Section 1 of 4What a Fuel Cell Does
A fuel cell converts chemical energy directly into electrical energy by reacting a fuel with oxygen, without a combustion step.
Fuel
- Fuel Hydrogen gas is the most common fuel, supplied continuously from a storage tank or from another chemical process.
Oxidant
- Oxidant Oxygen from the air is the second reactant, and it combines with hydrogen to form water.
Products
- Products The only product of a hydrogen fuel cell is water, so no carbon dioxide is released where the cell is used.
Continuous supply
- Continuous supply Unlike a battery, a fuel cell does not run down, because reactants are supplied constantly and products are removed.
Section 2 of 4How the Reaction Produces Current
The electrolyte separates the two gases, forcing the electrons to travel around an external circuit.
1
Hydrogen is supplied to one side of the cell and oxygen to the other side, with an electrolyte between them.
2
Hydrogen gives up electrons, which flow through an external circuit as an electric current that can power a motor.
3
The electrons return to the other side of the cell, where oxygen gains them and combines with hydrogen ions to form water.
4
Overall, 2H₂(g) + O₂(g) → 2H₂O(l), with the energy released leaving the cell as electricity and some heat.
| Feature | Hydrogen fuel cell | Combustion engine | Rechargeable battery |
|---|---|---|---|
| Energy conversion | Chemical to electrical, directly | Chemical to heat, then to motion | Chemical to electrical, directly |
| Emissions at point of use | Water only | Carbon dioxide and other pollutants | None |
| Refuelling or recharging | Refuelling takes minutes | Refuelling takes minutes | Recharging often takes hours |
Section 3 of 4Advantages and Disadvantages
Advantages and Disadvantages
Clean use
- Clean use No carbon dioxide or sulfur dioxide is produced where the fuel cell is used, which improves local air quality.
Quiet running
- Quiet running A fuel cell has few moving parts and produces little noise or vibration compared with an engine.
Hydrogen storage
- Hydrogen storage Hydrogen is a highly flammable gas that is difficult to store and transport safely.
Production
- Production Most hydrogen is currently made from natural gas, which releases carbon dioxide and reduces the overall environmental benefit.
Cost
- Cost Platinum catalysts and specialised components make many fuel cells expensive compared with batteries and engines.
Section 4 of 4Exam Checklist & Tips
5.2.2 – Key Points for the Exam
- Fuel cell A device that converts the chemical energy of a fuel into electrical energy while fuel and oxygen are supplied.
- Electrolyte A substance that allows ions to move between the two sides of a fuel cell while keeping the reactant gases apart.
- Combustion Burning a fuel in oxygen, which releases energy as heat and light rather than directly as electricity.
- Point of use The place where a fuel is consumed, such as inside a vehicle, where emissions are judged.
- Catalyst A substance that speeds up a reaction without being used up, such as the platinum used in some fuel cells.
Common Mistakes
- Describing a fuel cell as a store of energy like a battery;
- a fuel cell generates electricity from reactants supplied from outside.
- Claiming hydrogen fuel cells produce no emissions at all;
- producing the hydrogen can still release carbon dioxide.
- Saying a fuel cell burns hydrogen with a flame; the reaction generates electricity directly.
- Forgetting that fuel and oxygen must be supplied continuously for a fuel cell to keep working.
Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.