Construction and application of thermogalvanic hydrogels
0 Abstract
Low-grade heat (below 373 Kelvins) is abundant and ubiquitous, yet the lack of cost-effective recovery technologies frequently impedes its effective utilization. The advent of thermogalvanic hydrogel thermocells has garnered significant attention due to their high thermopower, inherent flexibility, low cost, and scalability. Thermogalvanic hydrogels have significantly enhanced their thermoelectric performance, resulting in the development of functional materials that exhibit flexibility, stretchability, self-healing, and frost resistance. However, there are substantial challenges in developing multifunctional thermogalvanic hydrogels that combine high power density and efficiency with practical applicability. This review discusses the synthesis of the novel redox couple, improving the performance of electrolytes to increase thermopower, creating electrodes with extensive surface areas for better current density and flexibility, and optimizing thermocell structure design to improve performance further. This comprehensive review aims to propel progress toward higher performance levels and broader applications of thermogalvanic hydrogel thermocells.
Keywords
INTRODUCTION
Low-grade heat energy from natural and anthropogenic sources (such as solar radiation, industrial plants, automobiles, and the human body) is abundant but underutilized, leading to significant energy wastage[1-7]. Consequently, developing and effectively using this thermal energy has become a meaningful way to promote sustainable development[8-12]. As a critical component of green energy, thermoelectric materials can convert thermal energy into electricity[13-19]. They also play an essential role in improving energy efficiency and reducing environmental burdens[20-23]. Although inorganic solid-state thermoelectric materials have been the subject of extensive research[24-27], their constituent elements are often rare and expensive. They typically generate thermopower at the microvolt level, hindering their widespread commercialization[28]. Ionic thermoelectric materials, such as thermogalvanic hydrogels, are intriguing because their thermopower is two to three orders of magnitude higher than their inorganic counterparts[21,29-31].
Thermogalvanic hydrogel materials have significant prospects for low-grade thermal energy harvesting because of their low cost, excellent scalability, flexibility, and high thermopower[32-39]. The high thermopower implies the generation of high voltages with a slight temperature gradient, thereby simplifying device design and integration[40-43]. Regarding thermogalvanic hydrogel preparation techniques, researchers can access diverse methods. Hydrogels can be broadly categorized into physically and chemically crosslinked types based on their crosslinking methods[44-46]. The physical crosslinking method mostly uses hydrogen bonds, van der Waals forces, and other non-covalent interactions to keep the hydrogel network structure stable[47-50]. Conversely, chemical crosslinking enhances the structural integrity of hydrogels by forming covalent bonds[51-53]. Each of these crosslinking techniques possesses unique advantages and application scenarios[54,55]. Through meticulous design and optimization, thermogalvanic hydrogels are poised to emerge as pivotal components in thermal energy harvesting technologies[2,15,56,57].
The principle of thermogalvanic hydrogels converting thermal energy into electrical energy differs from traditional electronic materials[58-60]. The principle in thermogalvanic hydrogels is mainly based on the thermogalvanic effect[61]. This effect is based on the redox reaction of redox couples in ionic conductors at the electrode interface under the thermal drive[40,62]. Owing to the disparity in reaction entropy between anions and cations, the ions undergo oxidation reactions in the high-temperature region and reduction reactions in the low-temperature region. The reaction generates a thermal voltage between the two electrodes, typically reaching several millivolts per Kelvin.
In recent years, significant progress has been achieved in thermogalvanic hydrogels. For example, by adding chaotropic cations and highly soluble amide derivatives to the redox couple electrolyte, the thermopower of thermogalvanic hydrogels was as high as 6.5 mV·K-1[63]. Furthermore, efforts to improve the output performance of electrodes by utilizing a large specific surface area to promote reaction kinetics have also proven effective[64]. In addition to the significant progress in performance, functional thermogalvanic hydrogels with properties such as flexibility, stretchability, toughness, antifreeze and self-healing are crucial for practical applications. Compared to traditional solid-state thermocells, hydrogel thermocells - extensively investigated in recent years - are more competitive and practical, as illustrated in Figure 1. However, thermogalvanic hydrogels still face challenges, including low energy conversion efficiency, low power density and stability issues in integrated devices for practical applications. In recent years, some excellent reviews have been made on thermogalvanic hydrogels. This paper aims to provide new insights to guide future research toward more efficient single thermocells and higher-performance low-grade heat-harvesting devices, among other directions.
Figure 1. Comparisons of (A) thermopower and (B) merits between inorganic thermoelectric materials and hydrogel thermocells. The materials used in hydrogel thermocells are abundant, inexpensive, and low-toxic. Furthermore, hydrogel thermocells exhibit excellent flexibility and stretchability, which can meet the needs of flexible wearable electronics[33,63,65-69].
THERMOGALVANIC HYDROGEL FUNCTIONALITY AND PERFORMANCE INFLUENCING FACTORS
Working principle
Thermogalvanic hydrogels represent a new class of materials exhibiting thermoelectric properties, and their thermoelectric conversion mechanism is mainly based on the thermogalvanic effect[56,70-72]. This phenomenon refers to generating electric potential under a temperature difference. The hydrogel thermocell is mainly composed of two electrodes and an electrolyte [Figure 2]. The two electrodes are in complete contact with the electrolyte containing the redox couple, and they are connected by an external circuit[73,74]. Applying a temperature difference across the two electrodes of a thermocell causes the redox couple to undergo oxidation at the anode and reduction at the cathode[75-77]. During this process, the reduced substances return to the anode for oxidation through convection, diffusion, and migration within the electrolyte. Meanwhile, the oxidized substances migrate towards the cathode. This ensemble of reactions and migratory processes constitutes a sustained current loop[78,79]. Ideally, as long as the composition of the electrolyte does not degrade, this cyclic reaction can theoretically continue indefinitely[80]. This property endows thermogalvanic hydrogels with tremendous potential for energy conversion and storage applications, thereby presenting a novel trajectory for advancing renewable energy technologies[32,78,81-86].
Figure 2. The thermocell working diagram (A) [Fe(CN)6]3-/[Fe(CN)6]4-. Reproduced with permission[33]. Copyright 2022, Royal Society of Chemistry; (B) Fe2+/Fe3+. Reproduced with permission[42]. Copyright 2024, Royal Society of Chemistry; (C) Cu/Cu2+. Reproduced with permission[71]. Copyright 2024, Royal Society of Chemistry; (D) I-/I3- Reproduced with permission[72]. Copyright 2016, American Chemical Society.
Factors influencing the thermoelectric properties of thermogalvanic hydrogels
Firstly, the material itself has a significant impact on its thermoelectric performance. Hydrogel is a three-dimensional network structure polymer with high porosity and water content, and it can store a large number of ions[87,88]. With their unique structure, hydrogel thermocells act as an electrolyte to conduct ions and maintain the movement of charge carriers. Additionally, they serve as solidifying matrices for the electrodes, addressing the issue of electrolyte leakage typically associated with liquid electrolytes[89]. Therefore, the physical and chemical properties of hydrogels (such as molecular composition, crosslinking density, pore size, etc.) influence the conductivity, specific heat capacity, thermal expansion coefficient, and thermogalvanic effect of thermocells. These properties directly determine the energy density and power density of the redox couples. For instance, the polymer chain structure of hydrogels directly influences their thermoelectric characteristics, including chain length, degree of branching, and copolymer composition ratio. Longer-chain polymers usually conduct heat more efficiently, while optimizing the copolymer composition ratio can enhance thermocell performance. Secondly, the crosslinking density plays a determinant role in establishing the network density of the hydrogel, thereby significantly influencing its thermoelectric properties. A high crosslinking density can enhance the mechanical strength of the thermocell but might decrease its thermal conductivity. On the other hand, a low crosslinking density can augment thermal conductivity but may compromise mechanical strength. Finally, the conductivity of hydrogels is crucial to their thermoelectric properties, as excellent conductivity is essential for converting thermal energy into electrical energy. The conductivity of hydrogels can be enhanced by adding conductive fillers or constructing conductive networks.
Hydrogel thermocells generate a potential difference at specific temperatures, which is critical in determining their output power. This potential difference is a consequence of the redox reactions occurring at that particular temperature. The potential difference directly affects the amount of energy that a redox couple can deliver, significantly influencing its power output. The redox reactions can be expressed as:
Thermopower refers to the reaction thermodynamics and is determined by the thermopower of redox species in the electrolytes. In thermocells, the thermopower parameter is also called the temperature coefficient. It is defined as the ratio of the voltage difference to the temperature change and describes the ability to generate electromotive force unit temperature difference. This parameter is an essential factor in determining the power output of the device. Thermopower can be calculated using[90]:
where n represents the number of electrons transferred in a redox reaction, F is the Faraday constant,
Thermal conductivity represents another pivotal factor that influences the performance of thermocells. If the electrolyte has good thermal conductivity, it can conduct heat more efficiently, reducing the temperature difference between the electrodes. This reduction may result in lower output power from the thermocell. It is well known that heat conduction depends on lattice vibrations. The amorphous and inhomogeneous structure of polymers makes their lattice vibrations discontinuous, resulting in generally low thermal conductivity. Hydrogel thermocells usually use polymers as matrices, leveraging the lattice vibrational discontinuities between the aggregated and entangled nanofiber networks of the polymer and the embedded ions; therefore, their thermal conductivity is generally lower.
STRATEGIES TO IMPROVE THERMOELECTRIC PROPERTIES
Enhancement of thermopower
The main factors restricting the widespread application of thermocells are their low output power, conversion efficiency, and stability[91]. To address these challenges, numerous researchers are actively working on solutions across multiple levels[92-96].
Thermopower primarily depends on the entropy change of the redox species during the reaction, and the entropy change is determined by the thermodynamics of the reaction. Specifically, the entropy change is related to the solvation structure entropy difference (∆S) and the concentration ratio difference (∆C) between the redox species, indicating that increasing ∆S or ∆C can effectively enhance thermopower[30]. Generally, the ∆S value of the redox species can be increased by regulating the interaction between the redox species and its solvent environment through special additives. Additives reorient and complicate the solvation structure of the redox species, thereby increasing the thermopower. Recent studies have demonstrated that the absolute value of the charge of the redox couples and the types of solvents surrounding it significantly influence the magnitude of ∆S[29]. Charge transfer leads to a change in entropy. Alterations in the charge of redox couples with large absolute charges modify the electron distribution and ionization state within the molecule, thereby increasing the complexity of the molecular internal structure. This increased complexity raises the entropy difference and the thermopower. For instance, the thermopower of [Fe(CN)6]3-/[Fe(CN)6]4- is approximately ~ 1.3 mV·K-1, much larger than that of I-/I3- at ~ 0.6 mV·K-1.
To amplify the entropy difference, organic solvents with different numbers of donors (DN) can be introduced to the aqueous solution[66,97,98]. The dimensions of the solvation shell are contingent upon the electron density of the surrounding solvation molecules. Meanwhile, the size of the solvation shell is inversely proportional to its entropy. Jiao et al., Lazar et al., and He et al. reported a series of non-liquid electrolyte thermocells exhibiting high thermopower, with a maximum value of 2.65 mV·K-1[99-101]. Li et al. significantly enhanced the performance of the K4Fe(CN)6/K3Fe(CN)6 redox couple within an organic hydrogel thermocell [Figure 3][102]. This change also influenced the structure of the solvent shell around the redox couples during the reaction, which led to a difference in the concentration of the redox anions. Consequently, the thermopower was substantially increased from 1.27 to 2.30 mV·K-1. Owing to the high compressibility and inherent stretchability, these thermocells were subsequently assembled into self-powered strain sensors. These sensors could monitor the movement of the human body under various stretches and pressures in real-time with high sensitivity.
Figure 3. (A) The schematic operation mechanism for organogel-based thermocells. The thermopower (B) and conductivity (C) of thermocells. (D) Illustration showing the wearable application scenario of organized-based thermocells for self-powered strain sensing harvesting body heat. (E) The diagram of finger bending activity with a self-powered tensile sensor. (F) The thumb and index finger grasping bottle schematic with a self-powered pressure sensor. Reproduced with permission[102]. Copyright 2023, Wiley-VCH Verlag.
To enhance the thermopower beyond improving ∆S, an alternative approach involves increasing ∆C. However, the redox couple cannot indefinitely sustain the concentration gradient between the hot and cold ends, as this state is thermodynamically unstable and will eventually revert to a uniform state. When the electrolyte reaches a steady state, ∆C is zero. Han et al. reported a new concept using methylcellulose to capture I3- at the hot side and then release I3- at the cold side, creating a concentration gradient of free I3- in the thermocell, leading to an increase and reversal of the thermopower[31]. Zhou et al. successfully increased the concentration difference between the hot and cold ends by introducing α-cyclodextrin (α-CD) and potassium chloride (KCl)[72]. Due to the host-guest interaction, α-CD bound to I3- at the cold end, thereby reducing the relative concentration of I3-. Consequently, the equilibrium spontaneously shifted to the right at the cold end, where oxidation of 3I- to I3- occurred. Concurrently, with the temperature elevation, the
Figure 4. (A) A schematic figure of the thermocell combined with α-CD and I3-/I- redox pair; (B) Increase of ionic conductivity and power factor of the thermocell with α-CD (4 mM); (C) Comparison of the thermopower values after the addition of α-CD, KCl, and (D) estimated concentration of uncomplexed I3- in the electrolyte solution with simulated thermopower values (Se) at 10/40 oC. Reproduced with permission[72]. Copyright 2016, American Chemical Society. α-CD: α-cyclodextrin; KCl: potassium chloride.
In addition to the methods above, introducing electrolyte additives is another advantageous strategy[2,31,33,40]. Cations with strong chaotropic properties can combine with redox ions, crystallizing redox substances. This raises the ∆S of the redox electrolyte and amplifies the ∆C, significantly improving the thermopower of the thermocell. Yu et al. added guanidinium ions (Gdm+) to the electrolyte. These ions selectively caused [Fe(CN)6]4- to crystallize at the cold end, which led to the formation of thermosensitive crystals [Figure 5A and B][30]. Owing to the concentration gradient, these thermosensitive crystals moved toward the hot end and dissolved there. This phenomenon generated a substantial concentration disparity between the two electrodes, resulting in higher thermopower[103]. This was attributed to the fact that [Fe(CN)6]4- had a higher charge density compared to [Fe(CN)6]3-, and it interacted more strongly with Gdm+. Yu et al. corroborated this hypothesis through comprehensive experimental and simulation analyses[30]. At the low-temperature electrode (293 K), [Fe(CN)6]4- crystallized almost completely, with a concentration ratio of [Fe(CN)6]3-/[Fe(CN)6]4- of approximately 0.02. Conversely, near the high-temperature electrode (343 K), the [Fe(CN)6]4- crystals dissolved rapidly, bringing the [Fe(CN)6]3-/[Fe(CN)6]4- concentration ratio to about 0.94. More importantly, the large concentration gradient of the redox couple significantly amplified the thermogalvanic effect. Adding Gdm+ caused the thermopower to rise from 1.4 to 3.73 mV·K-1, nearly 2.5 times than before. In addition to increasing ∆C, the crystals generated at the cold end could also inhibit thermal convection in the system, decreasing thermal conductivity [Figure 5C]. Due to the synergistic optimization of these parameters, its Carnot-relative efficiency reached 11.1%. Liu et al. combined stretch-induced crystallization with thermoelectric effect and proposed a high-strength quasi-solid stretchable polyvinyl alcohol thermogalvanic thermocell (SPTC) [Figure 5D][63]. Thus, the SPTC system had a large thermopower of 6.5 mV·K-1, a high specific output power density of 1969 μW m-2 K-2. The extraordinary thermoelectric and mechanical properties of the SPTC were superior to those of most reported quasi-solid stretchable thermogalvanic thermocells [Figure 5E].
Figure 5. (A) Schematic of Gdm+ induced [Fe(CN)6]4- crystallization and enhanced Se effect in K4Fe(CN)6/K3Fe(CN)6 electrolyte; (B) The photo shows the 0.4 M K4Fe(CN)6/K3Fe(CN)6 electrolyte before and after the addition of Gdm+; (C) The open-circuit voltage of the liquid [Fe(CN)6]3-/[Fe(CN)6]4- system after the addition of guanidinium chloride. Reproduced with permission[30]. Copyright 2020, American Association for the Advancement of Science; (D) A schematic illustration of the Gdm+ contribution to the thermogalvanic effect was enhanced in the thermocell; (E) The thermopower and conductivity of thermocells. Reproduced with permission.[63] Copyright 2023, Wiley-VCH Verlag.
Enhancement of ionic conductivity
The conductivity of hydrogel thermocells is generally three orders of magnitude lower than that of traditional inorganic thermoelectric materials. Consequently, research efforts have been directed towards enhancing their conductivity[104-106]. A primary approach to improving conductivity is to increase the solubility of redox couples within the electrolyte. However, this strategy could lead to an increase in the viscosity of the electrolyte, thereby intensifying mass transfer resistance and diminishing overall conductivity. Additionally, the thermopower of certain redox couples is concentration-dependent and can decline rapidly at high concentrations. For example, the thermopower could decrease by over 400% when the ion pair concentration increases from 0.01 to 2 M in I-/I3- systems[107]. To solve these problems, researchers have proposed methods to optimize the properties of thermoelectric materials through electrode modification. For example, they proposed the creation of composite electrodes featuring three-dimensional pores or using copper electrodes to create three-dimensional layered electrodes through oxidation-etching-reduction processes. Im et al. used carbon nanotube aerogel sheets coated on the surface with Pt nanoparticles as electrodes, increasing the conversion efficiency to 3.95%[108]. Wei et al. used MXene as the core of the electrode structure. They put polyaniline on the carbon nanotubes to make a three-layer thin film with substantial electrochemically active space [Figure 6][64]. This ternary composite flexible thin film electrode demonstrated superior performance in thermoelectrochemistry due to its porous and layered architecture. The synergistic interaction between MXene and polyaniline facilitated ion and charge transport at the electrolyte-electrode interface, resulting in an output power density of up to 13.15 mW·cm-2 at a temperature difference of ∆T = 40 K. These methods primarily rely on increasing the specific surface area of the electrodes to reduce electron transmission resistance, thereby increasing the current density of the thermocell.
Figure 6. (A) Diagram illustrating the process of creating a flexible ternary composite thin film electrode that has a porous layer structure; (B) Schematics of the configuration and size information of the square thermocell; (C) Optical images of the as-fabricated square thermocell; (D) Demonstration of using the square thermocell to light up two LED bulbs under the temperature difference of
THERMOGALVANIC HYDROGEL APPLICATIONS
Thermogalvanic hydrogels, as a multifunctional advanced material, have demonstrated their unique application potential across various fields[109-118]. In the energy sector, these materials can be utilized for waste heat recovery, solar energy conversion and bioenergy conversion, potentially leading to significant energy savings and environmental benefits[65,119]. Thermogalvanic hydrogels have been proven indispensable in sensor technology, enabling temperature sensing, stress sensing, and other functions[89,120,121]. Additionally, thermogalvanic hydrogels have exhibited important application value in high-tech areas such as flexible electronics and smart nanodevices[63,122,123].
Energy field
As global energy demands continue to rise, finding efficient ways to reduce energy consumption and lower industrial energy usage has become a key focus for researchers[79,124]. Kong et al. prepared a tellurium nanowire (Te-NW)-doped poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS)/polyvinyl alcohol hydrogel [Figure 7A][83]. After integration with a power management unit, the hydrogel module can drive small electronic devices such as commercial calculators and light-emitting diodes, showing good application potential in human body thermal energy harvesting. Xu et al. designed a PEDOT:PSS-polyacrylamide (PAAm) double network hydrogel as a high-performance interfacial photothermal material for solar steam power generation[125]. The hydrogel has good mechanical properties, long-term reliability and stability, and high-efficiency photothermal conversion performance. Meanwhile, Xu et al. developed a novel, cost-effective, highly porous hydrogel composite integrated with synchronous photothermal and photocatalytic effects by a facile two-step immobilization approach[126]. The effective recovery of waste heat generated during the production process is crucial. However, efficient heat dissipation and waste heat recovery are often contradictory and difficult to achieve simultaneously. To address this issue, Pu et al. designed and developed a thermogalvanic hydrogel through collaborative innovation [Figure 7B][127]. This hydrogel film was laminated onto heat-generating elements to facilitate effective evaporative cooling and convert a portion of waste heat into electricity. More uniquely, the hydrogel could absorb moisture from the surrounding air and spontaneously recycle it. The effective recovery of waste heat generated during production processes was critical for increasing energy efficiency and decreasing industrial energy consumption. Specifically, attaching the hydrogel membrane with a thickness of just 2 mm to a phone battery successfully reduced the operating temperature by 20 oC at a discharge rate of 2.2 C, while retrieving electricity of 5 μW. This method facilitated spontaneous recycling by adsorbing water molecules from the environment when the device was unused. Thus, it provided a novel energy conservation and environmental protection solution.
Figure 7. (A) Demonstration of thermoelectric system in powering electronic devices. Reproduced with permission[83]. Copyright 2023, Elsevier B.V; (B) A hydrogel thermocell on a mobile phone battery is demonstrated to achieve effective evaporative cooling while converting the waste heat into electrical energy. Reproduced with permission[127]. Copyright 2020, American Chemical Society. TEG: Thermoelectric generator; TG: thermogravimetric.
Temperature sensing
As an emerging material, thermogalvanic hydrogels have demonstrated significant potential as temperature sensors[89,128-130]. Researchers can monitor temperature fluctuations by measuring voltage differences, as the open-circuit voltage of hydrogels directly relates to temperature changes. Temperature sensors based on thermogalvanic hydrogels have gained widespread attention due to their rapid response and high precision. Wang et al. engineered a hydrogel-based electronic skin with dual-mode temperature and strain-sensing capabilities that harvest thermal energy from human body heat [Figure 8A][131]. They selected the iodine/triiodide (I-/I3-) redox couple. By combining the thermogalvanic and piezoresistive effects, self-powered temperature and strain sensing are successfully demonstrated through encapsulation and integration of the hydrogels into human tissue. Wu et al. have made a significant breakthrough. By connecting ten pairs of p-n thermoelectric modules in series, they constructed a system capable of real-time fire monitoring
Figure 8. (A) Conceptual schematic diagram of the thermogalvanic hydrogel patch resembling skin for self-powered temperature and strain sensing. Reproduced with permission[131]. Copyright 2024, Springer Nature; (B) A wireless fire monitoring system tracks the real-time voltage change during a fire. Reproduced with permission[132]. Copyright 2020, American Chemical Society. PVA: Polyvinyl alcohol.
Passive wearable sensors
Wearable sensors are another promising application area for thermogalvanic hydrogels[71,110,133-135]. Shen et al. reported for the first time a novel non-toxic redox couple of SO42-/SO32- as a high-performance, p-type thermogalvanic electrolyte ion that delivered a high Seebeck coefficient of 1.63 mV·K-1 at the redox couple concentration of 0.1 M[77]. Moreover, this PVA-SO4/32- hydrogel-based quasi-solid-state device showed the capacity for grabbing body heat to operate small electronics. Liu et al. have developed a tough and stretchable thermogalvanic hydrogel with superior thermoelectric properties [Figure 9A][63]. This hydrogel utilized the stretching-induced crystallization process and the thermogalvanic effect. With the help of freeze-thaw cycles and stretching steps, polymer chains crystallize and arrange themselves in the stretching direction, creating a layered anisotropic network. The researchers added guanidinium chloride to the [Fe(CN)6]3-/[Fe(CN)6]4- system to change the solvent shell around the [Fe(CN)6]4-. The chaotropic cations promote the crystallization of [Fe(CN)6]4- and enhance the reversibility of the redox reaction, thereby increasing the thermopower. The thermogalvanic hydrogel got a high thermopower of 6.5 mV·K-1 and a specific output power density of 1,969 µW·m-2·K-2. This represented a nearly fivefold increase compared to the thermopower of liquid [Fe(CN)6]3-/[Fe(CN)6]4- (1.4 mV·K-1) and a fourfold increase in the output power. The stretching caused the polymer chains to densify, which increased the mechanical properties. The hydrogel had a breaking strength of 19 MPa and a toughness of 163.4 MJ·m-3. The thermogalvanic hydrogels were also integrated as an array to harvest low-grade thermal energy from the environment and conduct strain sensing and health monitoring. This device generated enough electrical power to drive small medical devices, contributing to the promotion of green, sustainable, and wearable electronics in the Internet of Things era. Yang et al. constructed a solar thermogalvanic hydrogel[136]. The innovative system used external sunlight-induced by swinging arms to dynamically change the temperature difference of the gel during the thermoelectric conversion process [Figure 9B]. Han et al. developed a smart glove that integrates multiple thermal sensor arrays[121]. By monitoring all sensing nodes, the smart glove can feel the temperature and touch the position of an object, showcasing the potential application of flexible thermal sensor arrays in an intelligent environment [Figure 9C].
Figure 9. (A) Hydrogel thermocells are designed for wearable, flexible electronics and human health monitoring applications. Reproduced with permission[63]. Copyright 2023, Wiley-VCH Verlag; (B) Schematic illustration of the self-powered PTE patch for human motion detection, the main features of movements such as walking, jogging, and running are recognized by the PTE patch. Reproduced with permission[136]. Copyright 2023, Elsevier B.V; (C) Photographs of a smart glove integrated with multiple thermal sensor arrays. Reproduced with permission[121]. Copyright 2023, Open Access. iTE: Ionic thermoelectric.
Coupling with catalysis
In addition to applications in energy, temperature sensing, and passive wearable sensors, thermoelectric materials can also be used in catalysis[137]. Wang et al. reported an in situ photocatalytically enhanced redox reaction that generates hydrogen and oxygen to realize a continuous concentration gradient of redox ions in thermogalvanic devices[65]. They used polyacrylic acid (PAA) as a hydrogel matrix, FeCN4-/3- as redox ions, and WO3 (Ov-WO3) and ZnIn2S4 as the photocatalysts (Sv-ZIS) for O2 production and H2 production. An O2-evolution photocatalyst aided the forward reaction from FeCN3- to FeCN4- and facilitated H2O to O2 production, resulting in a high FeCN4- concentration on the hot side[138]. The H2-evolution photocatalyst converted the FeCN4- to FeCN3- and facilitated H2 production from H2O, increasing the amount of FeCN3- on the cold side[139]. The authors also used in-situ Raman spectroscopy to test the species distribution of the system. They found that high concentration difference distribution can indeed be achieved and can exist stably, achieving the effect of stable and efficient operation of the thermoelectric battery under light. In addition to theoretical feasibility, the system exhibited a thermopower of 8.2 mV·K-1 in actual application tests and a solar-to-hydrogen efficiency of up to 0.4%. A large-area generator (112 square centimeters) consisting of 36 units yielded an open circuit voltage of 4.4 volts and a power of 20.1 milliwatts, as well as 0.5 millimoles of hydrogen and 0.2 millimoles of oxygen after six hours of outdoor operation.
CONCLUSION AND OUTLOOK
In summary, thermal energy conversion technology based on the thermogalvanic effect is a relatively cutting-edge form of energy conversion. In this field, various thermoelectric materials play an essential role. However, thermoelectric materials have faced challenges in their widespread application in industry, households, and other scenarios. The emerging hydrogel thermocell has the advantages of high thermopower, low cost, low thermal conductivity, excellent flexibility, and scalability, which may be an ideal choice for thermoelectric materials working at room temperature and expanding the application range of thermoelectric materials. In recent years, they have made considerable progress by optimizing electrodes, designing electrolytes, and improving device modules, but there are still some challenges.
Firstly, the underlying mechanism of hydrogel thermocells has yet to be fully understood. The associated theoretical framework has yet to be completely established. There is an urgent need for effective theoretical guidance to facilitate material optimization and design for practical applications. Consequently, the current research should focus on deepening the understanding of its mechanism and promoting the development of a comprehensive and systematic research framework.
Secondly, the efficiency of hydrogel thermocells should be improved. The three parameters of thermopower, conductivity, and thermal conductivity within the thermocell are interdependent, presenting an extreme challenge in achieving efficient improvement. For example, tuning the solvation structure of the redox couple can increase the thermopower but simultaneously decrease the conductivity. Although optimizing electrodes can accelerate reactions and enhance conductivity without lowering thermopower, thermodynamic constraints impose limits on conversion efficiency. Future research should focus on the synergistic optimization of electrolytes and electrodes to circumvent these issues. This approach aims to decouple the three parameters, thereby improving the efficiency of a single thermocell.
Finally, given the high thermopower characteristic of thermocells, their application potential in flexible sensor and refrigeration fields deserves further exploration. In the field of flexible sensor technology, hydrogel thermocells have been combined with various sensor types, such as pressure, humidity, light, and power sensors. These integrated systems are poised to find utility in various applications, such as electronic skin, monitoring human activities, personalized healthcare, and facilitating advanced human-computer interfaces.
DECLARATIONS
Authors’ contributions
Literature review, data collection, data analysis and interpretation, and manuscript drafting: Liu W, Fang Y
Conceptualization and content development: Lyu X, Peng X, Luo ZZ
Critical revision of the manuscript, supervision, and project administration: Luo ZZ, Zou Z
Availability of data and materials
Not applicable.
Financial support and sponsorship
This study was supported in part by the National Natural Science Foundation of China (52102218, 51972061, 22203015, and 52273032), the National Key Research and Development Program of China (2020YFA0710303), the Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China (2021ZZ127), and the Scientific Research Foundation of Fujian University of Technology (GY-Z21014, GY-Z17073). The authors also acknowledge the Minjiang Scholar Professorship (GXRC-21004), the State Key Laboratory of Structure Chemistry (20240010), and the Natural Science Foundation of Fujian Province of China (2021J01594 and 2022J01088).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
REFERENCES
1. Forman C, Muritala IK, Pardemann R, Meyer B. Estimating the global waste heat potential. Renew Sustain Energy Rev 2016;57:1568-79.
2. Duan J, Yu B, Liu K, et al. P-N conversion in thermogalvanic cells induced by thermo-sensitive nanogels for body heat harvesting. Nano Energy 2019;57:473-9.
3. Wang H, Zhuang X, Xie W, et al. Thermosensitive-CsI3-crystal-driven high-power I-/I3- thermocells. Cell Rep Phys Sci 2022;3:100737.
4. Gao X, Chen G, Sun J, Dong S, Cui G. A review on realizing rechargeable batteries based on SOCl2/SO2 electrolyte systems. MetalMat 2024;1:e19.
5. Wu M, Sun K, He J, et al. Hierarchically 3D fibrous electrode for high-performance flexible AC-line filtering in fluctuating energy harvesters. Adv Funct Mater 2023;33:2305039.
6. Zhang D, Sia SA, Solco SFD, Xu J, Suwardi A. Energy harvesting through thermoelectrics: topological designs and materials jetting technology. Soft Sci 2023;3:1.
7. Liu Z, Cheng H, Le Q, Chen R, Li J, Ouyang J. Giant thermoelectric properties of ionogels with cationic doping. Adv Energy Mater 2022;12:2200858.
8. Yang Y, Lee SW, Ghasemi H, et al. Charging-free electrochemical system for harvesting low-grade thermal energy. Proc Natl Acad Sci U S A 2014;111:17011-6.
9. Rahimi M, Straub AP, Zhang F, et al. Emerging electrochemical and membrane-based systems to convert low-grade heat to electricity. Energy Environ Sci 2018;11:276-85.
10. Wu M, Cui H, Cai S, et al. Weak electron-phonon coupling and enhanced thermoelectric performance in n-type PbTe-Cu2 Se via dynamic phase conversion. Adv Energy Mater 2023;13:2203325.
11. Soo XYD, Tan SY, Cheong AKH, et al. Electrospun PEO/PEG fibers as potential flexible phase change materials for thermal energy regulation. Exploration 2024;4:20230016.
13. Yang R, Li X, Guo W, et al. New thermoelectric semiconductors Pb5Sb12+xBi6-xSe32 with ultralow thermal conductivity. Chin J Struct Chem 2024;43:100268.
14. Xu C, Sun Y, Zhang J, Xu W, Tian H. Adaptable and wearable thermocell based on stretchable hydrogel for body heat harvesting. Adv Energy Mater 2022;12:2201542.
15. Liu Y, Zhang S, Zhou Y, et al. Advanced wearable thermocells for body heat harvesting. Adv Energy Mater 2020;10:2002539.
16. Cao T, Shi X, Li M, et al. Advances in bismuth-telluride-based thermoelectric devices: progress and challenges. eScience 2023;3:100122.
17. Fan Y, Xie C, Li J, et al. Engineering thermoelectric performance of α -GeTe by ferroelectric distortion. Energy Environ Mater 2024;7:e12535.
18. Hong M, Sun S, Lyu W, et al. Advances in printing techniques for thermoelectric materials and devices. Soft Sci 2023;3:29.
19. Shen L, Liu M, Liu P, et al. A lamellar-ordered poly[bi(3,4-ethylenedioxythiophene)-alt-thienyl] for efficient tuning of thermopower without degenerated conductivity. Soft Sci 2023;3:20.
20. He J, Tritt TM. Advances in thermoelectric materials research: looking back and moving forward. Science 2017;357:eaak9997.
21. Zong Y, Li H, Li X, et al. Bacterial cellulose-based hydrogel thermocells for low-grade heat harvesting. Chem Eng J 2022;433:134550.
22. Guo M, Cui H, Guo W, et al. Achieving superior thermoelectric performance in Ge4Se3 Te via symmetry manipulation with I-V-VI2 alloying. Adv Funct Mater 2024;34:2313720.
23. Liu Z, Cheng H, He H, Li J, Ouyang J. Significant enhancement in the thermoelectric properties of ionogels through solid network engineering. Adv Funct Mater 2022;32:2109772.
24. Rehan M, Cho A, Jeong I, et al. Defect engineering in earth-abundant Cu2 ZnSnSe4 absorber using efficient alkali doping for flexible and tandem solar cell applications. Energy Environ Mater 2024;7:e12604.
25. Ming H, Luo ZZ, Chen Z, et al. Chemical pressure-driven band convergence and discordant atoms intensify phonon scattering leading to high thermoelectric performance in SnTe. J Am Chem Soc 2024;Online ahead of print.
26. Chen Z, Cui H, Hao S, et al. GaSb doping facilitates conduction band convergence and improves thermoelectric performance in n-type PbS. Energy Environ Sci 2023;16:1676-84.
27. Satoh N, Otsuka M, Kawakita J, Mori T. A hierarchical design for thermoelectric hybrid materials: Bi2Te3 particles covered by partial Au skins enhance thermoelectric performance in sticky thermoelectric materials. Soft Sci 2022;2:15.
28. He W, Wang D, Wu H, et al. High thermoelectric performance in low-cost SnS0.91Se0.09 crystals. Science 2019;365:1418-24.
29. Dupont MF, MacFarlane DR, Pringle JM. Thermo-electrochemical cells for waste heat harvesting - progress and perspectives. Chem Commun 2017;53:6288-302.
30. Yu B, Duan J, Cong H, et al. Thermosensitive crystallization-boosted liquid thermocells for low-grade heat harvesting. Science 2020;370:342-6.
31. Han Y, Zhang J, Hu R et al. High-thermopower polarized electrolytes enabled by methylcellulose for low-grade heat harvesting. Sci Adv 2022;8:eabl5318.
32. Lu X, Xie D, Zhu K, et al. Swift assembly of adaptive thermocell arrays for device-level healable and energy-autonomous motion sensors. Nanomicro Lett 2023;15:196.
33. Zhang D, Mao Y, Ye F, et al. Stretchable thermogalvanic hydrogel thermocell with record-high specific output power density enabled by ion-induced crystallization. Energy Environ Sci 2022;15:2974-82.
34. Li Q, Han C, Wang S, et al. Anionic entanglement-induced giant thermopower in ionic thermoelectric material Gelatin-CF3SO3K-
35. Shi X, Ma L, Li Y, et al. Double hydrogen-bonding reinforced high-performance supramolecular hydrogel thermocell for self-powered sensing remote-controlled by light. Adv Funct Mater 2023;33:2211720.
36. Liu C, Wang S, Feng SP, Fang NX. Portable green energy out of the blue: hydrogel-based energy conversion devices. Soft Sci 2023;3:10.
37. Li T, Zhang X, Lacey SD, et al. Cellulose ionic conductors with high differential thermal voltage for low-grade heat harvesting. Nat Mater 2019;18:608-13.
38. Han CG, Qian X, Li Q, et al. Giant thermopower of ionic gelatin near room temperature. Science 2020;368:1091-8.
39. Zhang J, Bai C, Wang Z, Liu X, Li X, Cui X. Low-grade thermal energy harvesting and self-powered sensing based on thermogalvanic hydrogels. Micromachines 2023;14:155.
40. Duan J, Feng G, Yu B, et al. Aqueous thermogalvanic cells with a high Seebeck coefficient for low-grade heat harvest. Nat Commun 2018;9:5146.
41. Lin Y, Hsu C, Hong S, et al. Highly conductive triple network hydrogel thermoelectrochemical cells with low-grade heat harvesting. J Power Sources 2024;609:234647.
42. Hu J, Wei J, Li J, Bai L, Liu Y, Li Z. Double selective ionic gel with excellent thermopower and ultra-high energy density for low-quality thermal energy harvesting. Energy Environ Sci 2024;17:1664-76.
43. Li Q, Yu D, Wang S, et al. High thermopower of agarose-based ionic thermoelectric Gel through micellization effect decoupling the cation/anion thermodiffusion. Adv Funct Mater 2023;33:2305835.
44. Zhang Z, Fu H, Li Z, et al. Hydrogel materials for sustainable water resources harvesting & treatment: synthesis, mechanism and applications. Chem Eng J 2022;439:135756.
45. Li L, Wu P, Yu F, Ma J. Double network hydrogels for energy/environmental applications: challenges and opportunities. J Mater Chem A 2022;10:9215-47.
46. Huang H, Dong Z, Ren X, et al. High-strength hydrogels: fabrication, reinforcement mechanisms, and applications. Nano Res 2023;16:3475-515.
47. Yan X, Huang H, Bakry AM, Wu W, Liu X, Liu F. Advances in enhancing the mechanical properties of biopolymer hydrogels via multi-strategic approaches. Int J Biol Macromol 2024;272:132583.
48. Wang Y, Xiang Y, Huang Q, et al. High-strength ionic hydrogel constructed by metal-free physical crosslinking strategy for enhanced uranium extraction from seawater. Chem Eng J 2024;479:147875.
49. Yang J, Chen Y, Zhao L, Zhang J, Luo H. Constructions and properties of physically cross-linked hydrogels based on natural polymers. Polym Rev 2023;63:574-612.
50. GhavamiNejad A, Ashammakhi N, Wu XY, Khademhosseini A. Crosslinking strategies for 3D bioprinting of polymeric hydrogels. Small 2020;16:e2002931.
51. Yuan Y, Shen S, Fan D. A physicochemical double cross-linked multifunctional hydrogel for dynamic burn wound healing: shape adaptability, injectable self-healing property and enhanced adhesion. Biomaterials 2021;276:120838.
52. Ettoumi F, Huang H, Xu Y, et al. Supramolecular assembly of dual crosslinked nanocomposite polysaccharides hydrogel: integration of injectable, self-healing, and pH-responsive platform for sustained delivery of polyphenols. Food Hydrocoll 2024;154:110108.
53. Li W, Wang X, Liu Z, et al. Nanoconfined polymerization limits crack propagation in hysteresis-free gels. Nat Mater 2024;23:131-8.
54. Zhan W, Zhang H, Lyu X, Luo Z, Yu Y, Zou Z. An ultra-tough and super-stretchable ionogel with multi functions towards flexible iontronics. Sci China Mater 2023;66:1539-50.
55. Gong Y, Yu L, Lyu X, et al. A mechanically robust, self-healing, and adhesive biomimetic camouflage ionic conductor for aquatic environments. Adv Funct Mater 2023;33:2305314.
56. Zhang D, Fang Y, Liu L, et al. Boosting thermoelectric performance of thermogalvanic hydrogels by structure engineering induced by liquid nitrogen quenching. Adv Energy Mater 2024;14:2303358.
57. Sang S, Bai C, Wang W, et al. Finger temperature-driven thermogalvainc gel-based smart pen: utilized for identity recognition, stroke analysis, and grip posture assessment. Nano Energy 2024;123:109366.
58. Cheng H, Le Q, Liu Z, Qian Q, Zhao Y, Ouyang J. Ionic thermoelectrics: principles, materials and applications. J Mater Chem C 2022;10:433-50.
59. Yu M, Li H, Li Y, et al. Ionic thermoelectric gels and devices: progress, opportunities, and challenges. EnergyChem 2024;6:100123.
60. Qian X, Ma Z, Huang Q, Jiang H, Yang R. Thermodynamics of ionic thermoelectrics for low-grade heat harvesting. ACS Energy Lett 2024;9:679-706.
61. Li Z, Jiang J, He X, Wang C, Niu Y. Recent progress on the thermoelectric effect for electrochemistry. J Mater Chem A 2024;12:13623-46.
62. Liu Y, Cui M, Ling W, et al. Thermo-electrochemical cells for heat to electricity conversion: from mechanisms, materials, strategies to applications. Energy Environ Sci 2022;15:3670-87.
63. Liu L, Zhang D, Bai P, et al. Strong tough thermogalvanic hydrogel thermocell with extraordinarily high thermoelectric performance. Adv Mater 2023;35:e2300696.
64. Wei S, Ma J, Wu D, et al. Constructing flexible film electrode with porous layered structure by MXene/SWCNTs/PANI ternary composite for efficient low-grade thermal energy harvest. Adv Funct Mater 2023;33:2209806.
65. Wang Y, Zhang Y, Xin X, et al. In situ photocatalytically enhanced thermogalvanic cells for electricity and hydrogen production. Science 2023;381:291-6.
66. Liu Y, Zhang Q, Odunmbaku GO, et al. Solvent effect on the Seebeck coefficient of Fe2+/Fe3+ hydrogel thermogalvanic cells†. J Mater Chem A 2022;10:19690-8.
67. Jia B, Wu D, Xie L, et al. Pseudo-nanostructure and trapped-hole release induce high thermoelectric performance in PbTe. Science 2024;384:81-6.
68. Liu H, Shi X, Pan L, et al. Rational triple optimizations boost near-room-temperature thermoelectric performance of BiSe. Acta Mater 2024;280:120343.
69. Liu Y, Jiang Q, Zhang J, et al. Green synthesis of air-stable tellurium nanowires via biomolecule-assisted hydrothermal for thermoelectrics†. Mater Adv 2020;1:1125-33.
70. Buckingham MA, Zhang S, Liu Y, Chen J, Marken F, Aldous L. Thermogalvanic and thermocapacitive behavior of superabsorbent hydrogels for combined low-temperature thermal energy conversion and harvesting. ACS Appl Energy Mater 2021;4:11204-14.
71. Yang X, Zhang Z, Khan SA, et al. Thermogalvanic organohydrogel-based non-contact self-powered electronics for advancing smart agriculture. J Mater Chem C 2024;12:3298-305.
72. Zhou H, Yamada T, Kimizuka N. Supramolecular thermo-electrochemical cells: enhanced thermoelectric performance by host-guest complexation and salt-induced crystallization. J Am Chem Soc 2016;138:10502-7.
73. Liu Y, Yin L, Chen S, et al. A hydrogel thermoelectrochemical cell with high self-healability and enhanced thermopower both induced by zwitterions. J Mater Chem A 2024;12:18582-92.
74. Hsu C, Lin Y, Hong S, et al. 3D printed gelatin methacrylate hydrogel-based wearable thermoelectric generators. Adv Sustain Syst 2024;8:2400039.
75. Xu T, Tao Y, Qian Y, et al. Semi-solid thermo-electrochemical cell based wearable power generator for body heat harvesting. Adv Funct Mater 2024;34:2316068.
76. Jia Y, Zhang S, Li J, et al. Coordination enhanced high-seebeck coefficient n-type gel-based thermocells for low-grade energy harvesting and n-p type connected devices. J Power Sources 2024;602:234400.
77. Shen X, Wu J, Hua Z, Liu G. p-n conversion of thermogalvanic cells by harnessing the micellization of thermoresponsive diblock copolymers. ACS Appl Energy Mater 2023;6:10147-54.
78. Peng P, Zhou J, Liang L, et al. Regulating thermogalvanic effect and mechanical robustness via redox ions for flexible quasi-solid-state thermocells. Nano Micro Lett 2022;14:81.
79. Kim T, Lee JS, Lee G, et al. High thermopower of ferri/ferrocyanide redox couple in organic-water solutions. Nano Energy 2017;31:160-7.
81. Han C, Zhu Y, Yang L, et al. Remarkable high-temperature ionic thermoelectric performance induced by graphene in gel thermocells. Energy Environ Sci 2024;17:1559-69.
82. Zhu Y, Han C, Chen J, et al. Ultra-high performance of ionic thermoelectric-electrochemical gel cells for harvesting low grade heat. Energy Environ Sci 2024;17:4104-14.
83. Kong S, Huang Z, Hu Y, et al. Tellurium-nanowire-doped thermoelectric hydrogel with high stretchability and seebeck coefficient for low-grade heat energy harvesting. Nano Energy 2023;115:108708.
84. Liu Y, Chen X, Dong X, Liu A, Ouyang K, Huang Y. Recurrently gellable and thermochromic inorganic hydrogel thermogalvanic cells. Sci Adv 2024;10:eadp4533.
85. Chen J, Shi C, Wu L, et al. Environmentally tolerant ionic hydrogel with high power density for low-grade heat harvesting. ACS Appl Mater Interfaces 2022;14:34714-21.
86. Chen B, Chen Q, Xiao S, Feng J, Zhang X, Wang T. Giant negative thermopower of ionic hydrogel by synergistic coordination and hydration interactions. Sci Adv 2021;7:eabi7233.
87. Li N, Liu W, Zheng X, et al. Antimicrobial hydrogel with multiple pH-responsiveness for infected burn wound healing. Nano Res 2023;16:11139-48.
88. Wang L, Chen P, Pan Y, et al. Injectable photocurable Janus hydrogel delivering hiPSC cardiomyocyte-derived exosome for post-heart surgery adhesion reduction. Sci Adv 2023;9:eadh1753.
89. Li X, Xiao X, Bai C, et al. Thermogalvanic hydrogels for self-powered temperature monitoring in extreme environments. J Mater Chem C 2022;10:13789-96.
90. Quickenden TI, Mua Y. A review of power generation in aqueous thermogalvanic cells. J Electrochem Soc 1995;142:3985-94.
92. Yang P, Liu K, Chen Q, et al. Wearable thermocells based on gel electrolytes for the utilization of body heat. Angew Chem Int Ed Engl 2016;55:12050-3.
93. Jin L, Greene GW, Macfarlane DR, Pringle JM. Redox-active quasi-solid-state electrolytes for thermal energy harvesting. ACS Energy Lett 2016;1:654-8.
94. Yu B, Duan J, Li J, et al. All-day thermogalvanic cells for environmental thermal energy harvesting. Research 2019;2019:2460953.
95. Meng FL, Gao M, Ding T, Yilmaz G, Ong WL, Ho GW. Modular deformable steam electricity cogeneration system with photothermal, water, and electrochemical tunable multilayers. Adv Funct Mater 2020;30:2002867.
96. Schönig M, Schuster R. Sensitive and fast measurement of surface temperature with a thermogalvanic cell. Appl Phys Lett 2020;116:091601.
97. Inoue D, Niwa H, Nitani H, Moritomo Y. Scaling relation between electrochemical seebeck coefficient for Fe2+/Fe3+ in organic solvent and its viscosity. J Phys Soc Jpn 2021;90:033602.
98. Fang R, Li X, Khan S A et al. Anhydrous thermogalvanic Gel for simultaneous waste heat recovery and thermal management of electronics. ACS Appl Polym Mater 2023;5:4628-35.
99. Jiao N, Abraham TJ, Macfarlane DR, Pringle JM. Ionic liquid electrolytes for thermal energy harvesting using a cobalt redox couple. J Electrochem Soc 2014;161:D3061-5.
100. Lazar MA, Al-Masri D, MacFarlane DR, Pringle JM. Enhanced thermal energy harvesting performance of a cobalt redox couple in ionic liquid-solvent mixtures. Phys Chem Chem Phys 2016;18:1404-10.
101. He J, Al-Masri D, MacFarlane DR, Pringle JM. Temperature dependence of the electrode potential of a cobalt-based redox couple in ionic liquid electrolytes for thermal energy harvesting. Faraday Discuss 2016;190:205-18.
102. Li J, Chen S, Han Z, et al. High performance bacterial cellulose organogel-based thermoelectrochemical cells by organic solvent-driven crystallization for body heat harvest and self-powered wearable strain sensors. Adv Funct Mater 2023;33:2306509.
103. Liang Y, Ka-ho Hui J, Morikawa M, Inoue H, Yamada T, Kimizuka N. High positive seebeck coefficient of aqueous i-/i3- thermocells based on host-guest interactions and LCST behavior of PEGylated α-Cyclodextrin. ACS Appl Energy Mater 2021;4:5326-31.
104. Artyukhov D, Kiselev N, Gorshkov N, et al. Harvesting waste thermal energy using a surface-modified carbon fiber-based thermo-electrochemical cell. Sustainability 2021;13:1377.
105. Kang TJ, Fang S, Kozlov ME, et al. Electrical power from nanotube and graphene electrochemical thermal energy harvesters. Adv Funct Mater 2012;22:477-89.
106. Abraham TJ, Tachikawa N, MacFarlane DR, Pringle JM. Investigation of the kinetic and mass transport limitations in thermoelectrochemical cells with different electrode materials. Phys Chem Chem Phys 2014;16:2527-32.
107. Laux E, Uhl S, Journot T, Brossard J, Jeandupeux L, Keppner H. Aspects of protonic ionic liquid as electrolyte in thermoelectric generators. J Electron Mater 2016;45:3383-9.
108. Im H, Kim T, Song H, et al. High-efficiency electrochemical thermal energy harvester using carbon nanotube aerogel sheet electrodes. Nat Commun 2016;7:10600.
109. Tian C, Bai C, Wang T, et al. Thermogalvanic hydrogel electrolyte for harvesting biothermal energy enabled by a novel redox couple of SO4/32- ions. Nano Energy 2023;106:108077.
110. Li J, Wang Z, Khan SA, Li N, Huang Z, Zhang H. Self-powered information conversion based on thermogalvanic hydrogel with interpenetrating networks for nursing aphasic patients. Nano Energy 2023;113:108612.
111. Li J, Xu T, Ma Z, et al. Self-healable and stretchable PAAc/XG/Bi2Se0.3Te2.7 hybrid hydrogel thermoelectric materials. Energy Environ Mater 2024;7:e12547.
112. Fu M, Wu Z, Liu X, Yuan Y, Lai X, Yue K. Highly stretchable ionic hydrogels with enhanced thermoelectric performance and flame retardancy for intelligent fire protection. J Mater Chem A 2024;12:27588-97.
113. Hu Q, Li H, Chen X, et al. Strong tough ionic organohydrogels with negative‐thermopower via the synergy of coordination interaction and hofmeister effect. Adv Funct Mater 2024;34:2406968.
114. Lyu X, Lin Z, Huang C, et al. Tough and elastic hydrogel thermocells for heat energy utilization. Chem Eng J 2024;493:152887.
115. Zhang L, Shi X, Yang Y, Chen Z. Flexible thermoelectric materials and devices: From materials to applications. Mater Today 2021;46:62-108.
116. Yang M, Hu Y, Wang X, et al. Chaotropic effect-boosted thermogalvanic ionogel thermocells for all-weather power generation. Adv Mater 2024;36:e2312249.
117. Ma X, Wang W, Cui X, et al. Machine learning assisted self-powered identity recognition based on thermogalvanic hydrogel for intelligent security. Small 2024;20:e2402700.
118. Lu X, Mo Z, Liu Z, et al. Robust, efficient, and recoverable thermocells with zwitterion-boosted hydrogel electrolytes for energy-autonomous and wearable sensing. Angew Chem Int Ed Engl 2024;63:e202405357.
119. Zhao J, Wu X, Yu H, et al. Regenerable aerogel‐based thermogalvanic cells for efficient low-grade heat harvesting from solar radiation and interfacial solar evaporation systems. EcoMat 2023;5:e12302.
120. Liang L, Lv H, Shi XL, et al. A flexible quasi-solid-state thermoelectrochemical cell with high stretchability as an energy-autonomous strain sensor. Mater Horiz 2021;8:2750-60.
121. Han Y, Wei H, Du Y, et al. Ultrasensitive flexible thermal sensor arrays based on high-thermopower ionic thermoelectric hydrogel. Adv Sci 2023;10:e2302685.
122. Fu M, Sun Z, Liu X, et al. Highly stretchable, resilient, adhesive, and self‐healing ionic hydrogels for thermoelectric application. Adv Funct Mater 2023;33:2306086.
123. Wang Z, Xue R, Zhang H, et al. A hydrogel electrolyte toward a flexible zinc-ion battery and multifunctional health monitoring electronics. ACS Nano 2024;18:7596-609.
124. Wu G, Xue Y, Wang L, Wang X, Chen G. Flexible gel-state thermoelectrochemical materials with excellent mechanical and thermoelectric performances based on incorporating Sn2+/Sn4+ electrolyte into polymer/carbon nanotube composites†. J Mater Chem A 2018;6:3376-80.
125. Zhao Q, Liu J, Wu Z, et al. Robust PEDOT:PSS-based hydrogel for highly efficient interfacial solar water purification. Chem Eng J 2022;442:136284.
126. Xu X, Liu Q, Qiu J, et al. Photothermal-photocatalytic bifunctional highly porous hydrogel for efficient coherent sewage purification-clean water generation. Desalination 2025;597:118364.
127. Pu S, Liao Y, Chen K, et al. Thermogalvanic hydrogel for synchronous evaporative cooling and low-grade heat energy harvesting. Nano Lett 2020;20:3791-7.
128. Fu X, Zhuang Z, Zhao Y, et al. Stretchable and self-powered temperature-pressure dual sensing ionic skins based on thermogalvanic hydrogels. ACS Appl Mater Interfaces 2022;14:44792-8.
129. Tian Y, Yang X, Li K, et al. High-performance ionic thermoelectric materials and emerging applications of ionic thermoelectric devices. Mater Today Energy 2023;36:101342.
130. Sun W, Zhang P, Lin X, et al. Heat source recognition sensor mimicking the thermosensation function of human skin. The Innovation 2024;5:100673.
131. Wang Z, Li N, Yang X, Zhang Z, Zhang H, Cui X. Thermogalvanic hydrogel-based e-skin for self-powered on-body dual-modal temperature and strain sensing. Microsyst Nanoeng 2024;10:55.
132. Wu X, Gao N, Zheng X, et al. Self-powered and green ionic-type thermoelectric paper chips for early fire alarming. ACS Appl Mater Interfaces 2020;12:27691-9.
133. Zhang Y, Wang H, Ahmed Khan S, et al. Deep-learning-assisted thermogalvanic hydrogel fiber sensor for self-powered in-nostril respiratory monitoring. J Colloid Interface Sci 2025;678:143-9.
134. Chen L, Lou J, Rong X, et al. Super-stretching and high-performance ionic thermoelectric hydrogels based on carboxylated bacterial cellulose coordination for self-powered sensors. Carbohydr Polym 2023;321:121310.
135. Tian C, Khan SA, Zhang Z, Cui X, Zhang H. Thermoelectric hydrogel electronic skin for passive multimodal physiological perception. ACS Sens 2024;9:840-8.
136. Yang H, Ahmed Khan S, Li N, Fang R, Huang Z, Zhang H. Thermogalvanic gel patch for self-powered human motion recognition enabled by photo-thermal-electric conversion. Chem Eng J 2023;473:145247.
137. Zhang Y, Li S, Zhang J, et al. Thermoelectrocatalysis: an emerging strategy for converting waste heat into chemical energy. Natl Sci Rev 2024;11:nwae036.
138. Qi Y, Zhang J, Kong Y, et al. Unraveling of cocatalysts photodeposited selectively on facets of BiVO4 to boost solar water splitting. Nat Commun 2022;13:484.
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Liu, W.; Fang, Y.; Lyu, X.; Peng, X.; Luo, Z. Z.; Zou, Z. Construction and application of thermogalvanic hydrogels. Soft Sci. 2024, 4, 44. http://dx.doi.org/10.20517/ss.2024.59
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