Enhancement of p-type thermoelectric power factor by low-temperature calcination in carbon nanotube thermoelectric films containing cyclodextrin polymer and Pd

Abstract

The p-type properties of carbon nanotubes (CNTs) in organic thermoelectric devices need urgent improvement for large-scale, low-grade thermal energy applications. Here, we present a suitable approach to significantly enhance the power factor (PF) by increasing the electrical conductivity through the low-temperature calcination-induced pyrolysis of the insulating γ-cyclodextrin polymer (PγCyD), which is used as a solubilizer of film-like CNTs. The low-temperature calcination method, which can be used to realize good electrical contact between CNT bundles, shows enhancement behavior as a universal phenomenon for not only PγCyD but also other commonly used polymers for CNT films. To moderate the calcination temperature, the Pd catalyst was added, and the optimal temperature was reduced from 340 °C to 250 °C. Consequently, the PF value of the CNT film was 570 μW m−1 K−2, which was found to be more than twice that of the original CNT film. In addition, we demonstrated the energy harvesting capability of a thermoelectric generator based on this p-type CNT film; a thermoelectric generator with 10 p-type thermoelectric elements showed a maximum power output of 10.3 μW with a temperature difference of 75 °C, which is comparable to the maximum power output of some of the best single-component organic thermoelectric devices demonstrated to date. This outstanding output power shows that easy-to-handle CNT films with low-temperature heat treatment can open new avenues for the development of thermoelectric generators.

Water‐resistant organic thermoelectric generator with >10 μW output

“…A pure CNT film without any added dispersant or surfactant in the aerobic oxidation state of the reference sample yielded a positive S value of 62.3 ± 0.2 μV K −1 , indicating p-type properties. 52 In contrast, the S values for the gemini-surfactant-doped CNT films were all negative, indicating that most of the charge carriers in the CNTs had switched from p-type to n-type. These S values decreased from −31.3 ± 2.7 to −43.2 ± 1.5 μV K −1 with the increasing spacer length from 2 to 12.…”

“…Figure 3 and Table 1 summarize the results of a TE performance analysis of the doped CNT films formed using gemini surfactants with different spacer chains. A pure CNT film without any added dispersant or surfactant in the aerobic oxidation state of the reference sample yielded a positive S value of 62.3 ± 0.2 μV K −1 , indicating p‐type properties 52 . In contrast, the S values for the gemini‐surfactant‐doped CNT films were all negative, indicating that most of the charge carriers in the CNTs had switched from p‐type to n‐type.…”

Surfactant-Wrapped n-Type Organic Thermoelectric Carbon Nanotubes for Long-Term Air Stability and Power Characteristics

“…Figure 4 summarizes the S, σ, and PF values of the surfactant/CNT films at 345 K. The S value of the pristine CNTs was 62.3 μV K −1 , indicating that the CNTs exhibited ptype properties owing to aerobic oxidation, which is in agreement with previous results. 39 In addition, the pristine CNT film possessed a positive S value owing to oxygen doping and was p-type, where the charge carriers were holes. In contrast, the CNT films prepared using the various cationic surfactant solutions exhibited negative S values regardless of the length of the monomeric or dimeric tails.…”

n-Type thermoelectric behavior in oxyethylene surfactant/carbon nanotubes

“…This finding is consistent with the carrier properties of CNTs in air (S = 62.3 mV K À1 ). 29 By contrast, the S values of the C 12 EO 4 /CNT film were consistently negative (range = À62.5 to À52.7 mV K À1 ), regardless of the measurement temperature. C 12 EO 4 , a carrier for NTs, acts as an n-type dopant that converts the charge carriers from holes to electrons, and its s value is higher than that of EO 4 at all measurement temperatures.…”