Continuous Electrical Power from Chemical Reactions
A fuel cell converts chemical energy from a fuel directly to electrical energy. Unlike conventional closed batteries that run down, fuel cells generate continuous electricity as long as an external supply of fuel and oxidant is maintained.
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 (Animated)
Hydrogen Fuel Cell Half-Equations
Assessed Fuel Cell Half-Equations
You must be able to deduce and balance redox half-equations for both hydrogen and direct methanol fuel cells. Proton exchange membrane (PEM) micro-mechanisms 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 |
Methanol Fuel Cells Produce Carbon Dioxide
Unlike pure hydrogen fuel cells which only produce water, direct methanol fuel cells do emit CO₂ (CH₃OH + 1.5O₂ → CO₂ + 2H₂O). However, they still offer higher thermal efficiency and lower particulate emissions than combustion engines.
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 of Fuel Cell Technology
- Zero direct point-of-use emissions: H₂ fuel cells produce only pure water vapour.
- High thermodynamic efficiency: 60–80% conversion efficiency compared to 25–30% for thermal engines.
- Silent operation: Absence of reciprocating mechanical components.
- Continuous operation: Rapid refuelling with external reactants.
Commercialisation Challenges for Fuel Cells
- Hydrogen storage: Low volumetric energy density requires high-pressure (700 bar) or cryogenic storage.
- Hydrogen source: Steam methane reforming currently produces fossil-derived CO₂.
- Electrocatalyst cost: Platinum and ruthenium catalysts remain expensive.
- Distribution infrastructure: Limited refuelling network.
Point-of-Use vs Lifecycle Carbon Neutrality
Hydrogen fuel cells achieve zero emissions at the tailpipe, but the overall environmental impact depends heavily on the hydrogen production pathway. If hydrogen is derived from steam reforming of methane, substantial fossil CO₂ is emitted upstream. Only "green hydrogen" produced via water electrolysis using renewable electricity provides true lifecycle carbon neutrality.
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