In an alkaline water electrolyser, the electrodes are immersed in a liquid alkaline electrolyte (e.g. an aqueous 20-30% KOH solution). The two electrodes are separated by a diaphragm, which keeps the product gases apart from each other but is permeable to the hydroxide ions and water molecules. The following reaction scheme shows the oxygen evolution reaction which takes place at the surface of the anode in the anode-containing half cell of the alkaline water electrolyser:
4 OH^?→O₂+2 H₂O+4 e^?

In a polymer electrolyte membrane (PEM) water electrolyser (also referred to as a “proton exchange membrane” (PEM) water electrolyser), a solid polymer electrolyte is used which is responsible for proton transport from the anode to the cathode while electrically insulating the electrodes from each other, and for separating the product gases. The following reaction scheme shows the oxygen evolution reaction which takes place at the surface of the anode in the anode-containing half cell of the PEM water electrolyser:
2 H₂O→4 H⁺+O₂+4 e^?

Due to its complexity, the oxygen evolution reaction has slow kinetics, which is why a significant overpotential is needed at the anode side for producing oxygen at reasonable rates. Typically, PEM water electrolysers are operated at a voltage of about 1.5 to 2 V (vs. RHE (“reversible hydrogen electrode”)).

As the pH is very acidic (PEM: pH of less than 2) and a high overpotential has to be applied, the materials which are present in the anode side of a PEM water electrolyser need to be very corrosion resistant.