Electrochemistry of the Lead Acid Accumulators July 2012 Schema of working http://sycomoreen.free.fr

Cathode

⊕

Anode

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≃ 2,1 V

electrolyte

Cathode : Solid (s) in lead Pb with layer of lead dioxide PbO2 and/or of PbSO4. It is the positive pole of the battery. Anode : Solid (s) in lead Pb, with a layer of lead sulfate PbSO4. It is the negative pole of the battery. Electrolyte : aqueous sulfuric acid 2 H 3O + SO42− , H 2O ; this liquid is also containing some additives. Other possibilities are gelled electrolyte or absorbed glass materials. Voltage : about 2,1 V

i>0

Chemical reactions while discharging 2− 4

+

⊕

−

Cathodic reduction : PbO2 s + SO + 4 H 3O + 2e → PbSO4 s + 6 H 2O Anodic oxydization : Pbs + SO42− → PbSO4 s + 2e− Net reaction

2− 4

Consumer

e−

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H 2O

+

: PbO2 s + Pbs + 2 SO + 4 H 3O → 2 PbSO4 s + 6 H 2O

Commentaries : the discharge shows a formation of water and a consumption of sulphuric acid. About the numbers of oxydization, the cathodic Lead goes from +IV in PbO2 to +II in PbSO4, in the same time, the anodic Lead goes from 0 in Pb to +II in PbSO4 ; it explains the transfer of 2 electrons from the anode towards the cathode. The discharge entails some deposits of lead sulfate on the electrodes, which is few conductive : it decreases the performances of the battery (by increasing its internal resistance due to sulfation). Generator i>0 Chemical reactions while charging − 2− 4

+

Cathodic oxydization : PbSO4 s + 6 H 2O → PbO2 s + SO + 4 H 3O + 2e Anodic reduction Net reaction

⊕

−

: PbSO4 s + 2e − → Pbs + SO42− 2− 4

: 2 PbSO4 s + 6 H 2O → PbO2 s + Pbs + 2 SO + 4 H 3O

+

e

H 2O

Commentaries : the charge provokes a water consumption and the formation of sulphuric acid. The cathodic lead goes from +II in PbSO4 to +IV in PbO2 , while the anodic lead comes from +II in PbSO4 to 0 in Pb ; it justifies the transfer of 2 electrons from the cathode towards the anode. The charge suppresses the sulphate lead deposits on the electrodes. The lead/acid accumulators are simple, cheap and very heavy. Fully charged, they can briefly provide enormous currents. They are used as stationary batteries, or as starting devices for motors. These batteries especially fear the deep discharges (problem of permanent sulfation which imposes a desulfation of maintenance for the electrodes).

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