Explain why a hydrogen fuel cell can be considered an alternative to a rechargeable battery, and give one reason why it might not be a suitable replacement in all applications.

A hydrogen fuel cell can be considered an alternative because it generates electricity electrochemically from hydrogen and oxygen, similar to how a rechargeable battery generates electricity from the movement of ions between electrodes. Unlike a rechargeable battery, a fuel cell can keep producing electricity as long as fuel is supplied, so it can be used for continuous power. However, it may not be suitable for all applications because it requires a safe and reliable supply of hydrogen gas, which is difficult to store and transport safely, whereas rechargeable batteries can be easily carried and recharged with electricity.

The answer shows the comparison of the underlying electrochemical processes and the practical advantage of continuous operation. It also identifies a key limitation—hydrogen supply and safety—that prevents fuel cells from replacing batteries in every context.