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 1960’s.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 Nafion’s 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 160°C • 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 160°C • 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 FC’s, Molten Carbonate FC’s, Alkaline FC’s • PEM FC’s 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 • Mauiritz, Kenneth A. Moore, Rober B. “State of Understanding of Nafion”. Chem.Rev 104, 4535-4595, 2004. • Banerjee, Shoibal. Curtin, Dennis E. “Nafion Perfluorinated Membranes in Fuel Cells”. Journal of Fluorine Chemistry:125, 1211-1216, 2004. • Perma Pure LLC. Frequently Asked Questions. 2006. 1 April 2008 <http://www.permapure.com/FAQs.htm>. • Mauritz, Kenneth A. and Robert B. Moore. "State of Understanding of Nafion." Chem. Rev. 104 (2004): 4535-4585. • DuPont. What is a Fuel Cell? 2008. 1 April 2008 <http://www2.dupont.com/Fuel_Cells/en_US/tech_info/about_more.html>. • DuPont. Safe Handling and Use of Perfluorosulfonic Acid Products. 2006. 1 April 2008 <http://www2.dupont.com/Fuel_Cells/en_US/assets/downloads/dfc301.pdf>. • DuPont. DuPont Nafion PFSA Membranes. 2006. 1 April 2008 <http://www2.dupont.com/Fuel_Cells/en_US/assets/downloads/dfc101.pdf>.