Perfluorosulfonic acid membrane

Until recently (i.e., till early 1990s), most of the efforts to develop DMFCs has been with sulfuric acid as the electrolyte. The recent success with a proton conducting membrane (perfluorosulfonic acid membrane) in PEMFCs has steered DMFC research toward the use of this electrolyte. The positive feature of a liquid feed to a DMFC is that it eliminates the humidification subsystem, as required for a PEMFC with gaseous reactants. Another positive point is that the DMFC does not require the heavy and bulky fuel processor. Two problems continue to be nerve-wracking in the projects to develop DMFCs (1) the exchange current density for methanol oxidation, even on the... 

Cation-exchange membranes, perfluorosulfonic acid ionomer, 125... 

Keywords Analysis Cross leak degradation High temperature membrane Perfluorosulfonic acid PFSA Reinforcement... 

The principle of operation is shown in Fig. 2. Chlorine gas is produced at the anode (especially optimized dimensionally stable anode) with an anolyte feed concentration of 14 wt % HCl. Anode and cathode are separated by a cation exchange membrane (perfluorosulfonic acid polymer, PFSA, e.g., Nafion of DuPont). The ODC is based on a conductive carbon cloth which operates simultaneously as a gas diffusion layer because a suitable material is incorporated. The oxygen reduction reaction (5) takes place in three-phase boundaries of a thin, porous catalyst layer on the surface. 

Membrane. Perfluorosulfonic acid (PFSA) is the most commonly used membrane material [4], PFSA membranes are relatively strong and stable in both oxidative and reductive environments, since the structure of PFSA is based on a PTFE backbone. The conductivity of a well-humidified PFSA membrane can be as high as 0.2s cm. As is well known, fuel cell operation at elevated temperatures can increase the rates of reaction, reduce problems related to catalyst poisoning, reduce the use of expensive catalysts, and minimize problems due to electrode flooding. Unfortunately, a PFSA membrane must be kept hydrated to retain its proton conductivity. Moreover, a PFSA membrane is alcohol permeable if it is used in DAFCs. Because of the disadvantages of PFSA membranes, many alternatives have been proposed [106]. Five categories of membranes are classified (1) perfluorinated, (2) partially fluorinated, (3) non-fluorinated, (4) non-fluorinated composite, and (5) others. 

The electrolyte is a perfluorosulfonic acid ionomer, commercially available under the trade name of Nafion . It is in the form of a membrane about 0.17 mm (0.007 in) thick, and the electrodes are bonded directly onto the surface. The elec trodes contain veiy finely divided platinum or platinum alloys supported on carbon powder or fibers. The bipolar plates are made of graphite or metal. 

The PEM (proton exchange membrane) material is a perfluorosulfonic acid polymer film. Several manufacturers make PEMs in one form or another. We used one made by du Pont called Nation 117. Nation 117 is a transparent polymer film about 175 microns (0.007 inches) thick. Dow Chemical Co., Asahi Chemical Co., and Chloride Engineers Ltd. make something similar. A patent describing how one PEM manufacturer s film is processed is listed in the references section at the end of this article. 

High Temperature Operation of the PEMFC The first generation of commercial PEMFCs will use presently known components, consisting of a perfluorosulfonic acid membrane as electrolyte and catalyst compositions as cited above. The electrolyte determines that the fuel cell needs to be operated at fully humidified conditions and limits the operating temperature to 80-90 °C. 

Geiger, A. B., Newman, J. and Prausnitz, J. M. 2001. Phase equilibria for water-methanol mixtures in perfluorosulfonic-acid membranes. AlChE Journal 47 445-452. 

Ren, X. and Gottesfeld, S. 2001. Electro-osmotic drag of water in poly(perfluorosulfonic acid) membranes. Journal of the Electrochemical Society 148 A87-A93. 

Paddison, S. J. and Elliott, J. A. 2006. On the consequences of side chain flexibility and backbone conformation on hydration and proton dissociation in perfluorosulfonic acid membranes. Physical Chemistry Chemical Physics 8 2193-2203. 

Adjemian, K. T., Srinivasan, S., Benzieger, J. and Bocarsly, A. B. 2002. Investigation of PEMFC operation above 100°C employing perfluorosulfonic acid silicon oxide composite membranes. Journal of Power Sources 109 356-364. 

Futerko and Hsing presented a thermodynamic model for water vapor uptake in perfluorosulfonic acid membranes.The following expression was used for the membrane—internal water activity, a, which was borrowed from the standard Flory—Huggins theory of concentrated polymer solutions ... 

Nafion and Other Poly(perfluorosulfonic acid) Membranes 4590... 

This review will outline the materials requirements for advanced alternative proton exchange membranes for fuel cells, assess recent progress in this area, and provide directions for the development of next-generation materials. The focus will be on the synthesis of polymeric materials that have attached ion conducting groups. State-of-the-art Nation and its commercially available perfluorosulfonic acid relatives will initially be discussed. Other chain-growth co-... 

The current state-of-the-art proton exchange membrane is Nafion, a DuPont product that was developed in the late 1960s primarily as a permselective separator in chlor-alkali electrolyzers. Nation s poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical stability, which is also important in fuel cell applications. 

Like many other fluoropolymers, Nafion is quite resistant to chemical attack, but the presence of its strong perfluorosulfonic acid groups imparts many of its desirable properties as a proton exchange membrane. Fine dispersions (sometimes incorrectly called solutions) can be generated with alcohol/water treatments. Such dispersions are often critical for the generation of the catalyst electrode structure and the MEAs. Films prepared by simply drying these dispersions are often called recast Nafion, and it is often not realized that its morphology and physical behavior are much different from those of the extruded, more crystalline form. 

J.T. Hinatsu, M. Mizuhata, H. Takenaka, Water uptake of perfluorosulfonic acid membranes from liquid water and water vapor, J. Electrochem. Soc. 141, 1493 (1994)... 

T.D. Gierke, Ionic Clustering in Nation Perfluorosulfonic Acid Membranes and Its Relationship to Hydroxyl Rejection and Chlor-Alkali Current Efficiency, Paper presented at the Electrochemical Society Fall Meeting, Atlanta, GA (1977). 

Perfluorosulfonic acid polymers, for example, Nafion, or ionic and cross-linked polystyrene derivatives, are the best known examples of ion-exchange membrane materials (see also Section 2.6.4). 

Solid Polymer Electrolyte Fuel Cell Here, there is no apparent liquid solution, or high-temperature ionic conductor. The usual ionic solution between the electrodes is replaced by a well-humidified membrane made of a perfluorosulfonic acid polymer that conducts protons. 

The perfluorosulfonic acid family of membranes includes a range of alternative products from different manufacturers, using for example different side chains, but generally the inherent characteristics are similar as follows ... 

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