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Hydrogen + Oxygen (from the air) → Electricity + Water Vapor

A fuel cell is a device that converts chemical energy  into electrical energy.

A PEM (Proton Exchange Membrane) cell uses hydrogen gas and oxygen (from the air) as fuel. The product of the reaction in the cell is water releasing  electricity, and heat.

This is a much cleaner way to produce energy than burning coal or internal combustion engines which produce poisonous bi-products.

Since oxygen is readily available in the atmosphere we only need to supply the fuel cell with hydrogen which can be made by electrolysis

The main parts of a PEM Fuel Cell:

The anode
At the anode the platinum catalyst cause the hydrogen atom to lose its electron and turn into a hydrogen ion (H+). The channels in the anode spread the hydrogen gas equally over the surface..

The Cathode
At the cathode electrons combine with the hydrogen ions to form hydrogen which join with oxygen atoms to form water.

The PEM
The key to the cell is the Polymer Electrolyte Membane (PEM) which allows the hydrogen ions to pass though it but blocks the electrons.
The electrons have to move along an external wire (as in a normal battery cell) to get from the anode to the cathode.

The catalyst is a special material that speeds up the reaction between  oxygen and hydrogen. It is usually made of tiny platinum particles very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM.

Due to the high energetic content of hydrogen and high efficiency of fuel cells (55%), this great technology can be used in many applications like transport (cars, buses, forklifts, etc)  and backup power to produce electricity during a failure of the electricity grid.

Advantages fuel cells over internal combustion engines: