Exceptional n-type thermoelectric ionogels enabled by metal coordination and ion-selective association

Cation-Anchoring-Induced Efficient n-Type Thermo-Electric Ionogel with Ultra-High Thermopower

Ionogels have emerged as promising candidates for low-grade thermal energy harvesting due to their leak-free electrolytes, exceptional flexibility, thermal stability, and high thermopower. While substantial progress in the thermoelectric performance of p-type ionogels, research on n-type ionic materials lags behind. Striking a harmonious balance between high mechanical performance and thermoelectric properties remains a formidable challenge. This work presents an advanced n-type ionogel system integrating polyethylene glycol diacrylate (PEGDA), hydroxyethyl methacrylate (HEMA), 1-allyl-3-methylimidazolium chloride ([AMIM]Cl), and bacterial cellulose (BC) through a rational design strategy. The synergistic combination of photo-polymerization and hydrogen-bonding networks effectively immobilizes imidazolium cations while enabling rapid chloride ion transport, creating a pronounced cation-anion mobility disparity that yields a substantial negative ionic Seebeck coefficient of -7.16 mV K⁻¹. Furthermore, BC's abundant hydroxyl groups establish multivalent hydrogen bonds within the ternary polymer matrix, endowing the composite with exceptional mechanical properties-notably a tensile strength of 3.2 MPa and toughness of 4.1 MJ m⁻3. Moreover, the ionogel exhibits sensitive responses to stimuli such as pressure, strain, and temperature. The thermoelectric modules fabricated can harness body heat to illuminate a bulb, showcasing great potential for low-grade energy harvesting and ultra-sensitive sensing.

Thermoelectric Composites Based on Porous Laser-Induced Graphene and Ion Hydrogel

Despite the rapid development of single-modal flexible sensors, there is an urgent need to develop self-powered multimodal flexible sensing devices to eliminate power constraints. This work reports stretchable thermoelectric composites based on porous laser-induced graphene foams and an ion hydrogel, aiming to create a self-powered sensor that can detect temperature changes and strain with high accuracy. The self-powered strain sensor based on 3D porous laser-induced graphene (LIG) foam exhibits a high maximum sensitivity of 105.9 for strain up to 30%, a low detection limit of 0.071%, and good stability over 5,000 cycles at 30% strain. With an increased Seebeck coefficient of -189.90 μV/K, the sensor can also detect temperatures in the range of -10-100 °C with a resolution of 0.1 °C. The thermoelectric power generation array with integrated units can achieve an output voltage of 104.18 mV for a temperature difference of 20 °C. By combining the electronic thermoelectric material LIG and the ionic thermoelectric material NKKC/PFF, the dual-parameter sensors demonstrate high potential in human health monitoring, smart storage, and bathroom systems. The reported thermoelectric composites can be further utilized in temperature-strain decoupled sensing for battery monitoring, smart garments, and medical applications.

Thermoelectric Conversion Eutectogels for Highly Sensitive Self-Powered Sensors and Machine Learning-Assisted Temperature Identification

Endowing flexible sensors with self-powering capabilities is of significant importance. However, the thermoelectric conversion gels reported so far suffer from the limitations of insufficient flexibility, signal distortion under repetitive deformation, and insufficient comprehensive performance, which seriously hinder their wide application. In this work, we designed and prepared eutectogels by an ionic liquid and a polymerizable deep eutectic solvent (PDES), which exhibit good mechanical properties, adhesion, and excellent thermoelectric conversion and thermoelectric response performance. The Seebeck coefficient (Si) can reach 30.38 mV K-1 at a temperature difference of 10 K. To amplify the self-powered performance of individual gel units, we assembled them into arrays and further prepared temperature sensors. The combination of the K-means clustering algorithm of machine learning can filter out the noise of traditional thermoelectric sensors and improve the consistency of signals, thereby enabling the prediction of absolute temperature under the conditions of 10 or 20 K temperature difference. This study also demonstrates potential application of these eutectogels in thermoelectric self-powered sensing.

Enhancing the Thermoelectric Performance of Sustainable Cellulose-Based Ionogels Through Water Content Regulation

Ionogels are widely studied as promising ionic thermoelectric (i-TE) materials to harvest low-grade waste heat into electrical energy due to their huge thermopower and good ionic conductivity, providing a feasible way to sustainable development. Herein, a p-type i-TE cellulose ionogel (CIG) based on Soret effect is prepared by dissolving cellulose in an ionic liquid (IL) and subsequent water-absorbing induced gelation. Its morphological structure and IL distribution are intuitively investigated through cryo-focused ion beam-scanning electron microscope. Experimental characterizations and molecular dynamic simulation studies elucidate that the regulation of water content induces the hydration of 1-butyl-3-methylimidazolium cation and the swelling of CIG, which greatly promotes the ions diffusion and expands the difference in mobility between anions and cations. The proposed CIG exhibits superior thermoelectric properties: an ionic conductivity of 51.2 mS cm-1, an ionic Seebeck coefficient of 20.7 mV K-1, and an ionic figure of merit zTi of 2.36 at 30 °C, respectively. A CIG-based i-TE device is designed and assembled to demonstrate its great potential for wearable body heat-to-electricity conversion. The cellulose skeleton in CIG is completely biodegradable in nature and the used IL is recyclable and reusable, providing a green and sustainable strategy for energy harvesting.

Harnessing Thermoelectric Power in Self-Healing Wearables: A Review

Wearable thermoelectric generators are sustainable devices that generate electricity from body heat to provide a continuous power supply for electronic devices. In healthcare, they are particularly valuable for powering wireless devices that transmit vital health signals, where maintaining an uninterrupted power source is a significant challenge. However, these generators are prone to failure over time or due to mechanical damage caused by mechanical stress or environmental factors, which can lead to the loss of critical healthcare data. To address these issues, the integration of self-healing capabilities alongside flexibility and longevity is essential for their reliable operation. To our knowledge, this review is one of the first to look in depth at self-healing materials specifically designed for wearable thermoelectric generators. It explores the latest innovations and applications in this field highlighting how these materials can improve the reliability and lifetime of such systems.

Piezoelectric-Augmented Thermoelectric Ionogels for Self-Powered Multimodal Medical Sensors

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.

Unlocking new possibilities in ionic thermoelectric materials: a machine learning perspective

The high thermopower of ionic thermoelectric (i-TE) materials holds promise for miniaturized waste-heat recovery devices and thermal sensors. However, progress is hampered by laborious trial-and-error experimentations, which lack theoretical underpinning. Herein, by introducing the simplified molecular-input line-entry system, we have addressed the challenge posed by the inconsistency of i-TE material types, and present a machine learning model that evaluates the Seebeck coefficient with an R 2 of 0.98 on the test dataset. Using this tool, we experimentally identify a waterborne polyurethane/potassium iodide ionogel with a Seebeck coefficient of 41.39 mV/K. Furthermore, interpretable analysis reveals that the number of rotatable bonds and the octanol-water partition coefficient of ions negatively affect Seebeck coefficients, which is corroborated by molecular dynamics simulations. This machine learning-assisted framework represents a pioneering effort in the i-TE field, offering significant promise for accelerating the discovery and development of high-performance i-TE materials.

Sea Cucumber-Inspired Polyurethane Demonstrating Record-Breaking Mechanical Properties in Room-Temperature Self-Healing Ionogels

Practical applications of existing self-healing ionogels are often hindered by the trade-off between their mechanical robustness, ionic conductivity, and temperature requirements for their self-healing ability. Herein, this challenge is addressed by drawing inspiration from sea cucumber. A polyurethane containing multiple hydrogen-bond donors and acceptors is synthesized and used to fabricate room-temperature self-healing ionogels with excellent mechanical properties, high ionic conductivity, puncture resistance, and impact resistance. The hard segments of polyurethane, driven by multiple hydrogen bonds, coalesce into hard phase regions, which can efficiently dissipate energy through the reversible disruption and reformation of multiple hydrogen bonds. Consequently, the resulting ionogels exhibit record-high tensile strength and toughness compared to other room-temperature self-healing ionogels. Furthermore, the inherent reversibility of multiple hydrogen bonds within the hard phase regions allows the ionogels to spontaneously and efficiently self-heal damaged mechanical properties and ionic conductivity multiple times at room temperature. To underscore their application potential, these ionogels are employed as electrolytes in the fabrication of electrochromic devices, which exhibit excellent and stable electrochromic performance, repeatable healing ability, and satisfactory impact resistance. This study presents a novel strategy for the fabrication of ionogels with exceptional mechanical properties and room-temperature self-healing capability.

Transparent, mechanically robust, conductive, self-healable, and recyclable ionogels for flexible strain sensors and electroluminescent devices

A mechanically robust, self-healable, and recyclable PVP-based ionogel was achieved through a simple one-pot photoinitiated polymerization process. This ionogel exhibits a combination of excellent properties, including transparency, high mechanical strength, good ionic conductivity, self healability, and recyclability. A wearable resistive strain sensor based on the ionogel is successfully assembled and demonstrated accurate response to human motion. Moreover, a flexible electroluminescent device has been fabricated based on our ionogel, which can maintain optimal luminescence functionality even when subjected to bending. Considering the simple preparation method and excellent applications, we believe that our PVP-based ionogel has promising applications in many fields such as in wearable devices, electronic skin, implantable materials, robotics and human-machine interfaces.

Metal-Halogen Interactions Inducing Phase Separation for Self-Healing and Tough Ionogels with Tunable Thermoelectric Performance

Ionic liquid-based thermoelectric gels become a compelling candidate for thermoelectric power generation and sensing due to their giant thermopower, good thermal stability, high flexibility, and low-cost production. However, the materials reported to date suffer from canonical trade-offs between self-healing ability, stretchability, strength, and ionic conductivity. Herein, a self-healing and tough ionogel (PEO/LiTFSI/EmimCl) with tunable thermoelectric properties by tailoring metal-halogen bonding interactions, is developed. Different affinities between polymer matrix and salts are exploited to induce phase separation, resulting in simultaneous enhancement of ionic conductivity and mechanical strength. Molecular dynamics (MD) simulations and spectroscopic analyses show that Cl- ions impair the lithium-ether oxygen coordination, leading to changes in chain conformation. The migration difference between cations and anions is thus widened and a transition from n-type to p-type thermoelectric ionogels is realized. Furthermore, the dynamic interactions of metal-ligand coordination and hydrogen bonding yield autonomously self-healing capability, large stretchability (2000%), and environment-friendly recyclability. Benefiting from these fascinating properties, the multifunctional PEO-based ionogels are applied in sensors, supercapacitors, and thermoelectric power generation modules. The strategy of tuning solvation dominance to address the trade-offs in thermoelectric ionogels and optimize their macroscopic properties offers new possibilities for the design of advanced ionogels.

Self-Healing Ionogel-Enabled Self-Healing and Wide-Temperature Flexible Zinc-Air Batteries with Ultra-Long Cycling Lives

Hydrogel-based zinc-air batteries (ZABs) are promising flexible rechargeable batteries. However, the practical application of hydrogel-based ZABs is limited by their short service life, narrow operating temperature range, and repair difficulty. Herein, a self-healing ionogel is synthesized by the photopolymerization of acrylamide and poly(ethylene glycol) monomethyl ether acrylate in 1-ethyl-3-methylimidazolium dicyanamide with zinc acetate dihydrate and first used as an electrolyte to fabricate self-healing ZABs. The obtained self-healing ionogel has a wide operating temperature range, good environmental and electrochemical stability, high ionic conductivity, satisfactory mechanical strength, repeatable and efficient self-healing properties enabled by the reversibility of hydrogen bonding, and the ability to inhibit the production of dendrites and by-products. Notably, the self-healing ionogel has the highest ionic conductivity and toughness compared to other reported self-healing ionogels. The prepared self-healing ionogel is used to assemble self-healing flexible ZABs with a wide operating temperature range. These ZABs have ultra-long cycling lives and excellent stability under harsh conditions. After being damaged, the ZABs can repeatedly self-heal to recover their battery performance, providing a long-lasting and reliable power supply for wearable devices. This work opens new opportunities for the development of electrolytes for ZABs.