Nafion microphase separation

Schematic representation of microphase separation in Nafion and SPEEKK. (From Kreuer, K. D. 2001. Journal of Membrane Science 185 29-39.)...

Fig. 22. Chemical structure of Nafion11 and schematic illustration for microphase separation in Nafion " membrane.

In these circumstances, when the microphase-separated morphology is clearly evidenced by numerous experiments, which can be however ambiguously interpreted, modeling and simulation attract much attention as a means to clarify and extend the concept of Nafion morphology and ionic conductivity. Such studies provide a valuable tool for a deeper understanding of the complex properties of water-containing Nafion systems at a fundamental level. 

Figure 15.1. Isosurface between the hydrophilic and hydrophobic domains showing a microphase-separated water-containing Nafion system at hydration level corresponding to h = 4H2O molecules per SO3H group. There are continuous passes in the water-rich regions from any side of the system to any other its side. Adapted from Ref. [Ill...

Figure 15.10. Snapshots from full atomistic MD simulations showing microphase-separated structure of hydrated Nafion, at hydration levels of (a) /i = 4 and (b) h = 20. Spherical domains were cut out from the cubic MD simulation box. For visual clarity, three-dimensional Connolly surfaces were generated for the subsystem of hydrophobic atoms. The equilibrium Nafion structure consists of water-filled pores (diameter 40 A) that are connected by channels having diameter of order 10 A. This pore-channel nanostructure provides energetically favorable pathways through the nonpolar fluorocarbon matrix of the membrane for water and other mobile species.

A modem FC used in transportation and other applications is shown in Fig. 2. Its key elements are the electrodes, the catalyst, and the proton exchange membrane (PEM) the cell is fueled by hydrogen or methanol at the anode and oxygen or air at the cathode. The membrane electrode assembly (MEA) that is the heart of ECs includes the proton exchange membrane, a polymer modified to include ions, typically sulfonic groups an ionomer In the presence of water, ionomers self-assem-ble into microphase separated domains that allow the movement of in one direction only, from the anode to the cathode. The membrane performance was first demonstrated by Nafion, the ionomer made by DuPont, which consists of a perflu-orinated backbone and pendant chains terminated by sulfonic groups, -SOs . Nafion was the major component in the PEMEC developed by General Electric for... 

Fig. 13 Clustering of hydrophilic domains in Nafion. At low temperatures, the sulfonic acid groups cluster to reduce the repulsive interactions with the PTFE matrix. Entropy drives the sulfonic acid groups to disperse at high temperatures. Water absorption increases the fraction of hydrophilic species inducing microphase separation...

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