Upscaling Thermoelectrics: Micron-Thick, Half-a-Meter-Long Carbon Nanotube Films with Monolithic Integration of p- and n-Legs

Abstract

In order for organic thermoelectrics to successfully establish their own niche as energy-harvesting materials, they must reach several crucial milestones, including high performance, long-term stability, and scalability. Performance and stability are currently being actively studied, whereas demonstrations of large-scale compatibility are far more limited and for carbon nanotubes (CNTs) are still missing. The scalability challenge includes material-related economic considerations as well as the availability of fast deposition methods that produce large-scale films that simultaneously satisfy the thickness constraints required for thermoelectric modules. Here we report on true solutions of CNTs that form gels upon air exposure, which can then be dried into micron-thick films. The CNT ink can be extruded using a slot-shaped nozzle into a continuous film (more than half a meter in the present paper) and patterned into alternating n- and p-type components, which are then folded to obtain the finished thermoelectric module. Starting from a given n-type film, differentiation between the n and p components is achieved by a simple postprocessing step that involves a partial oxidation reaction and neutralization of the dopant. The presented method allows the thermoelectric legs to seamlessly interconnect along the continuous film, thus avoiding the need for metal electrodes, and, most importantly, it is compatible with large-scale printing processes. The resulting thermoelectric legs retain 80% of their power factor after 100 days in air and about 30% after 300 days. Using the proposed methodology, we fabricate two thermoelectric modules of 4 and 10 legs that can produce maximum power outputs of 1 and 2.4 μW, respectively, at a temperature difference ΔT of 46 K.

Figure 1

(a) Relevant properties of CNTs and CPs for the fabrication of large-scale OTEGs. (b) Schematic representation of the fabrication process described in this work. The inset photograph shows a 1-cm-thick gel, which forms after deposition. (c) Cross section of a folded film and photograph of a finished module. (d) Photograph of a ≈60-cm-long CNT film fabricated with the method depicted in part b.

Figure 2

Photographs of CNT gels deposited in circular (a and b) and rectangular (c and d) Petri dishes. The dry buckypaper in part d is 10 cm × 10 cm. (e) SEM micrograph of a representative CNT mat. (f) TEM micrograph of a bundle within the CNT mat.

Figure 3

(a) Electrical conductivity, (b) Seebeck coefficient, and (c) power factor of CNT films prepared using different carbon-to-potassium molar ratios. Solid lines are a guide for the eye. The phrase “previous work” corresponds to work by Dörling et al.

Figure 4

Thermoelectric performance of CNT films aged under air (a) at room temperature and (b) at 180 °C. Films were kept under these conditions and only moved for characterization. Solid lines are a guide to the eye. Shadow areas mark the change from negative to positive Seebeck coefficient.

Figure 5

Oxidation of CNT films. (a) Seebeck coefficient of representative samples dedoped using different oxidation methods and time intervals, ranging from 5 min (left pyramid) and 10 min (right pyramid) to 20 min (diamond). (b) FTIR spectra of a CNT film treated with H₂O₂ + UV for different time intervals. Measurements were taken on a free-standing film. Shaded areas show the distribution density for each method. We measured samples per time condition.

Figure 6

Thermoelectric characterization of the OTEG modules. (a) Seebeck coefficient of the fabricated modules. (b) Current and power of the 10-leg module at different hot-side temperatures. (c) Power versus load resistance of both modules at a ΔT value of 46 °C.