A device that converts the chemical energy in a fuel, usually hydrogen, into electrical energy. A hydrogen fuel cell resembles a battery because it contains an anode, a cathode and an electrolyte, but needs a constant supply of fuel and oxygen. At the anode, a catalytic reaction splits the hydrogen into protons (ions) and electrons. The hydrogen ions travel to the cathode where they combine with oxygen to form water. The electrons travel through their own circuit and produce a direct electric current.