PEM water electrolysis. Illustration of a cell, or electrolyzer, for proton exchange membrane (PEM) water electrolysis. Two electrodes, a negative cathode (green) and positive anode (purple), are placed in water (blue) and separated by a solid polymer electrolyte (SPE, grey). When a voltage is applied to the system the water at the anode splits into gaseous oxygen (purple spheres), electrons (grey spheres) and hydrogen ions (white spheres). The electrons travel through the anode and the external electrical circuit that is powering the the cell to the cathode. The hydrogen ions also travel to the cathode side of the cell via the proton conducting membrane. On the cathode side the hydrogen ions and electrons combine to form gaseous hydrogen (green spheres). PEM water electrolysis could be used to produce hydrogen in energy storage systems for renewable energy sources.