Nafion

Nafion® is a sulfonated tetrafluorethylene copolymer discovered in the late 1960s by Walther Grot of proton exchange membrane (PEM) fuel cells because of its excellent thermal and mechanical stability.

The chemical basis of Nafion's superior conductive properties remain a focus of research. Protons on the SO₃H (electrons. Nafion can be manufactured with various cationic conductivities.

Nomenclature and molecular weight

Nafion can be produced as both a powder resin and a copolymer and has therefore acquired several IUPAC names. 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
• tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer

The ion-exchange resins are usually described in terms of their ion exchange capacity (IEC) which is inversely proportional to the equivalent weight.

Preparation

Nafion derivatives are first synthesized by the copolymerization of carboxylic acid to give the olefinated structure.^[4]

The resulting product is an -SO₂F-containing electrodes, or repair damaged membranes.^[2]

Properties

The combination of the stable Teflon backbone with the acidic sulfonic groups gives Nafion its desirable characteristics:^[5]

• It is highly conductive to cations, making it ideal for many membrane applications.
• It resists chemical attack. According to alkali metals (particularly sodium) can degrade Nafion under normal temperatures and pressures.
• The Teflon backbone interlaced with the ionic sulfonate groups gives Nafion a high operating temperature, e.g. up to 190 °C.
• It is a trifluoromethanesulfonic acid, CF₃SO₃H, although Nafion is a weaker acid by at least three orders of magnitude..
• It is selectively and highly permeable to water. The degree of hydration of the Nafion membrane directly affects its ion conductivity and overall morphology.

Structure/morphology

The morphology of Nafion membranes is a matter of continuing study to allow for greater control on its properties. Other properties must be related to the Nafion structure such as water management, hydration stability at high temperatures, electro-osmotic drag, as well as the mechanical, thermal, and oxidative stability .

  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'^[3]) with a 40 Å (4 nm) diameter held within a continuous fluorocarbon lattice. Narrow channels about 10 Å (1 nm) in diameter interconnect the clusters, which explains the transport properties.^[2][3][7]

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.^[3] Consistency between the models include a network of ionic clusters; the models differ in the cluster geometry and distribution. Although no model was yet 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.^[3]

Applications

Nafion's superior properties allowed for broad application. Nafion has found use in fuel-cells, electrochemical devices, chlor-alkali production, metal-ion recovery, water plating, surface treatment of metals, batteries, sensors, Donnan dialysis cells, drug release, gas drying or humidifaction, and super-acid catalysis for the production of fine chemicals (Gelbard, 2005).^[2][3][5][8] 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

  Chlorine and sodium/potassium hydroxide are among the most produced commodity chemicals in the United States. Modern production methods produce Cl₂ and NaOH/KOH from the electrolysis of 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, although economical factors also played a part. 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.^[2][3][5]

The figure to the right shows a chlor-alkai 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^– and^–OH ions.^[2]

Proton exchange membrane (PEM) for fuel cells

Although fuel cells have been used since the 1960's 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 cells 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.^[2] Fuel cells are expected to find strong use in the transportation industry.

Superacid catalyst for fine chemical production

Nafion, as a oxidation. New applications are constantly being discovered.^[8] 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 synthesis, can promote polyalkylation:^[9]

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:^[9]

Catalysis of Protection groups
Nafion-H increases reaction rates of protection via dihydropyran or o-trialkylsilation of alcohols, phenol, and carboxylic acids.^[8]

Isomerization
Nafion can catalyze a 1,2-hydride shift.^[8]

Recently scientists have been able to immobilize lipophilic salts. Nafion is able to maintain a structure and pH to provide a stable environment for the enzymes. Application has included catalytic oxidation of adenine dinucleotides.^[8]

Sensors

Nafion has found use in the production of sensors, which application in ion-selective, metallicized, optical, and biosensors. What makes Nafion especially interesting is its demonstration in biocompatibility. Nafion has been shown to be stable in cell cultures as well as the human body, and there is considerable research towards the production of higher sensitivity glucose sensors.^[2]

References

1. ^ Church, Steven. "Del. firm installs fuel cell", The News Journal, January 6, 2006, p. B7. 
2. ^ ^{a b c d e f g h i j} Heitner-Wirguin, C. (1996). "Recent advances in perfluorinated ionomer membranes: structure, properties and applications". Journal of Membrane Science 120: 1–33. doi:10.1016/0376-7388(96)00155-X .
3. ^ ^{a b c d e f g h i j} Mauritz, K. A., Moore, R. B. (2004). "State of Understanding of Nafion". Chemical Reviews 104: 4535–4585. doi:10.1021/cr0207123.
4. ^ Connolly, D.J.; Longwood; Gresham, W. F. (1966). "Fluorocarbon Vinyl Ether Polymers". U.S. Patent 3,282,875 .
5. ^ ^{a b c}
6. ^ Schuster, M., Ise, M., Fuchs, A., Kreuer, K.D., Maier, J. (2005). "Proton and Water Transport in Nano-separated Polymer Membranes". Germany: Max-Planck-Institut für Festkörperforschung, n.d..
7. ^ Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. (1981). "The morphology in nafion perfluorinated membrane products, as determined by wide- and small-angle x-ray studies". Journal of Polymer Science: Polymer Physics Edition 19 (11): 1687–1704. doi:10.1002/pol.1981.180191103.
8. ^ ^{a b c d e} Gelbard, Georges (2005). "Organic Synthesis by Catalysis with Ion-Exchange Resins". Industrial & Engineering Chemistry Research 44: 8468–8498. doi:10.1021/ie0580405.
9. ^ ^{a b} El-Kattan, Y.; McAtee, J.; Nafion-H. (2001) Nafion-H. In Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons.

10. "Fluorinated Ionomers" William Andrew 2007 : http://www.williamandrew.com/titles/1541.htm