Positive electrodes lead sulfate

At the cathode, or positive electrode, lead dioxide [1309-60-0] Pb02, reacts with sulfuric acid to form lead sulfate [7446-14-2] PbSO, and water in the discharging reaction... 

As shown in reaction formula for the discharging of battery, at the negative electrode, metallic lead reacts with the sulfate ions in water solution to produce lead sulfate and release electrons (Formula 1). At the positive electrode, lead dioxide reacts also with the sulfate ions in solution to produce lead sulfate and water (Formula 2). For the charging of the battery, the inverse reactions occur at the negative and positive electrodes. 

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. 

Paste Mixing. The active materials for both positive and negative plates are made from the identical base materials. Lead oxide, fibers, water, and a dilute solution of sulfuric acid are combined in an agitated batch mixer or reactor to form a pastelike mixture of lead sulfates, the normal, tribasic, and tetrabasic sulfates, plus PbO, water, and free lead. The positive and negative pastes differ only in additives to the base mixture. Organic expanders, barium sulfate [7727-43-7] BaSO carbon, and occasionally mineral oil are added to the negative paste. Red lead [1314-41 -6] or minium, Pb O, is sometimes added to the positive mix. The paste for both electrodes is characterized by cube weight or density, penetration, and raw plate density. 

PbO TbSO H20. If the temperature is elevated to >57° C, the result is coarse tetrabasic lead sulfate, 4PbO TbSO, crystals. This is especially critical for the positive electrode where tribasic sulfate converts readily to Pb02 during battery formation but tetrabasic sulfate does not (93). 

Lead oxide (PbO) (also called litharge) is formed when the lead surface is exposed to oxygen. Furthermore, it is important as a primary product in the manufacturing process of the active material for the positive and negative electrodes. It is not stable in acidic solution but it is formed as an intermediate layer between lead and lead dioxide at the surface of the corroding grid in the positive electrode. It is also observed underneath lead sulfate layers at the surface of the positive active material. 

The charge-discharge equation of the positive electrode corresponds to curve B in Fig. 1 and the corresponding Eq. (15). But here also the Pb2+ activity is now determined by the solubility of lead sulfate, and Eq. (15) has to be modified into ... 

After an initial overshoot, a constant potential is attained at the electrode (Fig. 16.4) this is governed by the kinetics of lead sulfate crystallization. After some time, the potential begins to shift in the positive direction, slowly at first bnt then rapidly. This... 

The characteristic of the lead-acid battery is that both electrodes are based on the chemistry of lead. The discharge-charge process is known as the double sulfate reaction, with the positive and negative electrodes being the seats of a dissolving-precipitating (and not some kind of solid-state ion transport or film formation) mechanism of the lead sulfate. The cell, the electrode reactions and the cell reaction are ... 

Although several hypotheses have been proposed, the mechanisms of electrode degradation involved in shedding and inactivation processes are still not clear. The method of material preparation plays a substantial role here. For example, positive material prepared by oxidation of needle like crystals of tetrabasic lead sulfate HPbO PbS04) maintains the latter s morphology and the electrode s superior performance during cycling of stationary cells." ... 

The positive electrode is made of lead dioxide (Pb02) and is reduced to lead sulfate (PbS04), while sponge metallic lead (Pb) is oxidized to lead sulfate at the negative electrode. The electrolyte is sulfuric acid (H2SO4), which provides the sulfate ion (S04 ) for the discharge reactions. 

It is evident from the curves in Fig. 2.11 that for polarization up to —0.50 V (curves 1—5) opening the circuit results in rapid drop of the electrode potential to the Pb/PbS04 equilibrium value. When the electrode potential is more positive than —0.50 V and the circuit is opened, a potential arrest is observed before reaching the Pb/PbS04 plateau. This difference in electrode behaviour is related to the alkalization of the solution in the pores of the PbS04 layer and the subsequent local formation of PbO and basic lead sulfates. 

The electrons from reaction [4] are transferred through the external electric circuit to the positive electrode, where lead dioxide (Pb02) is reduced by reacting with two electrons (e ), as well as hydrogen and bisulfate ions, forming lead sulfate and water. The reaction is [5]... 

Reaction [6] is called the double sulfate reaction because sulfuric acid reacts at both electrodes. A resistive layer of lead sulfate crystals forms on both the positive and negative electrode surfaces. The equilibrium cell voltage is the sum of the two electrode reactions ... 

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