📘 Key Principle
A fuel cell converts chemical energy → electrical energy directly, bypassing the inefficient thermal step of combustion engines. Much higher efficiency.
Half-Equations (Alkaline Fuel Cell)
| Electrode | Half-Equation | Process |
|---|---|---|
| Anode (−) | H₂ + 2OH⁻ → 2H₂O + 2e⁻ | Oxidation |
| Cathode (+) | O₂ + 2H₂O + 4e⁻ → 4OH⁻ | Reduction |
| Overall | 2H₂ + O₂ → 2H₂O | Only product = water! |
Fuel Cells vs Combustion Engines
| Feature | Fuel Cell | Combustion Engine |
|---|---|---|
| Efficiency | ~60–80% | ~25–30% |
| Emissions | H₂O only (zero carbon) | CO₂, CO, NOx, particulates |
| Noise | Silent | Loud |
| Moving parts | Very few | Many (pistons, valves) |
Challenges of Hydrogen
- Storage: Very low energy density as a gas → needs high-pressure tanks (700 bar) or cryogenic liquid (−253 °C)
- Production: Most H₂ currently made by steam reforming of CH₄ → still produces CO₂
- "Green" H₂: Electrolysis of water using renewable electricity → truly zero-carbon, but expensive
- Infrastructure: Lack of refuelling stations; Pt catalysts are costly