Having it both ways with rechargeable batteries

Redox reactions can be reversed to regenerate the original reactants, allowing people to make rechargeable batteries. Nickel-cadmium (Ni-Cad) and lithium batteries fall into this category, but the most familiar type of rechargeable battery is probably the automobile battery.

The ordinary automobile battery, or lead storage battery, consists of six cells connected in series. The anode of each cell (where oxidation takes place) is lead, and the cathode (where reduction takes place) is lead dioxide (PbO2). The electrodes are immersed in a sulfuric acid (H2SO4) solution. When you start your car, the following cell reactions take place:

✓ Anode: Pb(s) + H₂SO₄(aq) → PbSO₄(s) + 2H⁺ + 2 e^–

✓ Cathode: 2 e^– + 2 H⁺ + PbO₂(s) + H₂SO₄(aq) → PbSO₄(s) + 2 H₂O(l)

✓ Overall reaction: Pb(s) + PbO2(s) + 2 H₂SO₄(aq) → 2 PbSO₄ + 2 H₂O(l)

When this happens, both electrodes become coated with solid lead(II) sulfate, and the sulfuric acid is used up.

After you start the automobile, the alternator or generator takes over the job of producing electricity (for spark plugs, lights, and so on) and also recharges the battery. During charging, the automobile battery acts like an electrolytic cell.

The alternator reverses both the flow of electrons into the battery and the original redox reactions, and it regenerates the lead and lead dioxide:

2 PbSO₄(s) + 2 H₂O(l) → Pb(s) + PbO₂(s) + 2 H₂SO₄(aq)

The lead storage battery can be discharged and charged many times. But the shock of running over bumps in the road or into the curb flakes off a little of the lead(II) sulfate and eventually causes the battery to fail.