Giant Thermopower of Hydrogen Ion Enhanced by a Strong Hydrogen Bond System

P-N Series Integrated Ionic Thermoelectric Generator Based on Cation/Anion-Selective Hydrogels for Body Heat Harvesting

Low-grade heat accounts for a significant portion of the heat, but is often overlooked and not utilized. Ionic thermoelectric generators (i-TEGs) are an ideal choice for harvesting low-grade heat due to their simple design and operation. However, challenges such as ion mobility regulation and the need for series integration still hinder their performance. In this work, an interesting approach is introduced to improve the thermoelectric efficiency of i-TEG by utilizing ion-selective hydrogels containing sodium chloride (NaCl). The anionic polymer in the cation-selective hydrogel is attractive to sodium ions, while the cationic polymer in the anion-selective hydrogel is attractive to chloride ions. This regulates the thermal mobility of ions in the hydrogel without polymer modification or the addition of some additives, thus enhancing or changing the Seebeck coefficient (S). By connecting 12 pairs of the P-type and N-type i-TEGs, a voltage of 1.54 V and an output power of 21 µW can be achieved on the surface of the body skin. This work opens a new horizon for obtaining high-performance i-TEGs through ion mobility regulation by ion-selective hydrogels, and highlights the prospect of P-N series integrated i-TEGs from hydrogels for body heat harvesting.

Recent Progress in Polymer Gel-Based Ionic Thermoelectric Devices: Materials, Methods, and Perspectives

Polymer gel-based ionic thermoelectric (i-TE) devices, including thermally chargeable capacitors and thermogalvanic cells, represent an innovative approach to sustainable energy harvesting by converting waste heat into electricity. This review provides a comprehensive overview of recent advancements in gel-based i-TE materials, focusing on their ionic Seebeck coefficients, the mechanisms underlying the thermodiffusion and thermogalvanic effects, and the various strategies employed to enhance their performance. Gel-based i-TE materials show great promise due to their flexibility, low cost, and suitability for flexible and wearable devices. However, challenges such as improving the ionic conductivity and stability of redox couples remain. Future directions include enhancing the efficiency of ionic-electronic coupling and developing more robust electrode materials to optimize the energy conversion efficiency in real-world applications.

Bio-inspired thermoelectric cement with interfacial selective immobilization towards self-powered buildings

Buildings and infrastructure significantly contribute to global energy consumption and CO2 emissions. Transforming cement, the most widely used construction material, into a functional medium for heat harvesting presents a promising avenue to offset the energy demands of buildings. The disparity in diffusion rate between cations and anions within cement pore solution due to variations in interactions with pore walls, endows cement with inherent ionic thermoelectric properties. However, the isolation of pores by the dense cement matrix hinders the rapid transportation of ions with superior diffusion rates, impeding the enhancement of mobility difference between ions and limiting the enhancement of Seebeck coefficient. Inspired by the stem structure of plants, we present a cement-polyvinyl alcohol (PVA) composite (CPC) featuring aligned cement and PVA hydrogel layers. While PVA hydrogel layers provide ion diffusion highways for OH- ions, cement-PVA interfaces establish strong coordination bonds with Ca2+ ions and weaker interactions with OH- ions, enabling selective immobilization, which amplifies the diffusion rate disparity between Ca2+ and OH-. The CPC's multilayer structure yields abundant interfaces, providing ample interaction sites that maximize the contribution of cement ions to thermoelectric performance. The as-prepared composite achieves an impressive Seebeck coefficient of -40.5 mV/K and a figure of merit (ZT) of 6.6 × 10-2. Due to the engineered multilayer structure, the CPC also demonstrates superior mechanical strength and intrinsic energy storage potential, which has been assembled into a self-powered architecture. The biomimetic structure and interfacial selective immobilization mechanism may pave the way for the design and fabrication of high-performance ionic thermoelectric materials.

A Multifunctional MXene/PVA Hydrogel as a Continuous Ionic Thermoelectric Generator and a Strain/Temperature Sensor

This research reports a continuous output ionic thermoelectric (i-TE) system based on MXene/PVA (polyvinyl alcohol) hydrogel, by utilizing thermo-diffusion of Cu2+ and Cl- ions and the redox reaction involving Cu/Cu2+ at the electrode interfaces. The thermopower of the i-TE system can be independently tuned to a value of -3.13 mVK-1 by adjusting the ion diffusivity via MXene (Ti3C2Tx). The i-TE system demonstrates a rapid response time of less than 100 s, outperforming any other polyelectrolyte-based system. Crucially, the i-TE system achieves continuous current output when equipped with copper electrodes, facilitated by the redox reaction involving Cu/Cu2+, and maintains stable long-term outputs across a range of resistances from 1 kΩ to 1 MΩ. A three-serial-connected i-TE module demonstrates an output voltage of 26 mV with 6 °C temperature difference, confirming the feasibility of creating an array of i-TE devices for substantial energy output. Beyond energy harvesting, the MXene/PVA hydrogel serves as multifunctional strain/temperature sensors, capable of detecting mechanical strains via the piezoresistive effect and locating finger contact points via the ionic thermoelectric effect.

Intricate Ionic Behaviors in High-Performance Self-Powered Hydrothermal Chemical Generator Using Water and Iron (III) Gate

Utilizing the ionic flux to generate voltage output has been confirmed as an effective way to meet the requirements of clean energy sources. Different from ionic thermoelectric (i-TE) and hydrovoltaic devices, a new hydrothermal chemical generator is designed by amorphous FeCl3 particles dispersing in MWCNT and unique ferric chloride or water gate. In the presence of gate, the special ion behaviors enable the cell to present a constant voltage of 0.60 V lasting for over 96 h without temperature difference. Combining the differences of cation concentration, humidity and temperature between the right and left side of sample, the maximum short-circuit current and power output can be obtained to 168.46 µA and 28.11 µW, respectively. The generator also can utilize the low-grade heat to produce electricity wherein Seebeck coefficient is 6.79 mV K-1. The emerged hydrothermal chemical generator offers a novel approach to utilize the low-grade heat, water and salt solution resources, which provides a simple, sustainable and low-cost strategy to realize energy supply.

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.

Thermoelectrics and thermocells for fire warning applications

Historically, fire disasters have killed numerous human lives, and caused tremendous property loss. Fire warning systems play a vital role in predicting fire risks, and are strongly desired to effectively prevent the disaster occurrence and significantly reduce the loss. Among the developed fire warning systems, thermoelectrics (TEs) and thermocells (TECs)-based fire warning materials are extremely important and indispensable in future research, owing to their unique capability of direct conversion between heat and electricity. Here, we present this review of the recent progress of TEs and TECs in fire warning field. Firstly, a brief introduction of existing fire warning systems is provided, including the mechanisms and features of various types. Then, the mechanisms of electronic TE (eTE), ionic TE (iTE) and TEC are elucidated. Next, the basic principles for the material preparation and device fabrication are discussed in their dimension sequence. Subsequently, some important advances or examples of TE fire warnings are highlighted in details. Finally, the challenges and prospects are outlooked.

Mixed Ionic-Electronic Conducting Hydrogels with Carboxylated Carbon Nanotubes for High Performance Wearable Thermoelectric Harvesters

Mixed ionic-electronic conducting (MIEC) thermoelectric (TE) materials offer higher ionic conductivity and ionic Seebeck coefficient compared to those of purely ionic-conducting TE materials. These characteristics make them suitable for direct use in thermoelectric generators (TEGs) as the charge carriers can be effectively transported from one electrode to the other via the external circuit. In the present study, MIEC hydrogels are fabricated via the chemical cross-linking of polyacrylamide (PAAM) and polydopamine (PDA) to form a double network. In addition, electrically conducting carboxylated carbon nanotubes (CNT-COOH) are dispersed evenly within the hydrogel via sonication and interaction with the PDA. Moreover, the electrical properties of the hydrogel are further improved via the in situ polymerization of polyaniline (PANI). The presence of CNT-COOH facilitates the ionic conductivity and enhances the ionic Seebeck coefficient via ionic-electronic interactions between sodium ions and carboxyl groups on CNT-COOH, which can be observed in X-ray photoelectron spectroscopy results, thereby promoting the charge transport properties. As a result, the optimum device exhibits a remarkable ionic conductivity of 175.3 mS cm-1 and a high ionic Seebeck coefficient of 18.6 mV K-1, giving an ionic power factor (PFi) of 6.06 mW m-1 K-2 with a correspondingly impressive ionic figure of merit (ZTi) of 2.65. These values represent significant achievements within the field of gel-state organic TE materials. Finally, a wearable module is fabricated by embedding the PAAM/PDA/CNT-COOH/PANI hydrogel into a poly(dimethylsiloxane) mold. This configuration yields a high power density of 171.4 mW m-2, thus highlighting the considerable potential for manufacturing TEGs for wearable devices capable of harnessing waste heat.