A Promising Approach to Enhanced Thermoelectric Properties Using Carbon Nanotube Networks

Thermoelectric materials are of great interest for applications as heat pumps and power generators, which can realize conversion between thermal and electrical energy without moving mechanical components or hazardous working fluids. Especially since the combustion of fossil fuel has caused very alarming environmental problems, the conversion of waste heat to electric power by means of thermoelectric devices has become more urgent. The performance of thermoelectric materials is quantified by a figure of merit, given by ZT1⁄4 SsT/k, where S, s, T, and k are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. To have a high ZT value at room temperature, high S, high s, and low k are required. Various attempts have been made to enhance the efficiency of thermoelectric materials. However, there is a strong correlation of these three parameters according to theWiedemann–Franz law, which makes it a very challenging task. Therefore, very slow improvement was achieved in the past, until some promising approaches were reported recently, which involve quantum-well structures, crystals with complex electronic structures, thin and multilayer films and so-called phonon-glass/electroncrystal compound materials. Synthesizing composites was considered an effective strategy to achieve improved material performances by combining the advantages of each component. Synergistic enhancement effects have been found in some research fields, such as dielectric permittivity and thermal conductivity. However, for thermoelectric materials, it was believed impossible to enhance thermoelectric properties through composites, because early theoretical numerical simulations indicated that the Seebeck coefficient and the figure of merit of the composites could not be higher than the maximum of one of its components. Recently, when we tried to use carbon nanotubes (CNTs) to improve the transport properties of polyaniline (PANI, which is a typical kind of conductive polymer) through composites, we found that the thermoelectric performance of the composites could be remarkably enhanced compared with both of their bulk parent samples, which is not consistent with the early theoretical conclusions about composites. Morphology characterization showed that the PANI component uniformly coated the surface of each individual CNTand of each CNT bundle. It is a composite with a low-dimensional network structure. Here, we explore the reason for these observations and surmise that this method may be extended to be a facile and general strategy to synthesize nanocomposites with enhanced thermoelectric properties. The CNT/PANI nanocomposites used in the study were fabricated by a simple two-step method, that is, the formation of a thick CNT network and the polymerization of the PANI component. First, a freestanding CNTnetwork made of randomly entangled individual CNTs and CNT bundles was obtained by filtering a uniform CNT suspension through a microporous membrane with the aid of vacuum. Second, the in situ chemical polymerization approach was used to synthesize a PANI layer uniformly coated on the prepared CNT network. Details of the process are given in the Experimental section. Macroscopically, the pristine CNT sheet surface looks very smooth and shiny. From scanning electron microscopy (SEM) observations, it was apparent that individual CNTs and their bundles randomly intertwined together to form a good CNT network (Fig. 1A). The original CNTdiameters were in the range of about 15–30 nm. After PANI polymerization (0.2 M aniline), the diameters increased to about 60–200 nm. Figure 1B indicates that PANI formed a wholly uniform coating layer on the surface of the CNTs. It is notable that the framework of the network was retained after the PANI coating process in the liquid phase. This is indeed a well-formed randomly distributed nanostructural system. Macroscopically, its surface became a little rough, however, it still remained flexible (inset of Fig. 1B). It can be rolled up, bent, or twisted easily, and even folded without cracking. This mechanical nature is superior to that of conventional fragile thermoelectric films. The transmission electron microscopy (TEM) images in Figure 1C and 1D further demonstrate the nanostructure of individual CNTs and CNT bundles coated with PANI, where some PANI coating was destroyed through intensive ultrasonication during the specimen preparation. Clear interfaces (indicated by white arrows) can be seen around the outer walls of the CNTs. The thickness of the PANI coating layer is about 50–90 nm. These observations provide strong evidence that the PANI chains grow on the outer walls of the CNTs. The formation mechanism can be understood briefly as follows. In themixed solution, aniline hydrochlorides are oxidized by ammonium peroxidisulfate (APS) to form insoluble oligomers and polymers of polyaniline. The p-bonded surface of the CNTs interacts strongly with the conjugated structure of polyaniline, which facilitates the deposition of polymer chains at the surface of the CNTs, forming a tubular coating layer. Here the individual CNTs and the CNT network serve as the core and the template, respectively.