Environmentally Tolerant Ionic Hydrogel with High Power Density for Low-Grade Heat Harvesting

Abstract

Harvesting low-grade heat by an ionic hydrogel thermoelectric generator (ITEG) into useful electricity is promising to power flexible electronics. However, the poor environmental tolerance of the ionic hydrogel limits its application. Herein, we demonstrate an ITEG with high thermoelectric properties, as well as excellent capabilities of water retention, freezing resistance, and self-regeneration. The obtained ITEG can maintain the original water content at ambient conditions (302 K, 65% relative humidity (RH)) for 7 days and keep unfreezing at a low temperature (253 K). It can even be self-regenerated and recovered to its original state after a water loss in high-temperature conditions. Furthermore, a high ionic Seebeck coefficient of 11.3 mV K–1 and an impressive power density of 167.90 mW m–2 are achieved under a temperature difference of 20 K. A high power density of 60.00 mW m–2 can also be maintained even at 258 K. After drying and regeneration, ITEG-re could even exhibit a higher ionic Seebeck coefficient of 11.8 mV K–1. Successful lighting of light-emitting diodes (LEDs) and charging of capacitors demonstrate the great potential of ITEG to provide continuous energy supply for powering flexible electronics.

Soret Effect of Ionic Liquid Gels for Thermoelectric Conversion

“…The ionogels should have a high ionic conductivity and high ionic Seebeck coefficient to have high thermoelectric properties. A couple of methods were recently reported to increase the ionic conductivity and/or the ionic Seebeck coefficient. − The ZT i value of ionogels was rapidly increased to >6 …”

Wearable Thermoelectric Generators: Materials, Structures, Fabrications, and Applications

“…[ 108 ] Apart from the liquid‐state i ‐TE materials, the TE performances of quasi‐solid‐state ionic liquid polymer gel was also reported and the thermopower of the ionic gel can be tuned from −4 to 14 mV K −1 by changing the compositions. [ 108 ] Chen et al also successfully demonstrated an ionic hydrogel TEG by solvent displace method and achieved an output thermopower of 11.3 mV K −1 together with an impressive power density of 167.90 mW m −2 under the temperature gradient of 20 K. [ 19 ] In that work, authors also demonstrated the practical applications of the TEG by lighting an light‐emitting diode (LED) light with a temperature gradient of 40 K. [ 19 ] Further research on ionic liquid polymer gels achieved high thermopower of 26.1 mV K −1 . [ 113 ] In addition, Zhang et al also prepared a flexible composite hydrogel containing NaCl as ion provider and successfully achieved a rather high thermopower of 34.27 mV K −1 and power density of 730 mW m −2 .…”

“…thermopower of 11.3 mV K À1 together with an impressive power density of 167.90 mW m À2 under the temperature gradient of 20 K. [19] In that work, authors also demonstrated the practical applications of the TEG by lighting an light-emitting diode (LED) light with a temperature gradient of 40 K. [19] Further research on ionic liquid polymer gels achieved high thermopower of 26.1 mV K À1 . [113] In addition, Zhang et al also prepared a flexible composite hydrogel containing NaCl as ion provider and successfully achieved a rather high thermopower of 34.27 mV K À1 and power density of 730 mW m À2 .…”

“…Organic TE materials and ionic thermoelectric (i-TE) materials have attracted increasing research interests thanks to their potential applications in flexible TE devices. [19,20] In the meantime, thin-film-based and fiber-based TEGs are reported for wearable applications due to their excellent tensile, bending and inplane shear properties. [21] Despite the aforementioned advantages, the applications of wearable TEGs also meet many challenges.…”

Evolution of Thermoelectric Generators: From Application to Hybridization

“…TEGs are difficult to obtain significant temperature gradients in practical applications for the limitations of temperature resistance properties of flexible materials and application scenarios. [296][297][298][299][300][301][302][303][304][305][306][307][308][309][310][311][312][313][314][315] Accordingly, the previous section suggests that flexible TEGs at smaller temperature gradients are capable of capturing the vertical temperature gradient between the hot and cold ends by optimizing the design structure to provide high power density. Moreover, more stacking of thermoelectric legs per unit area is achieved by optimizing the design.…”