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Lead acid battery Electrochemistry

- May 15, 2018 -

Lead acid battery Electrochemistry


Discharge


In the discharged state both the positive and negative plates become lead(II) sulfate (PbSO4),  and the electrolyte loses much of its dissolved sulfuric acid and becomes primarily water. The discharge process is driven by the conduction of electrons from the negative plate back into the cell at the positive plate in the external circuit.


Negative plate reaction

  • Pb(s) + HSO
    4
    (aq) → PbSO
    4
    (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

  • PbO
    2
    (s) + HSO
    4
    (aq) + 3H+
    (aq) + 2e → PbSO
    4
    (s) + 2H
    2
    O(l)

The total reaction can be written as

  • Pb(s) + PbO
    2
    (s) + 2H
    2
    SO
    4
    (aq) → 2PbSO
    4
    (s) + 2H
    2
    O(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.


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