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 (H₂SO₄ ). The negative electrode of a fully charged battery is composed of sponge lead (Pb) and the positive electrode is composed of lead dioxide (PbO₂ ).

Negative bateery plate reaction

Pb (s) + HSO^-₄− (aq) → PbSO₄(s) + H⁺ (aq) + 2e^- −

Release of two conducting electrons gives lead electrode a net negative charge

As electrons accumulate they create an electric field which attracts hydrogen ions and repels sulfate ions, leading to a double-layer near the surface. The hydrogen ions screen the charged electrode from the solution which limits further reactions unless charge is allowed to flow out of electrode.

Positive plate reaction

PbO₂(s) + HSO₄(aq) + 3H⁺(aq) + 2e^- − → PbSO₄(s) + 2H₂O(l)

The total reaction can be written as

Pb(s) + HSO₂(aq) + 2H₂SO₄−(aq) → 2PbSO₄(s) + 2H₂O(l)

The sum of the molecular masses of the reactants is 642.6 g/mol, so theoretically a cell can produce two faradays of charge (192,971 coulombs ) from 642.6 g of reactants, or 83.4 ampere-hours per kilogram (or 13.9 ampere-hours per kilogram for a 12-volt battery). For a 2 volts cell, this comes to 167 watt-hours per kilogram of reactants, but a lead–acid cell in practice gives only 30–40 watt-hours per kilogram of battery, due to the mass of the water and other constituent parts