How Does a Fuel Cell Generate Electricity?

In a proton-exchange membrane (PEM) fuel cell, hydrogen gas enters the anode side, where a platinum catalyst strips electrons from hydrogen atoms. The freed electrons travel through an external circuit (generating electricity), while the hydrogen protons pass through a polymer membrane to the cathode. At the cathode, oxygen from air combines with the protons and electrons to form water — the only byproduct. A single fuel cell produces about 0.6–0.7 volts; cells are stacked in series to reach useful voltages. Toyota's Mirai fuel-cell car stacks 330 cells to produce 128 kW. Fuel cells are also used for stationary power, backup systems, and space missions — NASA's Apollo spacecraft used them for both electricity and drinking water.