Nafion

1
Nafion

• EMAC 276
• Professor John Blackwell

2
Introduction

• Nafion is a copolymer of tetrafluoroethylene
  (TFE) and sulfonic acid (-SO3-H)
  containing-perfluorinated vinyl ether.
• Nafion is the first ionomer polymer that
  contains ionized components and conducts ionic
  species.

Fig.1. Chemical structure of Nafion.1
3
History

• Developed by DuPont from studies on
  copolymerizations of TFE and perfluorinated vinyl
  ether monomers in early 1960s.2
• Initially marketed as material for membrane
  separator of chloralkali cells used in production
  of chlorine and sodium hydroxide.
• Used by NASA and military as membrane material of
  proton-exchanged-membrane (PEM) fuel cells.

4
Synthesis

• Nafion is synthesized via copolymerization of TFE
  and perfluorinated vinyl ether monomers.
• Comonomer perfluorinated vinyl ether with
  sulfonyl fluoride groups derived from reactions
  of tetrafluoroethylene with sulfur trioxide.2

Fig 2. Synthesis Process for Nafion membrane
comonomer PSEPVE.2
5
Synthesis

• Ratio of sulfonic acid-containing comonomer and
  TFE monomer allows control over Nafions ionic
  conductivity.
• Nafion is characterized by equivalent weight
  (EW), which indicates the number of grams of dry
  Nafion per mole of sulfonic acid.

Fig.1. Chemical structure of Nafion.1
6
Processing

• Copolymerization produces sulfonyl fluoride
  (-SO2F) precursor form.
• To use as ionomer, sulfonyl fluoride is converted
  into sulfonate (-SO3-Na) then into sulfonic acid
  (-SO3-H).1
• Processed as thermoplastic via extrusion to
  produce Nafion films.
• Nafion dispersion can be casted to prepare
  membranes.
• Nafion also available as resins or dispersion.

7
Structure/Properties

• Resistant to chemicals and corrosion
• Extremely acidic and conductive
• Thermally stable up to 160C
• Excellent ion transport
• The properties of Nafion are due to its three
  distinct regions a polytetrafluoroethylene
  backbone, oxygen and fluorocarbon side chains,
  and sulfonic acid ions on the end of those side
  chains.3

8
Fluorocarbon Backbone

• Gives great resistance to chemicals and corrosion
  which is common with fluorine-containing polymers
• Stabilizes the entire polymer, leading to no
  significant thermal problems up to 160C
• And due to the high electronegativity the
  fluorine, the backbone helps improve the acidity
  cause by the sulfonic acid groups3

9
Sulfonic Acid Ions

• Even though the sulfonic acid is immobilized, it
  is still able to react with other materials and
  transfer water, hydrogen, and ions through the
  material
• It is thought this is achieved through a
  mechanism in which continuous channels between
  two chains act as conduction pathways that allow
  for ion transport to occur (shown to the right)4

10
Applications for Nafion

• All applications involve the transport of ions
• Electrochemical devices
• Surface treatment of metals
• Metal-ion recovery
• Water electrolysis
• Plating
• Batteries
• Sensors
• Drug release
• Fuel Cells

Honda FCX uses PEM fuel cells.
11
Fuel Cells

• A fuel cell (FC)
• is an energy conversion device (e.g. engine) that
  reacts a fuel (hydrogen) and oxygen (air) to
  produce an electric current.
• has externally supplied reactants, unlike
  batteries.
• provides clean and nearly emissions free energy
  with water and heat as byproducts.
• can be used repeatedly, there is no package to
  throw away (environmentally friendly).

12
Fuel Cell Background

• All fuel cells react a fuel and oxygen to produce
  electricity, but differ in the medium or
  electrolyte where the ions are transported.
• The kind of electrolyte determines all of the
  important characteristics of the fuel cell
  (operating temperature, materials used, and
  variety of fuels that are applicable).
• Proton Exchange Membrane fuel cells use proton
  conducting polymers.

13
Fuel Cell Components

• The anode conducts the electrons that are freed
  from the hydrogen molecules so that they can be
  used in an external circuit, and also disperses
  hydrogen gas over the surface of the catalyst.
• The cathode distributes oxygen to the surface of
  the catalyst. It also conducts the electrons back
  from the external circuit to the catalyst, where
  they can recombine with the hydrogen ions and
  oxygen to form water.
• The electrolyte is the proton exchange membrane -
  Nafion
• The catalyst facilitates the reaction of oxygen
  and hydrogen. It is usually made of platinum
  nanoparticles very thinly coated onto carbon
  paper or cloth.

14
PEM Fuel Cell
15
Chemistry of Nafion Membrane
16
PEM Requirements for Fuel Cells

• A Proton exchange membrane should exhibit
• High protonic conductivity
• Low electronic conductivity
• Low permeability to fuel and oxidant
• Low water transport through diffusion and
  electro-osmosis
• Oxidative and hydrolytic stability
• Good mechanical properties in both dry and
  hydrated states
• Capability for fabrication into MEAs
• Cost effective
• With the exception of cost, Nafion is an
  excellent fit for a PEM. These characteristics,
  coupled with commercial availability is what
  makes it the industry standard.

17
Cost

• Nafion is extremely expensive
• 130 to 300 per 0.3m2 x 1-10mm thickness for
  films
• 5-7 for 10-50 grams in pellet form,
• 50-125per 25mL in solution form
• The extremely high costs of Nafion, along with
  other expensive fuel cell components (such as
  platinum catalysts) is what is inhibiting the
  widespread adoption of fuel cells.

18
Substitutes and Competitors

• Other types of fuel cells that use different
  components such as Solid Oxide FCs, Molten
  Carbonate FCs, Alkaline FCs
• PEM FCs are best for portable and transportation
  applications
• Low cost membrane alternatives (e.g. sulfonated
  polyethers)
• Have not reached performance levels of Nafion

19
Environmental Concerns

• Success in the automotive fuel cell industry
  would lead to huge strides in cleaner, safer, and
  quieter exhaust5
• However, processing leads to toxic fumes forming
  at high temperatures, endangering workers and the
  environment
• Also Nafion is not biodegradable in any way and
  must be disposed of either by landfill or
  incineration in special alkaline scrubbing
  facilities6,7

20
Conclusion

• Nafion was one of the first ionomers produced and
  has been heavily researched for the past 40
  years.
• Excellent resistive properties coupled with the
  ability to be an ion transport membrane make it a
  valuable material for many different
  applications.
• The major drawback is its high cost, which has
  forced researchers to try to find cheaper
  alternatives.
• Nafion has proven to be successful in a few
  markets (e.g. Chlor-alkali), but further
  widespread adoption is heavily dependent on
  economic factors especially in the energy
  sector.

21
Reference

1. Mauiritz, Kenneth A. Moore, Rober B. State of
   Understanding of Nafion. Chem.Rev 104,
   4535-4595, 2004.
2. Banerjee, Shoibal. Curtin, Dennis E. Nafion
   Perfluorinated Membranes in Fuel Cells. Journal
   of Fluorine Chemistry125, 1211-1216, 2004.
3. Perma Pure LLC. Frequently Asked Questions. 2006.
   1 April 2008 lthttp//www.permapure.com/FAQs.htmgt.
4. Mauritz, Kenneth A. and Robert B. Moore. "State
   of Understanding of Nafion." Chem. Rev. 104
   (2004) 4535-4585.
5. DuPont. What is a Fuel Cell? 2008. 1 April 2008
   lthttp//www2.dupont.com/Fuel_Cells/en_US/tech_info
   /about_more.htmlgt.
6. DuPont. Safe Handling and Use of
   Perfluorosulfonic Acid Products. 2006. 1 April
   2008 lthttp//www2.dupont.com/Fuel_Cells/en_US/asse
   ts/downloads/dfc301.pdfgt.
7. DuPont. DuPont Nafion PFSA Membranes. 2006. 1
   April 2008 lthttp//www2.dupont.com/Fuel_Cells/en_U
   S/assets/downloads/dfc101.pdfgt.