📘 IB Understanding
A fuel cell can be used to convert chemical energy from a fuel directly to electrical energy. Unlike batteries, fuel cells generate electricity continuously as long as external reactants (fuel + oxidant) are supplied.
How a Fuel Cell Works
A fuel cell is an electrochemical device that converts chemical energy into electrical energy through controlled redox reactions at two separated electrodes. Unlike combustion engines, there is no thermal step, which makes fuel cells significantly more efficient.
Simplified Hydrogen Fuel Cell
Hydrogen Fuel Cell Half-Equations
📋 Exam Note
You must be able to deduce half-equations for both hydrogen and methanol fuel cells. Proton exchange membrane details will not be assessed.
In Acidic Conditions
| Electrode | Half-Equation | Process |
|---|---|---|
| Anode | H₂(g) → 2H⁺(aq) + 2e⁻ | Oxidation |
| Cathode | ½O₂(g) + 2H⁺(aq) + 2e⁻ → H₂O(l) | Reduction |
| Overall | H₂(g) + ½O₂(g) → H₂O(l) | Only product = water |
In Alkaline Conditions
| Electrode | Half-Equation | Process |
|---|---|---|
| Anode | H₂(g) + 2OH⁻(aq) → 2H₂O(l) + 2e⁻ | Oxidation |
| Cathode | O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq) | Reduction |
| Overall | 2H₂(g) + O₂(g) → 2H₂O(l) | Only product = water |
Methanol Fuel Cell
Methanol (CH₃OH) can also be used as a fuel in a direct methanol fuel cell (DMFC). The advantage is that methanol is a liquid at room temperature, making storage and transport much easier than hydrogen gas.
| Electrode | Half-Equation (Acidic) |
|---|---|
| Anode | CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻ |
| Cathode | ¾O₂ + 6H⁺ + 6e⁻ → 3H₂O |
| Overall | CH₃OH + ¾O₂ → CO₂ + 2H₂O |
⚠️ Key Difference
Unlike hydrogen fuel cells, methanol fuel cells do produce CO₂. However, they still offer advantages over combustion engines in terms of efficiency and lower overall emissions.
Fuel Cell vs Primary (Voltaic) Cell
| Fuel Cell | Primary Cell (Battery) | |
|---|---|---|
| Reactant supply | Continuous external supply | Fixed amount sealed inside |
| Lifespan | Runs as long as fuel is supplied | Stops when reactants are used up |
| Recharging | Not needed (just add more fuel) | Cannot be recharged |
| Efficiency | 60-80% | Varies (typically lower) |
Advantages and Challenges
✅ Advantages
- Zero direct emissions (H₂ cells produce only water)
- Higher efficiency (60-80% vs 25-30% for combustion)
- Silent operation (no moving mechanical parts)
- Continuous power as long as fuel is supplied
❌ Challenges
- H₂ storage: needs high-pressure tanks (700 bar) or cryogenic cooling
- Production: most H₂ from steam reforming of CH₄ (still produces CO₂)
- Cost: platinum catalysts are very expensive
- Infrastructure: limited refuelling stations worldwide
💬 Deep Think
Hydrogen fuel cells produce zero emissions at the point of use, but the hydrogen itself must be produced. If it comes from steam reforming of methane, the overall process still generates CO₂. Only "green hydrogen" (produced by electrolysis using renewable electricity) is truly carbon-neutral. This is a common exam discussion point.