Why do electrons flow through an external circuit in a 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.
In a fuel cell, electrons flow through an external circuit because of the electrochemical process occurring at the electrodes. Specifically, a fuel (such as hydrogen) is supplied to the anode, where it undergoes oxidation, a process that produces hydrogen ions and releases electrons.1
Because the anode and cathode are separated by an electrolyte that conducts ions but not electrons, the electrons are forced to travel through an external circuit to reach the cathode. At the cathode, an oxidizer (such as oxygen) is supplied, and the electrons arriving from the external circuit are absorbed to complete the reaction, typically producing water. This movement of electrons from the reducing agent at the anode to the oxidizing agent at the cathode through the external circuitry is what generates the electric current.12