Editing Nafion

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'''Nafion''' is a brand name for a sulfonated [[tetrafluoroethylene]] based [[fluoropolymer]]-[[copolymer]] synthesized in 1962 by Dr. Donald J. Connolly at the DuPont Experimental Station in Wilmington Delaware (U.S. Patent 3,282,875). Additional work on the polymer family was performed in the late 1960s by Dr. Walther Grot of [[DuPont]].<ref>{{cite news
| first=Steven | last=Church
| page=B7 | title= Del. firm installs fuel cell
| date=January 6, 2006 | newspaper=[[Delaware News-Journal|The News Journal]]
}}</ref> Nafion is a brand of the [[Chemours]] company. It is the first of a class of synthetic polymers with ionic properties that are called [[ionomer]]s. Nafion's unique ionic properties are a result of incorporating perfluorovinyl ether groups terminated with sulfonate groups onto a tetrafluoroethylene ([[Polytetrafluoroethylene|PTFE]]) backbone.<ref name=":0">{{Cite journal|last1=Kusoglu|first1=Ahmet|last2=Weber|first2=Adam Z.|date=2017-02-08|title=New Insights into Perfluorinated Sulfonic-Acid Ionomers|journal=Chemical Reviews|language=en|volume=117|issue=3|pages=987–1104|doi=10.1021/acs.chemrev.6b00159|pmid=28112903|issn=0009-2665|doi-access=free}}</ref><ref name="Heitner-Wirguin1996">{{cite journal
| author= Heitner-Wirguin, C.
| title= Recent advances in perfluorinated ionomer membranes: structure, properties and applications
| journal=[[Journal of Membrane Science]]
| year=1996 | volume=120 | issue= 1
| pages= 1–33
| doi = 10.1016/0376-7388(96)00155-X
}}</ref><ref name="Mauritz2004" />  Nafion has received a considerable amount of attention as a [[proton conductor]] for [[Proton exchange membrane fuel cell|proton exchange membrane (PEM) fuel cells]] because of its excellent chemical and mechanical stability in the harsh conditions of this application.

The chemical basis of Nafion's ion-conductive properties remain a focus of extensive research.<ref name=":0" /> Ion conductivity of Nafion increases with the level of hydration. Exposure of Nafion to a humidified environment or liquid water increases the amount of water molecules associated with each sulfonic acid group. The hydrophilic nature of the ionic groups attract water molecules, which begin to solvate the ionic groups and dissociate the protons from the -SO<sub>3</sub>H ([[sulfonic acid]]) group. The dissociated protons "hop" from one acid site to another through [[Grotthuss mechanism|mechanisms]] facilitated by the water molecules and [[Hydrogen bond|hydrogen bonding]].<ref name=":0" /> Upon hydration, Nafion phase-separates at nanometer length scales resulting in formation of an interconnected network of hydrophilic domains which allow movement of water and [[Cation|cations]], but the [[Artificial membrane|membranes]] do not conduct [[Electron|electrons]] and minimally conduct [[Anion|anions]] due to permselectivity (charge-based exclusion). Nafion can be manufactured with or exchanged to alternate cation forms for different applications (e.g. lithiated for Li-ion batteries) and at different equivalent weights (EWs), alternatively considered as ion-exchange capacities (IECs), to achieve a range of cationic conductivities with trade-offs to other physicochemical properties such as water uptake and swelling.

==Nomenclature and molecular weight==
Nafion can be produced as both a powder [[resin]] and a [[copolymer]]. It has various chemical configurations and thus several chemical names in the [[IUPAC]] system. Nafion-H, for example, includes the following systematic names:
* From [[Chemical Abstracts]]: ethanesulfonyl fluoride, 2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-, with tetrafluoroethylene
* {{not a typo|tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic}} acid copolymer

The [[molecular weight]] of Nafion is variable due to differences in processing and solution morphology.<ref name="Heitner-Wirguin1996"/><ref name="Mauritz2004">{{cite journal|last2=Moore|first2=Robert B.|year=2004|title=State of Understanding of Nafion|journal=Chemical Reviews|volume=104|issue=10|pages=4535–4586|doi=10.1021/cr0207123|pmid=15669162|last1=Mauritz|first1=Kenneth A.}}</ref> The structure of a Nafion unit illustrates the variability of the material; for example, the most basic [[monomer]] contains chain variation between the [[ether]] groups (the z subscript). Conventional methods of determining molecular weight such as [[light scattering]] and [[gel permeation chromatography]] are not applicable because Nafion is insoluble, although the molecular weight has been estimated at 10<sup>5</sup>–10<sup>6</sup> Da.<ref name="Heitner-Wirguin1996"/><ref name="Mauritz2004"/> Instead, the equivalent weight (EW) and material thickness are used to describe most commercially available membranes. The EW is the number of grams of dry Nafion per mole of sulfonic acid groups when the material is in the acid form.<ref name="Mauritz2004"/> Nafion membranes are commonly categorized in terms of their EW and thickness.<ref name=":0" /><ref name=":1">{{Cite web|title=nafion membrane, chemours nafion, proton exchange membrane|url=https://www.nafion.com:443/en/products/sulfonic-membranes|access-date=2021-04-22|website=www.nafion.com|language=en}}</ref> For example, Nafion 117 indicates an extrusion-cast membrane with 1100 g/mol EW and 0.007&nbsp;inches (7 thou) in thickness.<ref name=":1" /> In contrast to equivalent weight, conventional [[ion-exchange resins]] are usually described in terms of their [[ion exchange]] capacity (IEC), which is the multiplicative inverse or reciprocal of the equivalent weight, i.e., IEC = 1000/EW.

==Preparation==
Nafion derivatives are first synthesized by the copolymerization of [[tetrafluoroethylene]] (TFE) (the monomer in Teflon) and a derivative of a perfluoro (alkyl vinyl ether) with sulfonyl acid fluoride.  The latter reagent can be prepared by the [[pyrolysis]] of its respective [[oxide]] or [[carboxylic acid]] to give the olefinated structure.<ref>{{cite journal |author1=Connolly, D.J. |author2=Longwood |author3=Gresham, W. F. |year=1966 |title=Fluorocarbon Vinyl Ether Polymers |journal=[[Google Patents]] |id={{US patent|3282875}}}}</ref>

The resulting product is an -SO<sub>2</sub>F-containing [[thermoplastic]] that is [[extrusion|extruded]] into films. Hot aqueous NaOH converts these sulfonyl fluoride (-SO<sub>2</sub>F) groups into sulfonate groups (-SO<sub>3</sub><sup>−</sup>Na<sup>+</sup>). This form of Nafion, referred to as the neutral or salt form, is finally converted to the acid form containing the sulfonic acid (-SO<sub>3</sub>H) groups.  Nafion can be dispersed into solution by heating in aqueous alcohol at 250&nbsp;°C in an [[autoclave]] for subsequent casting into [[thin film]]s or use as polymeric binder in electrodes.  By this process, Nafion can be used to generate composite films, coat [[electrode]]s, or repair damaged membranes.<ref name="Heitner-Wirguin1996"/>

==Properties==
The combination of the stable PTFE backbone with the acidic sulfonic groups gives Nafion its characteristics:<ref name=":0" /><ref name="PermaPure2004">{{cite web|author=Perma Pure LLC |year=2004 |title=Nafion: Physical and Chemical Properties |work=Technical Notes and Articles |url=http://www.permapure.com/tech-notes/key-concepts/nafion-physical-and-chemical-properties/ |url-status=dead |archive-url=https://web.archive.org/web/20130928005427/http://www.permapure.com/tech-notes/key-concepts/nafion-physical-and-chemical-properties/ |archive-date=September 28, 2013 }}<!--Minor error in both formulas : eliminate O between S and F : O=S=O--></ref>
* It is highly conductive to cations, making it suitable for many membrane applications.<ref name=":0" />
* It resists chemical attack.  According to Chemours, only [[alkali metal]]s (particularly sodium) can degrade Nafion under normal temperatures and pressures.
* The PTFE backbone interlaced with the ionic sulfonate groups gives Nafion a high chemical stability temperature (e.g. 190&nbsp;°C) but a softening point in the range of 85-100 °C give it a moderate [[operating temperature]], e.g. up to 100&nbsp;°C, with additional challenges in all applications due to the loss of water above 100 °C.
* It is a [[superacid]] catalyst. The combination of fluorinated backbone, sulfonic acid groups, and the stabilizing effect of the polymer matrix make Nafion a very strong acid, with pK<sub>a</sub> ~ -6.<ref>{{Cite journal|author=Schuster, M., Ise, M., Fuchs, A., Kreuer, K.D., Maier, J.|url=http://www.fkf.mpg.de/maier/people/kreuer/microstructure-transport1.pdf|title=Proton and Water Transport in Nano-separated Polymer Membranes|journal=Le Journal de Physique IV|volume=10|publisher=Germany: Max-Planck-Institut für Festkörperforschung|year=2005|issue=PR7 |pages=Pr7-279-Pr7-281|doi=10.1051/jp4:2000756|issn=1155-4339|archive-url=https://web.archive.org/web/20070611035636/http://www.fkf.mpg.de/maier/people/kreuer/microstructure-transport1.pdf|archive-date=2007-06-11|url-status=bot: unknown}}</ref> In this respect Nafion resembles the [[trifluoromethanesulfonic acid]], CF<sub>3</sub>SO<sub>3</sub>H, although Nafion is a weaker acid by at least three orders of magnitude.
* It is selectively and highly permeable to water.
* Its proton conductivity up to 0.2 [[Siemens (unit)|S]]/cm depending on temperature, hydration state, thermal history and processing conditions.<ref>{{Cite journal|last1=Sone|first1=Yoshitsugu|last2=Ekdunge|first2=Per|last3=Simonsson|first3=Daniel|date=1996-04-01|title=Proton Conductivity of Nafion 117 as Measured by a Four-Electrode AC Impedance Method|url=https://iopscience.iop.org/article/10.1149/1.1836625/meta|journal=Journal of the Electrochemical Society|language=en|volume=143|issue=4|pages=1254|doi=10.1149/1.1836625|bibcode=1996JElS..143.1254S|issn=1945-7111|url-access=subscription}}</ref><ref name=":0" />
* The solid phase and the aqueous phase of Nafion are both permeable to gases,<ref>{{Cite journal|title = Gas Permeation through Nafion. Part 1: Measurements|journal = The Journal of Physical Chemistry C|date = 2015-10-28|doi = 10.1021/acs.jpcc.5b04155|first1 = Maximilian|last1 = Schalenbach|first2 = Tobias|last2 = Hoefner|first3 = Paul|last3 = Paciok|first4 = Marcelo|last4 = Carmo|first5 = Wiebke|last5 = Lueke|first6 = Detlef|last6 = Stolten|volume=119|issue = 45|pages=25145–25155}}</ref><ref>{{Cite journal|title = Gas Permeation through Nafion. Part 2: Resistor Network Model|journal = The Journal of Physical Chemistry C|date = 2015-10-14|doi = 10.1021/acs.jpcc.5b04157|first1 = Maximilian|last1 = Schalenbach|first2 = Michael A.|last2 = Hoeh|first3 = Jeff T.|last3 = Gostick|first4 = Wiebke|last4 = Lueke|first5 = Detlef|last5 = Stolten|volume=119|issue = 45|pages=25156–25169}}</ref> which is a drawback for energy conversion devices such as artificial leaves, fuel cells, and water electrolyzers.

==Structure/morphology==
The morphology of Nafion membranes is a matter of continuing study to allow for greater control of its properties. Other properties such as water management, hydration stability at high temperatures, [[Electro-osmosis|electro-osmotic drag]], as well as the mechanical, thermal, and oxidative stability, are affected by the Nafion structure. A number of models have been proposed for the morphology of Nafion to explain its unique transport properties.<ref name=":0" />

[[Image:cluster network model.png|thumb|Cluster-network model]]
The first model for Nafion, called the '''cluster-channel''' or '''cluster-network model''', consisted of an equal distribution of sulfonate ion clusters (also described as 'inverted [[micelles]]'<ref name="Mauritz2004"/>) with a 40 [[angstrom|Å]] (4 [[nanometer|nm]]) diameter held within a continuous fluorocarbon lattice.  Narrow channels about 10 Å (1&nbsp;nm) in diameter interconnect the clusters, which explains the transport properties.<ref name="Heitner-Wirguin1996"/><ref name="Mauritz2004"/><ref>{{Cite journal | doi = 10.1002/pol.1981.180191103| title = The morphology in nafion perfluorinated membrane products, as determined by wide- and small-angle x-ray studies| journal = Journal of Polymer Science: Polymer Physics Edition| volume = 19| issue = 11| pages = 1687–1704| year = 1981| last1 = Gierke | first1 = T. D.| last2 = Munn | first2 = G. E.| last3 = Wilson | first3 = F. C.| bibcode = 1981JPoSB..19.1687G}}</ref>

The difficulty in determining the exact structure of Nafion stems from inconsistent solubility and crystalline structure among its various derivatives. Advanced morphological models have included a '''core-shell model''' where the ion-rich core is surrounded by an ion poor shell, a '''rod model''' where the sulfonic groups arrange into crystal-like rods, and a '''sandwich model''' where the polymer forms two layers whose sulfonic groups attract across an aqueous layer where transport occurs.<ref name="Mauritz2004"/> Consistency between the models include a network of ionic clusters; the models differ in the cluster geometry and distribution. Although no model has yet been determined fully correct, some scientists have demonstrated that as the membrane hydrates, Nafion's morphology transforms from the cluster-channel model to a rod-like model.<ref name="Mauritz2004"/>

A cylindrical-water channel model<ref name="Schmidt-Rohr & Chen">{{Cite journal | doi = 10.1038/nmat2074| title = Parallel cylindrical water nanochannels in Nafion fuel-cell membranes| journal = Nature Materials| volume = 7| issue = 1| pages = 75–83| year = 2007| last1 = Schmidt-Rohr | first1 = K. | last2 = Chen | first2 = Q. | pmid=18066069}}</ref> was also proposed based on simulations of small-angle X-ray scattering data and solid state nuclear magnetic resonance studies. In this model, the sulfonic acid functional groups self-organize into arrays of hydrophilic water channels, each ~ 2.5&nbsp;nm in diameter, through which small ions can be easily transported. Interspersed between the hydrophilic channels are hydrophobic polymer backbones that provide the observed mechanical stability. Many recent studies, however, favored a phase-separated nanostructure consisting of locally-flat, or ribbon-like, hydrophilic domains based on evidence from direct-imaging studies<ref>{{Cite journal|last1=Allen|first1=Frances I.|last2=Comolli|first2=Luis R.|last3=Kusoglu|first3=Ahmet|last4=Modestino|first4=Miguel A.|last5=Minor|first5=Andrew M.|last6=Weber|first6=Adam Z.|date=2015-01-20|title=Morphology of Hydrated As-Cast Nafion Revealed through Cryo Electron Tomography|journal=ACS Macro Letters|language=en|volume=4|issue=1|pages=1–5|doi=10.1021/mz500606h|pmid=35596390 |issn=2161-1653|doi-access=free}}</ref> and more comprehensive analysis of the structure and transport properties.<ref name=":0" /><ref>{{Cite journal|last1=Kreuer|first1=Klaus-Dieter|last2=Portale|first2=Giuseppe|date=2013-11-20|title=A Critical Revision of the Nano-Morphology of Proton Conducting Ionomers and Polyelectrolytes for Fuel Cell Applications|url=http://doi.wiley.com/10.1002/adfm.201300376|journal=Advanced Functional Materials|language=en|volume=23|issue=43|pages=5390–5397|doi=10.1002/adfm.201300376|s2cid=94579140 |url-access=subscription}}</ref> 

==Applications==
Nafion's properties make it suitable for a broad range of applications. Nafion has found use in [[fuel cell]]s, electrochemical devices, chlor-alkali production, metal-ion recovery, water [[electrolysis]], [[plating]], surface treatment of metals, batteries, [[sensor]]s, [[Donnan dialysis cell]]s, drug release, gas drying or humidification, and [[superacid]] catalysis for the production of fine chemicals.<ref name="Heitner-Wirguin1996"/><ref name="Mauritz2004"/><ref name="PermaPure2004"/><ref name="Gelbard2005">{{cite journal
| author=Gelbard, Georges
| title=Organic Synthesis by Catalysis with Ion-Exchange Resins
| journal=[[Industrial & Engineering Chemistry Research]]
| year=2005 | volume=44 | issue= 23| pages= 8468–8498
| doi=10.1021/ie0580405
}}</ref> Nafion is also often cited for theoretical potential (i.e., thus far untested) in a number of fields. With consideration of Nafion's wide functionality, only the most significant will be discussed below.

===Chlor-alkali production cell membrane===
{{Main|Chloralkali process}}
[[Image:Chlor alkali cell.png|250 px|thumb|A chlor-alkali cell]]
Chlorine and sodium/potassium hydroxide are among the most produced commodity chemicals in the world. Modern production methods produce Cl<sub>2</sub> and NaOH/KOH from the electrolysis of [[brine]] using a Nafion membrane between half-cells. Before the use of Nafion, industries used [[Mercury (element)|mercury]] containing sodium amalgam to separate sodium metal from cells or [[asbestos]] diaphragms to allow for transfer of sodium ions between half cells; both technologies were developed in the latter half of the 19th century. The disadvantages of these systems is worker safety and environmental concerns associated with mercury and asbestos, economical factors also played a part, and in the diaphragm process chloride contamination of the hydroxide product. Nafion was the direct result of the chlor-alkali industry addressing these concerns; Nafion could tolerate the high temperatures, high electrical currents, and corrosive environment of the electrolytic cells.<ref name="Heitner-Wirguin1996"/><ref name="Mauritz2004"/><ref name="PermaPure2004"/>

The figure to the right shows a chlor-alkali cell where Nafion functions as a membrane between half cells. The membrane allows sodium ions to transfer from one cell to the other with minimal electrical resistance. The membrane was also reinforced with additional membranes to prevent gas product mixing and minimize back transfer of Cl<sup>−</sup> and <sup>−</sup>OH ions.<ref name="Heitner-Wirguin1996"/>

===Proton exchange membrane (PEM) for fuel cells===
Although fuel cells have been used since the 1960s as power supplies for satellites, recently they have received renewed attention for their potential to efficiently produce clean energy from hydrogen. Nafion was found effective as a membrane for [[proton exchange membrane]] (PEM) [[fuel cell]]s by permitting hydrogen ion transport while preventing electron conduction.  Solid Polymer Electrolytes, which are made by connecting or depositing electrodes (usually noble metal) to both sides of the membrane, conduct the electrons through an energy requiring process and rejoin the hydrogen ions to react with oxygen and produce water.<ref name="Heitner-Wirguin1996"/>  Fuel cells are expected to find strong use in the transportation industry.

===Superacid catalyst for fine chemical production===
Nafion, as a [[superacid]], has potential as a catalyst for [[organic synthesis]].  Studies have demonstrated catalytic properties in [[alkylation]], [[isomerization]], [[oligomerization]], [[acylation]], [[ketalization]], [[esterification]], [[hydrolysis]] of [[sugars]] and [[ethers]], and [[oxidation]]. New applications are constantly being discovered.<ref name="Gelbard2005"/> These processes, however, have not yet found strong commercial use. Several examples are shown below:

====Alkylation with alkyl halides====
Nafion-H gives efficient conversion whereas the alternative method, which employs [[Friedel-Crafts reaction|Friedel-Crafts synthesis]], can promote polyalkylation:<ref name="El-Kattan2001">El-Kattan, Y.; McAtee, J.; Nafion-H. (2001) "Nafion-H". In ''Encyclopedia of Reagents for Organic Synthesis.'' John Wiley & Sons, {{ISBN|978-0-470-01754-8}}.</ref>
::[[Image:nafion alkylation halides.png|300 px|Alkyl Halide Reaction]]

====Acylation====
The amount of Nafion-H needed to catalyze the acylation of benzene with aroyl chloride is 10–30% less than the Friedel-Crafts catalyst:<ref name="El-Kattan2001" />
::[[Image:nafion acylation benzene.png|350 px|Acylation of Benzene]]

====Catalysis of protection groups====
Nafion-H increases [[reaction rate]]s of [[protective group|protection]] via dihydropyran or o-trialkylsilation of alcohols, phenol, and carboxylic acids.<ref name="Gelbard2005"/>
::[[Image:nafion protection.png|200 px|Catalysis of protection groups]]

====Isomerization====
Nafion can catalyze a [[1,2-rearrangement|1,2-hydride shift]].<ref name="Gelbard2005"/>
::[[Image:nafion isomerize.png|350 px|Isomerization via Nafion]]

It is possible to immobilize [[enzyme]]s within the Nafion by enlarging pores with [[lipophilic]] salts. Nafion maintains a structure and pH to provide a stable environment for the enzymes. Applications include catalytic oxidation of [[adenine dinucleotide]]s.<ref name="Gelbard2005"/>

===Sensors===
Nafion has found use in the production of [[sensor]]s, with application in ion-selective, metallized, optical, and [[biosensor]]s.  What makes Nafion especially interesting is its demonstration in [[biocompatibility]]. Nafion has been shown to be stable in [[cell culture]]s as well as the human body, and there is considerable research towards the production of higher sensitivity [[glucose]] sensors.<ref name="Heitner-Wirguin1996"/>

===Antimicrobial surfaces===
Nafion surfaces show an exclusion zone against bacteria colonization.<ref>{{cite journal |last1=Cheng |first1=Yifan |last2=Moraru |first2=Carmen I. |date=2018 |title=Long-range interactions keep bacterial cells from liquid-solid interfaces: Evidence of a bacteria exclusion zone near Nafion surfaces and possible implications for bacterial attachment. |journal=ColloidsSurf. B: Biointerfaces|volume=162 |pages=16–24 |doi=10.1016/j.colsurfb.2017.11.016 |pmid=29132042 |doi-access=free }}</ref> Moreover, layer-by-layer coatings comprising Nafion show excellent antimicrobial properties.<ref>{{cite journal |last1=Gibbons |first1=Ella N. |last2=Winder |first2=Charis |last3=Barron |first3=Elliot |display-authors=etal |date=2019 |title=Layer by Layer Antimicrobial Coatings Based on Nafion, Lysozyme, and Chitosan |journal=Nanomaterials |volume=9 |issue=1563 |pages=1563 |doi=10.3390/nano9111563|pmid=31689966 |pmc=6915488 |doi-access=free }}</ref>

=== Dehumidification in spacecraft===
The [[SpaceX Dragon 2]] human-rated spacecraft uses Nafion membranes to dehumidify the cabin air. One side of the membrane is exposed to the cabin atmosphere, the other to the vacuum of space. This results in dehumidification since Nafion is permeable to water molecules but not air. This saves power and complexity since cooling is not required (as needed with a condensing dehumidifier), and the removed water is rejected to space with no additional mechanism needed.<ref>{{cite conference |url=https://ttu-ir.tdl.org/bitstream/handle/2346/86364/ICES-2020-333.pdf |title=Development of the Crew Dragon ECLSS |author1=Jason Silverman |author2=Andrew Irby |author3=Theodore Agerton |conference=International Conference on Environmental Systems |year=2020}}</ref>

== Modified Nafion for PEM fuel cells ==

Normal Nafion will dehydrate (thus lose proton conductivity) when the temperature is above ~80&nbsp;°C. This limitation troubles the design of fuel cells because higher temperatures are desirable for better efficiency and CO tolerance of the platinum catalyst. Silica and zirconium phosphate can be incorporated into Nafion water channels through ''in situ'' chemical reactions to increase the working temperature to above 100&nbsp;°C.{{Citation needed |date=July 2024}}

==References==
{{reflist}}

==External links==
*[http://fuelcellsetc.com/2012/07/nafion-membrane-thickness-for-electrolyze/ What Nafion Membrane is Right for an Electrolyzer / Hydrogen Generation?]
*[http://www.nafion.mysite.com Homepage of Walther G. Grot]
*[https://web.archive.org/web/20071026181150/http://www.williamandrew.com/title.php?id=9 Walther G. Grot: "Fluorinated Ionomers"]
*[http://pubs.rsc.org/en/content/articlelanding/1990/ft/ft9908600409#!divAbstract Isotopic effects on Nafion conductivity]
*[https://www.academia.edu/2636724/The_effect_of_membrane_thickness_on_the_factors Membrane thickness on conductivity_of_Nafion]
*[http://www.hydrogen.energy.gov/pdfs/progress07/v_d_6_korzeniewski.pdf Nafion hydration]
*{{webarchive |url=https://web.archive.org/web/20070922083747/http://nafion.totallyexplained.com/ |date=22 September 2007 |title=Nafion Totally Explained}}

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