Fuel Cells
Summary: A fuel cell converts chemical energy directly into electrical energy. The hydrogen-oxygen fuel cell combines H₂ and O₂ to produce electricity with water as the only waste product. Unlike batteries, fuel cells don’t run down as long as fuel is supplied. Tags: igcse chemistry fuel-cells electrochemistry energy Created: 2026-07-14 Last Updated: 2026-07-14
What Is a Fuel Cell?
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel directly into electrical energy. Unlike a battery:
| Battery (Cell) | Fuel Cell |
|---|---|
| Chemical energy stored INSIDE | Fuel supplied CONTINUOUSLY from outside |
| Eventually runs down / goes flat | Keeps producing electricity as long as fuel + O₂ are supplied |
| Rechargeable or disposable | Does not need recharging |
| Contains finite reactants | Reactants flow in continuously |
The Hydrogen-Oxygen Fuel Cell
This is the only type required for IGCSE (0620/0971). It combines hydrogen and oxygen to produce water and electricity.
The Overall Reaction
2H₂(g) + O₂(g) → 2H₂O(l)
This is exactly the same reaction as burning hydrogen in oxygen, but in a fuel cell the energy is released as electrical energy, not heat and light.
Structure
A fuel cell consists of:
- Two platinum/polymer electrodes (anode and cathode) separated by an electrolyte (potassium hydroxide solution or a proton-exchange membrane)
- H₂ is fed to one electrode, O₂ to the other
- The electrolyte allows ions to pass through but prevents the gases from mixing
Electrode Reactions (Alkaline Electrolyte — KOH)
At the anode (negative electrode — oxidation): H₂(g) + 2OH⁻(aq) → 2H₂O(l) + 2e⁻
At the cathode (positive electrode — reduction): O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
Overall: 2H₂(g) + O₂(g) → 2H₂O(l)
Electrode Reactions (Acid Electrolyte — Proton-Exchange Membrane)
At the anode (negative — oxidation): H₂(g) → 2H⁺(aq) + 2e⁻
At the cathode (positive — reduction): O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)
Overall: 2H₂(g) + O₂(g) → 2H₂O(l) — same overall reaction in both types
Advantages of Hydrogen Fuel Cells
| Advantage | Explanation |
|---|---|
| Only waste product is water | No CO₂, NOₓ, SO₂, or particulates — zero emission at point of use |
| Higher efficiency | Converts chemical → electrical energy directly, not via heat (avoids Carnot efficiency limit). ~50-60% efficient vs ~25-30% for petrol engines |
| Doesn’t run down | As long as H₂ and O₂ are supplied, electricity is produced continuously |
| No moving parts | Quiet operation, less maintenance |
| Lightweight for energy output | Higher energy density than batteries |
Disadvantages of Hydrogen Fuel Cells
| Disadvantage | Explanation |
|---|---|
| Hydrogen production is energy-intensive | Most H₂ comes from steam reforming of methane (fossil fuel) or electrolysis of water (needs electricity, often from fossil fuels) |
| Hydrogen storage is difficult | H₂ is a gas at room temperature — must be compressed at high pressure (700 bar) or liquefied at −253°C (energy-intensive) |
| Hydrogen is highly flammable | Forms explosive mixtures with air (4–75% concentration range) |
| Expensive catalysts | Platinum or palladium catalysts are costly |
| Infrastructure not established | No widespread hydrogen refuelling network |
Comparison: Fuel Cell vs Petrol Engine
| Factor | Hydrogen Fuel Cell | Petrol/Diesel Engine |
|---|---|---|
| Emissions at vehicle | H₂O only | CO₂, CO, NOₓ, SO₂, particulates, unburnt hydrocarbons |
| Efficiency | ~50–60% | ~25–30% (most energy lost as heat) |
| Fuel source | H₂ (must be manufactured) | Crude oil (finite, non-renewable) |
| Refuelling | Hydrogen stations (rare) | Petrol stations (everywhere) |
| Noise | Quiet | Noisy |
| Range | ~300-400 miles (similar to petrol) | ~400 miles |
| Cost | Expensive (platinum catalyst) | Mature technology (cheaper) |
Environmental Impact
”Is hydrogen a clean fuel?”
At point of use: Yes — the only product is water. Zero greenhouse gas or pollutant emissions.
Overall (lifecycle): It depends how the hydrogen is made:
- Green hydrogen: Made by electrolysis of water using renewable electricity (solar/wind) → truly clean
- Blue hydrogen: Made from methane — CO₂ produced is captured/stored (carbon capture) → partially clean
- Grey hydrogen: Made from methane without carbon capture → releases CO₂, just like burning fossil fuels
Currently, ~95% of hydrogen is “grey” — made from methane.
Common Question Types
Type 1: Write the Overall Equation for the Hydrogen Fuel Cell (1-2 marks)
- Answer: 2H₂ + O₂ → 2H₂O
- Often worth easy marks
Type 2: Give Advantages of Fuel Cells (2-3 marks)
- Frequency: ~30% of papers
- Water is the only waste product
- Do not run down (unlike batteries)
- Higher efficiency than combustion engines
- Less pollution
Type 3: Give Disadvantages of Fuel Cells (2-3 marks)
- Hydrogen is flammable/difficult to store safely
- Hydrogen is usually made from fossil fuels (methane)
- Expensive catalysts
- Limited refuelling infrastructure
Type 4: Comparison (3-4 marks)
- “Compare and contrast a hydrogen fuel cell vehicle with a petrol-powered vehicle”
- Must give BOTH advantages and disadvantages for full marks
Common Mistakes
- Confusing a fuel cell with a battery: Fuel cells don’t store energy — they convert fuel
- Saying hydrogen is “found naturally”: H₂ must be manufactured (from water or methane)
- Forgetting the catalyst: Platinum electrodes are needed for the electrode reactions to occur at a reasonable rate
- Saying fuel cells are “100% clean”: Only at the point of use — hydrogen production may not be clean
- Writing OH⁻ on wrong side of half equations: In alkaline electrolyte, OH⁻ is consumed at the anode, produced at the cathode
Key Facts to Memorize
- Overall equation: 2H₂ + O₂ → 2H₂O
- Fuel cells do not run down — electricity produced as long as fuel is supplied
- Only waste product = water
- H₂ is obtained from methane (steam reforming) or electrolysis of water
- Contains a platinum catalyst bonded to porous electrodes
- More efficient than petrol engines (converts chemical energy directly to electrical energy, not via heat)
- Hydrogen is difficult to transport and store (compressed gas or cryogenic liquid)
Related Notes
- Electrolysis Applications — Electrolysis of water to produce H₂
- Electrolysis of Aqueous Solutions — Competition at electrodes; half equations
- Redox Reactions — Oxidation and reduction in fuel cells
- Ionic Equations and Half Equations — Writing half equations for fuel cells
- Energetics of Reactions — Exothermic reactions; bond energy
- Climate Change and Greenhouse Gases — Why reducing CO₂ emissions matters
- Composition of Air — O₂ from air is used in fuel cells
- IGCSE-Chem-Index
Past Paper Sources
- 0971/42 May/June 2023 Q8(c): Write equation for fuel cell, give one advantage (3 marks)
- 0620/43 Oct/Nov 2019 Q7(d): Explain why fuel cells are considered cleaner than petrol engines (3 marks)
- 0971/32 May/June 2021 Q6(e): Give disadvantage of hydrogen as a fuel for cars (2 marks)
- 0620/42 Feb/March 2022 Q8(f): Compare fuel cells with rechargeable batteries (3 marks)
IGCSE Chemistry (0620/0971) wiki. Modern topic — increasingly common in exams from 2020 onwards.