Proton Exchange Membrane

A proton exchange membrane or polymer electrolyte 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 membrane electrode assembly (MEA) of a proton exchange membrane fuel cell or of a proton exchange membrane electrolyser : 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 the fluoropolymer (PFSA) Nafion, a DuPont product. 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.

PEM fuel cells use a solid polymer membrane (a thin plastic film) as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons.

Proton exchange membrane fuel cells (PEMFC) are believed to be the best type of fuel cell as the vehicular power source to eventually replace the gasoline and diesel internal combustion engines. They are being considered for automobile applications because they typically have an operating temperature of ~80 °C and a rapid start up time. PEMFCs operate at 40–60% efficiency and can vary the output to match the demands. First used in the 1960s for the NASA Gemini program, PEMFCs are currently being developed and demonstrated for systems ranging from 1 W to 2 kW.

PEMFCs contain advantages over other types of fuel cells such as solid oxide fuel cells (SOFC). PEMFCs operate at a lower temperature, are lighter, and more compact, which makes them ideal for applications such as cars. However, some disadvantages are: the ~80 °C operating temperature is too low for cogeneration like in SOFCs and that the electrolyte for PEMFCs must be water saturated.

The fuel for the PEMFC is hydrogen and the charge carrier is the hydrogen ion (proton). At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons. The hydrogen ions permeate across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power. Oxygen, usually in the form of air, is supplied to the cathode and combines with the electrons and the hydrogen ions to produce water. The reactions at the electrodes are as follows:

Anode reaction: 2H₂ → 4H+ + 4e-
Cathode reaction: O₂ + 4H+ + 4e- → 2H₂O
Overall cell reaction: 2H₂ + O₂ → 2H₂O