In a hydrogen fuel cell, the cathode serves as the positive electrode where reduction occurs, while the anode functions as the negative electrode where oxidation takes place. The two electrodes are separated by an electrolyte that permits the passage of ions but not electrons. As a result, electrons are compelled to flow through an external circuit, generating an electric current.

At the cathode, oxygen gas ($O_2$) from the atmosphere is introduced. This oxygen reacts with hydrogen ions ($H^+$) that have migrated through the electrolyte from the anode, as well as the electrons that have traversed the external circuit. The reaction at the cathode is classified as a reduction reaction since the oxygen is gaining electrons. The balanced chemical equation for this process is given by:

This equation indicates that one molecule of oxygen gas combines with four hydrogen ions and four electrons to produce two molecules of water. The reaction is exothermic, meaning it releases energy, which is harnessed to power the fuel cell.

The only byproduct of this reaction is water, which makes hydrogen fuel cells an exceptionally clean energy source. However, it is important to note that the hydrogen gas utilized at the anode is frequently derived from natural gas, a fossil fuel. Consequently, while the operation of the fuel cell itself is environmentally friendly, the overall production process may still contribute to greenhouse gas emissions.