U.S. Department of Energy: Energy Efficiency and Renewable Energy: Hydrogen, Fuel Cells & Infrastructure Technologies Program
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A fuel cell is a device that uses hydrogen (or hydrogen-rich fuel) and oxygen to create electricity. Fuel cells are more energy-efficient than combustion engines and the hydrogen used to power them can come from a variety of sources. If pure hydrogen is used as a fuel, fuel cells emit only heat and water, eliminating concerns about air pollutants or greenhouse gases.

Fuel cell shown with its inputs and outputs. Hydrogen input on left, oxygen input on right, water and heat outputs on the back, with an electrical circuit going around the top.