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 . Additional work on the polymer family was performed 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. 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. For example, Nafion 117 indicates an extrusion-cast membrane with 1100 g/mol EW and 0.007&nbsp;inches (7 thou) in thickness.

The resulting product is an -SO<sub>2</sub>F-containing thermoplastic that is 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 films or use as polymeric binder in electrodes. By this process, Nafion can be used to generate composite films, coat electrodes, or repair damaged membranes.

  • It is highly conductive to cations, making it suitable for many membrane applications. 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 S/cm depending on temperature, hydration state, thermal history and processing conditions. 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-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.

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. 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 and more comprehensive analysis of the structure and transport properties.

Applications

Nafion's properties make it suitable for a broad range of applications. Nafion has found use in fuel cells, electrochemical devices, chlor-alkali production, metal-ion recovery, water electrolysis, plating, surface treatment of metals, batteries, sensors, Donnan dialysis cells, drug release, gas drying or humidification, and superacid catalysis for the production of fine chemicals. 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

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 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.

::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: Moreover, layer-by-layer coatings comprising Nafion show excellent antimicrobial properties.

Biomedical applications of Nafion

A review of the literature describes the investigation of Nafion in a wide range of biomedical applications, including bioelectronic systems, energy harvesting, sensors, wearable electronics, tissue engineering, lab-on-a-chip platforms, implants, controlled drug delivery systems and antimicrobial surface coatings.

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.

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.

References

  • What Nafion Membrane is Right for an Electrolyzer / Hydrogen Generation?
  • Homepage of Walther G. Grot
  • Walther G. Grot: "Fluorinated Ionomers"
  • Isotopic effects on Nafion conductivity
  • Membrane thickness on conductivity_of_Nafion
  • Nafion hydration