrogen reaction on lead proceeds with great hindrances, i.e. at high overpotential. Hence, the competing reaction of lead sulfate reduction to lead proceeds with high coulombic efficiency and kinetic stability. This, in turn, ensures stable performance of the lead—acid battery. 

About 40Z of the world s lead production goes into the manufacture of the lead acid battery, and it is by far the dominant technology in rechargeable battery systems with market size of about 12 billion. The earliest reference on this battery is from 1854 (5), while the real commercial breakthrough came with Plante s experiments in 1859(4). Gaston Plante had been investigating the effect of polarization on different metals and he noticed the unique behavior of lead plates in dilute sulphuric acid as the electrolyte, and the rest is history. 

To simulate corrosion in lead-acid battery environments, Dacres et al. [118] and others [119,120] anoically polarized test materials at 1.226 V (versus mercury/mercurous sulfate reference electrode) in sulfuric acid solutions (of 1.285 specific gravity) at 50, 60, and/or 70°C. At 1.226 V, lead and water are oxidized to lead dioxide (Pb02) and molecular oxygen (O2), respectively [122,123]. About one third of the total anodic current is consumed in the oxidation of lead under these conditions [120]. 

The curves represented by Eq. (24) are linearized when plotted semilogarithmically and are called Tafel lines. The constant b represents the slope of the Tafel hne and means the potential difference that causes a current increase of one decade. Tafel lines are important tools when reactions are considered that occur at high overvoltages, since such a linearization allows quantitative considerations. They are often used with lead-acid batteries, since polarization of the secondary reactions hydrogen evolution and oxygen evolution is very high in this system (cf.. Fig. 1.24). 

As in lead-acid batteries, oxygen evolution cannot be avoided at the positive electrode, it already occurs at open circuit and is increased with a more positive polarization of the positive electrode. 

The Thevenin equivalent circuit is the simplest combination, since it is the association of an ideal voltage source and a resistor connected in series. This is a much more realistic way of modeling a lead-acid battery. Indeed, the resistor illustrates the voltage drop due to the current passing through the components of the battery. In the case of LABs, this instantaneous voltage drop mainly results from the low electrical conductivity of electrolyte and is proportional to the current. But, such a simple combination does not account for the polarization of the electrodes happening later on, when the battery is operated. 

FIGURE 24.31 Polarization characteristics vs. state of charge for prismatic lead acid battery. (a) during discharge (b) during charge. 

Figure 20.7 The potential of the two electrodes in a lead-acid battery during charge and discharge. Note that the polarity of the cell is not changed, (for details see P. Moran and E. Gileadi, J. Chem. Educ., 66 (1989) 912.

If the lifetime of Li-based batteries (the term lithium ion batteries for batteries with polar Li-compounds as negative electrodes is very unfortunate) can be further enhanced, they will be also of importance for electrotraction. The classical battery type used in automobiles, viz, the lead-acid accumulator, is distinctly superior in terms of long-time stability but possesses too low an energy content per unit weight as to drive automobiles. Driving car of sensible size and performance with this alone requires a battery weight on the order of 11, (This problem is not removed by using Ni-Cd accumulators,) Much effort has been undertaken to develop a sodium-sulphur cell. In the Na-S cells ... 

The corrosion processes affecting lead when polarized within the Pb02 potential region (Eh > +0.95 V vs Hg/Hg2S04 electrode) are of major practical interest. Lead electrodes operate in this potential interval when used as positive electrodes in a lead—acid storage battery or as nonsoluble anodes during the refining of some metals. 

The potentials of the two electrodes of the lead—acid cell are measured vs. a reference electrode. Thus, the lead—acid cell turns into a three-electrode cell. During measuring the potential of the two electrodes of the LA cell, the reference electrode should not be polarized, i.e. its potential should remain constant. The most common reference electrodes are hydrogen, cadmium, mercury-mercurous sulfate and silver-silver sulfate electrodes. Cadmium sticks are widely used in industrial quality control laboratories to measure the electrode potentials of the manufactured batteries. Cadmium does not form poorly soluble cadmium sulfate, which is the reason why during the measurement the electrolyte in the cell absorbs a few Cd ion impurities that do not affect the performance of the battery, however. 

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