Electrochemistry of the Lead-Acid Cell

Lead-Acid Storage Batteries DOE-HDBK-1084-95 OPERATON AND CONSTRUCTION Rev. 0 Page 13 Batteries OPERATION AND CONSTRUCTION The following paragraphs describe the general operation and construction of lead-acid batteries. Lead-Acid Battery Active Materials The active materials in a battery are those that participate in the electrochemical charge/discharge reaction. These materials include the electrolyte and the positive and negative electrodes. As mentioned earlier, the electrolyte in a lead-acid battery is a dilute solution of sulfuric acid (H SO ). The negative electrode of a fully charged battery is 2 4 composed of sponge lead (Pb) and the positive electrode is composed of lead dioxide (PbO ). 2 Electrochemistry of the Lead-Acid Cell All lead-acid batteries operate on the same fundamental reactions. As the battery discharges, the active materials in the electrodes (lead dioxide in the positive electrode and sponge lead in the negative electrode) react with sulfuric acid in the electrolyte to form lead sulfate and water. On recharge, the lead sulfate on both electrodes converts back to lead dioxide (positive) and sponge lead (negative), and the sulfate ions (SO ) are driven back into the 4 ₂- electrolyte solution to form sulfuric acid. The reactions involved in the cell follow. At the positive electrode PbO + 3H + HSO + 2e PbSO + 2H O (1.685 V). (2) 2 4 4 2 + - - Discharge $ % Charge At the negative electrode Pb + HSO PbSO + H + 2e (0.356 V). (3) 4 4 - Discharge + - $ % Charge For the overall cell PbO + Pb + 2H SO 2PbSO + 2H O (2.041 V). (4) 2 2 4 4 2 Discharge $ % Charge Therefore the maximum open-circuit voltage that can be developed by a single lead-acid cell is 2.041 V.