Proton-exchange membrane

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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.

Contents

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 Nafion 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) which is permeable to protons when it is saturated with water, but it does not conduct electrons.

History

Leonard Niedrach (left) and Thomas Grubb (right), inventors of proton-exchange membrane technology. Grubb Niedrach photo.jpg
Leonard Niedrach (left) and Thomas Grubb (right), inventors of proton-exchange membrane technology.

Early proton-exchange membrane technology was developed in the early 1960s by Leonard Niedrach and Thomas Grubb, chemists working for the General Electric Company. [6] Significant government resources were devoted to the study and development of these membranes for use in NASA's Project Gemini spaceflight program. [7] A number of technical problems led NASA to forego the use of proton-exchange membrane fuel cells in favor of batteries as a lower capacity but more reliable alternative for Gemini missions 1-4. [8] An improved generation of General Electric's PEM fuel cell was used in all subsequent Gemini missions, but was abandoned for the subsequent Apollo missions. [9] The fluorinated ionomer Nafion, which is today the most widely utilized proton-exchange membrane material, was developed by DuPont plastics chemist Walther Grot. Grot also demonstrated its usefulness as an electrochemical separator membrane. [10]

In 2014, Andre Geim of the University of Manchester published initial results on atom thick monolayers of graphene and boron nitride which allowed only protons to pass through the material, making them a potential replacement for fluorinated ionomers as a PEM material. [11] [12]

Fuel cell

PEMFCs have some 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 in 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. [13] [14] [15]

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.

Applications

The primary application of proton-exchange membranes is in PEM fuel cells. These fuel cells have a wide variety of commercial and military applications including in the aerospace, automotive, and energy industries. [16] [17]

Early PEM fuel cell applications were focused within the aerospace industry. The then-higher capacity of fuel cells compared to batteries made them ideal as NASA's Project Gemini began to target longer duration space missions than had previously been attempted. [16]

The automotive industry as well as personal and public power generation are the largest markets for proton-exchange membrane fuel cells today. [18] PEM fuel cells are popular in automotive applications due to their relatively low operating temperature and their ability to start up quickly even in below-freezing conditions. [19] As of March 2019 there were 6,558 fuel cell vehicles on the road in the United States, with the Toyota Mirai being the most popular model. [20] California leads the United States in hydrogen refueling stations with 43, with the California Energy Commission having access to $20 million per year in funding until 2023 to expand coverage. [21] PEM fuel cells have seen successful implementation in other forms of heavy machinery as well, with Ballard Power Systems supplying forklifts based on the technology. [22] The primary challenge facing automotive PEM technology is the safe and efficient storage of hydrogen, currently an area of high research activity. [19]

Polymer electrolyte membrane electrolysis is a technique by which proton-exchange membranes are used to decompose water into hydrogen and oxygen gas. [23] The proton-exchange membrane allows for the separation of produced hydrogen from oxygen, allowing either product to be exploited as needed. This process has been used variously to generate hydrogen fuel and oxygen for life-support systems in vessels such as US and Royal Navy submarines. [16] A recent example is the construction of a 20 MW Air Liquide PEM electrolyzer plant in Québec. [24] Similar PEM-based devices are available for the industrial production of ozone. [25]

See also

Related Research Articles

<span class="mw-page-title-main">Electrochemical cell</span> Electro-chemical device

An electrochemical cell is a device that generates electrical energy from chemical reactions. Electrical energy can also be applied to these cells to cause chemical reactions to occur.

<span class="mw-page-title-main">Fuel cell</span> Device that converts the chemical energy from a fuel into electricity

A fuel cell is the electrochemical cell that converts the chemical energy of a fuel and an oxidizing agent into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requiring a continuous source of fuel and oxygen to sustain the chemical reaction, whereas in a battery the chemical energy usually comes from substances that are already present in the battery. Fuel cells can produce electricity continuously for as long as fuel and oxygen are supplied.

An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dissolving, the substance separates into cations and anions, which disperse uniformly throughout the solvent. Solid-state electrolytes also exist. In medicine and sometimes in chemistry, the term electrolyte refers to the substance that is dissolved.

A regenerative fuel cell or reverse fuel cell (RFC) is a fuel cell run in reverse mode, which consumes electricity and chemical B to produce chemical A. By definition, the process of any fuel cell could be reversed. However, a given device is usually optimized for operating in one mode and may not be built in such a way that it can be operated backwards. Standard fuel cells operated backwards generally do not make very efficient systems unless they are purpose-built to do so as with high-pressure electrolysers, regenerative fuel cells, solid-oxide electrolyser cells and unitized regenerative fuel cells.

<span class="mw-page-title-main">Nafion</span> Brand name for a chemical product

Nafion is a brand name for a sulfonated tetrafluoroethylene based fluoropolymer-copolymer discovered in the late 1960s by Dr. Walther Grot of DuPont. Nafion is a brand of the Chemours company. It is the first of a class of synthetic polymers with ionic properties that are called ionomers. Nafion's unique ionic properties are a result of incorporating perfluorovinyl ether groups terminated with sulfonate groups onto a tetrafluoroethylene (PTFE) backbone. Nafion has received a considerable amount of attention as a proton conductor for proton exchange membrane (PEM) fuel cells because of its excellent chemical and mechanical stability in the harsh conditions of this application.

<span class="mw-page-title-main">Proton-exchange membrane fuel cell</span> Power generation technology

Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable fuel-cell applications. Their distinguishing features include lower temperature/pressure ranges and a special proton-conducting polymer electrolyte membrane. PEMFCs generate electricity and operate on the opposite principle to PEM electrolysis, which consumes electricity. They are a leading candidate to replace the aging alkaline fuel-cell technology, which was used in the Space Shuttle.

<span class="mw-page-title-main">Proton conductor</span> Type of electrolyte

A proton conductor is an electrolyte, typically a solid electrolyte, in which H+ are the primary charge carriers.

An ionomer is a polymer composed of repeat units of both electrically neutral repeating units and ionized units covalently bonded to the polymer backbone as pendant group moieties. Usually no more than 15 mole percent are ionized. The ionized units are often carboxylic acid groups.

<span class="mw-page-title-main">Electrolysis of water</span> Electricity-induced chemical reaction

Electrolysis of water is using electricity to split water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, or remixed with the oxygen to create oxyhydrogen gas, for use in welding and other applications.

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<span class="mw-page-title-main">Membrane electrode assembly</span>

A membrane electrode assembly (MEA) is an assembled stack of proton-exchange membranes (PEM) or alkali anion exchange membrane (AAEM), catalyst and flat plate electrode used in fuel cells and electrolyzers.

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Hydrogen purity or hydrogen quality describes the presence of impurities in hydrogen when used as a fuel gas. Impurities in hydrogen can interfere with the proper functioning of equipment that stores, distributes, or uses hydrogen fuel.

An anion exchange membrane (AEM) is a semipermeable membrane generally made from ionomers and designed to conduct anions but reject gases such as oxygen or hydrogen.

<span class="mw-page-title-main">Alkaline anion exchange membrane fuel cell</span>

An alkaline anion exchange membrane fuel cell (AAEMFC), also known as anion-exchange membrane fuel cells (AEMFCs), alkaline membrane fuel cells (AMFCs), hydroxide exchange membrane fuel cells (HEMFCs), or solid alkaline fuel cells (SAFCs) is a type of alkaline fuel cell that uses an anion exchange membrane to separate the anode and cathode compartments.

Richard Yeo Swee Chye is an American scientist with 17 U.S. patents, best known for his research on disposable diapers.

Alkaline water electrolysis is a type of electrolyzer that is characterized by having two electrodes operating in a liquid alkaline electrolyte solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH). These electrodes are separated by a diaphragm, separating the product gases and transporting the hydroxide ions (OH−) from one electrode to the other. A recent comparison showed that state-of-the-art nickel based water electrolyzers with alkaline electrolytes lead to competitive or even better efficiencies than acidic polymer electrolyte membrane water electrolysis with platinum group metal based electrocatalysts.

High Temperature Proton Exchange Membrane fuel cells (HT-PEMFC), also known as High Temperature Polymer Electrolyte Membrane fuel cells, are a type of PEM fuel cells which can be operated at temperatures between 120 and 200°C. HT-PEM fuel cells are used for both stationary and portable applications. The HT-PEM fuel cell is usually supplied with hydrogen-rich gas like reformate gas formed by reforming of methanol, ethanol, natural gas or LPG.

<span class="mw-page-title-main">Anion exchange membrane electrolysis</span> Splitting of water using a semipermeable membrane

Anion exchange membrane(AEM) electrolysis is the electrolysis of water that utilises a semipermeable membrane that conducts hydroxide ions (OH−) called an anion exchange membrane. Like a proton-exchange membrane (PEM), the membrane separates the products, provides electrical insulation between electrodes, and conducts ions. Unlike PEM, AEM conducts hydroxide ions. The major advantage of AEM water electrolysis is that a high-cost noble metal catalyst is not required, low-cost transition metal catalyst can be used instead. AEM electrolysis is similar to alkaline water electrolysis, which uses a non-ion-selective separator instead of an anion-exchange membrane.

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. ↑ USpatent 5266421,Townsend, Carl W.&Naselow, Arthur B.,"Enhanced membrane-electrode interface",issued 2008-11-30, assigned to Hughes Aircraft
  4. ↑ Gabriel Gache (17 December 2007). "New Proton Exchange Membrane Developed – Nafion promises inexpensive fuel-cells". Softpedia . Retrieved 18 July 2008.
  5. ↑ Nakhiah Goulbourne. "Research Topics for Materials and Processes for PEM Fuel Cells REU for 2008". Virginia Tech. Archived from the original on 27 February 2009. Retrieved 18 July 2008.
  6. ↑ Grubb, W. T.; Niedrach, L. W. (1 February 1960). "Batteries with Solid Ion‐Exchange Membrane Electrolytes: II . Low‐Temperature Hydrogen‐Oxygen Fuel Cells". Journal of the Electrochemical Society. 107 (2): 131. doi:10.1149/1.2427622. ISSN   1945-7111.
  7. ↑ Young, George J.; Linden, Henry R., eds. (1 January 1969). Fuel Cell Systems. Advances in Chemistry. Vol. 47. WASHINGTON, D.C.: AMERICAN CHEMICAL SOCIETY. doi:10.1021/ba-1965-0047. ISBN   978-0-8412-0048-7.
  8. ↑ "Barton C. Hacker and James M. Grimwood. On the Shoulders of Titans: A History of Project Gemini. Washington, D. C.: National Aeronautics and Space Administration. 1977. Pp. xx, 625. $19.00". The American Historical Review. April 1979. doi:10.1086/ahr/84.2.593. ISSN   1937-5239.
  9. ↑ "Collecting the History of Proton Exchange Membrane Fuel Cells". americanhistory.si.edu. Smithsonian Institution. Retrieved 19 April 2021.{{cite web}}: CS1 maint: url-status (link)
  10. ↑ Grot, Walther. "Fluorinated Ionomers - 2nd Edition". www.elsevier.com. Retrieved 19 April 2021.{{cite web}}: CS1 maint: url-status (link)
  11. ↑ Hu, S.; Lozado-Hidalgo, M.; Wang, F.C.; et al. (26 November 2014). "Proton transport through one atom thick crystals". Nature . 516 (7530): 227–30. arXiv: 1410.8724 . Bibcode:2014Natur.516..227H. doi:10.1038/nature14015. PMID   25470058. S2CID   4455321.
  12. ↑ Karnik, Rohit N. (26 November 2014). "Breakthrough for protons". Nature. 516 (7530): 173–174. Bibcode:2014Natur.516..173K. doi: 10.1038/nature14074 . PMID   25470064. S2CID   4390672.
  13. ↑ 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; 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 (4): 1276. doi:10.1039/C4EE02280G.
  14. ↑ Jiangshui Luo, Olaf Conrad; Ivo F. J. Vankelecom (2013). "Imidazolium methanesulfonate as a high temperature proton conductor" (PDF). Journal of Materials Chemistry A . 1 (6): 2238. doi:10.1039/C2TA00713D.
  15. ↑ Jiangshui Luo; Jin Hu; Wolfgang Saak; Rüdiger Beckhaus; Gunther Wittstock; Ivo F. J. Vankelecom; Carsten Agert; Olaf Conrad (2011). "Protic ionic liquid and ionic melts prepared from methanesulfonic acid and 1H-1,2,4-triazole as high temperature PEMFC electrolytes" (PDF). Journal of Materials Chemistry . 21 (28): 10426–10436. doi:10.1039/C0JM04306K.
  16. 1 2 3 "Collecting the History of Proton Exchange Membrane Fuel Cells". americanhistory.si.edu. Smithsonian Institution. Retrieved 19 April 2021.{{cite web}}: CS1 maint: url-status (link)
  17. ↑ "Could This Hydrogen-Powered Drone Work?". Popular Science. 23 May 2015. Retrieved 7 January 2016.
  18. ↑ Barbir, F.; Yazici, S. (2008). "Status and development of PEM fuel cell technology". International Journal of Energy Research. 32 (5): 369–378. doi: 10.1002/er.1371 . ISSN   1099-114X. S2CID   110367501.
  19. 1 2 Li, Mengxiao; Bai, Yunfeng; Zhang, Caizhi; Song, Yuxi; Jiang, Shangfeng; Grouset, Didier; Zhang, Mingjun (23 April 2019). "Review on the research of hydrogen storage system fast refueling in fuel cell vehicle". International Journal of Hydrogen Energy. 44 (21): 10677–10693. doi:10.1016/j.ijhydene.2019.02.208. ISSN   0360-3199. S2CID   108785340.
  20. ↑ "Fact of the Month March 2019: There Are More Than 6,500 Fuel Cell Vehicles On the Road in the U.S." Energy.gov. Retrieved 19 April 2021.
  21. ↑ "Alternative Fuels Data Center: Hydrogen Basics". afdc.energy.gov. Retrieved 19 April 2021.
  22. ↑ "Material Handling - Fuel Cell Solutions | Ballard Power". www.ballard.com. Retrieved 19 April 2021.
  23. ↑ Carmo, Marcelo; Fritz, David L.; Mergel, Jürgen; Stolten, Detlef (22 April 2013). "A comprehensive review on PEM water electrolysis". International Journal of Hydrogen Energy. 38 (12): 4901–4934. doi:10.1016/j.ijhydene.2013.01.151. ISSN   0360-3199.
  24. ↑ "Air Liquide invests in the world's largest membrane-based electrolyzer to develop its carbon-free hydrogen production". www.newswire.ca. Air Liquide. 25 February 2019. Retrieved 28 August 2020.
  25. ↑ ,"PEM (proton exchange membrane) low-voltage electrolysis ozone generating device",issued 2011-05-16


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