| Summary: | Numerous alternative energy sources are being researched for sustainable energy applications, but their overall benefit is still too costly for them to be considered viable. Commonly produced temperature gradients created by the environment, or are man-made, can be converted into useful energy by using thermoelectric materials. Inorganic semiconductors are the most commonly used thermoelectric materials, but have raised concerns due to toxicity issues, rarity of heavy elements used, and high fabrication temperatures. These concerns have led research efforts into electrically conductive polymer composites prepared in ambient conditions from aqueous solutions. By combining polymer latex with carbon nanotubes (CNT), electrical conductivity can resemble metals while thermal conductivity remains similar to polymers. Using different CNT stabilizers for these fully organic composites can tailor the thermoelectric properties and harvest thermal gradients from previously inconceivable places (e.g., body heat converted into a voltage).
A semiconducting CNT stabilizer, meso-tetra(4-carboxyphenyl) porphine (TCPP), was used to investigate the influence stabilizers have on composite thermoelectric properties. As TCPP was compared to a similar system containing an insulating stabilizer, sodium deoxycholate (DOC), the multi-walled carbon nanotube (MWNT)-filled composites showed a 5x increase in the Seebeck coefficient (S). TCPP did not have a distinct effect on the electrical conductivity (σ), demonstrating the tailorability of S with this molecule.
An intrinsically conductive polymer, poly(3,4-ethylenedioxythiophene) :poly(styrene sulfonate) (PEDOT:PSS), was used to stabilize highly conductive double-walled carbon nanotubes (DWNT) and demonstrate the promise of fully organic composites as thermoelectric materials. This combination of CNT and stabilizer produced metallic electrical conductivity (200,000 S m-1) and power factors (S2σ) within an order of magnitude of commonly used semiconductors (~400 μW m-1 K-2). Electrical conductivity was doubled by stabilizing single-walled carbon nanotubes (SWNT) with PEDOT:PSS in a thin film without the insulating polymer latex.
To further demonstrate the tailorability of polymer composites, a dual stabilizer approach using semiconducting and intrinsically conductive stabilizers was used. This approach effectively provided the high electrical conductivity from PEDOT:PSS and the enhanced Seebeck coefficients of TCPP. By using multiple stabilizers for CNTs within the same composite, power factors among the highest reported for fully organic composites are achieved (~500 μW m-1 K-2). These water-based, flexible composites are becoming real competition as their conversion efficiencies, when normalized by density, are similar to commonly used semiconductors.
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