Researchers Offer New Model of Structure and Function of Nafion Membranes in Fuel Cells

12 December 2007

The structure of Nafion, according to the new study. Click to enlarge.

Researchers at the US Department of Energy’s Ames Laboratory have developed a new model to explain the structure and function of Nafion proton exchange membranes (PEM) in fuel cells. Understanding Nafion’s structure may enable other scientists to build similar fuel-cell membrane materials that are less expensive or have different properties, such as higher operating temperatures.

The model proposed by Ames Laboratory scientists Klaus Schmidt-Rohr and Qiang Chen, and detailed in the 9 December issue of the journal Nature Materials, looked specifically at Nafion, a widely used perfluorinated polymer film that stands out for its high selective permeability to water and protons. Schmidt-Rohr, who is also a professor of chemistry at Iowa State University, suggests that Nafion has a closely packed network of nanoscale cylindrical water channels running in parallel through the material.

From nuclear magnetic resonance (NMR), we know that Nafion molecules have a rigid backbone structure with hair-like ‘defects’ along the chain, but we didn’t know just how these molecule were arranged. Some have proposed spheroidal water clusters, others a web-like network of water channels. Our theory is that these hydrophobic backbone structures cluster together to form long rigid cylinders about 2.5 nanometers in diameter with the hydrophilic ‘hairs’ to the inside of the water-filled tubes.

—Klaus Schmidt-Rohr

Nafion crystallites (approximately 10 vol%), which form physical crosslinks that are crucial for the mechanical properties of Nafion films, are elongated and parallel to the water channels, with cross-sections of approximately 5 nm².

Though the cylinders in different parts of the sample may not align perfectly, they do connect to create water channels passing through the membrane material, which can be 10s of microns thick. Its this structure of relatively wide diameter channels, densely packed and running mostly parallel through the material that helps explain how water and protons can so easily diffuse through Nafion, “almost as easily as water passing through water,” said Schmidt-Rohr.

To test their theory, the team used mathematical modeling of small-angle X-ray and neutron scattering (SAXS/SANS). X-ray or neutron radiation is scattered by a sample and the resulting scattering pattern is analyzed to provide information about the size, shape and orientation of the components of the sample on the nanometer scale.

Using multidimensional Fourier transformation, they showed that the model of long, densely packed channels closely matches the known scattering data of Nafion. Mathematical modeling of other proposed structures, in which the water clusters have other shapes or connectivities, did not match the measured scattering curves.

Our model also helps explain how conductivity continues even well below the freezing point of water. While water would freeze in the larger channels, it would continue to diffuse in the smaller-diameter pores.

—Klaus Schmidt-Rohr

Additional analysis is needed to determine how the cylinders connect through the membrane. The research is funded by the Department of Energy’s Office of Basic Energy Sciences and conducted by Ames Laboratory’s Materials Chemistry and Biomolecular Material Program.

Resources

• Klaus Schmidt-Rohr and Qiang Chen. “Parallel cylindrical water nanochannels in Nafion fuel-cell membranes”, Nature Materials Published online: 9 December 2007 | doi:10.1038/nmat2074