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 Schmidt-Rohr said.
To unlock the structure mystery, Schmidt-Rohr turned to mathematical modeling of small-angle X-ray and neutron scattering, or SAXS/SANS. X-ray or neutron radiation is scattered by the 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 an algorithm known as multidimensional Fourier transformation, Schmidt-Rohr was able to show that his 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, Schmidt-Rohr said. While water would freeze in the larger channels, it would continue to diffuse in the smaller-diameter pores.
Schmidt-Rohr added that additional analysis is needed to determine how the cylinders connect through the membrane.
|Contact: Kerry Gibson|