4.3.6Additional lessons
Fuel Cells — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 58 of 98
Part 1 of 6How a Hydrogen Fuel Cell Works
A fuel cell is a source of electricity that does not run down while it is supplied with fuel.
Concept
- A fuel cell converts the chemical energy of a fuel directly into electrical energy through a redox reaction,
- as long as fuel and oxygen are supplied.
Anode
- Hydrogen is oxidised at the negative electrode: H₂(g) → 2H⁺(aq) + 2e⁻ in an acidic membrane cell.
Cathode
- Oxygen is reduced at the positive electrode: O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l).
Overall
- The overall reaction 2H₂(g) + O₂(g) → 2H₂O(l) is the same as hydrogen combustion, but the energy appears as electricity.
Part 2 of 6Flow of Charge
1
Hydrogen gas is fed to the anode, where a catalyst helps it split into hydrogen ions and electrons.
2
The electrolyte or proton exchange membrane lets H⁺ ions pass through to the cathode but not electrons.
3
Electrons are forced around the external circuit from anode to cathode, doing electrical work on the way.
4
At the cathode oxygen, hydrogen ions and electrons combine to form water, which leaves the cell as the only product.
| Electrode | Reaction type | Half-equation |
|---|---|---|
| Anode (−) | Oxidation | H₂ → 2H⁺ + 2e⁻ |
| Cathode (+) | Reduction | O₂ + 4H⁺ + 4e⁻ → 2H₂O |
Part 3 of 6Fuel Cells Compared with Combustion
Fuel Cells Compared with Combustion
Efficiency
- Fuel cells convert a larger fraction of the energy into useful work than engines, which lose much of the energy as heat.
Emissions
- A hydrogen fuel cell emits only water at the point of use, with no CO₂, CO or soot.
Continuous supply
- Unlike a battery, it does not need recharging while fuel is supplied, though it contains no stored charge.
Moving parts
- Fuel cells have few moving parts, so they run quietly.
Part 4 of 6Limitations
Limitations
Hydrogen source
- Most hydrogen is currently made from fossil fuels, which produce CO₂ unless renewable energy is used.
Storage
- Hydrogen is a gas of low density, so it must be compressed or liquefied, which needs energy and strong tanks.
Cost
- Catalysts such as platinum and membranes make fuel cells expensive.
Safety
- Hydrogen is flammable and forms explosive mixtures with air, requiring careful handling.
Part 5 of 6Worked Example
Question
- A hydrogen fuel cell consumes 1.00 g of hydrogen.
- Calculate the mass of water formed.
- (Mr H₂ = 2.02, Mr H₂O = 18.02)
- 2H₂ + O₂ → 2H₂O, so 1 mol H₂ gives 1 mol H₂O.
- Amount of H₂ = 1.00 ÷ 2.02 = 0.495 mol.
- Mass of H₂O = 0.495 × 18.02 = 8.92 g.
Answer 8.92 g of water
Part 6 of 6Exam Checklist & Tips
4.3.6 – Key Points for the Exam
- Fuel cell A device which converts the chemical energy of a fuel directly to electrical energy through a continuous redox reaction.
- Anode
- The electrode where oxidation occurs;
- in a fuel cell it is the negative electrode.
- The electrode where oxidation occurs;
- Cathode
- The electrode where reduction occurs;
- in a fuel cell it is the positive electrode.
- The electrode where reduction occurs;
- Proton exchange membrane A membrane that allows H⁺ ions to pass but blocks electrons and gases.
Common Mistakes
- Writing electrons flowing through the electrolyte; they flow through the external circuit.
- Saying that a fuel cell stores energy like a battery.
- Mixing up the electrodes, writing oxidation at the cathode.
- Claiming hydrogen fuel cells are carbon free regardless of how the hydrogen was produced.
Last updated:
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.