Batteries: Dry Cell, Lead Storage Cell, & Fuel Cells

Three classic batteries illustrate the whole story of electrochemical power: the dry (Leclanché) cell, the lead storage cell, and the fuel cell. The dry cell is a use-once primary cell, the lead-acid battery is a rechargeable secondary cell, and the fuel cell generates power continuously as long as fuel is fed in.

Understanding their chemistry explains everyday facts: why ordinary cells go flat and can't be revived, why car batteries need topping up and can be recharged, and why spacecraft carry hydrogen–oxygen fuel cells instead of heavy batteries.

Key Concepts

• Dry cell: zinc can anode (oxidised to Zn2+), MnO2 + carbon cathode, NH4Cl/ZnCl2 paste electrolyte; 1.5 V; primary — the reactions are not practically reversible. The alkaline variant uses KOH electrolyte and lasts longer.
• Lead storage cell: Pb anode, PbO2 cathode, H2SO4 electrolyte; each cell gives 2 V, so a car battery is six cells in series = 12 V; secondary — recharging reverses the discharge reaction.
• Fuel cell (alkaline H2–O2): hydrogen oxidised at the anode, oxygen reduced at the cathode, product is pure water; E° = 1.23 V; runs as long as gases are supplied.
• Capacity is rated in ampere-hours (Ah): a 60 Ah battery delivers 60 A for one hour, or 1 A for 60 hours.

Formulas

• Lead-acid discharge: Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O (PbSO4 coats both electrodes; charging reverses it).
• Fuel cell net: 2H2 + O2 → 2H2O, E°cell = 1.23 V.
• Dry cell (simplified): Zn + 2MnO2 + 2NH4Cl → Zn(NH3)2Cl2 + 2MnOOH.

Worked Example

Compute the standard EMF of the alkaline hydrogen–oxygen fuel cell from its half-reactions:

1. Cathode (reduction): O2 + 2H2O + 4e− → 4OH−, E° = +0.40 V.
2. Anode (oxidation): 2H2O + 2e− → H2 + 2OH−, E° = −0.83 V (written as a reduction).
3. E°cell = E°cathode − E°anode = 0.40 − (−0.83) = 1.23 V.
4. The reverse reaction — electrolysing water — needs at least 1.23 V, showing fuel cells and electrolysers are the same chemistry run in opposite directions.

Common Misconceptions

• "Dry cells can be recharged." Ordinary zinc–carbon cells are primary; forcing current through them is ineffective and can make them leak or burst.
• "A fuel cell stores its fuel inside." It doesn't — hydrogen and oxygen are fed continuously from external tanks; stop the feed and power stops.
• "A car battery is one 12 V cell." It is six 2 V lead-acid cells wired in series inside one case.

Real-World Applications

• Dry/alkaline cells: remotes, toys, clocks — cheap portable primary power.
• Lead-acid batteries: car starters, UPS systems, inverters — high surge current and rechargeability.
• Fuel cells: Apollo spacecraft drank their fuel cells' water product; today they back up telecom towers and power some buses.

Primary cells spend their chemistry once, secondary cells rewind it with a charger, and fuel cells outsource storage to external tanks. Pick the battery by the job: shelf life and cheapness (dry), rechargeable muscle (lead-acid), or continuous clean power (fuel cell).