Unique Approach Addresses Growing Challenge in Materials Science

Fabricating Improved Multifunctional Composites for Energy Conversion and Storage Devices

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.

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.

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.

More reading:
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/