(Nanowerk News) Fuel-cell cars are reaching commercial viability in today's
increasingly eco-conscious society, but despite their promise, even scientists
have struggled to explain just how the fuel-cell's central component – the
proton exchange membrane – really works.
However, a team of researchers at the U.S. Department of Energy's Ames
Laboratory has offered a new model that provides the best explanation to
date for the membrane's structure and how it functions. And armed with that
information, scientists should be able to build similar fuel-cell membrane
materials that are less expensive or have different properties, such as higher
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 Dec. 9,2007 issue of the journal Nature Materials ("Parallel cylindrical water nanochannels in Nafion fuel-cell membranes"),
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-
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-
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.
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.
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.