4.2 Hydrogen–oxygen fuel cells

2026 Syllabus Objectives

Core:

  • State that a hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity with water as the only chemical product

Supplement:

  • Describe the advantages and disadvantages of using hydrogen–oxygen fuel cells in comparison with gasoline/petrol engines in vehicles

What is a Fuel Cell? ⚡

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A fuel cell is a device that continuously converts chemical energy into electrical energy using a combustion reaction. Unlike simple electrochemical cells (batteries) that lose their power and need recharging, a fuel cell operates continuously as long as the reactants are fed into the electrodes.

Key Principle 🔑

In electrolysis, electrical energy is used to make a chemical reaction happen (decomposition - endothermic process). A fuel cell reverses this process by using chemical reactions to produce electrical energy.

Hydrogen-Oxygen Fuel Cells

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Basic Operation

A hydrogen-oxygen fuel cell uses the reaction between hydrogen and oxygen to generate electricity. This is a much more efficient way of changing chemical energy into electrical energy compared to ordinary batteries.

Key features:

  • Operates continuously with no need for recharging
  • Supplies energy as long as reactants are fed to the electrodes
  • Uses hydrogen as fuel and oxygen from air
  • Water is the only product

Overall Reaction

The hydrogen-oxygen fuel cell produces electricity through the following reaction:

2H2(g)+O2(g)→2H2O(l)2\mathrm{H}_2(g) + \mathrm{O}_2(g) \rightarrow 2\mathrm{H}_2\mathrm{O}(l)

This reaction releases a large amount of energy, with water as the only chemical product.

Structure of a Hydrogen-Oxygen Fuel Cell

The fuel cell consists of:

  • Negative electrode (cathode) - where hydrogen is fed in
  • Positive electrode (anode) - where oxygen is fed in
  • Electrolyte - allows ion movement between electrodes
  • Carbon electrodes - conduct electrons through external circuit

How it works:

  1. Hydrogen gas (H2\mathrm{H}_2) is pumped in from a tank to the negative electrode
  2. Oxygen (O2\mathrm{O}_2) from air enters at the positive electrode
  3. The reaction produces electricity and water vapor (H2O\mathrm{H}_2\mathrm{O})
  4. Electrons flow through an external circuit (e.g., to power an electric motor)
  5. Water vapor is released as exhaust

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