What a fuel cell is
Definition
Fuel cell
An electrochemical cell that produces electricity from the reaction of a continuously supplied fuel with oxygen.
- A fuel cell produces electricity directly from a chemical reaction between a fuel and oxygen.
- The fuel and the oxygen are supplied continuously from outside, so a fuel cell keeps working for as long as they are fed in.
- An ordinary battery is different, because it holds a fixed amount of chemicals and stops working once they are used up.
- A fuel cell does not burn its fuel in a flame and has no moving parts.
- Instead, the reaction is split between two electrodes, so much of the energy released is transferred as an electric current rather than as heat and light.
The hydrogen-oxygen fuel cell
Definition
Hydrogen-oxygen fuel cell
A fuel cell in which hydrogen reacts with oxygen to generate electricity, forming water as its only chemical product.
- The most widely used fuel cell is the hydrogen-oxygen fuel cell, which uses hydrogen as its fuel.
- The oxygen is usually taken from the air, so only the hydrogen has to be stored and carried.
- The cell contains two electrodes separated by an electrolyte, with hydrogen gas fed to one electrode and oxygen gas fed to the other.
- The electrodes are porous and coated with a catalyst such as platinum, which speeds up the reactions on their surfaces.
- As hydrogen and oxygen react at the electrodes, electrons flow from one electrode to the other through an external circuit, and this flow is the electric current.
- The only chemical product is water, which leaves the cell as liquid water or water vapour.
- The overall reaction is hydrogen+oxygen→water\text{hydrogen} + \text{oxygen} \rightarrow \text{water}hydrogen+oxygen→water, or as a symbol equation 2H2(g)+O2(g)→2H2O(l)2\text{H}_2(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2\text{H}_2\text{O}(\text{l})2H2(g)+O2(g)→2H2O(l).
- This is the same overall reaction as burning hydrogen, but in a fuel cell much of the energy is released as electrical energy instead of as heat.
- A fuel cell is the reverse of the electrolysis of water, because electrolysis uses electricity to split water into hydrogen and oxygen while a fuel cell joins them to release electricity.
Uses of hydrogen-oxygen fuel cells
- Fuel cells are used wherever a steady, long-lasting supply of electricity is needed without burning a fuel.
- Vehicles: fuel-cell cars, buses and lorries use the electricity to run an electric motor.
- Spacecraft: fuel cells supplied electricity on the Apollo missions and the Space Shuttle.
- Back-up power: fuel cells provide emergency electricity for buildings such as hospitals and for mobile phone masts.
- In every one of these uses, the waste product is only water, so the cell releases nothing harmful where it is working.
Example
Electricity and water in space
- The Apollo spacecraft carried tanks of liquid hydrogen and liquid oxygen.
- Fuel cells combined the two gases to generate electricity for the spacecraft's lights, radios and computers.
- The water produced by the cells was pure enough for the astronauts to drink.
- This was only possible because water is the only chemical product of a hydrogen-oxygen fuel cell.
Common Mistake
- Do not say that a fuel cell burns hydrogen, because the hydrogen reacts at the electrodes without a flame.
- Do not give carbon dioxide as a product, because the fuel contains no carbon and water is the only chemical product.
- Do not confuse a fuel cell with electrolysis, because electrolysis uses electricity to break compounds down while a fuel cell produces electricity.
Exam technique
Stating what a fuel cell does
- A complete statement names both reactants, hydrogen and oxygen, says that electricity is produced, and says that water is the only chemical product.
- Write electricity or electrical energy rather than just energy, because energy alone would also describe the combustion of hydrogen.
Self review
- Which two substances react in a hydrogen-oxygen fuel cell?
- What is the only chemical product of a hydrogen-oxygen fuel cell?
- What is the balanced symbol equation for the overall reaction in the cell?
- How is a fuel cell different from an ordinary battery?
- How does the energy released by a fuel cell differ from the energy released when hydrogen burns?
