Fuel cells and corrosion
A fuel cell turns a fuel's combustion energy straight into electricity. Rusting is a tiny electrochemical cell: iron is oxidised at one spot, oxygen reduced at another.
The hydrogen-oxygen fuel cell
A fuel cell is a galvanic cell that turns the energy of combustion of a fuel such as hydrogen, methane or methanol directly into electrical energy. H₂-O₂ fuel cells powered the Apollo space missions, and the water they made was used by the astronauts for drinking.
- 1H₂ and O₂ are bubbled through porous carbon electrodes into a concentrated aqueous solution of sodium hydroxide (NaOH). Finely divided platinum or palladium in the electrodes speeds up the electrode reactions.
- 2Cathode: O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
- 3Anode: 2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻
- 4★Overall: 2H₂(g) + O₂(g) → 2H₂O(l)
- 5The cell keeps working as long as the gases are supplied; it never needs recharging.
★ key step
Why fuel cells are useful
- High efficiency, about 70%, against about 40% for a thermal power plant, because chemical energy becomes electricity directly, without first becoming heat.
- Pollution-free: the H₂-O₂ cell gives out only water.
- Continuous: it runs as long as fuel is supplied.
- Fuel cells are being developed for buses, cars and backup power.
Mechanism of rusting
Corrosion slowly coats a metal with its oxide or other salts: the rusting of iron, the tarnishing of silver and the green coating on copper and bronze are examples. In rusting, a drop of water on iron, with CO₂ from air dissolved in it as H₂CO₃ (a source of H⁺), sets up a tiny galvanic cell.
- 1Anode (one spot on the iron): 2Fe(s) → 2Fe²⁺(aq) + 4e⁻; E°(Fe²⁺/Fe) = −0.44 V
- 2The electrons travel through the metal to another spot, which acts as the cathode.
- 3Cathode: O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l); E° = +1.23 V
- 4★Overall: 2Fe(s) + O₂(g) + 4H⁺(aq) → 2Fe²⁺(aq) + 2H₂O(l); E°cell = 1.23 − (−0.44) = 1.67 V
- 5Air oxidises Fe²⁺ further to Fe³⁺, which comes out as rust, Fe₂O₃·xH₂O.
★ key step
How to draw the rusting cell
- Draw a horizontal strip labelled iron, with a drop of water sitting on it.
- Mark one spot under the drop as the anode, with Fe → Fe²⁺ + 2e⁻.
- Mark another spot, near the edge of the drop where air reaches easily, as the cathode, with O₂ + 4H⁺ + 4e⁻ → 2H₂O.
- Draw an arrow through the iron for electrons moving from the anode to the cathode.
- Show rust, Fe₂O₃·xH₂O, forming in the drop. Label the water drop, H⁺ and O₂.
Preventing corrosion
| Method | How it works | Examples |
|---|---|---|
| Barrier protection | Keeps air and moisture away from the metal surface | Paint, oil or grease on gates, cycles and tools; coating with chemicals such as bisphenol |
| Covering with another metal | A layer of a metal such as tin or zinc covers the iron; zinc keeps protecting even when scratched, because it corrodes first | Galvanised (zinc-coated) buckets and roof sheets; tin-coated cans |
| Sacrificial electrode | A more reactive metal such as Mg or Zn, joined to the iron, is oxidised instead of the iron | Magnesium blocks on buried pipelines; zinc blocks on ship hulls |
Brass and copper kitchen vessels are coated with tin (kalai) by the traditional kalaiwala, a metal barrier that also keeps copper out of sour food. Galvanised buckets and the zinc blocks on fishing boats use zinc as the metal that corrodes first.
3 marks: “Write the electrode reactions and the overall reaction of the H₂-O₂ fuel cell. Give two advantages of fuel cells.” Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻. Anode: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻. Overall: 2H₂ + O₂ → 2H₂O. Advantages: high efficiency (about 70%), no pollution, works continuously while fuel is supplied.
3 marks: “Explain the rusting of iron as an electrochemical process, with equations.” Anode: 2Fe → 2Fe²⁺ + 4e⁻. Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O, with H⁺ from H₂CO₃. Overall: 2Fe + O₂ + 4H⁺ → 2Fe²⁺ + 2H₂O, E°cell = 1.67 V. Fe²⁺ is then oxidised by air to Fe³⁺, forming rust, Fe₂O₃·xH₂O.
2 marks: “Why does a scratched galvanised iron sheet not rust?” Zinc (E° = −0.76 V) is more easily oxidised than iron (E° = −0.44 V). Where the iron is exposed, zinc acts as the anode and corrodes instead, so the iron is protected.
Rust needs air and water; dissolved salts and acids speed it up. Fuel cell: hydrogen in, water out, power on.