Oxygen-Rich Polymer Polyethylene Glycol-Functionalized Single-Walled Carbon Nanotubes Toward Air-Stable n‑Type Thermoelectric Materials

It is crucial for thermoelectric (TE) devices to obtain both p-type and n-type materials and control charge carrier density. However, n-type thermoelectric materials are quite deficient and have lower thermoelectric properties. We report one oxygen-rich polymer named polyethylene glycol (PEG) for converting p-type single-walled carbon nanotubes (SWCNTs) to air-stable n-type thermoelectric materials. When pristine SWCNTs were doped with 2 mg·mL^–1 PEG in an ethanol solution, the optimal Seebeck coefficient of PEG/SWCNT composites reached −50.8 μV·K^–1. The result of ultraviolet photoelectron spectroscopy demonstrated that the lone pair of oxygen atoms in the PEG chain has electron transferability to SWCNTs. According to the hard and soft acid and base theory, sodium hydroxide (NaOH) was further introduced to improve air stability and thermoelectric performance of doped SWCNTs. As a result, PEG/NaOH/SWCNT composites achieved the highest power factor of 173.8 μW·m^–1·K^–2 at 300 K. Meanwhile, their final changes in electrical conductivity and the Seebeck coefficient are less than 8% in the investigation of air stability over two months. Inspired by this finding, we fabricated the TE generator composed of the pristine p-type SWCNTs and n-type PEG/NaOH/SWCNT composites. The maximum output power of this robust TE device reached 5.3 μW at a temperature gradient of 76 K, which is superior to many reported TE devices. Moreover, the experimental procedure is attractive as a sustainable process for materials preparation. Our study has indicated that the oxygen-rich polymer-functionalized SWCNTs have huge potential for developing air-stable n-type carbon-based thermoelectric materials.