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For catalysts in fuel cells and electrodes in batteries, engineers would
like to manufacture metal films that are porous, to make more surface area
available for chemical reactions, and highly conductive, to carry off the
electricity. The latter has been a frustrating challenge.
But Cornell University chemists have now developed a
way to make porous metal films with up to 1,000 times the electrical
conductivity offered by previous methods. Their technique also opens the door
to creating a wide variety of metal nanostructures for engineering and
biomedical applications, the researchers said.
The results of several years of experimentation are described online in Nature Materials.
“We have reached unprecedented levels of control on composition, nanostructure
and functionality—for example, conductivity—of the resulting materials, all
with a simple ‘one-pot’ mix-and-heat approach,” said senior author Ulrich
Wiesner, the Spencer T. Olin Professor of Engineering.
The new method builds on the “sol-gel process,” already familiar
to chemists. Certain compounds of silicon mixed with solvents will
self-assemble into a structure of silicon dioxide (i.e., glass) honeycombed
with nanometer-scaled pores. The challenge facing the researchers was to add
metal to create a porous structure that conducts electricity.
About 10 years ago, Wiesner’s research group, collaborating with the Cornell
Fuel Cell Institute, tried using the sol-gel process with the catalysts that
pull protons off of fuel molecules to generate electricity. They needed
materials that would pass high current, but adding more than a small amount of
metal disrupted the sol-gel process, explained Scott Warren, first author of
the Nature Materials paper.
Warren, who was then a PhD student in Wiesner’s group and is now a
researcher at Northwestern University, hit on the idea of using an amino acid
to link metal atoms to silica molecules, because he had realized that one end
of the amino acid molecule has an affinity for silica and the other end for
metals.
“If there was a way to directly attach the metal to the silica sol-gel
precursor then we would prevent this phase separation that was disrupting the
self-assembly process,” he explained.
The immediate result is a nanostructure of metal, silica and carbon, with
much more metal than had been possible before, greatly increasing conductivity.
The silica and carbon can be removed, leaving porous metal. But a silica-metal
structure would hold its shape at the high temperatures found in some fuel
cells, Warren
noted, and removing just the silica to leave a carbon-metal complex offers
other possibilities, including larger pores.
The researchers report a wide range of experiments showing that their
process can be used to make “a library of materials with a high degree of
control over composition and structure.” They have built structures of
almost every metal in the periodic table, and with additional chemistry can
“tune” the dimensions of the pores in a range from 10 to 500 nm. They
have also made metal-filled silica nanoparticles small enough to be ingested
and secreted by humans, with possible biomedical applications. Wiesner’s group
is also known for creating “Cornell dots,” which encapsulate dyes in
silica nanoparticles, so a possible future application of the sol-gel process
might be to build Graetzel solar cells, which contain light-sensitive dyes.
Michael Graetzel of the École Polytechnique Fédérale de Lausanne and innovator
of the Graetzel cell is a coauthor of the new paper. The measurement of the
record-setting electrical conductivity was performed in his laboratory.
