Proton exchange membrane

A proton exchange membrane (PEM) is a semipermeable membrane generally made from ionomers and designed to conduct protons while being impermeable to gases such as oxygen or hydrogen. This is their essential function when incorporated into a proton exchange membrane fuel cell: separation of reactants and transport of protons. PEMs can be made from either pure polymer membranes or from composite membranes where other materials are embedded in a polymer matrix. One of the most common and commercially available PEM materials is Nafion which is made by DuPont. While nafion is an ionomer with a perfluorinated backbone like Teflon, there are many other structural motifs used to make ionomers for proton exchange membranes. Many use polyaromatic polymers while others use partially fluorinated polymers.

Proton exchange membranes are primarily characterized by proton conductivity (σ), methanol permeability (P), and thermal stability.