A fuel cell is an electrochemical device that combines hydrogen fuel and oxygen from the air to produce electricity, heat, and water.  Fuel cells do not utilize combustion, and produce little if any hazardous effluents.  Fuel cells convert fuel gases directly into electricity, and can be operated at much higher efficiencies than many other types of electric generators. 
A typical polymer electrolyte membrane (PEM) fuel cell includes a membrane electrode assembly (MEA) comprising an ion conducting membrane (the PEM) with an anode electrode disposed on one side of the ion conducting membrane and a cathode electrode disposed on the other side of the ion conducting membrane.  Hydrogen is reduced into hydrogen ions and electrons at the anode electrode.  The electrons provide an electrical current to drive an external load and the hydrogen ions pass through the membrane.  At the cathode electrode, oxygen combines with the hydrogen ions to form water as a byproduct.  Fuel cell operation depends in part on the degree of transportation of gases, liquids, electrons, and ions through the materials that form the layers of the MEA.