A finely-tuned carbon nanotube thin film has the potential to act as a thermoelectric power generator that captures and uses waste heat to generate electricity, according to researchers at the US Department of Energy's National Renewable Energy Laboratory (NREL).

This research could help guide the development of thermoelectric devices based on either single-walled carbon nanotube (SWCNT) films or composites containing these nanotubes. Because more than half of the energy consumed worldwide is ultimately lost as waste heat, thermoelectric power generation is emerging as a potentially important future renewable energy technology.

"There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that's what we feel this paper does," said Andrew Ferguson, a research scientist in NREL's Chemical and Materials Science Center and co-lead author with Jeffrey Blackburn of a paper in Nature Energy.

This work is a collaboration between NREL, Yong-Hyun Kim's group at the Korea Advanced Institute of Science and Technology and Barry Zink's group at the University of Denver. The other authors from NREL are Azure Avery (now an assistant professor at Metropolitan State University of Denver), Ben Zhou, Elisa Miller, Rachelle Ihly, Kevin Mistry and Sarah Guillot.

Nanostructured inorganic semiconductors have demonstrated promise for improving the performance of thermoelectric devices. But inorganic materials can run into problems when the semiconductor needs to be lightweight, flexible or irregularly shaped, because they are often heavy and lack the required flexibility. By contrast, carbon nanotubes, which are organic, are lighter and more flexible.

"There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that's what we feel this paper does."Andrew Ferguson, NREL

How useful a particular SWCNT is for thermoelectrics, however, depends on whether the nanotube is metallic or semiconducting, and both types are produced simultaneously in current SWCNT synthesis processes. A metallic nanotube would harm devices such as a thermoelectric generator, whereas a semiconductor nanotube actually enhances performance. Furthermore, as with most optical and electrical devices, the electrical band gap of the semiconducting SWCNT affects the thermoelectric performance as well.

Fortunately, Blackburn, a senior scientist and manager of NREL's Spectroscopy and Photoscience group, has built up quite a bit of expertise in separating semiconducting nanotubes from metallic ones. and his methods were critical to the research. "We are at a distinct advantage here that we can actually use that to probe the fundamental properties of the nanotubes," said Ferguson.

To generate highly-enriched semiconducting samples, the researchers extracted nanotubes from polydisperse soot using polyfluorene-based polymers. The semiconducting SWCNTs were then prepared on a glass substrate to create a film, which was soaked in a solution of the oxidant triethyloxonium hexachloroantimonate (OA) as a doping step. Doping increases the density of charge carriers that flow through the film to conduct electricity.

The researchers found the samples that performed best were exposed to higher concentrations of OA, but not the highest. They also discovered an optimum diameter for the carbon nanotubes that ensured the best thermoelectric performance.

When it comes to thermoelectric materials, a trade-off exists between thermopower (the voltage obtained when subjecting a material to a temperature gradient) and electrical conductivity, because thermopower decreases with increasing conductivity. The researchers discovered, however, that the carbon nanotube films could retain large thermopowers even at very high electrical conductivities.

Furthermore, the researchers found that their doping strategy, while dramatically increasing the electrical conductivity, actually decreased the thermal conductivity. This unexpected result represents another benefit of using carbon nanotubes for thermoelectric power generation, since the best thermoelectric materials must have high electrical conductivity and thermopower, while maintaining low thermal conductivity.

This story is adapted from material from NREL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.