Perfluoro sulfonic acid

Apart from the problems of low electrocatalytic activity of the methanol electrode and poisoning of the electrocatalyst by adsorbed intermediates, an overwhelming problem is the migration of the methanol from the anode to the cathode via the proton-conducting membrane. The perfluoro-sulfonic acid membrane contains about 30% of water by weight, which is essential for achieving the desired conductivity. The proton conduction occurs by a mechanism (proton hopping process) similar to what occurs... 

Figure 7. Minimum energy conformation of a two-side-chain fragment of a perfluoro sulfonic acid polymer (Dow) with six water molecules, showing the dissociation of both acidic protons.

The strong acidity and high hydrophilicity of the perfluoro-sulfonic acid group result in a membrane of high water content and low electric resistance. Since the fixed-ion concentration in the sulfonic acid membrane is also low, current efficiency is less than 80% with caustic concentrations of 17% or more (26). The chemical stability of perfluorosulfonic acid group is excellent. Because of its low pKa value, the membrane can be exposed to solutions of pH 1. 

The water content of a perfluoro sulfonic acid membrane and a perfluorocarboxylic acid membrane illustrated in Fig. 18 clearly... 

T.A. Zawodzinski, S. Gottesfeld, S. Shoichet, T.J. McCarthy, The contact-angle between water and the surface of perfluoro-sulfonic acid membranes. Journal of Applied Electrochemistry 1993,23(1), 86-88. 

Synonyms C9-C10 alkanes, perfluoro-, sulfonic acid, ammonium salts Definition Ammonium salt of a sulfonated, fluorinated alkane with 9 or 10 carbon atoms in alkyl chain Uses Surfactant in cosmetics... 

J. T. Hinatsu, M. Mizuhata, and H. Takenaka, Water-Uptake of Perfluoro-sulfonic Acid Membranes from Liquid Water and Water-Vapor, Journal of the Electrochemical Society, 141, 1493 (1994). 

Gierke model of nanoclusters in perfluoro-sulfonic acid materials. 

Perfluoro-sulfonic acid membranes are also prone to incorporate foreign cations. The incorporation of di-valent cations (in particular Ca " and Mg +) has a strong negative impact on membrane conductivity and hydroxides precipitate. Therefore, calcium and magnesium contents in the saturated brine must be less than a threshold value of 0.02 mg/L. 

Liu J, Suraweera N, Keffer D J, Cui S and Paddison S J (2010), On the relationship between polymer electrolyte structure and hydrated morphology of perfluoro-sulfonic acid membranes ,/P/zy5 Chem C, 114,11279-11292. 

K. Xu, H. Oh, M.A. Hickner, Q. Wang, Highly conductive aromatic ionomers with perfluoro-sulfonic acid side chains for elevated temperature fuel cells. Macromolecules 44 (12) (2011) 4605 609. 

N.D. (2010) Changes in the chemical stmcture and properties of a perfluoro-sulfonated acid membrane induced by fuel-cell operation. J. Appl. Polym. Sci.,... 

Many different types of polymer membranes containing fixed ionic groups have been explored since. Up to today, the materials of choice are perfluoro sulfonic acid membranes of the Nafion-type (Fig. 2), for the first time described by (jrot [9]. These materials, similar products have later been developed by other companies (Asahi Glass, Asahi Kasei, Solvay, 3M, Gore, etc.), have been continuously improved for their application in fuel cells. A comprehensive overview of membrane development can be found in [10]. 

Polymer Electrolyte Fuel Cells, Membrane-Electrode Assemblies, Fig. 2 Chemical structure of perfluoro-sulfonic acid polymer... 

Membrane Electrolytes, from Perfluoro Sulfonic Acid (PFSA) to Hydrocarbon lonomers... 

Copolymers composed of poly(tetrafluoroethylene) and poly(trifluoroethylene) with perfluoro sulfonic acid ether side chains. 

Low cost (final target for electric vehicle apphcations would be cheaper than US 10/m ) need to be taken into account. While a number of PEMs have been developed, no single membranes fulfill aU of these requirements. Currently, the most promising PEMs are perfluoro sulfonic acid (PFSA) ionomers. Another candidate second to the PFSA ionomers is non-fluorinated (or in some cases only slightly fluorinated) hydrocarbon ionomers. The aim of this chapter is to review the most recent progress on these two classes of ionomer membranes for low-temperature fuel cell applications... 

Fig. 7.1 General chemical structure of the perfluoro sulfonic acid (PFSA)...

Fig. 7.2 Recent research trends in perfluoro sulfonic acid (PFSA) membranes...

Fig. 7.6 Possible degradation mechanisms of perfluoro sulfonic acid (PFSAs) involving oxidative radical species...

One of the significant differences between hydrocarbon ionomers and perfluoro-sulfonic acid polymers is add groups. The pKa value of benzenesulfonic acid (PhSOsH) is 2.5 and that of trifluoromethanesulfonic acid (CF3SO3H) is —13. The pKa value was estimated to be ca. 1 for sulfonated polyether ketone and ca. —6 for Nafion membranes [78]. Therefore, the effective proton concentration and proton mobility should be lower in the hydrocarbon ionomer membranes. Without appropriate molecular design such as multiblock copolymer and sulfonic acid clusters as mentioned above, hydrocarbon ionomer membranes lack well-developed ionic channels due to less pronounced hydrophilic and hydrophobic phase separation, which causes the lower proton conductivity at low humidity. 

---