Thermoelectric transport in nanocarbon-based composites

Post date: Apr 16, 2019 6:48:27 PM

Since the discoveries of graphene and carbon nanotubes (CNTs), there have been many exciting scientific and technological developments for nanocarbon materials. However, their potential use in thermoelectric (TE) energy conversion has been relatively unexplored, mainly due to their intrinsically high thermal conductivities and low Seebeck coefficients, both of which are detrimental to TE performance. 

Fig.1. SEM images of (a) SWCNT:PDMS composite [2], (inset: a highly-flexible ~ 3mm thick sample) and (b) three-dimensional graphene (3DG) [1]

Recent reports have shown promising TE properties for nanocarbon materials. A large Seebeck coefficient of ~ 150 μV/K room-temperature has been reported for semiconducting single-walled CNTs (SWCNTs) networks, which is comparable to those of the state-of-the-art TE materials. Appropriate doping of SWCNTs has increased the value much further to ~ 400 μV/K. The thermal conductivities were measured to be merely 1 ~ 3 W/mK for the SWCNT networks embedded in polymers due to the suppressed thermal transport at the CNT junctions and the low thermal conductivity of the polymer matrix. Graphene nanoribbons, if properly doped with desired scattering characteristics, can also achieve a large magnitude Seebeck coefficient greater than 100 μV/K in both n-type and p-type regimes. Our three-dimensional graphene composites with polyaniline showed excellent power factor ~ 82  μW m-1 K-2, which is one of the highest among organic-inorganic hybrid composites up to date.[1] All these recent works suggest that nanocarbon-based hybrid composites can be excellent TE materials, with a large thermoelectric figure of merit zT potentially exceeding those of the state-of-the-art TE materials, in addition to their other unique advantages demonstrated previously.

Fig. 2. Conceptual figure of carrier tunneling transport through a junction between two adjacent carbon nanotubes [2]

Despite these exciting recent results, however, fundamental thermoelectric transport in nanocarbon-based composites has not been fully understood. Most of the theoretical works have relied upon the ‘internal transport’ inside the nanocarbons to explain the experimental data. The impact of junction transport between nanocarbons has been largely neglected. Our recent work on CNT networks embedded in polydimethylsiloxane (PDMS) elastomer found that the energy-dependent carrier tunneling at the junctions between CNTs dominates over the internal transport in determining the macroscopic thermoelectric properties.[2] Our theory also shows that a very large figure of merit zT that is comparable to those of the state-of-the-art inorganic TE materials, may be possible if the CNTs are properly doped and appropriate junction parameters are achieved in the composite.[unpublished] This work prompts further investigation of the thermoelectric transport and carrier dynamics in nanocarbon-based hybrid composites to realize the full potential of them for thermoelectric energy conversion.

[1] Y.-Y. Hsieh, Y. Zhang, L. Zhang, Y. Fang, S. N. Kanakaraaj, J.-H. Bahk and V. Shanov, "High Thermoelectric Power-factor Composites Based on Flexible Three-dimensional Graphene and Polyaniline," Nanoscale (Advance Article) (2019). DOI: 10.1039/C8NR10537E

[2] R. Prabhakar, M. S. Hossain, W. Zheng, P. K. Athikam, Y. Zhang, Y.-Y. Hsieh, E. Skafidas, Y. Wu, V. Shanov, and J.-H. Bahk, "Tunneling-Limited Thermoelectric Transport in Carbon Nanotube Networks Embedded in Poly(dimethylsiloxane) Elastomer," ACS Appl. Energy Mater. (2019). DOI: 10.1021/acsaem.9b00227