A fuel cell that uses hydrogen fuel works through a process that is the reverse of the before mentioned water electrolysis. Instead of using water and electricity to produce hydrogen and oxygen, fuel cells use hydrogen fuel and oxygen from the air to produce water, usually as steam. In general, a typical fuel cell consists of a thin electrolyte material, typically a semi-permeable membrane, in between two porous electrodes, the cathode and anode. Molecular hydrogen is delivered to the anode via a gas flow, where the anode catalyst oxidizes the hydrogen, producing hydrogen cations and electrons. Hydrogen cations pass through the electrolyte/membrane from the anode to the cathode. The electrons in the system cannot transfer from the anode to the cathode through the layers of the electrolyte, but only through an external electrical circuit. It is this movement of electrons that produces the electric current. At the cathode, molecular oxygen combines with the hydrogen protons and electrons to form water. Figure shows this process in a single unit cell.