Strong Tough Thermogalvanic Hydrogel Thermocell With Extraordinarily High Thermoelectric Performance

Thermogalvanic hydrogels for low-grade heat harvesting and health monitoring

Direct conversion of ubiquitous heat energy into electricity is crucial for the development of green and sustainable power sources and self-powered electronic devices. Compared with traditional semiconductor thermoelectric materials, emerging thermogalvanic hydrogels offer high thermopowers, excellent intrinsic flexibilities, and low manufacturing costs, making them highly promising for low-grade thermal energy harvesting, self-powered flexible electronics, and wearable health monitoring devices. This review summarizes the recent advancements in thermogalvanic hydrogels, focusing on the strategies employed to enhance their thermoelectric properties and mechanical performances and expand their operational temperature ranges. We also explore their potential applications in low-grade heat harvesting for powering electronic devices and wearable applications. This review will provide valuable insights and guidance for the development and application of high-performance thermogalvanic hydrogels by systematically analyzing the potential of thermogalvanic hydrogels for flexible energy supply systems, outlining the performance enhancement mechanisms, and further discussing the current challenges and opportunities.

High-Performance Temperature Sensors for Early Warning Utilizing Flexible All-Inorganic Thermoelectric Films

The demand for highly sensitive temperature-response materials is critical for the advancement of intelligent temperature sensing and fire warning systems. Despite notable progress in thermoelectrical (TE) materials and devices, designing TE materials suitable for wide-range temperature monitoring across diverse scenarios remains a challenge. In this study, we introduce a TE temperature sensor for fire warnings and hot object recognition, utilizing an all-inorganic TE film composite of reduced graphene oxide (rGO)/Te nanowires (Te NWs). The resulting all-inorganic TE film, annealed at a high temperature, exhibits distinct response ratios to varying temperature changes, enabling consistently sensitive thermosensation. The robust linear relationship between open circuit voltage and temperature difference establishes it as an effective thermoreceptor for enhanced temperature alerts. Furthermore, we demonstrate that the assembled TE sensor provides rapid high-temperature warnings with adjustable threshold voltages (1-7 mV), achieving an ultrafast response time of approximately 4.8 s at 1 mV threshold voltage. Additionally, this TE sensor can be integrated with the gloves to monitor high-temperature objects in various scenarios, such as the brewed milk in daily life and heating reactors in industrial applications. These results offer perspectives for future innovations in intelligent temperature monitoring.

Long-Term Serviceable Ionic Thermoelectric Hydrogel with Temperature and Moisture Dual-Driven Waste Energy Harvesting Capability

Despite the substantial progress in developing high-performance quasi-solid hydrogels based on ionic thermophoretic migration, ionic thermoelectric materials (i-TEs) show unsatisfactory long-lasting stability caused by ionic migration failures and de-electrolytes. In this work, by enriching oxygen-containing functional groups in the gel network and constructing oriented ionic transport nanochannels, an innovative approach is presented to reach long-term service and reusability for i-TEs without sacrificing their TE properties. The as-prepared hydrogel with thermopower of 17.0 ± 1.0 mV K-1 stables at 82% of its original performance when immersed in the electrolyte. Notably, even after being air-dried for 135 days, its thermopower returns to 87% of the original value through replenishing electrolyte solution and its 3D shape fully recovers. Meanwhile, the dual-driven nature for moisture and temperature as well as the pH sensitivity of this network is systematically investigated. The maximum output voltage of a single sample reaches 0.215 V at a ΔT of 3.7 K, and it works continuously for more than 26 h. This study offers a new approach to overcoming the short-term service bottleneck of i-TEs and provides a practical scheme for the multi-source drive of self-powered TE equipment.

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.

Development of Ternary Hydrogel Electrolytes for Superior Gel Thermocells: Exceptional Anti-Drying, Anti-Freezing, and Mechanical Robustness

Gel thermocells (GTCs) provide a safe, facile, and scalable solution for harvesting waste heat to power ubiquitous electronics. However, achieving a harmonious integration of high power density, wide-temperature-range stability, and mechanical robustness in GTCs remains a significant challenge. In this work, a novel ternary gel thermocell (TGTC) is proposed and fabricated by integrating ferro/ferricyanide (Fe(CN)6 3-/4-) redox couples, thermosensitive crystallizing agents guanidinium chloride (GdmCl), and supporting electrolytes lithium chloride (LiCl) into natural nanocellulose hydrogels to enhance overall performance. GdmCl selectively induces Fe(CN)6 4- crystallization, increasing the concentration difference of redox pairs, resulting in improving thermopower and significantly increased fiber friction, while LiCl rapidly balances charges through electromigration promoting efficient ion transport and reconstructing hydrogen bond networks, contributing to an excellent output power density and the capture of water molecules, which are further elucidated by simulations, achieving synchronous enhancement of anti-drying, anti-freezing and mechanical properties. Consequently, the TGTC achieves a remarkable thermopower of 3.42 mV K-1, a maximum power density of 2.8 mW m-2 K-2, multiple continuous stable cycles at -20 °C, and an impressive strength of 3.06 MPa. Notably, this study elucidates the design principles and underlying mechanisms of ternary gel electrolytes, offering a practical strategy for advancing GTC technology.

Fatigue-resistant and super-tough thermocells

Wearable thermocells offer a sustainable energy solution for wearable electronics but are hindered by poor fatigue resistance, low fracture energy, and thermal inefficiencies. In this study, we present a high-strength, fatigue-resistant thermocell with enhanced thermoelectric performance through solvent exchange-assisted annealing and chaotropic effect-enhanced thermoelectric properties. The mechanical strength and toughness are improved by forming macromolecular crystal domains and entangling polymer chains. Guanidine ions, with strong chaotropic properties, optimize the solvation layer of redox ion couple, boosting thermoelectric efficiency. Compared to existing anti-fatigue thermocells, the current design exhibits a 20-fold increase in mechanical toughness (368 kJ m-2) and a 3-fold increase in Seebeck coefficient (5.4 mV K-1). With an ultimate tensile strength of 12 MPa, a fatigue threshold of 4.1 kJ m-2, and a specific output power density of 714 μW m-2 K-2, this thermocell outperforms existing designs, enabling more reliable and efficient wearable electronics and stretchable devices.

Aqueous power source integrated on a microfluidic chip

The growing demand for portable sensors for point-of-care (POC) and onsite health monitoring has led to significant interest in developing suitable power sources. In this study, we developed a microfluidic chip-integrated reverse electrodialysis (μRED) system for ecofriendly power generation with monolithic operation. Leveraging its fully ionic characteristic, μRED was successfully applied to an ionic diode, thereby demonstrating its capability for seamless integration. The feasibility of operating a bipolar electrode sensor without an external power supply was demonstrated, highlighting its broad applicability in electrochemical portable sensors. μRED has great potential for future applications, including electrochemical sensors for POC diagnostics and wearable devices.

MXene Hollow Microsphere-Boosted Nanocomposite Electrodes for Thermocells with Enhanced Thermal Energy Harvesting Capability

Thermal energy, constantly being produced in natural and industrial processes, constitutes a significant portion of energy lost through various inefficiencies. Employing the thermogalvanic effect, thermocells (TECs) can directly convert thermal energy into electricity, representing a promising energy-conversion technology for efficient, low-grade heat harvesting. However, the use of high-cost platinum electrodes in TECs has severely limited their widespread adoption, highlighting the need for more cost-effective alternatives that maintain comparable thermoelectrochemical performance. In this study, a nanocomposite electrode featuring Ti3C2Tx with hollow microsphere structures is rationally designed. This design addresses the restacking issue inherent in MXene nanosheets, increases the electrochemically active surface area, and modifies the original MXene surfaces with oxygen terminations, leading to improved redox kinetics at the electrode-electrolyte interface, particularly in n-type TECs employing Fe2+/3+ redox ions. The optimized n-type TEC achieved an output power of 84.55 μW cm-2 and a normalized power density of 0.53 mW m-2 K-2 under a ΔT of 40 K, outperforming noble platinum-based TECs by a factor of 5.5. An integrated device consisting of 32 TEC units with a p-n connection is also fabricated, which can be successfully utilized to power various small electronics. These results demonstrate the potential of MXene-based composite electrodes to revolutionize TEC technology by offering a cost-effective, high-performance alternative to traditional noble metal electrodes and contributing to efficient low-grade heat harvesting.

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.

Fracture-Resistant Stretchable Materials: An Overview from Methodology to Applications

Stretchable materials, such as gels and elastomers, are attractive materials in diverse applications. Their versatile fabrication platforms enable the creation of materials with various physiochemical properties and geometries. However, the mechanical performance of traditional stretchable materials is often hindered by the deficiencies in their energy dissipation system, leading to lower fracture resistance and impeding their broader range of applications. Therefore, the synthesis of fracture-resistant stretchable materials has attracted great interest. This review comprehensively summarizes key design considerations for constructing fracture-resistant stretchable materials, examines their synthesis strategies to achieve elevated fracture energy, and highlights recent advancements in their potential applications.

Advances in flexible ionic thermal sensors: present and perspectives

Ionic thermal sensors (ITSs) represent a promising frontier in sensing technology, offering unique advantages over conventional electronic sensors. Comprising a polymer matrix and electrolyte, these sensors possess inherent flexibility, stretchability, and biocompatibility, allowing them to establish stable and intimate contact with soft surfaces without inducing mechanical or thermal stress. Through an ion migration/dissociation mechanism similar to biosensing, ITSs ensure low impedance contact and high sensitivity, especially in physiological monitoring applications. This review provides a comprehensive overview of ionic thermal sensing mechanisms, contrasting them with their electronic counterparts. Additionally, it explores the intricacy of the sensor architecture, detailing the roles of active sensing elements, stretchable electrodes, and flexible substrates. The decoupled sensing mechanisms for skin-inspired multimodal sensors are also introduced based on several representative examples. The latest applications of ITS are categorized into ionic skin (i-skin), healthcare, spatial thermal perception, and environment detection, regarding their materials, structures, and operation modes. Finally, the perspectives of ITS research are presented, emphasizing the significance of standardized sensing parameters and emerging requirements for practical applications.

Fabricating a SFMA/BAChol/PAA/ZnCl2 Hydrogel with Excellent Versatile Comprehensive Properties and Stable Sensitive Freezing-Tolerant Conductivity for Wearable Sensors

Flexible wearable sensors have obtained tremendous interest in various fields and conductive hydrogels are a promising candidate. Nevertheless, the insufficient mechanical properties, the low electrical conductivity and sensitivity, and the limited functional properties prevent the development of hydrogels as wearable sensors. In this study, an SFMA/BAChol/PAA/ZnCl2 hydrogel was fabricated with high mechanical strength and versatile comprehensive properties. Specifically, the obtained hydrogel displayed excellent adhesion and mechanical stability, cryophylactic ability, stable sensitive freezing-tolerant conductivity, and feasible electrical conduction under a wide temperature range, demonstrating the high application potential as a flexible wearable sensor for movement behavior surveillance, even under harsh environments. Furthermore, the mechanical strength of the hydrogel could easily be regulated by varying the copolymer content. The molecular mechanisms of the hydrogel formation and the reversible adhesion during the wet-dry transition were proposed. The non-covalent interactions, including the electrostatic interaction, hydrogen bond interaction and hydrophobic association, and coordination interaction, were dynamically presented in the hydrogel network and hence supported the versatile comprehensive properties of the hydrogel. This study provides a strategy for designing novel hydrogels to promote the development of flexible sensors with stable sensitive freezing-tolerant conductivity.

Thermoelectric Materials and Devices for Advanced Biomedical Applications

Thermoelectrics (TEs), enabling the direct conversion between heat and electrical energy, have demonstrated extensive application potential in biomedical fields. Herein, the mechanism of the TE effect, recent developments in TE materials, and the biocompatibility assessment of TE materials are provided. In addition to the fundamentals of TEs, a timely and comprehensive review of the recent progress of advanced TE materials and their applications is presented, including wearable power generation, personal thermal management, and biosensing. In addition, the new-emerged medical applications of TE materials in wound healing, disease treatment, antimicrobial therapy, and anti-cancer therapy are thoroughly reviewed. Finally, the main challenges and future possibilities are outlined for TEs in biomedical fields, as well as their material selection criteria for specific application scenarios. Together, these advancements can provide innovative insights into the development of TEs for broader applications in biomedical fields.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Strain-Temperature Dual Sensor Based on Deep Learning Strategy for Human-Computer Interaction Systems

Thermoelectric (TE) hydrogels, mimicking human skin, possessing temperature and strain sensing capabilities, are well-suited for human-machine interaction interfaces and wearable devices. In this study, a TE hydrogel with high toughness and temperature responsiveness was created using the Hofmeister effect and TE current effect, achieved through the cross-linking of PVA/PAA/carboxymethyl cellulose triple networks. The Hofmeister effect, facilitated by Na+ and SO42- ions coordination, notably increased the hydrogel's tensile strength (800 kPa). Introduction of Fe2+/Fe3+ as redox pairs conferred a high Seebeck coefficient (2.3 mV K-1), thereby enhancing temperature responsiveness. Using this dual-responsive sensor, successful demonstration of a feedback mechanism combining deep learning with a robotic hand was accomplished (with a recognition accuracy of 95.30%), alongside temperature warnings at various levels. It is expected to replace manual work through the control of the manipulator in some high-temperature and high-risk scenarios, thereby improving the safety factor, underscoring the vast potential of TE hydrogel sensors in motion monitoring and human-machine interaction applications.

Recent advances in ionic thermoelectric systems and theoretical modelling

Converting waste heat from solar radiation and industrial processes into useable electricity remains a challenge due to limitations of traditional thermoelectrics. Ionic thermoelectric (i-TE) materials offer a compelling alternative to traditional thermoelectrics due to their excellent ionic thermopower, low thermal conductivity, and abundant material options. This review categorizes i-TE materials into thermally diffusive and thermogalvanic types, with an emphasis on the former due to its superior thermopower. This review also highlights the i-TE materials for creating ionic thermoelectric supercapacitors (ITESCs) that can generate significantly higher voltages from low-grade heat sources compared to conventional technologies. Additionally, it explores thermogalvanic cells and combined devices, discussing key optimization parameters and theoretical modeling approaches for maximizing material and device performance. Future directions aim to enhance i-TE material performance and address low energy density challenges for flexible and wearable applications. Herein, the cutting-edge of i-TE materials are comprehensively outlined, empowering researchers to develop next-generation waste heat harvesting technologies for a more sustainable future.

Ultrastrong, flexible thermogalvanic armor with a Carnot-relative efficiency over 8

Body heat, a clean and ubiquitous energy source, is promising as a renewable resource to supply wearable electronics. Emerging tough thermogalvanic device could be a sustainable platform to convert body heat energy into electricity for powering wearable electronics if its Carnot-relative efficiency (ηr) reaches ~5%. However, maximizing both the ηr and mechanical strength of the device are mutually exclusive. Here, we develop a rational strategy to construct a flexible thermogalvanic armor (FTGA) with a ηr over 8% near room temperature, yet preserving mechanical robustness. The key to our design lies in simultaneously realizing the thermosensitive-crystallization and salting-out effect in the elaborately designed ion-transport highway to boost ηr and improve mechanical strength. The FTGA achieves an ultrahigh ηr of 8.53%, coupling with impressive mechanical toughness of 70.65 MJ m-3 and substantial elongation (~900%) together. Our strategy holds sustainable potential for harvesting body heat and powering wearable electronics without recharging.

Recurrently gellable and thermochromic inorganic hydrogel thermogalvanic cells

Thermogalvanic cells (TGCs) draw great attention in the field of heat to electricity conversion, but TGCs were only in the form of liquid or organic gel. Here, we report an all-inorganic hydrogel TGC via simply mixing and stirring two inorganic salt solutions. Benefiting from the hydrogen bonds resultant framework and endogenous Fe2+/3+ redox couple, the TGC can recurrently pulverize-gel with completely holding its initial thermogalvanic performances after even 60 cycles. As the temperature and pH coregulating Fe3+ concentration and reversible transformation between Fe3+ and Fe(OH)3, we boost thermopower and realize thermochromism. This work provides a different perspective for TGCs and offers an avenue for future hydrogel materials research.

Robust, Efficient, and Recoverable Thermocells with Zwitterion-Boosted Hydrogel Electrolytes for Energy-Autonomous and Wearable Sensing

The rapid growth of flexible quasi-solid-state thermocells (TECs) provides a fresh way forward for wearable electronics. However, their insufficient mechanical strength and power output still hinder their further applications. This work demonstrates a one-stone-two-birds strategy to synergistically enhance the mechanical and thermoelectrochemical properties of the [Fe(CN)6]3-/4--based TECs. By introducing Hofmeister effect and multiple non-covalent interactions via betaine zwitterions, the mechanical strength of the conventional brittle gelatin hydrogel electrolytes is substantially improved from 50 to 440 kPa, with a high stretchability approaching 250 %. Meanwhile, the betaine zwitterions strongly affect the solvation structure of [Fe(CN)6]3- ions, thus enlarging the entropy difference and raising the thermoelectrochemical Seebeck coefficient from 1.47 to 2.2 mV K-1. The resultant quasi-solid-state TECs exhibit a normalized output power density of 0.48 mW m-2 K-2, showing a notable improvement in overall performance compared to their counterparts without zwitterion regulation. The intrinsic thermo-reversible property also allows the TECs to repeatedly self-recover through sol-gel transformations, ensuring reliable energy output and even recycling of TECs in case of extreme mechanical damages. An energy-autonomous smart glove consisting of eighteen individual TECs is further designed, which can simultaneously monitor the temperature of different positions on any touched object, demonstrating high potential in wearable applications.

Developing Air-Stable n-Type SWCNT-Based Composites with High Thermoelectric and Robust Mechanical Properties for Wearable Electronics

Flexible organic thermoelectric generators are gaining prominence in wearable electronics, leveraging body heat as an energy source. Their advancement is hindered by the scarcity of air-stable n-type organic materials with robust mechanical properties. This study introduces two new polymers (HDCN4 and HDCN8), created through polycondensation of paraformaldehyde and diamine-terminated poly(ethylene glycol) (PEGDA) with molecular weights of 4000 and 8000 g/mol into single-walled carbon nanotubes (SWCNTs). The resulting HDCN4/SWCNT and HDCN8/SWCNT composites show impressive power factors of 225.9 and 108.2 μW m-1 K-2, respectively, and maintain over 90% in air for over four months without encapsulation. The HDCN4/SWCNT composite also demonstrates significant tensile strength (33.2 MPa) and flexibility (up to 10% strain), which is currently the best mechanically n-type thermoelectric material with such a high power factor reported in the literature. A thermoelectric device based on HDCN4/SWCNT generates 4.2 μW of power with a 50 K temperature difference. Additionally, when used in wearable temperature sensors, these devices exhibit high mechanical reliability and a temperature resolution of 0.1 K. This research presents a viable method to produce air-stable n-type thermoelectric materials with excellent performance and mechanical properties.

Chaotropic Effect-Boosted Thermogalvanic Ionogel Thermocells for All-Weather Power Generation

Ionic thermocells convert heat into electricity and are promising power sources for electronic devices. However, discontinuous and small electricity output limits practical use under varying environmental conditions. Here, a thermogalvanic ionogel with a high Seebeck coefficient (32.4 mV K-1) is designed. Thermocells that combine thermogalvanic ionogel-based thermocells, which realize all-weather power generation via passive radiative cooling, are also developed. These thermocells generate electricity continuously under varying weather conditions and over a wide temperature range (-40 to 90 °C), with a normalized power density of 25.84 mW m-2 K-2. Advanced characterization shows that the chaotropic effect enhances the Seebeck coefficient, while the self-supplying temperature difference given the radiative cooling structure enables all-weather power generation. These results provide an effective strategy for developing practical thermocells suitable for diverse daily and seasonal variations.

Thermoelectric Hydrogel Electronic Skin for Passive Multimodal Physiological Perception

Electronic skins (e-skins) are being extensively researched for their ability to recognize physiological data and deliver feedback via electrical signals. However, their wide range of applications is frequently restricted by the indispensableness of external power supplies and single sensory function. Here, we report a passive multimodal e-skin for real-time human health assessment based on a thermoelectric hydrogel. The hydrogel network consists of poly(vinyl alcohol)/low acyl gellan gum with [Fe(CN)6]4-/3- as the redox couple. The introduction of glycerol and Li+ furnishes the gel-based e-skin with antidrying and antifreezing properties, a thermopower of 2.04 mV K-1, fast self-healing in less than 10 min, and high conductivity of 2.56 S m-1. As a prospective application, the e-skin can actively perceive multimodal physiological signals without the need for decoupling, including body temperature, pulse rate, and sweat content, in real time by synergistically coupling sensing and transduction. This work offers a scientific basis and designs an approach to develop passive multimodal e-skins and promotes the application of wearable electronics in advanced intelligent medicine.

Exploring the local solvation structure of redox molecules in a mixed solvent for increasing the Seebeck coefficient of thermocells

A thermocell is an emerging alternative to thermoelectric devices and exhibits a high Seebeck coefficient (Se) due to the large change of solvation entropy associated with redox reactions. Here, the Se of p-chloranil radicals/dianions (CA˙-/2-) in acetonitrile was drastically increased from -1.3 to -2.6 mV K-1 by the addition of ethanol, and the increment surpassed the estimation of the classical Born model with continuum solvent media. UV-vis spectroscopy and electrochemical measurements at various mixing ratios of acetonitrile to ethanol revealed that the strong hydrogen bonding between ethanol and oxygen atoms of CA2- forms a 4 : 1 solvent-ion pair, while the ethanol molecules binding to CA2- dissociate upon its oxidation to CA˙-. The local solvation structures of CA2- are in good agreement with density functional theory. This order-disorder transition of the local solvation structure around the CA˙-/2- ions produces a large entropy change and results in a large Se value. The tailored solvation structure of redox ions by hydrogen bonding is a versatile method applicable to a variety of redox pairs and solvents, contributing to the development of electrolyte engineering for thermocells.

Highly Antifreezing Thermogalvanic Hydrogels for Human Heat Harvesting in Ultralow Temperature Environments

Thermogalvanic hydrogels have been quickly developed and are widely used in thermal energy harvesting. However, the freezing behaviors of thermogalvanic hydrogels at subzero temperatures greatly limit their practical applications. Herein, we design an antifreezing thermogalvanic hydrogel based on [Fe(CN)6]3-/4- ions for thermoelectric power generation in ultralow temperature environments. The antifreezing thermogalvanic hydrogels show excellent flexibility at -80 °C owing to the hydrogen bonding between ethylene glycol and water molecules. Even after 500 cyclic tensile strains, the thermogalvanic hydrogels can still maintain excellent mechanical stability, and the Seebeck coefficient is as high as 1.43 mV/K, corresponding to a large retention rate of ∼95%. Moreover, we demonstrate a wearable thermoelectric shoe based on antifreezing thermogalvanic hydrogels for harvesting human thermal energy in a simulated winter environment of -30 °C, and the electricity can drive a green LED. This work provides important guidance for the design and optimization of antifreezing thermogalvanic hydrogels.

Stretchable and Durable Bacterial Cellulose-Based Thermocell with Improved Thermopower Density for Low-Grade Heat Harvesting

Low-grade heat exists ubiquitously in the environment, and gel-state thermogalvanic cells (GTCs) can directly convert thermal energy into electricity by a redox reaction. However, their low ionic conductivity and poor mechanical properties are still insufficient for their potential applications. Here, we designed a bacterial cellulose (BC) nanofiber-macromolecular entanglement network to balance the GTC's thermopower and mechanical properties. Therefore, the BC-GTC shows a Seebeck coefficient of 3.84 mV K-1, an ionic conductivity of 108.5 mS cm-1, and a high specific output power density of 1760 μW m-2 K-2, which are much higher than most current literature. Further connecting 15 units of BC-GTCs, the output voltage of 3.35 V can be obtained at a temperature gradient of 65 K, which can directly power electronic devices such as electronic calculators, thermohydrometers, fans, and light-emitting diodes (LEDs). This work offers a promising method for developing high-performance and durable GTC in sustainable green energy.

Design and Optimization Strategies for Flexible Quasi-Solid-State Thermo-Electrochemical Cells

Currently, efficient utilization of low-grade thermal energy is a great challenge. Thermoelectricity is an extremely promising method of generating electrical energy from temperature differences. As a green energy conversion technology, thermo-electrochemical cells (TECs) have attracted much attention in recent years for their ability to convert thermal energy directly into electricity with high thermal power. Within TECs, anions and cations gain and lose electrons, respectively, at the electrodes, using the potential difference between the hot and cold terminals of the electrodes by redox couples. Additionally, the anions and cations therein are constantly circulating and mobile via concentration diffusion and thermal diffusion, providing an uninterrupted supply of power to the exterior. This review article focuses mainly on the operation of TECs and recent advances in redox couples, electrolytes, and electrodes. The outlook for optimization strategies regarding TECs is also outlined in this paper.