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Since Honda and BMW have come out with some hydrogen cars, I figured I need to learn how these hydrogen cars and their fuel cells work. I knew that hydrogen automobiles have fuel cells, and that they don't run off of gasoline, but that's really about it. Here's what I found about how these cool alternative energy cars work:

A hydrogen car uses hydrogen as its primary source of power for locomotion. There are two main methods: combustion or fuel cell conversion. Fuel cell conversion is what has been in the news most recently and is expected to power the cars of the future. This is what the Honda Clarity hydrogen cars run off of.

In fuel cell conversion, the hydrogen is turned into electricity through fuel cells, which then powers electric motors. The proton exchange membrane fuel cell (PEMFC) is one of the most promising technologies. It transforms the chemical energy liberated during the electrochemical reaction of hydrogen and oxygen to electrical energy. A stream of hydrogen is delivered to the anode side of the membrane-electrode assembly. At the anode side it is catalytically split into protons and electrons.

The newly formed protons filter through the polymer electrolyte membrane to the cathode side. The electrons travel along a circuit to the cathode side of the membrane-electrode assembly. This creates the current output of the fuel cell. Meanwhile, a stream of oxygen is delivered to the cathode side of the membrane-electrode assembly. At the cathode side oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit for form water molecules.

Here are the steps that a hydrogen powered car goes through to power the vehicle:

1- Hydrogen is pumped into the anode part of the fuel cell.
2- Oxygen is usually taken in from the atmosphere by the positive cathode.
3- The negative anode, with the aid of the catalyst, splits the hydrogen cells and separates the electrons from it. The electrons are then passed into a separate external circuit to form electric current, which propels the car forward.
4- Meanwhile, the hydrogen cells, which are devoid of electrons, as well as those electrons that went into the external circuits return to combine with oxygen. The platinum catalyst facilitates the reaction between both the elements; as a result, water is formed. Water, besides some extra heat, is the byproduct of the reaction

Now let's look at some pictures of hydrogen cars!

I started a thread here for more info: Hydrogen fuel cost

And this is a must read: Honda FXC Clarity hydrogen car