Perfluorosulfonic acid membrane Nafion

The advent of Du Pont s inert perfluorosulfonic acid membranes, Nafion, in the late 1960,s made chlor-alkali production in a membrane cell a realistic possibility. The basic structure of Du Pont s fluoro ionomer is shown... 

Petersen, M. K. and Voth, G. A. 2006. Characterization of the solvation and transport of the hydrated proton in the perfluorosulfonic acid membrane Nafion. 110(37),... 

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

Gierke, T.D. "Ionic Clustering in Nafion Perfluorosulfonic Acid Membranes and its Relationship to Hydroxyl Rejection and Chlor-Alkali Efficiency", presented at the 152nd National Meeting of the Electrochemical Society, Atlanta, Ga.,... 

In April 1975, Asahi Chemical started operation of a membrane chlor-alkali plant with a capacity of 40,000 MT/Y of caustic soda using Nafion perfluorosulfonic acid membrane. In 1976, this membrane was replaced by perfluorocarboxylic acid membrane developed by Asahi Chemical. The total caustic production capacity of plants based on Asahi Chemical s membrane chlor-alkali technology using perfluorocarboxylic acid membrane will reach 520,000 MT/Y in 1982, at seven locations in various countries. 

Nafion (perfluorosulfonic acid) membranes are currently used in cells with a corrosive environment and high temperature. Many of these cells are designed with the solid polymer electrolyte (SPE) configuration. The merits of the solid polymer electrolyte technology will be discussed in the next section. 

Cation, anion, and water transport in ion-exchange membranes have been described by several phenomenological solution-diffusion models and electrokinetic pore-flow theories. Phenomenological models based on irreversible thermodynamics have been applied to cation-exchange membranes, including DuPont s Nafion perfluorosulfonic acid membranes [147, 148]. These models view the membrane as a black box and membrane properties such as ionic fluxes, water transport, and electric potential are related to one another without specifying the membrane structure and molecular-level mechanism for ion and solvent permeation. For a four-component system (one mobile cation, one mobile anion, water, and membrane fixed-charge sites), there are three independent flux equations (for cations, anions, and solvent species) of the form... 

Nafion polymers have been developed recently by the du Pont Company. They are perfluorosulfonic acid membranes mainly used as separators in electrochemical applications. The backbone of the poljrmer chains consists of perfluoroethylene units whereas the side chains are of the form - 0 - CF2 - CF... 

The current membranes of choice in such cells are perfluorosulfonic acid membranes, such as Nafion -1l7 manufactured by DuPont Chemical Co., which are highly conducting and show promise due to their high power and energy density capabilities. Some disadvantages of using Nafion -117 for use... 

W. Y. Hsu, J. R. Barkley, and P. Meakin, Ion Percolation and Insulator-to-Conductor Transition in Nafion Perfluorosulfonic Acid Membranes, Macromolecules, 13,198 (1980). 

Comparison of the conductivity of two perfluorosulfonic acid membranes in Fig. 4.8.11 shows that the conductivity increases [59] with increasing water content and also increasing ion-exchange capacity or concentration of fixed ionic groups. The Dow membrane has a higher EC and a shorter side-chain length than Nafion . 

FIGURE 4.8.11. Variation of proton conductivity of perfluorosulfonic acid membranes with water content at 30 C. Dow A DuPont Nafion 117 (revised from the figure in [59]). 

E.J. Hora and D.E. Maloney, Chemically Modified Nafion Perfluorosulfonic Acid Membranes as Separators in Chlor-Alkali CeXli, Abstract 441, Electrochemical Society Meeting, Atlanta, GA (1977). 

Verbrugge and Hill (1990) presented the theoretical representation and experimental data to characterize the proton and water in perfluorosulfonic acid membrane. The analysis describes the transport of water molecules carried along with the transport of proton across the membrane. Springer et al. (1991) presented a one-dimensional, isothermal model of FEMFC in which detailed considerations of the Nafion-117 membrane characteristics in terms of water content and water transport properties including water drag coefficient and diffusion coefficient are given. Their results demonstrated increased membrane resistance with current density. 

The most well-known and well-studied membrane materials for DMFCs are perfluorosulfonic acid membranes, such as Nafion (shown in Figure 5.2). These macromolecules combine two different functionalities in a single macromolecule first, the hydrophobic nature, which impacts the high chemical and thermal stability, and second, the hydrophilic sulfonic acid regions, which are responsible for the water update and ion exchange capability. In the presence of water, these membranes phase separate into hydrophobic and hydrophilic domains [5], and a significant body of work has been conducted into characterizing the phase-separated microstructure of perfluorosulfonic acid membranes. A detailed discussion is beyond the scope of this chapter, but the interested reader should consult the excellent review by Mauritz and Moore [6]. 

A number of companies have produced commercial perfluorosulfonic acid membranes, with trade names including Nafion, Flemion, Aciplex, Aquivion, and Fumion [8]. However, since methanol readily transports across these membranes, the efficiency of DMFCs operating with these membranes is rather low, as the methanol reacts at the cathode to produce carbon dioxide and water (reducing the coulombic efficiency of the cell). It is, therefore, important to modify the properties of these membranes to suppress as much as possible the methanol crossover. 

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 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. 

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