Proton Exchange Membrane (PEM) is a semi permeable membrane generally made from ionomers and designed to conduct protons while being impermeable to gases such as oxygen or hydrogen. Their essential function into a proton exchange membrane fuel cell is the separation of reactants and transport of protons. They can be made from either pure polymer membranes or from composite membranes where other materials are imbedded in a polymer matrix. Proton exchange membranes are primarily characterized by proton conductivity, methanol permeability and thermal stability. One of the most common and commercially available PEM materials is Nafion. Many use polyaromatic polymers while others use partially fluorinated polymers.