Fuel cells and corrosion

Thermal power plants burn fuel to make steam for turbines, which is inefficient and polluting. A fuel cell is a galvanic cell fed continuously with a fuel (hydrogen, methane, methanol) and oxidant, with products removed continuously.

The H₂-O₂ cell (used in the Apollo space programme, where its water was drunk by the astronauts) bubbles both gases through porous carbon electrodes into concentrated aqueous NaOH, with finely divided Pt or Pd as catalyst.

Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻. Anode: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻. Overall: 2H₂ + O₂ → 2H₂O. It runs as long as the gases are supplied.

Fuel cells are about 70% efficient against about 40% for thermal plants, and do not pollute.

Corrosion coats metals with oxides or other salts: rusting of iron, tarnishing of silver, the green coating on copper and bronze. It damages buildings, bridges and ships.

Rusting is electrochemical. At an anodic spot 2Fe → 2Fe²⁺ + 4e⁻; electrons travel through the metal to a cathodic spot where O₂ + 4H⁺ + 4e⁻ → 2H₂O, with H⁺ from carbonic acid (dissolved CO₂) or other acidic oxides. Overall 2Fe + O₂ + 4H⁺ → 2Fe²⁺ + 2H₂O, E°cell = 1.23 − (−0.44) = 1.67 V.

Air oxidises Fe²⁺ further to Fe³⁺, which deposits as rust, hydrated ferric oxide Fe₂O₃·xH₂O, releasing more H⁺.

Prevention: keep air and water off the surface with paint or chemicals (such as bisphenol); coat it with another metal (Sn, Zn); or attach a sacrificial electrode of a more reactive metal (Mg, Zn) that corrodes instead of the object.

Hydrogen economy: hydrogen burns to water only, so hydrogen made by splitting water with solar energy and used in fuel cells could replace fossil fuels; both steps rest on electrochemistry.