"If you turn the experiment around and put the hydrogen and oxygen in and take the electricity off, you have a fuel cell, " Roan said.

CNN: Bus showcases fuel cell technology

In the simplest terms, a fuel cell combines hydrogen and oxygen in a process that releases electricity, heat and water.

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Fuel cell electric vehicles generate their power from hydrogen and oxygen, and emit only water.

BBC: Nissan FCEV on display at a car show in Brussels in January

The fuel cell, which has been around for 50 years, generates electricity from hydrogen and oxygen.

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