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Although ionic hydrogels have been developed recently for innovative wearable electronics, they necessitate high humidity to diffuse ions in water and fast self‐healing, which negatively impacts their performance and stability in ambient conditions (e.g., dry environments). In this study, a series of p‐ and n‐type polymeric ionic gels (PIGs) with different ratios of ionic side chains are synthesized to allow only single‐type ions to pass through them. The results demonstrate that the repeatedly stretchable PIGs are transparent, thermally robust up to 125°C, and self‐healing. Among the series of PIGs, p‐ and n‐type PIGs with 75% ion moieties (P75 and N75) exhibit the optimum ionic conductivity (σi) (4.1 × 10−4 and 2.7 × 10−4 S cm−1) and ionic Seebeck coefficients (Si) (5.84, and ‐4.18 mV K−1) under ambient conditions (25°C and relative humidity (RH) of 30%), resulting in ZTi values of 1.87 × 10−3 and 1.18 × 10−3. Moreover, the σi and Si of PIGs are almost consistent under extremely low RH 10%. Accordingly, P(([EMIM+][SPA])0.75‐r‐MA0.25) (P75) and P(([APTA][TFSI−])0.75‐r‐MA0.25) (N75) are used to achieve stretchable ionic thermoelectrics (iTEs) with stable operability under ambient conditions (RH of 30%), satisfying all of the requirements. The iTEs with five pairs of p/n couples exhibit a thermovoltage of up to ∼0.8 V. [ABSTRACT FROM AUTHOR]

To efficiently harvest the abundant waste heat on earth is of great significance for sustainable development. Thermoelectric materials can be used to directly convert heat into electricity, and ionic thermoelectric materials like ionic liquids (ILs) are considered as the next‐generation thermoelectric materials. It is important to develop novel methods to improve the overall thermoelectric properties particularly the thermopower. Herein, the great enhancement in the thermopower of 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA) is reported that is an IL by introducing zeolitic imidazolate framework (ZIF‐8) that is a metal‐organic framework (MOF) for the first time. The presence of 40 wt.% ZIF‐8 can greatly increase the ionic thermopower of EMIM:DCA from 8.8 to 31.9 mV K−1 at room temperature, and the ZIF‐8/EMIM:DCA mixture at the ZIF‐8 loading of 10 wt.% can exhibit a ZTi value of 3.1, notably higher than that (0.59) of neat EMIM:DCA. The enhancement in the thermopower is attributed to the increase in the difference of the mobilities of EMIM+ and DCA− by ZIF‐8. Because DCA− is smaller while EMIM+ is larger than the pore size of ZIF‐8, the DCA− transport is hindered by ZIF‐8, while EMIM+ can bypass ZIF‐8. [ABSTRACT FROM AUTHOR]

The energy crisis is one of the most critical and urgent problems in modern society; thus, harvesting energy from ubiquitous low-grade heat energy with thermoelectric (TE) materials has become an available strategy in sustainable development. Recently, emerging ionic TE materials have been widely used to harvest low-grade heat energy, owing to their excellent performance in high ionic Seebeck coefficient, low thermal conductivity, and mechanical flexibility. However, the instability of ionic conductive materials in the underwater environment seriously suppresses underwater energy-harvesting, resulting in a waste of underwater low-grade heat energy. Herein, we developed a water-resistant TE ionogel (TEIG) with excellent long-term underwater stability utilizing a hydrophobic structure. Due to the hydrophobic polymer network and hydrophobic ionic liquid (IL), the TEIG exhibits high hydrophobicity and antiswelling capacity, which meets the requirement of environment stability for underwater thermoelectric application. Furthermore, the water resistance endows the TEIG with great thermoelectric performances in the underwater environment, including satisfactory ionic Seebeck coefficient, outstanding durability, and superior salt tolerance. Therefore, this investigation provides a promising strategy to design water-resistant TE materials, enabling a remarkable potential in harvesting low-grade heat energy under water. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

As an extended member of the thermoelectric family, ionic thermoelectrics (i‐TEs) exhibit exceptional Seebeck coefficients and applicable power factors, and as a result have triggered intensive interest as a promising energy conversion technique to harvest and exploit low‐grade waste heat (<130 °C). The last decade has witnessed great progress in i‐TE materials and devices; however, there are ongoing disputes about the inherent fundamentals and working mechanisms of i‐TEs, and a comprehensive overview of this field is required urgently. In this review, the prominent i‐TE effects, which set the ground for i‐TE materials, or more precisely, thermo‐electrochemical systems, are first elaborated. Then, TE performance, capacitance capability, and mechanical properties of such system‐based i‐TE materials, followed by a critical discussion on how to manipulate these factors toward a higher figure‐of‐merit, are examined. After that, the prevalent molding methods for assembling i‐TE materials into applicable devices are summarized. To conclude, several evaluation criteria for i‐TE devices are proposed to quantitatively illustrate the promise of practical applications. It is therefore clarified that, if the recent trend of developing i‐TEs can continue, the waste heat recycling landscape will be significantly altered. [ABSTRACT FROM AUTHOR]

A paradigm ionogel consisting of ionic liquid (IL) and PVDF-HFP composites is made, which inherently possesses dual-function ionic thermoelectric (iTE) and piezoelectric (PE) attributes. This study investigates an innovative "PE-enhanced iTEs" effect, wherein the ionic thermopower exhibits a 58% enhancement while the ionic conductivity arises more than 2× within a PE-induced internal electric field. By harnessing these multifaceted features, fully self-powered, multimodal sensors demonstrate their superior energy conversion capabilities, which possessed minimum sensitivities of 0.13 mV kPa -1 and 0.96 mV K -1 in pressure and temperature alterations, respectively. The PE augmentation of iTEs is maximized by ≈3× under rising water pressure. Their swift and sophisticated responses to various in vivo vital signs simultaneously in a hemorrhagic shock scenario, indicative of good prospects in the clinical medicine field are showcased., (© 2024 Wiley‐VCH GmbH.)

Self‐healable and stretchable thermoelectric (TE) materials provide new possibilities for self‐powered flexible wearable devices to self‐repair mechanical damage. However, developing high‐performance materials with such desirable TE and mechanical properties is a significant challenge. In this work, organic–inorganic ionic TE composites (OITCs) with an unprecedently high ionic TE figure of merit (ZTi = 3.74 at 80% relative humidity) and robust properties of simultaneous self‐healing and stretching are reported. The OITCs are developed by incorporating inorganic SiO2 nanoparticles (SiO2‐nps) in a polyaniline: poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid): phytic acid (PANI:PAAMPSA:PA) ternary polymer. The incorporated SiO2‐nps constructively interact with the hybrid polymer to provide autonomous self‐healability and stretchability while augmenting the mobile proton concentration in OITCs, which substantially improves their ionic TE properties (i.e., ionic Seebeck coefficient and ionic conductivity). Moreover, the OITCs remain repeatedly stretchable and self‐healable under severe external stresses (50 cycles of 100% strain and 25 cycles of cutting/healing) without degradation of their TE properties. Using the OITCs with multi‐walled carbon nanotube electrodes, an ionic TE supercapacitor (ITESC) with a maximum energy density of 19.4 mJ m−2 is demonstrated upon a temperature difference of 1.8 K. [ABSTRACT FROM AUTHOR]

Thermoelectric (TE) materials are significant for sustainable development because they can be used to directly harvest heat into electricity. Recently, ionic TE materials emerged as very promising materials mainly due to their high thermovoltage that can be higher than the Seebeck coefficient of electronic TE materials by 2–3 orders in magnitude. However, their conductivity is very low. Here, the significant improvement in the ionic conductivity and thus the overall TE properties of ionogels is reported by engineering their solid networks, which immobilize the ionic liquid in the ionogels. An antisolvent of poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) is added into the acetone solution of 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA) that is an ionic liquid and PVDF‐HFP prior to the ionogel formation. This can significantly change the solid networks formed by PVDF‐HFP and thus the microstructure of the EMIM:DCA/PVDF‐HFP ionogels, thereby facilitating ionic transport. As a result, the ionic conductivity of the ionogels can be increased from 7.0 to 17.6 mS cm−1. The ionogels can exhibit a high ionic figure of merit (ZTi) of 1.8 with the ionic Seebeck coefficient of 25.4 mV K−1 and the thermal conductivity of 0.190 W m−1 K−1. This is the highest recorded ZTi value for ionic conductors. [ABSTRACT FROM AUTHOR]

Organic‐based energy harvesting devices can contribute to a sustainable solution for the transition to renewable energy sources. The concept of ionic thermoelectrics (iTE) has been recently proposed and motivated by the high values of thermo‐voltage in electrolytes. So far, most research has focused on developing new electrolytes with high Seebeck coefficient. Despite the major role of the electrode materials in supercapacitors and batteries, the effect of various electrodes on energy harvesting in iTE devices has not been widely studied. In this work, the conducting polymer poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is investigated as the functional electrodes in iTE supercapacitors. Through investigating the thermo‐voltage of iTEs of the same electrolyte with varying composition of PEDOT electrodes, it is identified that the different PSS content greatly affects the overall thermo‐induced voltage coefficient, Seff (i.e., effective thermopower). The permselective polyanion in the electrode causes cation concentration differences at the electrode/electrolyte interface and contributes to an interfacial potential drop that is temperature dependent. As a result, the overall thermo‐voltage of the device possesses both an interfacial and a bulk contribution. The findings extend the fundamental understanding of iTE effect with functional electrodes, which could lead a new direction to enhance the heat‐to‐electricity conversion. [ABSTRACT FROM AUTHOR]

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 (S i ) 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 (ZT i ) 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., (© 2024 Wiley‐VCH GmbH.)

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., (© 2022 Wiley-VCH GmbH.)