April 30, 2012
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Utilizing this new method, Taylor was able to demonstrate a solar cell platform, a lithium battery, and a fuel cell membrane electrode assembly, all with good performance. Credit: Yale University
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A key problem in materials science is balancing the trade-offs
between different material properties: improving one property can have a
negative impact on others. Synthetic composites are often used to
address this problem. Designed to offer more independently “tunable”
performance, these composites take advantage of multiple materials’
properties within a single system, and have various applications,
including photovoltaic, battery and fuel cell technology.
Single-walled carbon nanotubes (SWNTs) have unique and
extraordinary properties that make them popular as starting points for
synthetic composites, used in combination with polymers. Yet these
nanotubes present their own challenges. When combined with a polymer,
they often spread poorly, resulting in a composite with a meager
conductivity in comparison to a pure SWNT network. The current
techniques used to overcome this problem limit themselves to the use of
conductive polymers that often do not disperse SWNTs well, which
dramatically limits the design freedom and extended applications of
composite materials.
Professor André Taylor,
Director of the Transformative Materials & Devices group at Yale
SEAS, has developed a scalable tandem Mayer rod coating technique that
preserves the electrical properties of these nanotubes when fabricating
SWNT and polymer composites. This novel approach eliminates the need to
use functional polymers that are capable of properly spreading the SWNTs
and thus loosens the design limitations for developing advanced
multifunctional composites.
Instead of immediately spreading the nanotubes within the desired
polymer for the final composite, the SWNTs are first dispersed using a
polymeric derivative of cellulose, sodium carboxymethyl cellulose (CMC).
The resulting film, which is transparent and contains well-dispersed
SWNTs suspended throughout the CMC, is coated onto glass slides. It is
transparent, but due to the CMC, nonconductive.
Conductivity is restored in the next step of the group’s technique,
where the CMC is removed by treating the film with acid. Removing the
CMC lets the nanotubes collapse onto each other, creating a dense
network of connected nanotubes with high conductivity. With this highly
conductive network of SWNTs on which to base a composite system, a
functional polymer can be selected and filled into the network based on
the intended application. The resulting films offer exceptional
electrical performance from the nanotube network and can be customized
for additional desired properties based the polymer that’s selected for
use.
Xiaokai Li, the lead author of the paper, states, “As the
challenges of generating more complex SWNT-based film systems require
engineers to impart new and transformative functionalities to materials
without sacrificing the conductivity or ease of manufacturing, our
technique provides the versatility to control nanoscale features and
functionality on the macroscopic level.”
What is truly unique about this approach, says Taylor, is that the
group was able to demonstrate a solar cell platform, a lithium battery,
and a fuel cell membrane electrode assembly, all with good performance.
“Normally these systems are made from individual layers, but by
using this tandem Mayer rod coating approach, we have been able to
create films that are asymmetric: electrically conductive on one side
dominated by the SWNT network and functional polymer (for ion transport,
etc.) on the other,” says Taylor. “This opens up a new range of
possibilities for advanced functional composites.”
The group’s next step is to design and process carbon nanotube
composite films using the same method specifically for next generation
flexible heterojunction solar cells.
Funding from the Semiconductor Research Corporation and the National Science Foundation supported this work.
Source: Yale University
Additional Information:
- Xiaokai Li, Forrest Gittleson, Marcelo Carmo, Ryan C. Sekol, and André D. Taylor. Scalable Fabrication of Multifunctional Freestanding Carbon Nanotube/Polymer Composite Thin Films for Energy Conversion. ACS Nano 2012 6 (2), 1347-1356.
- Transformative Materials & Devices group website at Yale SEAS: http://taylor.research.yale.edu/
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