Fuel cells — AQA GCSE Chemistry
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Fuel cells explained
Fuel cells covers 5 specification points, including Fuel cells are supplied by an external source… and The fuel is oxidised electrochemically within….
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Open a point below for its explanation and revision guidance.
Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air.
A fuel cell is a device that generates a potential difference continuously as long as it is supplied with fuel and oxygen. Unlike a battery, which stores a fixed amount of chemical energy, a fuel cell is fed from outside. In a hydrogen fuel cell, hydrogen is supplied from a cylinder or store and oxygen is supplied from the air. The hydrogen is oxidised at one electrode and oxygen is reduced at the other, so the cell keeps working while both inputs continue. For example, in a hydrogen-powered vehicle, hydrogen from a tank and oxygen from the air enter the fuel cell, which produces electricity to drive the motor. This statement establishes that the reactants are external, not stored inside the cell.
The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.
Inside a fuel cell, the fuel is oxidised at an electrode in an electrochemical reaction. Oxidation means the fuel loses electrons, and these electrons flow through an external circuit, creating a potential difference that can drive a current. In a hydrogen fuel cell, hydrogen is oxidised at the negative electrode, releasing electrons that travel through the external circuit to the positive electrode, where oxygen is reduced. The key idea is that the oxidation is electrochemical: it happens at an electrode surface and transfers electrons through the circuit rather than releasing energy only as heat. For example, hydrogen molecules are oxidised to hydrogen ions and electrons at one electrode, and the electron flow does electrical work in the external circuit.
The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water.
The overall chemical change in a hydrogen fuel cell is the oxidation of hydrogen to form water. Hydrogen is oxidised at one electrode and oxygen is reduced at the other, and the overall equation is 2H₂ + O₂ → 2H₂O. This means the only chemical product is water, which is why hydrogen fuel cells are described as producing no carbon dioxide at the point of use. The reaction releases energy electrochemically, producing a potential difference rather than heat alone. For example, in a hydrogen fuel cell vehicle, hydrogen from the tank and oxygen from the air combine to form water vapour, while electricity drives the motor. You should be able to state the overall reaction and identify hydrogen as the substance oxidised.
Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries.
A hydrogen fuel cell produces a voltage continuously while hydrogen and oxygen are supplied, because the reaction between them releases energy that is transferred electrically. Hydrogen enters the anode, where it is oxidised, and oxygen enters the cathode, where it is reduced; the only chemical product is water, so no carbon dioxide is released at the point of use. Unlike a rechargeable cell or battery, a fuel cell does not store its reactants internally and does not need recharging: it keeps working as long as fuel and oxygen flow. This makes it a potential alternative in vehicles and portable devices, but hydrogen must be made, stored and transported, which affects overall cost and environmental impact.
Students should be able to: evaluate the use of hydrogen fuel cells in comparison with rechargeable cells and batteries (HT only) write the half equations for the electrode reactions in the hydrogen fuel cell. AQA GCSE Chemistry 8462. GCSE exams Jun
Evaluating hydrogen fuel cells means weighing advantages, such as continuous electricity while hydrogen and oxygen are supplied and water as the only chemical product, against drawbacks, such as the energy and cost of producing, storing and transporting hydrogen. Rechargeable cells and batteries store reactants internally and must be recharged, but they are familiar and widely available. Higher Tier students must also write half equations showing what happens at each electrode: in acidic conditions, at the anode hydrogen is oxidised to hydrogen ions and electrons, and at the cathode oxygen and hydrogen ions are reduced to water. Balancing atoms and charges is essential for these HT equations.
Your focus
- State that a fuel cell is supplied with fuel and oxygen or air from external sources.
- Identify hydrogen as an example of a fuel used in fuel cells.
- Describe how the continuous supply of reactants allows a fuel cell to keep producing a potential difference.
Show all 15 objectives
- Describe the oxidation of fuel at an electrode in a fuel cell.
- Explain how electron loss during oxidation leads to a potential difference.
- Relate the electrochemical oxidation to the flow of electrons through an external circuit.
- State the overall reaction in a hydrogen fuel cell as hydrogen being oxidised to water.
- Write the balanced equation 2H₂ + O₂ → 2H₂O.
- Identify hydrogen as the fuel that is oxidised and water as the product.
- Describe how a hydrogen fuel cell produces a voltage while hydrogen and oxygen are supplied.
- Compare hydrogen fuel cells with rechargeable cells and batteries in terms of operation, products and environmental impact.
- Explain why a hydrogen fuel cell is described as a potential alternative rather than a straightforward replacement.
- Evaluate the use of hydrogen fuel cells compared with rechargeable cells and batteries, giving advantages and disadvantages.
- (HT only) Write balanced half equations for the anode and cathode reactions in a hydrogen fuel cell.
- (HT only) Check that half equations balance both atoms and charge.
Fuel cells exam tips
Marking Points
- A fuel cell requires a continuous external supply of fuel.
- Hydrogen is a common fuel used in fuel cells.
- Oxygen or air is supplied as the other reactant.
- The fuel and oxygen are fed into the cell from outside rather than being stored as the cell's only energy source.
- The cell produces a potential difference while the fuel and oxygen supplies continue.
- The fuel undergoes oxidation at an electrode within the fuel cell.
- Oxidation involves the loss of electrons from the fuel.
- The electrons flow through an external circuit, producing a potential difference.
- The electrochemical oxidation occurs at an electrode surface rather than by simple burning.
- The potential difference drives a current that can do useful work.
- The overall reaction in a hydrogen fuel cell is hydrogen plus oxygen forming water.
- Hydrogen is oxidised in the overall reaction.
- The balanced overall equation is 2H₂ + O₂ → 2H₂O.
- Water is the only chemical product of the overall reaction.
- The reaction produces a potential difference as well as water.
- A hydrogen fuel cell generates electrical energy continuously while hydrogen and oxygen are supplied, rather than storing energy internally like a rechargeable cell.
- At the anode hydrogen is oxidised and at the cathode oxygen is reduced, so the overall reaction produces water as the only chemical product.
- Because the product is water, a hydrogen fuel cell releases no carbon dioxide at the point of use, unlike a cell or battery recharged using electricity from burning fossil fuels.
- A rechargeable cell or battery must be recharged from an external supply, whereas a fuel cell is refuelled by supplying more hydrogen.
- Evaluating the alternative requires comparing both benefits, such as continuous operation and only water as a product, and drawbacks, such as producing, storing and transporting hydrogen.
- Hydrogen is often made by processes that use fossil fuels, so the overall environmental benefit depends on how the hydrogen is produced.
- An evaluation should include both advantages and disadvantages of hydrogen fuel cells compared with rechargeable cells and batteries, not just one side.
- A valid advantage is that a fuel cell produces electricity continuously while reactants are supplied and its only chemical product is water.
- A valid drawback is that hydrogen must be produced, stored and transported, which can use energy and may involve fossil fuels.
- (HT only) The anode half equation for hydrogen oxidation in acidic conditions is H₂ → 2H⁺ + 2e⁻.
- (HT only) The cathode half equation for oxygen reduction in acidic conditions is O₂ + 4H⁺ + 4e⁻ → 2H₂O.
- (HT only) Half equations must balance both atoms and charge, and the electrons lost at the anode must equal the electrons gained at the cathode when the equations are combined.
Examiner Tips
- 💡In explanations, name both inputs: the fuel, such as hydrogen, and oxygen or air.
- 💡Use the phrase external supply to make clear that the reactants are fed in continuously.
- 💡Contrast a fuel cell with a battery by stating that a battery stores reactants while a fuel cell is supplied from outside.
- 💡Use the term electrochemical to show that the oxidation occurs at an electrode and transfers electrons.
- 💡Link oxidation of the fuel to electron loss and then to electron flow in the external circuit.
- 💡Avoid saying the fuel burns; describe oxidation at the electrode instead.
- 💡Learn the balanced overall equation 2H₂ + O₂ → 2H₂O and use it when asked for the overall reaction.
- 💡State clearly that hydrogen is oxidised and water is produced.
- 💡If asked about environmental benefit, mention that the only product is water, avoiding claims beyond the specification.
- 💡When asked to compare, give at least one advantage and one disadvantage of hydrogen fuel cells relative to rechargeable cells or batteries.
- 💡Link the absence of carbon dioxide at the point of use to the product water, and mention that hydrogen production may still involve fossil fuels.
- 💡Use the terms anode, cathode, oxidation and reduction accurately when explaining how the fuel cell works.
- 💡For evaluation questions, structure your answer as advantages, disadvantages and a short overall judgement.
- 💡(HT only) When writing half equations, balance atoms first and then add electrons to balance charge.
- 💡(HT only) Check that the number of electrons lost at the anode equals the number gained at the cathode before combining the half equations.
Common Mistakes
- Treating a fuel cell as a battery that stores all its reactants internally; correct this by stating that fuel and oxygen are supplied continuously from external sources.
- Naming only hydrogen as the input and forgetting oxygen or air; correct this by stating that both a fuel and oxygen or air are supplied.
- Thinking the fuel cell must be recharged like a secondary cell; correct this by explaining that it operates as long as fuel and oxygen are supplied.
- Describing the fuel as burning in the fuel cell; correct this by explaining that the fuel is oxidised electrochemically at an electrode.
- Saying that oxidation is gain of electrons; correct this by stating that oxidation is loss of electrons.
- Claiming that the potential difference is produced without any electron transfer; correct this by linking the electron flow through the external circuit to the potential difference.
- Writing the product as hydrogen peroxide instead of water; correct this by stating that the overall product is water, H₂O.
- Forgetting to balance the equation; correct this by writing 2H₂ + O₂ → 2H₂O.
- Saying that hydrogen is reduced; correct this by stating that hydrogen is oxidised and oxygen is reduced.
- Saying a fuel cell stores hydrogen and oxygen inside itself like a battery stores chemicals; correct this by stating that reactants are supplied continuously from external stores.
- Claiming a hydrogen fuel cell produces carbon dioxide; correct this by stating that the only chemical product is water.
- Describing a fuel cell as needing to be recharged; correct this by explaining that it is refuelled with hydrogen and keeps producing electricity while reactants are supplied.
- Giving only advantages or only disadvantages in an evaluation; correct this by including both and reaching a supported judgement.
- (HT only) Writing a half equation that balances atoms but not charge; correct this by checking that the total charge is the same on both sides.
- (HT only) Putting the oxidation half equation at the cathode; correct this by placing hydrogen oxidation at the anode and oxygen reduction at the cathode.
- (HT only) Mixing acidic and alkaline half-equations for the same fuel cell; correct by using consistent conditions (e.g., both acidic).