Why must fuel and oxygen continue to be supplied to a fuel cell?

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Diagram of a proton exchange membrane (PEM) fuel cell showing labeled layers, including flow field plates, electrodes, and the proton exchange membrane, with arrows indicating hydrogen and oxygen inputs, electricity generation, and water and heat as outputs. — fuel cell Proton exchange membrane (PEM) fuel cell
The proton exchange membrane is one of the most advanced fuel cell designs. Hydrogen gas under pressure is forced through a catalyst, typically made of platinum, on the anode (negative) side of the fuel cell. At this catalyst, electrons are stripped from the hydrogen atoms and carried by an external electric circuit to the cathode (positive) side. The positively charged hydrogen ions (protons) then pass through the proton exchange membrane to the catalyst on the cathode side, where they react with oxygen and the electrons from the electric circuit to form water vapour (H₂O) and heat. The electric circuit is used to do work, such as power a motor.

Fuel and oxygen must be continuously supplied to a fuel cell because the device converts chemical energy directly into electricity through ongoing electrochemical reactions.12

Unlike a battery, which contains a finite amount of reactants that are depleted during use, the electrodes and electrolyte of a fuel cell are designed to remain unchanged by the chemical process. Consequently, the production of electric current depends entirely on the continuous availability of these external reactants. As long as fuel is supplied to the anode and an oxidizer like oxygen is supplied to the cathode, the system can generate power.12