In polymer electrolyte electrolysis the proton exchange membrane (PEM) is used to transfer the. Water is brought in contact with a bipolar plate; it circulates to the anode, where it is split into oxygen. Produced protons are transported through the PEM to the cathode. The electrons exit from the anode through the external power circuit, which provides the reaction's driving force (cell voltage). On the cathode side, the protons and electrons recombine to produce hydrogen. PEM separates produced gases, and electrodes are in direct contact with PEM to avoid undesired reactions. Electrodes are made of platinum, and of their alloys, they must be resilient to acids as PEM has the same properties as an aggressive acid.