U.S. Department of Energy: Energy Efficiency and Renewable Energy: Hydrogen, Fuel Cells & Infrastructure Technologies Program
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The Chemical Process

H2 and anode:
Hydrogen fuel (H2) is channeled to the anode, where the catalyst separates the hydrogen's negatively charged electrons from the positively charged protons.
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close up of fuel cell showing two hydrogen atoms, the anode, PEM, and cathode.

The membrane allows the positively charged protons to pass through to the cathode, but not the negatively charged electrons.

The negatively charged electrons must flow around the membrane through an external circuit. This flow of electrons forms an electrical current.
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Fuel cell close up. Two hydrogen atoms now broken into four electrons traveling up flow plate and four protons passing through anode.

At the cathode, the negatively charged electrons and positively charged hydrogen ions (protons) combine with oxygen to form water (H20) and heat.

Four protons moving through cathode and electrons coming down flow plate. Protons and electrons combining with two oxygen molecules to form water.Water being discharged through flow plate.