Perfluorosulfonic acid membranes chemical stability

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]. 

Now, a perfluorosulfonic acid membrane produced by DuPont Company named Naflon is commonly used in PEMFC industry. It has various excellent properties such as chemical stability, electrochemical properties, and thermostability. 

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. 

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. 

Perfluorosulfonic acid (PFSA) membranes as shown in Fig. 1 were first developed for fuel cells by DuPont as Naflon and installed into the Biosatellite spacecraft in 1967 [1,2]. Various types of PFSA polymers, such as Flemion , Aciplex , and Dow membrane, were developed subsequently. They have excellent chemical stability, high proton conductivity, and high water diffusivity in a wide range of temperatures, brought about by the nature of fluorinated compounds and these non-cross-linked structures [3-5]. 

As mentioned above, perfluorosulfonic acid (PFSA) membranes like Nation (Dupont) are CEMs of very high chemical and thermal stability. In a highly sophisticated composition, such membranes are used as separators in industrial chlor-alkali electrolysis. Most research in the area of SPE electrosynthesis has also been done using Nation membranes. 

The MEA is composed of three main parts, e.g., polymer electrolyte membrane (PEM), gas diffusion medium, and catalyst layer (CL). The membrane, with hydrophilic proton-conducting channels embedded in a hydrophobic structural matrix, plays a key role in the operation of PEFCs. The PEMs for PEFCs commonly use perfluorosulfonic acid (PFSA) electrolytes such as Nation , with the chemical structure shown in Fig. 2, because of its high proton conductivity as well as chemical and thermal stability [1]. The gas diffusion medium (GDM), including both the microporous layer (MPL) and the gas diffusion layer (GDL), which typically is based on carbon fibers, is also an important component. The GDM is designed with three distinct... 

In perfluorinated ionomers, a PTFE-based polymeric backbone offers chemical stability from the radical species or acid-base, which causes hydrolytic degradation of the polymer chain. Ionic conductivity is provided by pendant acidic moiety in carboxylate or sulfonate form. There are some reports on perfluorinated carboxylic acid (PFCA) materials, most of which are derived from Nafion [26-29]. However, PFCA is not suitable for fuel cell application due to its low proton conductivity. Perfluorosulfonic acid (PFSA) is the most favored choice among not only perfluorinated membranes but all other ionomers in fuel cell applications. Sulfonic acid form of Nafion is a representative PFSA and thus has been intensively studied since 1960s. Reported chemical structure of Nafion membrane is given in Fig. 13.8. 

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