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A fuel cell works by pumping hydrogen gas through the proton exchange
membrane. In the process, the hydrogen gives up electrons in the form
of electricity, then combines with oxygen gas to form water as the
by-product. It can also work in reverse � when current is applied,
water is split into its component gases, hydrogen and oxygen.
The model proposed by Ames Laboratory scientists Klaus Schmidt-Rohr
and Qiang Chen, and detailed in the 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,� Schmidt-Rohr said, �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 (water-hating) backbone
structures cluster together,� he continued, �to form long rigid
cylinders about 2.5 nanometers in diameter with the hydrophilic
�hairs� to the inside of the water-filled tubes.�
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 10�s of microns thick. It�s 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.
Ames Laboratory, celebrating its 60th anniversary in 2007, is operated
for the Department of Energy by Iowa State University. The Lab
conducts research into various areas of national concern, including
the synthesis and study of new materials, energy resources, high-speed
computer design, and environmental cleanup and restoration.
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