A proton-exchange membrane, or polymer-electrolyte membrane (PEM), is a semipermeable membrane generally made from ionomers and designed to conduct protons while acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen gas. 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 while blocking a direct electronic pathway through the membrane.
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.
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 (PEMFCs) are believed to be the most promising type of fuel cell to act as the vehicular power source replacement for gasoline and diesel internal combustion engines. They are being considered for automobile applications because they typically have a low operating temperature (~80 °C) and a rapid start-up time, including from frozen conditions. 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 from ~100 kW cars to a 59 MW power plant.
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. However, some fuel-cell cars, including the Toyota Mirai, operate without humidifiers, relying on rapid water generation and the high rate of back-diffusion through thin membranes to maintain the hydration of the membrane, as well as the ionomer in the catalyst layers. High-temperature PEMFCs operate between 100 °C and 200 °C, potentially offering benefits to electrode kinetics and heat management, and better tolerance to fuel impurities, particularly CO in reformate. These improvements potentially could lead to higher overall system efficiencies. However, these gains have yet to be realized, as the gold-standard perfluorinated sulfonic acid (PFSA) membranes lose function rapidly at 100 °C and above if hydration drops below ~100%, and begin to creep in this temperature range, resulting in localized thinning and overall lower system lifetimes. 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 + heat + electrical energy
The theoretical exothermic potential is +1.23 V overall.
Atomically thin material
PEM fuel cells have been used to power everything from cars to drones. 3,000 fuel cell cars will be sold or leased in 2016 globally, with 30,000 intended for 2017. Ballard Power Systems has developed a completely viable commercial market supplying forklifts.
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