Hydrogen-oxygen fuel cells

What a fuel cell is

A hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity, and water is the only chemical product.

That single sentence is the whole of the Core requirement for 4.2, and the phrase that carries the marks is the only chemical product. A fuel cell running on hydrogen produces no carbon dioxide, no carbon monoxide, no oxides of nitrogen, no unburnt hydrocarbons and no particulates.

The overall reaction is the same as burning hydrogen:

2H₂(g) + O₂(g) → 2H₂O(l)

The difference is that the energy comes out as electricity rather than as heat and light, because the hydrogen and the oxygen are kept apart and made to react through an external circuit.

A fuel cell is not a battery. A battery holds a fixed amount of chemicals and goes flat; a fuel cell keeps producing electricity for as long as hydrogen and oxygen are supplied to it.

The masses that react

The equation fixes the proportions. Mᵣ(H₂) = 2, Mᵣ(O₂) = 32 and Mᵣ(H₂O) = 18, so 2 × 2 = 4 g of hydrogen reacts with 32 g of oxygen and produces 2 × 18 = 36 g of water. Mass is conserved: 4 + 32 = 36.

The oxygen is taken from the air, so only the hydrogen has to be carried on board.

At the two electrodes

This detail is beyond what 0620 asks you to recall, but it makes the cell make sense and it uses the half-equations from 4.1.

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• At the negative electrode: 2H₂ → 4H⁺ + 4e⁻, so hydrogen is oxidised and releases electrons into the circuit.
• At the positive electrode: O₂ + 4H⁺ + 4e⁻ → 2H₂O, so oxygen is reduced and takes the electrons back.

Adding those two together cancels the electrons and the H⁺ ions and leaves 2H₂ + O₂ → 2H₂O, the overall equation above.

Fuel cells against petrol engines (Extended)

Everything below is Extended only. A Core candidate needs the definition and the equation but not this comparison.

Advantages

It produces no pollutants at the point of use. Water is the only chemical product, so a fuel cell vehicle emits no carbon dioxide, which contributes to climate change, and none of the carbon monoxide, oxides of nitrogen or particulates that damage health in cities.

It is more efficient. A fuel cell converts chemical energy directly into electrical energy. A petrol engine must first burn the fuel and then convert heat into movement, and most of the heat is lost. Roughly 60% of the chemical energy is usefully converted in a fuel cell against roughly 20 to 30% in a petrol engine, so a given amount of energy takes you considerably further.

Hydrogen carries more energy per gram than petrol. Hydrogen releases about 142 kJ per gram and petrol about 46 kJ per gram, so per gram hydrogen supplies 142 / 46 = 3.1 times as much energy.

It is quiet and reliable. A fuel cell has almost no moving parts, so it makes little noise and needs less maintenance than an engine full of pistons and valves.

Disadvantages

Hydrogen is hard to store and transport. It is a gas at room temperature with a very low density, so a useful amount must be compressed to high pressure or cooled to a liquid below −253 °C. Both take energy, cost money and take up space in the vehicle, and this is why hydrogen's advantage per gram does not translate into an advantage per litre of tank.

Hydrogen is highly flammable. It forms explosive mixtures with air over a wide range of concentrations, so storage and refuelling need careful engineering.

There are very few refuelling stations. Petrol stations are everywhere; hydrogen stations are rare, which limits how far such a vehicle can usefully travel.

Fuel cells are expensive. They rely on catalysts such as platinum, which is scarce and costly, and the cells have a limited working life.

The hydrogen has to be made first. Most hydrogen is currently manufactured from methane, which releases carbon dioxide, or by electrolysis of water, which needs a large amount of electricity. If that electricity is generated by burning fossil fuels then the pollution has been moved somewhere else rather than removed. A fuel cell is only genuinely clean if its hydrogen was made cleanly.

Performance falls at low temperatures, which matters in cold climates.

Answering the comparison question

The question is almost always "give one advantage and one disadvantage". The safest pair is:

• Advantage: water is the only product, so no carbon dioxide is released.
• Disadvantage: hydrogen is difficult to store, needing high pressure or very low temperatures.

Both are specific. Answers such as "it is better for the environment" or "it is expensive" are too vague to score.

Common mistakes

• Saying the fuel cell produces no products at all. It produces water.
• Saying hydrogen is burned in the cell. It reacts electrochemically and the energy is released as electricity.
• Calling a fuel cell a battery that needs recharging. It runs as long as it is fed.
• Claiming a fuel cell vehicle causes no pollution anywhere, without mentioning how the hydrogen was made.
• Saying hydrogen is easy to store because it is light. Its low density is exactly why storing it is difficult.
• Giving "it is cleaner" as the whole answer instead of naming water as the only product.

Check you have it

Question 1

Hydrogen is used as a fuel in rockets and is also used in hydrogen fuel cells.
Which statements are correct?
1 Both uses produce water vapour.
2 Burning hydrogen produces polluting gases.
3 A fuel cell produces electricity.

A1, 2 and 3B1 and 3 onlyC1 onlyD2 and 3 only

Question 2

Which statements about a hydrogen–oxygen fuel cell are correct?

A1 and 3B1 and 4C2 and 3D2 and 4

Question 3

Which statements about hydrogen are correct?
1 When hydrogen is burned, heat energy is released.
2 When hydrogen is used in a fuel cell, electrical energy is generated.
3 When hydrogen is used as a fuel, water is the only product.

A1, 2 and 3B1 and 2 onlyC1 onlyD3 only

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