A chemical cell and a hydrogen-oxygen fuel cell can both be used to produce a voltage. Describe how a hydrogen-oxygen fuel cell works and explain one way in which it differs from a chemical cell in terms of how long it can continue to produce a voltage.

A hydrogen‑oxygen fuel cell is a device that is continuously fed with hydrogen gas at the anode and oxygen gas at the cathode.
1. At the anode the hydrogen is oxidised:

2H₂ → 4H⁺ + 4e⁻

The electrons travel through the external circuit to the cathode.
2. At the cathode the oxygen is reduced, combining with the electrons and the protons that have travelled through the electrolyte to form water:

O₂ + 4H⁺ + 4e⁻ → 2H₂O

The overall reaction is 2H₂ + O₂ → 2H₂O.
3. The flow of electrons through the external circuit gives a voltage.
4. Unlike a simple chemical cell, which stops producing voltage when its reactants are exhausted, a fuel cell can keep generating voltage as long as hydrogen and oxygen are supplied, because the reactants are not consumed in the cell itself but are continually fed in.