Hydrogen-Oxygen Fuel CellsCambridge IGCSE Chemistry: Revision notes
Section 1
What Is a Hydrogen-Oxygen Fuel Cell?
A hydrogen-oxygen fuel cell is a device that uses hydrogen and oxygen gas to produce electricity directly, without burning the fuel.
- Hydrogen and oxygen react inside the cell to produce electricity
- Water is the only chemical product — no carbon dioxide or other pollutants are released
- The overall reaction is the same as hydrogen combustion (hydrogen + oxygen → water), but the energy is released as electrical energy rather than heat
Remember the key selling point of a fuel cell in one phrase: 'produces electricity with water as the only chemical product' — this is exactly how the spec phrases it.
Section 2
How Does Charge Move Inside the Cell?
A fuel cell works using the same general principles as electrolysis, but electricity is generated rather than supplied.
- Electrons flow through the external circuit, from the electrode where hydrogen is oxidised to the electrode where oxygen is reduced — this flow of electrons is the electric current
- At the electrodes, molecules lose or gain electrons: hydrogen loses electrons (is oxidised) and oxygen gains electrons (is reduced)
- Ions move through the electrolyte inside the cell to complete the circuit and maintain overall charge balance
Think of the fuel cell like a controlled version of the hydrogen 'pop test' — instead of the electrons' energy being released suddenly as heat and sound, it is channelled through a circuit as useful electrical energy.
Section 3
What Are the Ionic Half-Equations Involved?
The overall reaction in a hydrogen-oxygen fuel cell can be broken down into two half-equations, showing oxidation at one electrode and reduction at the other.
- Oxidation (loss of electrons) happens where hydrogen reacts, releasing electrons into the external circuit
- Reduction (gain of electrons) happens where oxygen reacts, taking in electrons from the external circuit
- Overall, the electrons removed at the oxidation electrode are exactly balanced by the electrons supplied at the reduction electrode
This pairing of oxidation at one electrode and reduction at the other is the same pattern seen in electrolysis, just running to generate current rather than consume it.
Don't mix up which electrode is which — oxidation (electron loss) always happens at the electrode where hydrogen reacts, not oxygen.
Section 4
Why Are Fuel Cells Used Instead of Petrol/Gasoline Engines?
Fuel cells are increasingly used as an alternative to conventional combustion engines, particularly in vehicles.
Advantages:
- Only water is produced, so there is no carbon dioxide or other harmful emissions from the vehicle itself
- Fuel cells can be more efficient at converting chemical energy into useful energy than a combustion engine, which wastes more energy as heat
Disadvantages:
- Hydrogen gas is difficult and expensive to store and transport safely, since it is highly flammable and takes up a large volume as a gas
- There is currently limited infrastructure (few refuelling stations) for hydrogen vehicles compared with petrol stations
- Producing hydrogen gas in the first place often requires energy, which may itself come from fossil fuels
A hydrogen fuel cell car only emits water vapour from its exhaust, but the hydrogen it uses must be produced, compressed and transported safely beforehand.
Must Know
- A hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as the only chemical product
- Electrons flow through the external circuit from the hydrogen (oxidation) electrode to the oxygen (reduction) electrode
- Ions move through the electrolyte inside the cell to complete the circuit
- Oxidation is loss of electrons (at the hydrogen electrode); reduction is gain of electrons (at the oxygen electrode)
- Advantage over petrol/gasoline engines: no carbon dioxide or pollutant emissions, only water
- Disadvantages: hydrogen is hard/costly to store and transport, and refuelling infrastructure is limited
That's the notes covered.
Carry on to the next subtopic.