How a Hydrogen Fuel Cell Works: Step-by-Step Science

The Reverse of Electrolysis

A hydrogen fuel cell is essentially an electrolyzer running in reverse. Where an electrolyzer uses electricity to split water into H₂ and O₂, a fuel cell combines H₂ and O₂ to produce electricity and water. The same fundamental electrochemistry applies.

PEM Fuel Cell Construction

The most common automotive fuel cell type is the Proton Exchange Membrane (PEM) cell. Its key components:

• Anode: Hydrogen flows in, platinum catalyst strips electrons, leaving H⁺ protons
• PEM Membrane: Allows H⁺ protons to pass through but blocks electrons, forcing them through the external circuit (producing electricity)
• Cathode: Oxygen flows in, combines with electrons (from circuit) and protons (from membrane) to form water

The Chemical Reaction

At the anode: H₂ → 2H⁺ + 2e⁻ (oxidation)
At the cathode: ½O₂ + 2H⁺ + 2e⁻ → H₂O (reduction)
Overall: H₂ + ½O₂ → H₂O + electricity + heat

Fuel Cell Stacks

A single PEM cell produces only about 0.6–0.7V at usable current densities — far too little for an electric motor. Automotive fuel cells stack hundreds of individual cells in series, achieving 100–300V DC output, which is then managed by power electronics.

Thermal Management

About 40–50% of hydrogen's energy becomes heat in a fuel cell. Sophisticated thermal management systems (coolant circuits, radiators) maintain the stack at optimal operating temperature (around 80°C) while rejecting waste heat.