Nafion ® Membrane

Currently, the most commonly used commercial ion-exchange polymer is Nafion^®. Nafion^® is a product of DuPont™. Nafion was be discovered by Walther Grot in the 1960s. Nafion^® is a kind of fluoropolymer-copolymer bases upon sulfonated tetrafluoroethylene (Teflon). The special ionic characters of Nafion^® are result in the molecule groups that embedded onto tetrafluoroethylene backbones. The critical molecule groups are incorporating perfluorovinyl ether groups and the molecule groups have sulfonate groups connect with terminals of the incorporating perfluorovinyl ether groups.

Many researchers paying attention on Nafion^® membrane, because Nafion^® membrane can be used to be a very important component for proton exchange membrane ( PEM ) fuel cells. Nafion^® membrane is as a key role in proton conductor. The Nafion^® membrane is widely used owing to the thermal and mechanical stability of Nafion^® membrane is very remarkable [22]. The molecule formula is shown in Figure 2.3.

Figure 2.4 Nafion^® polymer molecule structure 11

The excellent ion conductive characters of Nafion^® polymer is still a focal point of investigation. The sulfonic acid groups have protons on itself, and the protons on the groups can bounce to the other sulfonic acid groups from the initial group. The pores on the Nafion^® polymer permit cations to migrate in the polymer, but the polymers don’t allow anions or electrons to transfer. The protons or the other cations initially in the Nafion^® polymer can be replaced with several kinds of cations of conductors. The

property of Nafion^® membrane could also be changed through the process. Nafion^® powders and copolymer are available products through appropriate manufacturing method.

Nafion^® polymer has not the only one chemical name in the International Union of Pure and Applied Chemistry ( IUPAC ) system. Because of the chemical configurations of Nafion^® is not only one type, too. The molecular weight is also indeterminate owing to divergences of fabricating process and morphology of solution. Traditional measurement means such as, light scattering and gel permeation chromatography are useless for Nafion^® polymer, because Nafion^® polymer is unquestionably insoluble. Even though the molecular weight of common Nafion^® polymer has been estimated, it’s about 105-106 Da.

Furthermore, the other way to describe the common commercial Nafion^® product is the equivalent weight ( EW ) and the thickness of the membrane. Equivalent weight concerns the total amount of sulfonic acid group that is included in the Nafion^® membrane. The definition of EW is the weight of Nafion^® membrane that has total one mole of sulfonic acid group in the membrane. For instance, the EW of Nafion^® 117 is about 1150, and the thickness is about 183micrometer. The main factors effect the properties of Nafion^® are the stable tetrafluoroethylene backbone and the acidic sulfonic groups. For example, the conductivity of cations in the membrane is excellent, so the Nafion^® membrane is used in many applications that need selectively transparent of specific ions. And the stable property due to the tetrafluoroethylene backbone gives Nafion^® the capability to be operated in high temperature. But, one important phenomena should be careful about.

That is, although the Nafion^® polymer is good in the permittivity of water, but if there were too many water molecules in the membrane, the transmission of ions would be disturbed. For the operation of PEM fuel cells, the property should be concerned carefully, because the byproduct of the PEM fuel cells is just water.

The properties of Nafion^® polymer are important, and of course the actual structure of Nafion^® polymer is also very important. The study in the morphology of Nafion^®

membranes is a focal point to further understand the property and operation of Nafion^® membrane. Some properties must be illustrated by the research of the structure, such as the management of water in the membrane, the high temperature condition that affects the hydration stability, etc. And the most important, the electro-osmotic drag. It might be the direct factor concern the property of IPMC. Electro-osmotic drag is the phenomena that the liquid with polar property flow through a thin membrane or a material that has many porous in the structure due to the electro field that is applied on the material. And the polar liquid may also flow along the surfaces of any shape that had been charged. Not only macro-porous material can allow polar liquid to flow through by, the micro-porous or even the nano-porous materials, that include ionic trapping sites and permit the absorption of water, also allow the transport of polar liquid. In the field of electrochemistry, physics and vascular plant biology, electro-osmosis has another name:

electroendosmosis. Electro-osmosis was discovered by F.F. Reuss first in 1809, and now has several applications in the field of microfluidics and PEM fuel cells. The first structure model of Nafion^® polymer is composed of sulfonate ion micelles that distribute uniformly in the polymer structure, and the model is called the Cluster-Network Model or called the Cluster-Channel. Micelle is a kind of special molecule structure. The aggregation of surfactant molecules that suspend in a liquid colloid could be a spherical construction. The spherical construction often consist of hydrophilic head regions on the spherical surface that contact with liquid, and the hydrophobic tail regions are isolated in the center of micelles. But the micelles of Cluster-Network Model are called the inverted micelles. Because the position of hydrophilic head regions and the hydrophobic tail regions of Nafion^® molecule in the micelles are reversed. So, the center of the micelles permits the transmission of water and ions. A continuous fluorocarbon lattice held the micelles in the fixed positions. The clusters are about 40 Å in diameter and the channels have the diameter about 10 Å interconnect the micelles [23]. The concepts are illustrated

in the figure 2.4, and figure 2.5.

Figure 2.5 Structure of spherical micelle. 12

Figure 2.6 Cluster-network model for the morphology of hydrated Nafion^® polymer.13

The diversified derivatives of Nafion^® polymer cause the two major problems that increase the difficulties in determining the actual polymer structure of Nafion^® membrane.

The two problems are the uncertainty of the solubility and the inconsistent crystalline structure. The other Nafion^® advanced morphological models were also proposed. The categories have included a core-shell model, a rod model, and a sandwich model. The core-shell model is considered as the Nafion^® polymer structure separates into two parts:

ion-rich region and ion-poor region, and the ion-rich regions are the cores surrounded by hydrophilic head

-the ion-poor shells [24]. The rod model describes -the structure that -the arrangement of sulfonic groups in the Nafion^® membrane is the crystal-like rods. The third model is the sandwich model, the middle of the model is an aqueous layer where the water and ion transport occur, and the two layers on both sides are composed of sulfonic groups where the sulfonic groups attract each other across the middle aqueous layer. The models are different from each other, because the cluster geometry and the cluster distribution. Even though the models are not really match the actual structure very well. A demonstration has been announced by some scientists. The demonstration says that because the Nafion^® polymer is a kind of membrane hydrates, the morphology of Nafion^® polymer could be the cluster-channel model and then transfers to the rod-like model. The diagram of the models are shown in figure 2.6, figure 2.7, figure 2.8.

Figure 2.7 Separation of hydrophobic fluoro-carbon backbone polymer 14 and hydrophilic side-chains.

portion water/ methanol water/ methanol

Monomer Polymer

Figure 2.8 Structure element with sandwich structure. 15

Figure 2.9 Composition of basic structure elements to complex channel structures.16

Another recent structure model theory is the water channel model. Combine the simulation result, that according to the data of small-angle X-ray scattering [24], as shown in figure 2.9, and the observation result of solid state nuclear magnetic resonance [25].

water/methanol

core shell

shell

Figure 2.10 Schematic experimental set-ups for in situ measurements with synchrotron radiation.

17The conclusion is the water channel model. The concept is illustrated in figure 2.10.

Figure 2.11 Two views of an inverted-micelle cylinder, with the polymer backbones on the outside and the ionic side groups lining the water channel.18

Nafion® sample

water/methanol flow cell Synchrotron

Radiation DORIS 11 keV

position sensitive X-ray detector

× n

× n

× n

2.5 nm

This model gives several individual descriptions of Nafion^® polymer structure. First, the hydrophilic water channel is composed of sulfonic acid functional groups, and the construction of the channel is by the self-organization phenomenon of sulfonic acid functional groups. Second, the hydrophilic water channels are about 2.5 nm in the diameter, and the hydrophilic water channels could let the ions efficiently transport through. Third, the polymer backbone construct from hydrophobic polymer includes crystallites, and the quality of the backbone affects the stability of mechanical of membrane. The water channels are parallel to each other and the several crystallites also distribute in the construction. The hydrophilic sulfonic acid functional water/ion channels, hydrophobic fluoro-carbon backbone, and crystallites construct the structure of Nafion^® polymer as shown in figure 2.11 [25].

Figure 2.12 Diagram of the approximately hexagonal packing of 19 several inverted-micelle cylinders.

4 nm

50 nm

H₂O channel crystallite