Electrolysis uses electricity to split water. A fuel cell runs the idea backwards: it combines hydrogen and oxygen and releases the energy as electricity, not as a flame. It powers buses and cars with nothing but water from the exhaust, but the hydrogen has to come from somewhere.
A hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity, with water as the only chemical product. The overall reaction is 2H2 + O2 → 2H2O. Extended: compared with a petrol engine, a fuel-cell vehicle produces no carbon dioxide or other pollutants at the point of use and is more efficient, but hydrogen is difficult to store, is often made using fossil fuels or electricity, and there are few refuelling stations.
Hydrogen is fed to one electrode and oxygen, usually from the air, to the other. They react without burning, and the electrons transferred in the reaction flow round an external circuit.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the only chemical product of a hydrogen-oxygen fuel cell?
What does a hydrogen-oxygen fuel cell produce that is used to power a vehicle?
Which equation shows the overall reaction in a hydrogen-oxygen fuel cell?
How does a fuel cell differ from a battery?
Extended. Exam questions ask you to weigh up a fuel-cell car against a petrol (gasoline) car. Aim for specific points, not just "better for the environment".
| Advantages of fuel cells | Disadvantages of fuel cells |
|---|---|
| only water is produced; no carbon dioxide, carbon monoxide or oxides of nitrogen at the point of use | hydrogen is a gas that is hard to store; it needs large, strong, pressurised tanks |
| more efficient at converting the fuel's energy into useful energy | hydrogen is flammable and forms explosive mixtures with air |
| no burning of a non-renewable fossil fuel in the vehicle | most hydrogen is made from methane (a fossil fuel) or by electrolysis, which needs electricity |
| fewer moving parts and quieter | few hydrogen refuelling stations; fuel cells are expensive to make |
"Fuel cells produce no pollution" is not quite right. The vehicle emits only water, but making the hydrogen can release carbon dioxide, for example when it is produced from methane or when the electricity for electrolysis comes from burning fossil fuels. Say "no carbon dioxide at the point of use".
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
(Extended) Which is an advantage of a hydrogen fuel-cell car compared with a petrol car?
(Extended) Which is a disadvantage of using hydrogen fuel cells in vehicles?
(Extended) Most hydrogen is made from methane. Why does this reduce the environmental benefit of fuel-cell cars?
Hydrogen can also be burned in an engine. What is the main difference in a fuel cell?
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