Thermoelectric generators (TEGs) produce electric power from environmental heat energy and are expected to play a key role in powering the Internet of things. However, they require a heat source to create a stable and irreversible temperature gradient. Overcoming these restrictions will allow the use of TEGs to proliferate. Therefore, we propose heat source-free water-floating carbon nanotube (CNT) TEGs. Output voltage and power are generated by the temperature gradient in the CNT films in which water pumping via capillary action leads to evaporation-induced cooling in selected areas. Furthermore, the output voltage and power increase when the films are exposed to sunlight and wind flow. These water-floating CNT TEGs demonstrate a pathway for developing wireless monitoring systems for water environments.

This report presents n-type single-walled carbon nanotubes (SWCNT) films with ultra-long air stability using a cationic surfactant and demonstrates that the n-type Seebeck coefficient can be maintained for more than two years, which is the highest stability reported thus far to the best of our knowledge. Furthermore, the SWCNT films exhibit an extremely low thermal conductivity of 0.62 ± 0.08 W/(m·K) in the in-plane direction, which is very useful for thin-film TEGs. We fabricated all-carbon-nanotube TEGs, which use p-type SWCNT films and the n-type SWCNT films developed, and their air-stability was investigated. The TEGs did not degrade for 160 days and exhibited an output voltage of 24 mV, with a maximum power of 0.4 µW at a temperature difference of 60 K. These results open a pathway to enable the widespread use of carbon nanotube TEGs as power sources in IoT sensors.

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Thermoelectric generators (TEGs) produce electric power from environmental heat energy and are expected to play a key role in powering the Internet of things. However, they require a heat source to create a stable, irreversible temperature gradient. Overcoming these restrictions will allow the use of TEGs to proliferate. To this end, we propose heat source-free water-floating carbon nanotube (CNT) TEGs. Here, thermopower is generated by the temperature gradient in the CNT films in which water pumping via capillary action leads to water evaporation-induced cooling occurs in selected areas. Furthermore, the thermoelectric power increases when the films are exposed to sunlight and wind flow. These water-floating CNT TEGs demonstrate a pathway for developing wireless monitoring systems for water environments.

Water‐floating carbon nanotube thermoelectric generators (CNT‐TEGs) can effectively power Internet of Things (IoT) applications. CNT‐TEGs produce electricity by floating on water to generate a temperature gradient in the CNT films, where water pumping via capillary action causes evaporation‐induced cooling in selected areas. However, the amount of electricity required for target applications needs to be increased. Therefore, the wettability of CNT films is controlled via various treatments to increase water evaporation and temperature difference in the films. The contact angle of the water droplet on the film surface related to the wettability decreases upon atmospheric‐pressure plasma jet irradiation and increases upon waterproof spray treatment. Intermediate wettability is obtained by combining the two aforementioned treatments. Under the environmental conditions of light irradiation (1 kW m−2) and wind (3 m s−1) at a water temperature of 20 °C, the CNT‐TEGs with the combined treatment exhibit a highest output voltage of 3.9 mV, which is 30% larger than that of the pristine CNT‐TEG. At a water temperature of 60 °C, a highest output voltage of 13.1 mV is achieved in the pristine CNT‐TEG. The performance enhancement of CNT‐TEGs by controlling the wettability of CNT films under various environmental conditions can facilitate advancements in IoT technology.