Hydrogen-Oxygen Fuel Cells
Section: 4. Electrochemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
What is a Hydrogen-Oxygen Fuel Cell?
A hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity directly, with water as the only chemical product.
- Fuel (H₂) and oxidant (O₂, usually from air) are continuously supplied
- Electricity is generated for as long as fuel keeps flowing in
- No combustion takes place - the reaction occurs electrochemically, without burning
A Fuel Cell is the Reverse of Electrolysis
| Electrolysis of water | Hydrogen-oxygen fuel cell | |
|---|---|---|
| Energy | uses electrical energy | produces electrical energy |
| Reaction | 2H₂O → 2H₂ + O₂ | 2H₂ + O₂ → 2H₂O |
How the Fuel Cell Works
Hydrogen is supplied at the negative electrode; oxygen is supplied at the positive electrode. The two half-reactions are:
- At the negative electrode (oxidation): H₂(g) → 2H⁺(aq) + 2e⁻ - electrons flow out through the external circuit, which is the electric current produced
- At the positive electrode (reduction): O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)
Overall: 2H₂(g) + O₂(g) → 2H₂O(l)
Hydrogen is oxidised at one electrode and oxygen is reduced at the other, producing only water
Fuel Cell vs. Petrol Engine
Comparing a hydrogen-oxygen fuel cell vehicle with a petrol (gasoline) engine vehicle:
| Hydrogen fuel cell - advantages | Petrol engine - advantages |
|---|---|
| produces no carbon dioxide (only water) | can travel a greater distance on a full tank |
| generally more energy-efficient than combustion | far more filling stations available |
| Hydrogen fuel cell - disadvantages | Petrol engine - disadvantages |
|---|---|
| very few hydrogen filling stations exist | produces carbon dioxide, contributing to global warming |
| hydrogen is difficult and costly to store | produces other pollutants from combustion |
| hydrogen is highly flammable, so there is a risk of fire/explosion if the tank is damaged | a finite (non-renewable) fuel resource |
Worked Example: Matching Advantages to the Correct Vehicle
- Question: state one advantage of using the hydrogen-oxygen fuel cell, and one advantage of using the petrol engine, in a vehicle. [2]
- Answer: hydrogen fuel cell advantage - does not produce carbon dioxide gas, which causes global warming [1]; petrol engine advantage - can travel a greater distance after the fuel tank has been filled (petrol filling stations are also far more widespread) [1]
Common Mistakes
- ❌ Assuming a fuel cell has zero disadvantages because it produces no CO₂
- ✓ hydrogen is flammable, costly to store, and filling stations are scarce - always weigh advantages against disadvantages for both vehicle types
- ❌ Confusing a fuel cell with a battery
- ✓ a fuel cell needs a continuous supply of fuel (H₂) and oxidant (O₂) to keep producing electricity; a battery stores a fixed amount of chemical energy internally
- ❌ Writing the overall equation as H₂ + O₂ → H₂O
- ✓ the balanced equation is 2H₂ + O₂ → 2H₂O
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