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 acting as an electronic insulator and reactant barrier, e.g. to oxygen and hydrogen gas.[1] 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)[2] Nafion, a DuPont product. [3] While Nafon is an ionomer with a perfluorinated backbone like Teflon,[4] 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.[5]

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 (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.[6][7][8]

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

The theoretical exothermic potential is +1.23 V overall.

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.[9]

Commercial applications

PEM fuel cells have been used to power everything from cars to drones.[10][11] 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.

See also

References

  1. ↑ Alternative electrochemical systems for ozonation of water. NASA Tech Briefs (Technical report). NASA. 20 March 2007. MSC-23045. Retrieved 17 January 2015.
  2. ↑ Zhiwei Yang; et al. (2004). "Novel inorganic/organic hybrid electrolyte membranes" (PDF). Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 49 (2): 599.
  3. ↑ US patent 5266421, Townsend, Carl W. & Naselow, Arthur B., "US Patent 5266421 – Enhanced membrane-electrode interface", issued 2008-11-30, assigned to Hughes Aircraft
  4. ↑ Gabriel Gache (2007-12-17). "New Proton Exchange Membrane Developed – Nafion promises inexpensive fuel-cells". Softpedia. Retrieved 2008-07-18.
  5. ↑ Nakhiah Goulbourne. "Research Topics for Materials and Processes for PEM Fuel Cells REU for 2008". Virginia Tech. Retrieved 2008-07-18.
  6. ↑ Jiangshui Luo, Annemette H. Jensen, Neil R. Brooks, Jeroen Sniekers, Martin Knipper, David Aili, Qingfeng Li, Bram Vanroy, Michael Wübbenhorst, Feng Yan, Luc Van Meervelt, Zhigang Shao, Jianhua Fang, Zheng-Hong Luo, Dirk E. De Vos, Koen Binnemans and Jan Fransaer (2015). "1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells". Energy & Environmental Science. 8. doi:10.1039/C4EE02280G.
  7. ↑ Jiangshui Luo, Olaf Conrad & Ivo F. J. Vankelecom (2013). "Imidazolium methanesulfonate as a high temperature proton conductor". Journal of Materials Chemistry A. 1. doi:10.1039/C2TA00713D.
  8. ↑ Jiangshui Luo, Jin Hu, Wolfgang Saak, Rüdiger Beckhaus, Gunther Wittstock, Ivo F. J. Vankelecom, Carsten Agert and Olaf Conrad (2011). "Protic ionic liquid and ionic melts prepared from methanesulfonic acid and 1H-1,2,4-triazole as high temperature PEMFC electrolytes". Journal of Materials Chemistry. 21: 10426–10436. doi:10.1039/C0JM04306K.
  9. ↑ Hu, S.; Lozado-Hidalgo, M.; Wang, F.C.; et al. (26 November 2014). "Proton transport through one atom thick crystals" (PDF). Nature. 516: 227–30. doi:10.1038/nature14015 – via ArXiv. Lay summary – Nature: News & Views (11 December 2014).
  10. ↑ "Fuel Cell Vehicles" (PDF).
  11. ↑ "Could This Hydrogen-Powered Drone Work?". Popular Science. Retrieved 2016-01-07.

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