High thermal durability and thermoelectric performance with ultra-low thermal conductivity in n-type single-walled carbon nanotube films by controlling dopant concentration with cationic surfactant

Single-walled carbon nanotubes (SWCNTs) are promising candidates for use in thermoelectric generators (TEGs) to power Internet of Things (IoT) sensors. For practical applications, the major challenge for SWCNTs is improving the thermoelectric performance and thermal durability of n-type SWCNT films. Here, SWCNT inks were prepared using a dopant, which is a cationic surfactant of dimethyldioctadecylammonium chloride (DODMAC), by changing the mass ratio of DODMAC/SWCNT. The SWCNT films were fabricated by vacuum filtering, followed by heat treatment at 423 K. The in-plane thermoelectric properties were measured at 300 K, and the Seebeck coefficient changed from positive to negative values when the DODMAC/SWCNT was 10−2. The highest dimensionless figure-of-merit, ZT, of 1.0 × 10−2 was exhibited at a DODMAC/SWCNT of 80, which was close to saturation concentration. This ZT was achieved mainly because the thermal conductivity decreased significantly to 0.16 W/(m · K), and it is currently one of the highest values among those of n-type SWCNT films with various dopants. To demonstrate power generation, we fabricated a SWCNT-TEG consisting of n-type SWCNT films with the highest ZT. The SWCNT-TEG exhibited an output voltage of 24 mV and a maximum power of 0.9 μW at a temperature difference of 80 K. Furthermore, to investigate the thermal durability of n-type SWCNT films, thermal cycling tests were performed at temperatures ranging from 300 to 423 K. The SWCNT film with a DODMAC/SWCNT of 80 exhibited the highest durability. These findings will contribute to the widespread use of SWCNT-TEGs as power sources for IoT sensors.