Operating Principle
Direct Chemical-to-Electrical Energy Conversion
A fuel cell converts chemical energy directly into electrical energy via continuous electrochemical redox reactions, completely bypassing the Carnot efficiency limits of combustion heat engines.
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
Technology Challenges
Practical Limitations of Hydrogen Fuel Technology
- Storage: Low volumetric energy density as a gas → requires high-pressure carbon-fibre tanks (700 bar) or cryogenic liquid storage (−253 °C).
- Production: Majority of commercial H₂ is generated by steam methane reforming (
CH₄ + 2H₂O → CO₂ + 4H₂), which still generates carbon dioxide emissions. - "Green" H₂: Water electrolysis powered by renewable wind/solar is truly zero-emission, but currently more expensive.
- Infrastructure: Scarcity of refuelling stations; expensive platinum group metal catalysts.
Worked Example
Hydrogen Fuel Cell Half-Equations (Alkaline vs Acidic)
In alkaline electrolyte (OH⁻):
Anode (oxidation): H₂(g) + 2OH⁻(aq) → 2H₂O(l) + 2e⁻
Cathode (reduction): O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
In acidic electrolyte (H⁺):
Anode (oxidation): H₂(g) → 2H⁺(aq) + 2e⁻
Cathode (reduction): O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)
Overall reaction for both: 2H₂(g) + O₂(g) → 2H₂O(l)
Challenges of Hydrogen as a Fuel
| Challenge | Explanation |
|---|---|
| Storage | H2 is a gas with very low density. Must be compressed to high pressure or liquefied at -253 °C, both requiring energy |
| Production | Electrolysis of water requires electricity. Unless from renewables, this just shifts the carbon emissions |
| Infrastructure | Existing petrol stations and pipelines cannot be used for hydrogen without major modification |
| Safety | Hydrogen is highly flammable and forms explosive mixtures with air |
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