Proton exchange membrane

A proton exchange membrane or polymer electrolyte membrane (PEMfc) 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.

Fuel cell

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. On the other hand, high temperature PEMFCs operating between 100 °C and 200 °C offer further benefits such as improved electrode kinetics, simpler water and heat management, and better tolerance to fuel impurities, leading to higher overall system efficiencies. As a result, new anhydrous proton conductors, such as protic organic ionic plastic crystals (POIPCs) and protic ionic liquids, are actively studied for the development of suitable PEMs.

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

Atomically thin material

In 2014, Andre Geim of Manchester University published initial results on atom thick monolayers of graphene and boron nitride which allowed only protons to pass the material.

Commercial applications

In February, 2012 Belgian company Solvay announced the successful startup of a 1-megawatt PEM fuel cell system of 12,600 cells. Installed in Antwerp, it is fueled with the hydrogen created as a by-product of vinyl chloride manufacture.

Smaller PEM fuel cells have been used to power everything from cars to drones.

See also

• Electrolysis of water
• Alkali anion exchange membrane
• Isotope electrochemistry
• Membrane electrode assembly
• Artificial membrane
• Dry electrolyte
• Dynamic mechanical analysis
• Electroosmotic pump
• Gas diffusion electrode
• Roll-to-roll

External links

• Dry solid polymer electrolyte battery
• EC-supported STREP program on high pressure PEM water electrolysis