A pH-Sensitive Stretchable Zwitterionic Hydrogel with Bipolar Thermoelectricity

Amid growing interest in using body heat for electricity in wearables, creating stretchable devices poses a major challenge. Herein, a hydrogel composed of two core constituents, namely the negatively-charged 2-acrylamido-2-methylpropanesulfonic acid and the zwitterionic (ZI) sulfobetaine acrylamide, is engineered into a double-network hydrogel. This results in a significant enhancement in mechanical properties, with tensile stress and strain of up to 470.3 kPa and 106.6%, respectively. Moreover, the ZI nature of the polymer enables the fabrication of a device with polar thermoelectric properties by modulating the pH. Thus, the ionic Seebeck coefficient (Si ) of the ZI hydrogel ranges from -32.6 to 31.7 mV K-1 as the pH is varied from 1 to 14, giving substantial figure of merit (ZTi ) values of 3.8 and 3.6, respectively. Moreover, a prototype stretchable ionic thermoelectric supercapacitor incorporating the ZI hydrogel exhibits notable power densities of 1.8 and 0.9 mW m-2 at pH 1 and 14, respectively. Thus, the present work paves the way for the utilization of pH-sensitive, stretchable ZI hydrogels for thermoelectric applications, with a specific focus on harvesting low-grade waste heat within the temperature range of 25-40 °C.

Cellulose-Based Radiative Cooling and Solar Heating Powers Ionic Thermoelectrics

Cellulose opens for sustainable materials suitable for radiative cooling thanks to inherent high thermal emissivity combined with low solar absorptance. When desired, solar absorptance can be introduced by additives such as carbon black. However, such materials still shows high thermal emissivity and therefore performs radiative cooling that counteracts the heating process if exposed to the sky. Here, this is addressed by a cellulose-carbon black composite with low mid-infrared (MIR) emissivity and corresponding suppressed radiative cooling thanks to a transparent IR-reflecting indium tin oxide coating. The resulting solar heater provides opposite optical properties in both the solar and thermal ranges compared to the cooler material in the form of solar-reflecting electrospun cellulose. Owing to these differences, exposing the two materials to the sky generated spontaneous temperature differences, as used to power an ionic thermoelectric device in both daytime and nighttime. The study characterizes these effects in detail using solar and sky simulators and through outdoor measurements. Using the concept to power ionic thermoelectric devices shows thermovoltages of >60 mV and 10 °C temperature differences already at moderate solar irradiance of ≈400 W m^-2 .

Self-Healable and Stretchable Ionic-Liquid-Based Thermoelectric Composites with High Ionic Seebeck Coefficient

The advancement of wearable electronics, particularly self-powered wearable electronic devices, necessitates the development of efficient energy conversion technologies with flexible mechanical properties. Recently, ionic thermoelectric (TE) materials have attracted great attention because of their enormous thermopower, which can operate capacitors or supercapacitors by harvesting low-grade heat. This study presents self-healable, stretchable, and flexible ionic TE composites comprising an ionic liquid (IL), 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM:OTf); a polymer matrix, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP); and a fluoro-surfactant (FS). The self-healability of the IL-based composites originates from dynamic ion-dipole interactions between the IL, the PVDF-HFP, and the FS. The composites demonstrate excellent ionic TE properties with an ionic Seebeck coefficient (S_i ) of ≈38.3 mV K^-1 and an ionic figure of merit of ZT_i  = 2.34 at 90% relative humidity, which are higher than the values reported for other IL-based TE materials. The IL-based ionic TE composites developed in this study can maintain excellent ionic TE properties under harsh conditions, including severe strain (75%) and multiple cutting-healing cycles.