Thermogalvanic hydrogels for low-grade heat harvesting and health monitoring

Abstract

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.

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Wider impact

Thermogalvanic hydrogels hold considerable promise for thermoelectric power generation and self-powered wearable electronics owing to their high thermopowers, exceptional intrinsic flexibilities, and cost-effective fabrication methods. These materials have shown potential in flexible power sources, self-powered electronics, and smart health monitoring systems. This review discusses the recent advancements in thermogalvanic hydrogels for low-grade heat harvesting and wearable applications, focusing on strategies to enhance their thermoelectric performances, optimize the mechanical properties, and expand their operational temperature range. Furthermore, we explore their potential for integration into everyday applications. With advances in material synthesis and electronic processing technologies, thermogalvanic hydrogels are poised to play increasingly important roles in sustainable green energy and flexible self-powered electronics, with broad implications for scientific research and industrial applications.

1. Introduction

Globally, there is an abundant supply of low-grade thermal energy derived from various sources such as oceans, industrial waste, solar energy, geothermal energy, human body, and vehicle exhaust. Notably, over 50% of total energy derived from traditional fossil fuels is wasted in the air in the form of waste heat.^1,2 Therefore, recycling and reusing this waste heat can significantly alleviate the energy crisis and reduce the associated environmental damages.^3–5 Thermoelectric (TE) conversion technologies have gained substantial attention in waste heat recycling owing to their ability to directly convert heat into electricity, high efficiency and reliability, and excellent environmental friendliness.^6–10 TE materials are vital for effective heat-to-electricity conversion, and the design of TE devices is essential for their practical applications.^11,12 Traditional electron-conducting semiconducting TE materials rely on the Seebeck effect to convert heat into electricity, providing continuous electricity output under a stable temperature difference.^13–16 However, these materials generally have low thermopower, with a Seebeck coefficient (S_c) of less than 200 μV K⁻¹, along with challenges such as complex fabrication processes and high costs.^17–21 More importantly, the brittleness and limited flexibility of traditional semiconducting TE materials hinder their adaptation for flexible, multifunctional, and wearable applications.^17,22,23

In recent years, ion-conducting TE materials based on the Soret effect and thermogalvanic effect have emerged as promising alternatives for efficient thermoelectric conversion.^24–33 These materials offer high thermopower (several to dozens of mV K⁻¹), low cost, and easy fabrication process, making them highly significant for efficient thermoelectric conversion and flexible wearable electronics. Ionic TE materials based on the Soret effect (also known as the thermodiffusion effect) can achieve thermoelectric conversion through entropy-driven and directional ion migration induced by a temperature gradient. These ionic TE materials have demonstrated an ultrahigh thermopower exceeding 50 mV K⁻¹.^34–36 However, under a stable temperature gradient, ion migration in TE materials and electron migration in the external circuit gradually reach equilibrium, limiting the movement of carriers and inhibiting consistent electricity generation.^{31,32,37–39} In contrast, ionic TE materials based on the thermogalvanic effect can generate electricity through temperature-dependent entropy changes during charge migration between electrodes and redox pairs.^40–48 Though the thermopower of ionic TE materials based on the thermogalvanic effect is generally lower than that of the Soret effect, they can continuously generate electricity under stable temperature gradients. Moreover, these materials exhibit similar macro characteristics comparable to traditional p-/n-type semiconducting materials, making them highly promising for practical applications in daily life.^49–52

Traditional semiconducting TE materials face significant performance limitations due to the interdependence of thermopower, electrical conductivity, and thermal conductivity.^53–56 Aqueous thermogalvanic cells offer a cost-effective solution to decouple these factors and efficiently harvest low-grade waste heat.^57–60 For example, researchers achieved a high thermopower of 4.2 mV K⁻¹ in aqueous thermochemical cells by using specific ligands to modulate the solvation structure of K₃Fe(CN)₆/K₄Fe(CN)₆ redox pairs.⁶¹ Additionally, a eutectic solvent strategy was used to design high-performance aqueous thermogalvanic cells with excellent anti-freezing properties,⁶² achieving a high power density of 17.5 W m⁻² and a large energy density of 27 kJ m⁻² over two hours. While these advancements contribute to more efficient recovery and reuse of waste thermal energy, liquid thermogalvanic cells still face challenges such as poor mechanical flexibility and electrolyte leakage risks,^63–68 which hinder their broader application.

Hydrogels, with their three-dimensional crosslinked network and high water content, offer exceptional mechanical properties, including high tensile strength, toughness, flexibility, and stretchability.^69–76 These characteristics make hydrogels highly promising for ionic thermoelectric conversion, effectively addressing challenges such as electrolyte leakage and complex encapsulation, and enabling the development of flexible and adaptable devices. In recent years, thermogalvanic hydrogels advanced rapidly, with various optimization strategies employed to enhance their thermoelectric, mechanical, and thermal stability properties. As shown in Scheme 1, since the discovery of the thermogalvanic effect in 1885,⁷⁷ until 2016, Prof. Zhou's research group first introduced the thermogalvanic effect into quasi-solid-state hydrogels,⁷⁸ then thermogalvanic hydrogels have gradually gained favor among researchers due to their ability to prevent liquid leakage. Broadly speaking, any hydrogels exhibiting the thermogalvanic effect are considered thermogalvanic hydrogels. Over nearly a decade of development, thermogalvanic hydrogels have evolved from the initial thermogalvanic gels with simple function and structure into a series of multifunctional thermogalvanic gels, including synergetic thermogalvanic–thermodiffusion gels with ultrahigh thermopower,²⁶ anti-freezing thermogalvanic gels,⁷⁹ thermosensitive thermogalvanic gels with enhanced thermoelectric performance by thermosensitive crystallization,⁸⁰ p–n convertible thermogalvanic gels,⁵¹ and finally, highly fatigue-resistant and strong-tough thermogalvanic gels.⁸¹ These thermogalvanic hydrogels have seen rapid advancements in performance optimization, allowing for their design and development for various application fields. As shown in Fig. 1, these improvements highlight the significant potential of thermogalvanic hydrogels in low-grade thermal energy harvesting, self-powered electronics, and wearable health monitoring and sensing.^82–87

Scheme 1 Timeline of the development of thermogalvanic hydrogels.

Fig. 1 Potential applications of thermogalvanic hydrogels in low-grade heat harvesting, wearable electronics, and self-powered health monitoring systems.^82–87 Reproduced with permission.⁸² Copyright 2022, Wiley-VCH. Reproduced with permission.⁸³ Copyright 2020, Wiley-VCH. Reproduced with permission.⁸⁴ Copyright 2022, Springer Nature. Reproduced with permission.⁸⁵ Copyright 2024, the American Chemical Society. Reproduced with permission.⁸⁶ Copyright 2023, Springer Nature. Reproduced with permission.⁸⁷ Copyright 2021, Elsevier.

This review concludes recent advancements in thermogalvanic hydrogels for low-grade heat harvesting and health monitoring. It aims to provide a comprehensive overview of the optimization of thermoelectric properties and the development of self-powered, wearable applications. We systematically introduce the working mechanisms and thermoelectric evaluation criterion of thermogalvanic hydrogels. This review focuses on the design strategies for performance improvement, including enhancement in thermoelectric properties, mechanical strength, and operational temperature range. We also explore their applications in efficient low-grade heat harvesting for powering electronic devices, self-powered systems, and health monitoring. Finally, we address the challenges, opportunities, and prospects of thermogalvanic hydrogels, highlighting their significant potential in flexible energy supply and wearable applications.

2. Working mechanisms of thermogalvanic hydrogels

Fig. 2 illustrates the working mechanism and electrochemical/electrode potentials of thermogalvanic hydrogels with different thermoelectric properties, involving two main processes: redox reactions at the electrodes and mass transport within the electrolyte.^45,61 A typical thermogalvanic cell includes two identical electrodes and an electrolyte with redox ion pairs. The mechanism varies slightly depending on the electrolytes used, such as Fe(CN)₆^3−/4− and Fe^2+/3+ ion pairs.^79,88–91 This section focuses on the p-type thermogalvanic hydrogels with the Fe(CN)₆^3−/4− redox pair. When a temperature gradient is applied, redox reactions at the electrode–electrolyte interface create a potential difference (Fig. 2a). At the hot electrode, oxidation occurs: Fe(CN)₆⁴⁻ − e⁻ → Fe(CN)₆³⁻, releasing electrons that increase the electrochemical potential of the hot electrode (μ_h) and lower its electrode potential (E⁰(T_h)). The electrons then move to the cold electrode, where reduction happens: Fe(CN)₆³⁻ + e⁻ → Fe(CN)₆⁴⁻, attracting electrons and increasing its electrochemical potential (μ_c). This allows continuous redox reactions and generates a steady voltage. As shown in Fig. 2b, using an Fe^2+/3+ ion pair, n-type thermogalvanic hydrogels show opposite thermoelectric behaviors.^92–95

Fig. 2 Schematic of the mechanism of p-type (a) and n-type (b) thermogalvanic hydrogels.

Similar to traditional TE materials, thermogalvanic hydrogels have some performance evaluation parameters, such as power factor (PF) and the dimensionless figure of merit (ZT). These parameters can be calculated using eqn (1) and (2):^27,80

PF = S_c²σ (1)

ZT = S_c²σT/κ (2)

where S_c, σ, T, and κ represent the thermopower, electrical conductivity, absolute temperature, and thermal conductivity, respectively. Since the entire thermogalvanic cell operates under a temperature gradient, T_h and T_c represent the temperatures of the hot side and cold side, respectively. Therefore, the figure of merit (ZT) is usually expressed as an average value, as shown in eqn (3):

ZT_average = S_c²σ(T_c + T_h)/2κ (3)

The specific output power density (P_max/ΔT²) is an important metric for energy conversion performance. Within a certain temperature range, it is independent of ΔT, making it a key parameter for evaluating the output properties of thermogalvanic hydrogels. The specific output power density can be calculated using eqn (4):

P_max/ΔT² = V_ocI_sc/4ΔT² (4)

where V_oc and I_sc represent the open-circuit voltage and short-circuit current, respectively. The energy conversion efficiency (η) is another important parameter for evaluating the TE performance, which is defined as the ratio of the output power density (P_max) to the input power density (P_heat), as shown in eqn (5):^27,80

η = P_max/P_heat = P_max·d/k·ΔT (5)

The Carnot-relative efficiency (η_r) as a performance criterion of thermogalvanic hydrogels can be calculated using eqn (6):^27,96

η_r = η/(ΔT/T_hot) = P_max·d·T_hot/k·ΔT² (6)

where d is the electrode separation distance and κ is the effective thermal conductivity of the thermogalvanic hydrogel device.

3. Performance optimization of thermogalvanic hydrogels

The thermogalvanic effect, observed in redox pairs, originates from entropy changes during charge migration between the electrode and redox pair with temperature variations. The key electrolyte systems of thermogalvanic hydrogels such as typical p-type redox pairs including Fe(CN₆)^3−/4− ions and n-type redox ion pairs (e.g., I⁻/I₃⁻, Fe²⁺/Fe³⁺) all exhibit relatively large entropy changes in solvents.^{50,57,79,80,97} The TE performance of thermogalvanic hydrogels depends on the thermopower, effective electrical conductivity (σ_eff), and effective thermal conductivity (k_eff). Affected by mass transfer and chemical kinetics, the optimization effect of σ_eff and k_eff is limited and not obvious. By contrast, the thermopower can be effectively regulated and optimized by introducing ions with different properties, regulating the structure of the hydrogel network, and optimizing polymer–solvent–redox pair interactions.^98–102 Besides excellent thermoelectric properties, good mechanical properties and thermal stability are important for thermogalvanic hydrogels to perform efficient low-grade heat harvesting and self-powered wearable applications.

The polymer matrix not only provides a medium for ion migration but also enhances mechanical strength, toughness, and durability through rational crosslinking strategies such as double-network structures, dynamic covalent bonds, and hydrogen bonding, making the material more adaptable to complex wearable environments. Nanofillers including carbon nanotubes, graphenes, and metal nanoparticles can effectively improve the electrical conductivity, optimize the electrode–electrolyte interface, and facilitate the charge transport, thereby enhancing thermoelectric performance. Furthermore, the combination of molecular and structural engineering enables precise modifications of polymer chains, such as the introduction of highly crystalline regions and anisotropic networks, further improving the ionic conductivity and thermoelectric stability while ensuring long-term flexibility and durability. Collectively, these material optimization strategies work synergistically to significantly enhance the mechanical robustness and long-term usability of thermogalvanic hydrogels, providing crucial support for efficient low-grade heat energy harvesting and self-powered electronic devices. In the following sections, we will perform a detailed analysis from the perspective of performance enhancement strategies, revealing the intrinsic relationship between material design and application requirements.

3.1. Enhancement of thermoelectric performance

As one of the most important TE parameters, the thermopower of thermogalvanic hydrogels primarily depends on the reaction entropy of redox ion pairs, which is influenced by factors such as structural changes, solvation shells, and solvent interactions.^27,46,61,98 To enhance understanding, we provide an explanation of the term “chaotropic”. It refers to substances or effects that disrupt the ordered structure of a solvent, particularly water, by interfering with its hydrogen bond network. Chaotropic agents modulate the solution environment in multiple ways, significantly impacting molecular solubility, structural stability, and chemical reactivity. This principle has been ingeniously applied to thermoelectric materials, where optimizing ionic solvation shells and phase behavior has led to groundbreaking improvements in thermoelectric performance. For instance, chaotropic agents can form strong hydrogen bonds with water, enabling organic hydrogel thermocells to retain entropy elasticity even at subzero temperatures, thereby preventing ice formation. Given this, Gao et al. reported a high-performance thermogalvanic hydrogel using acrylamide (AM) as the monomer and ethylene glycol/water (EG/H₂O) as the binary solvent,⁷⁹ as shown in Fig. 3. As a chaotropic organic cosolvent, EG can efficiently destroy the strong hydrogen bonds among water molecules,^103,104 which not only enhance entropic elasticity and freezing resistance but also increase the entropy difference of the redox ions and improve the thermopower (Fig. 3a and b). Subsequently, to further improve the TE properties, a chaotropic comonomer of methyl chloride quaternized N,N-dimethylaminoethylacrylate (DMAEA-Q) was introduced into the precursor, which greatly improves the entropy difference of the Fe^2+/3+ ions and the thermopower of thermogalvanic hydrogels by increasing the solvation entropy difference, corresponding to a shift of absorbance band of Fe³⁺ ions from 225 nm to 232 nm (Fig. 3c), indicating a stronger ionic association and increased entropy difference. The optimized thermogalvanic hydrogels exhibit a high thermopower of 2.02 mV K⁻¹ and excellent anti-freezing properties. Similarly, by introducing 2-acrylamide-2-methylpropane sulfonic acid (AMPS) into a polyacrylamide (PAAm) thermogalvanic hydrogel, the strong interaction between AMPS and Fe(CN)₆⁴⁻ ions can increase the entropy difference of Fe(CN)₆^4−/3− redox pairs¹⁰² and increase the thermopower to 1.6 mV K⁻¹ and the specific output power density to 0.61 mW m⁻² K⁻².

Fig. 3 (a) Photograph of a soft and stretchable thermogalvanic hydrogel based on Fe^2+/3+ redox ion pairs. (b) Schematic of the anti-freezing mechanism. (c) UV-Vis spectrum of a mixture of DMAEA-Q and FeCl₃ in a binary solvent system of EG and water. Reproduced with permission.⁷⁹ Copyright 2021, Wiley-VCH. (d) Thermopower variation of thermogalvanic hydrogels (Fe^2+/3+) based on 6 mol% organic solvent additives with different DNs. (e) First solvation shell of Fe³⁺ in the electrolyte after adding TLS. (f) Dependence of thermopower on TLS concentration. Reproduced with permission.⁴⁶ Copyright 2022, the Royal Society of Chemistry. (g) Working mechanism diagram of the enhanced thermogalvanic effect induced by guanidine ions. (h) Photograph of the stretchable thermogalvanic hydrogel, showing excellent conductive property. (i) Variation in the thermopower and conductivity of thermogalvanic hydrogels (0.3 mol L⁻¹ Fe(CN₆)^3−/4−) with 0.5–4 mol L⁻¹ CH₆ClN₃. Reproduced with permission.⁸⁰ Copyright 2022, the Royal Society of Chemistry.

As mentioned earlier, the addition of EG into water solvent can improve the thermopower of thermogalvanic hydrogels with Fe²⁺/Fe³⁺ as redox ion pairs. Therefore, as we can imagine, different organic solvents with different properties may have different effects on TE properties. To explore the solvent effect, the researchers measured the TE properties of Fe²⁺/Fe³⁺ hydrogel electrolytes with different organic solvents that have different Gutmann donor numbers (DNs).⁴⁶ The results indicated that there is a strong negative correlation between solvent DN and thermopower due to the entropy change caused by the structure changes of the solvation shell (Fig. 3d and e). The tetramethylene sulfone (TLS) has the greatest effect on the thermopower improvement of thermogalvanic hydrogel based on Fe²⁺/Fe³⁺ ions, and the optimal thermopower is up to 2.49 mV K⁻¹ (Fig. 3f). TLS has a low Gutmann donor number (DN = 14.8), indicating a weak interaction with Fe³⁺. In the TLS–H₂O system, this weak coordination results in water molecules around Fe³⁺ being more tightly bound, thereby reducing the first solvation shell radius of Fe³⁺. In contrast, the first solvation shell radius of Fe²⁺ remains relatively unaffected due to its lower charge density. Consequently, the disparity in solvation shell radii between Fe²⁺ and Fe³⁺ increases. Since thermopower is governed by the entropy difference (ΔS) of the redox couple, defined as (S_c = ΔS/nF), and the entropy of an ion is inversely related to the solvation shell size, the reduced solvation shell of Fe³⁺ leads to an increased ΔS for the Fe²⁺/Fe³⁺ couple, thus boosting the thermopower to an optimal 2.49 mV K⁻¹. The thermoelectric optimization strategy in this work by regulating the solvent DN of organic solvents can be extended to other thermogalvanic electrolyte systems for efficient thermal energy harvest and utilization.

In recent years, the strategy of inducing redox ion pairs to form thermosensitive crystals, regulate the solvation shell, and expand the entropy difference of the redox pair has been extensively studied to enhance the thermoelectric properties.^{59,96,105–110} In 2022, Zhang et al. first introduced Gdm⁺ ions (from guanidine hydrochloride, CH₆ClN₃) into a thermogalvanic hydrogel for efficient low-grade heat energy harvest.⁸⁰ According to the Hofmeister series, Fe(CN)₆^4−/3− ions belong to the class of chaotropic anions, and some chaotropic cations such as guanidine ions (Gdm⁺) can rearrange the solvation shell of the redox pair and further improve the reaction entropy of the redox ion pair. Compared with Fe(CN)₆³⁻ ions, there is a stronger chaotrope–chaotrope interaction between Fe(CN)₆⁴⁻ and Gdm⁺ ions that can induce Fe(CN)₆⁴⁻ to form the thermosensitive crystal. As shown in Fig. 3g, during the process of heat-to-electricity conversion, the produced thermosensitive crystal occurs on the cold side and then dissolves on the hot side, increasing the reaction entropy difference of the Fe(CN)₆^3−/4− ion system, as well as enlarging the concentration differences of the redox ion pair between hot and cold sides. This can efficiently accelerate the reversible redox reaction and improve the thermopower. The optimized thermogalvanic hydrogels exhibit excellent mechanical and conductive properties, with high thermopower and conductivity of 4.4 mV K⁻¹ and 10.5 S m⁻¹, respectively (Fig. 3h and i). To further improve the mechanical and electrical cycling stability, the fabricated thermogalvanic hydrogels maintain excellent mechanical and TE stability with a high thermopower retention of over 90% after 1000 cycles at 200% strain. As a wearable electronic device, the designed TE array device can deliver a stable voltage of ∼0.4 V by harvesting human body heat. Under a relatively large ΔT of ∼40 K, five green LEDs can be directly lighted without any amplification system, showing huge application potential for self-powered wearable devices by human body heat harvesting.

The redox reactions and the thermopower of redox ions can be greatly improved by introducing special chaotropic ions and inducing redox ions to form thermosensitive crystals, whereas for quasi-solid thermogalvanic hydrogels, the thermosensitive crystals will precipitate on the surface of the hydrogel, and it seems that most crystals do not contribute to the improvement of TE performance. To address this issue, Zhang and collaborators further regulated the structure of thermogalvanic hydrogel threads by liquid nitrogen quenching (LNQ) for ultrafast cooling.¹¹¹ Specifically, the soaked thermogalvanic hydrogels with high temperature were directly frozen rapidly by LNQ, and the ultrafast cooling could effectively inhibit the diffusion of Fe(CN)₆^3−/4− and Gdm⁺ ions, allowing them to remain in their original positions. When it returns to room temperature, part of the thermosensitive crystal is trapped in the 3D network of the hydrogel, and only a small part of the crystal with small size precipitates on the surface of the thermogalvanic hydrogel. Through the LNQ process, the precipitation and growth of the thermosensitive crystals are efficiently inhibited, and the surface and interior of the thermogalvanic hydrogel are relatively smooth and homogeneous. Optimized by structure and composition with LNQ, the specific output power density (P_max/(ΔT)²) of the optimal thermogalvanic hydrogel threads is more than 40% higher than that of the thermogalvanic hydrogels prepared at room temperature.

In general, the same redox ion pair has a specific thermoelectric performance, and it is usually necessary to connect two redox ion systems with different thermoelectric characteristics (p- or n-type) in series in practical applications,^49,52,82 which is similar to traditional semiconductor arrays with π-type structures. However, it is not easy to find two opposite thermogalvanic systems with different TE characteristics, which brings a certain trouble to the array design of TE devices. For this purpose, based on I⁻/I₃⁻ redox pairs, Shen et al. developed a phase transition-enhanced hydrogel ionic thermoelectric cell (i-TEC) with excellent p-/n-type conversion characteristics.⁵¹ As shown in Fig. 4a, the hydrogel is copolymerized from methacrylic acid (MAA) and 3-dimethyl(methacryloxyethyl)ammonium propyl sulfonate (DMAPS), exhibiting superior thermosensitive behaviors. Through regulating the mass ratios (mMAA:mDMAPS) of monomers, the prepared hydrogels show different low and high critical solution temperatures (Fig. 4b). When the hot side temperature (T_h) exceeds the phase transition temperature (T_p), the hot side of n-type i-TEC exhibits obvious hydrophobic characteristics and attracts I₃⁻ ions, and the cold side with hydrophilic characteristics will repel I₃⁻ ions. For p-type i-TEC, the opposite is true. The hydrophilic hot side of p-type i-TEC repels I₃⁻ ions, while the hydrophobic cold side attracts I₃⁻ ions. This causes a large concentration difference of redox ion pairs on the cold and hot sides, thus increasing the thermopower. As shown in Fig. 4c and d, the optimized n-type and p-type i-TECs have relatively high thermopowers of 7.0 mV K⁻¹ and −6.3 mV K⁻¹, respectively (ΔT ∼ 15 K). When 10 pairs of p/n-type i-TECs are connected in series with the Z-shape structure, a high voltage of 1.8 V and a large output power of 85 μW can be achieved under ΔT ∼ 35 K. This breakthrough surpasses previous thermogalvanic hydrogels based on I₃⁻/I⁻ systems, showing great promise for low-grade heat harvesting applications.

Fig. 4 (a) Ionic thermoelectric cell (i-TEC) based on thermogalvanic hydrogels with enhanced thermoelectric performance via phase transition. The enlarged diagram illustrates the mechanisms of the n-type and p-type i-TECs. (b) Photographs of phase change hydrogel i-TEC at different temperatures. (c) Thermopower optimization via phase transition in n-type i-TEC. (d) Phase transition-induced p–n-type conversion of i-TECs. Reproduced with permission.⁵¹ Copyright 2024, Springer Nature.

Except for thermopower, electrical conductivity and thermal conductivity are also important parameters for thermoelectric materials (see eqn (2)). Since thermopower, electrical conductivity, and thermal conductivity are interdependent. The equivalent electrical conductivity of thermogalvanic cells depends on multiple factors such as ion concentration, diffusion coefficient, and charge transfer resistance at the electrode/electrolyte interface.^{63,68,96,112,113} Therefore, improving the electrical conductivity from the electrolyte perspective often adversely affects the other parameters. As an important part of thermoelectric devices, the utilization and selection of electrodes directly affect the reaction of redox ions and charge transfer between electrodes and hydrogel electrolytes.^{62,63,68,96,112–115} Nowadays, improving thermoelectric properties by electrode regulation has gained considerable attention and achieved huge progress.

As we know, noble metals have been widely used as electrode materials due to their high catalytic activity and electrical conductive properties. Broadly speaking, any conductor, such as iron or copper sheets, can be used as electrodes. However, these relatively reactive metals can undergo electrochemical reactions with redox ions, causing gradual corrosion of the electrodes and affecting the continuous thermoelectric conversion. Therefore, they are generally not considered suitable candidates for electrodes. At present, inert metal electrodes and carbon-based composites are often used for electrodes of thermogalvanic hydrogels.^115–117 Platinum (Pt), in particular, is favored in ionic thermoelectric devices due to its excellent chemical stability and high electrical conductivity. With Fe(CN)₆^3−/4− ions as a redox system and Pt as electrodes, the thermogalvanic hydrogels show a high P_max/(ΔT)² of ∼0.87 mW m⁻² K⁻² and a high Carnot-relative efficiency (η_r) of 0.417%.¹¹⁶ Additionally, the thermoelectric properties can be effectively improved by adjusting the electrode structure. As illustrated in Fig. 5a, hierarchical Au/Cu metal composites with a 3D structure and high roughness were designed by oxidation–etching–reduction methods.¹¹² With 3D hierarchical Au/Cu electrodes, the effective electrical conductivity and thermoelectric power density of thermogalvanic hydrogels can be efficiently enhanced. The 3D hierarchical structures of Au/Cu electrodes have a larger surface area, provide more active sites, and promote reversible redox reactions. With the increase in the roughness of the Cu foil, the charge transfer resistance between the electrode and hydrogel electrolyte was significantly reduced. With 3D Au/Cu electrodes, the thermogalvanic device exhibited a large current density of 11.5 A m⁻² and a power density of 320.7 mW m⁻², which were increased by 1083% and 1072%, respectively, compared to that with 2D Au/Cu electrodes. The optimized thermoelectric system achieved a high P_max/(ΔT)² of 8.9 mW m⁻² K⁻², much higher than previously reported works.

Fig. 5 (a) 3D hierarchical structure design of Au/Cu foils for enhancing power output. Reproduced with permission.¹¹² Copyright 2022, Wiley-VCH. (b) 3D-printed PEDOT:PSS electrodes with a porous structure for improving thermoelectric properties. Reproduced with permission.¹¹⁵ Copyright 2021, Elsevier.

Compared with the noble metal electrodes with relatively high cost, emerging carbon-based materials such as graphenes and carbon nanotubes have become more competitive due to their high surface area, low cost, and high conductivity. In 2013, Romano et al. optimized the porosity of composite electrodes by combining carbon nanotubes (CNTs) with reduced graphene oxide (rGO).¹¹⁷ The porous structure of composite electrodes improved the electroactive surface area, boosting the power density of thermogalvanic hydrogel with Fe(CN₆)^3−/4− ions to 337 mW m⁻². Chen and collaborators also explored the feasibility of conductive polymers as electrodes of thermogalvanic hydrogels.¹¹⁵ By 3D printed technology, they fabricated a porous PEDOT:PSS electrode for thermogalvanic hydrogels (Fig. 5b). Due to the open porous structure and excellent wettability of PEDOT:PSS polymers,^118,119 it demonstrated excellent thermoelectric performance in both n-type (Fe^2+/3+) and p-type (Fe(CN)₆^3−/4−) hydrogel electrolytes. Compared to the PEDOT:PSS thin-film electrode, the current density of the n-type thermogalvanic hydrogel using the 3D-printed PEDOT:PSS electrode is increased from 8.2 to 13.0 A m⁻², and the power density is increased from 12.2 to 25.0 mW m⁻². By using porous and low-contact-resistance PEDOT:PSS as the electrode material, energy loss is reduced, and ohmic resistance is minimized. By connecting 18 pairs of n-type and p-type thermogalvanic hydrogels in series through a Z-shaped structure, the wearable thermoelectric system can charge a commercial supercapacitor to 0.27 V and power low-power electronic devices by harvesting human body heat.¹¹⁵

3.2. Optimization of mechanical properties

In addition to excellent thermoelectric properties, good mechanical properties are also the key to ensuring the reliable and stable application of thermogalvanic hydrogels. However, based on redox ion pairs, thermogalvanic hydrogels often exhibit low fracture strength (<1.0 MPa), limiting their application in demanding environments.^{100,101,110,120–122} Recent studies have shown that the tensile strength and fracture toughness can be effectively improved by hydrogen bonding, hydrophilic/hydrophobic interactions, anisotropic 3D network, and dual-crosslinked network structures of thermogalvanic hydrogels.

Shi et al. reported a p-type supramolecular thermogalvanic hydrogel thermocell based on double hydrogen-bonding,¹⁰¹ which has excellent electrical conductivity and mechanical fatigue resistance (Fig. 6a). The supramolecular hydrogel was polymerized by using N-acryloyl glycinamide (NAGA) with an intramolecular hydrogen bond as a small molecular monomer and diacrylate-capped Pluronic (F68) modified with double-ended acrylate as a macromolecular crosslinker. The double hydrogen-bonding permeates the whole supramolecular hydrogel and endows it with excellent mechanical properties and high ionic conductivity. In addition, the ether bond on F68 can trap alkali metal ions, forming metal–organic coordination, which further improves the mechanical properties of the PNGA-F68 thermocell during the process of solvent exchange. The optimized PNGA-F68 thermocells have an ultrahigh Young's modulus (2.6 MPa) (Fig. 6b), high thermopower (−2.17 mV K⁻¹), and excellent fatigue resistance (∼3120 J m⁻²).

Fig. 6 (a) Double hydrogen-bonding strategy for enhancing the mechanical properties. (b) Stress–strain curves of the PNAGA-F68 supramolecular thermogalvanic hydrogel thermocell. Reproduced with permission.¹⁰¹ Copyright 2023, Wiley-VCH. (c) Schematic of the hydrophilic and hydrophobic interactions of copolymer networks for simultaneously improving the thermoelectric and mechanical properties of the p(SBMA-MMA) thermogalvanic hydrogel. Reproduced with permission.¹²¹ Copyright 2024, Wiley-VCH.

Similar to the p-type hydrogel thermocells, some methods can be applied to n-type thermogalvanic hydrogels. Shin et al. developed a copolymer p(SBMA-MMA) hydrogel network by using sulfonated betaine methacrylate (SBMA) and hydrophobic methyl methacrylate (MMA),¹²¹ as shown in Fig. 6c. The hydrophilic zwitterion SBMA helps to improve ionic conduction, while the hydrophobic MMA provides a strong skeleton that maintains structural stability even at elevated concentrations of electrolyte salts. During the solvent exchange process, the hydrophobic MMA clusters together for hydrophobic interactions, while hydrophilic SBMA preferentially solvates with H₂O molecules. Therefore, there is a phase separation structure in p(SBMA-MMA) hydrogels, which can greatly enhance the mechanical properties. In addition, the hydrophobic interaction between the MMA parts resulted in improved mechanical properties of the hydrogel, and the optimized p(SBMA-MMA) hydrogel thermocells have a high power density of 1.1 mW m⁻² K⁻² and a high elastic modulus of 1.6 MPa similar to human skin within the 0.1–2 MPa range. Additionally, by designing a dual-crosslinked network with two different polymer chains, combining their rigidity and stretchability, the strength and toughness of thermogalvanic hydrogels can be greatly improved.¹⁰²

In addition to the molecular engineering strategy to enhance the strength and toughness, the design of anisotropic polymer structures can efficiently improve the mechanical properties of thermogalvanic hydrogels.^{99,100,109,122} Chemically crosslinked polyvinyl alcohol (PVA) has been widely used as the matrix of quasi-solid hydrogel thermocells; however, the random and disordered network structure with isotropic characteristics makes the mechanical and conductive properties of thermocells relatively low. Inspired by the layered structure of natural muscle to improve the toughness and fatigue threshold of fibril (Fig. 7a),¹⁰⁰ Lei et al. first prepared physically crosslinked isotropic PVA thermogalvanic hydrogels by a freeze–thaw method and solvent exchange, and then obtained an anisotropic thermogalvanic hydrogel by periodically mechanical pre-stretching. The pre-stretching thermogalvanic hydrogels exhibit excellent tensile strength and fatigue-resistant properties owing to the oriented structure of PVA chains. This anisotropic-oriented structure not only enhances the mechanical properties but also greatly promotes ion transport and improves ionic conductivity.

Fig. 7 (a) High-strength thermogalvanic hydrogel with a hierarchical structure fabricated by bio-inspired mechanical training. Reproduced with permission.¹⁰⁰ Copyright 2023, Wiley-VCH. (b) Fabrication of the high-strength PVA thermogalvanic hydrogel with anisotropic polymer networks by unidirectional freezing based on the ice-template method. Reproduced with permission.⁹⁹ Copyright 2022, the American Chemical Society. (c) Pre-stretching and thermosensitive strategy for simultaneously improving the mechanical and thermoelectric properties of the PVA thermogalvanic hydrogel. Reproduced with permission.¹²² Copyright 2023, Wiley-VCH.

Unidirectional freezing based on the ice-template method can also realize anisotropic polymer networks of PVA hydrogels with high molecular weight. According to Flory's chain entanglement theory, the high molecular weight of the PVA polymer has an entanglement contribution toward mechanical properties and helps to promote the connection among the crystalline regions of PVA molecular chains. As shown in Fig. 7b,⁹⁹ when the bottom of the PVA aqueous solution is below the freezing point, the ice crystals grow unidirectionally from bottom to top, which promotes the orientation of PVA molecular chains and achieves physical cross-linking of PVA molecular chains through nanocrystalline domains with rich hydrogen bonds. After the PVA hydrogel returned to room temperature, the polymer orientation structure was retained. The orientated structure, as ionic conduction channels, improves both ionic conductivity and mechanical performance. It demonstrated excellent toughness (34 900 J m⁻²), a 400% increase in ionic conductivity, and a power density of 0.22 mW m⁻² K⁻², offering a versatile approach for developing high-performance thermogalvanic hydrogels with excellent mechanical properties. Although the above thermogalvanic hydrogels obtained high mechanical strength and toughness, their thermoelectric properties still need to be further improved, especially the thermopower. To simultaneously obtain excellent mechanical properties and also maintain high thermoelectric properties, Liu et al. combined stretching-induced structure orientation of PVA molecular chains for improving the mechanical properties and thermosensitive crystal enhancement strategy for optimizing the thermoelectric properties (Fig. 7c).¹²² The fabricated PVA thermogalvanic hydrogels formed a layered, anisotropic network using freeze-casting-assisted stretching, exhibiting a high fracture strength of 19.0 MPa and a toughness of 163.4 MJ m⁻³. The introduction of guanidinium ions (Gdm⁺) into the system induces significant changes in thermoelectric performance. First, according to the Hoffmann's effect, Fe(CN)₆⁴⁻ has a higher charge density than Fe(CN)₆³⁻, making it more likely to interact with Gdm⁺. This interaction disrupts the hydration shell of Fe(CN)₆⁴⁻, increasing the entropy difference (ΔS) of the Fe(CN)₆^4−/3− redox couple. Given that the Seebeck coefficient is proportional to ΔS, this process enhances thermopower. Second, Gdm⁺ promotes the formation of thermosensitive crystals of Fe(CN)₆⁴⁻. Under a temperature gradient, Fe(CN)₆⁴⁻ interacts with Gdm⁺, precipitating at the cold end while dissolving at the hot end. This dynamic phase transition accelerates the redox reaction kinetics and charge transport, further boosting the thermopower. Through these two mechanisms, Gdm⁺ successfully induces the formation of thermosensitive crystals of Fe(CN)₆⁴⁻, significantly enhancing the thermoelectric performance. The thermopower increases by approximately 6.5 mV K⁻¹, leading to an ultrahigh specific output power density of 1969.0 μW m⁻² K⁻².

3.3. Optimization of freeze and heat resistance

Thermogalvanic hydrogels often suffer from narrow operating temperature ranges, limiting their practical applications. For instance, when the working temperature is below 0 °C, the freezing of water molecules will make thermogalvanic hydrogels hard and brittle, with low electrical conductivity and poor thermoelectrical and mechanical properties, further limiting their application in low-temperature environments.^{79,90,97,123–126} In addition, high working temperatures will cause the evaporation of water molecules, and even the melting and decomposition of thermogalvanic hydrogels, thus destroying their long-term stability.¹²⁷ Therefore, developing thermogalvanic hydrogels with high freeze and heat resistance is crucial for expanding their applications.

The results show that the anti-freezing ability of hydrogel electrolytes can be greatly increased by exchanging organic solvents or directly using binary solvents. By introducing ethylene glycol (EG) as a co-solvent into the hydrogel electrolyte, the fabricated polyacrylamide (PAAm) thermogalvanic hydrogel exhibits high anti-freezing ability (−30 °C), which is attributed to the hydrogen bonds formed between water molecules and hydroxyls on the EG molecules.⁷⁹ The formation of hydrogen bonds induces EG to form a large number of molecular clusters with H₂O molecules, thus reducing the proportion of free water, the saturated vapor pressure of water, and the freezing point.^79,128,129 Similarly, by immersing PVA hydrogel into the mixture of glycerol (GL) and water and I⁻/I₃⁻ ion pairs, the freezing point of PVA thermogalvanic hydrogels dropped to −20 °C, showing good anti-freezing ability (Fig. 8a).¹²⁶ At a working temperature of −20 °C, the PVA thermogalvanic hydrogels still exhibit excellent flexibility, a high fracture strain of ∼300%, and a satisfactory thermopower of 0.41 mV K⁻¹.

Fig. 8 (a) Schematic and performance of the anti-freezing PVA/GL thermogalvanic hydrogel. Reproduced with permission.¹²⁶ Copyright 2022, the Royal Society of Chemistry. (b) Schematic of freeze-resistant thermogalvanic organogel and crosslinking network with macroscopic images. Reproduced with permission.⁹³ Copyright 2023, the American Chemical Society. (c) Schematic of gelatin/GTA-KCl-FeCN^4−/3− i-TE gel for the expanded temperature range. Reproduced with permission.¹²⁷ Copyright 2022, the Royal Society of Chemistry.

Despite these advancements, the evaporation of water molecules in thermogalvanic hydrogels still limits their long-term usability, and it is also difficult for thermogalvanic hydrogels to operate at lower temperatures due to the presence of water molecules. To address it, Zhang's group developed a type of anhydrous thermogalvanic gel by using a mixed dimethyl sulfoxide/ethylene glycol (DMSO/EG) solvent with Fe³⁺/Fe²⁺ ion pairs (Fig. 8b).⁹³ Due to the strong hydrogen bond between EG and DMSO, these gels prevent freezing at −80 °C, showing remarkable anti-freezing ability. In addition, the solvent evaporation greatly decreased at a relatively high temperature of ∼70 °C owing to the strong hydrogen bond. This water-free design strategy enables the thermogalvanic gels to maintain stable performance across a wide temperature range.

Gelatin, as another important and common hydrogel matrix, has been widely used in ionic thermoelectric materials and devices. However, due to the weak physical entanglement of the polymer chains, the gelatins have relatively low melting points, poor thermal stability, and a limited operating temperature range.^{26,43,108,112} To address it, Liu's group introduced glutaraldehyde (GTA) into a gelatin-based i-TE gel with KCl-Fe(CN)₆^4−/3− as the gel electrolyte,¹²⁷ as shown in Fig. 8c. GTA can efficiently improve the thermal stability of gelatin owing to the strong covalent bonds between gelatin and GTA, promoting the formation of stable interconnected porous structures in gel electrolytes, which greatly extends the operating temperature range (ΔT) from 9 K to 23 K. Additionally, the resulting gelatin/GTA polymer network increases the entropy difference between Fe(CN)₆^4−/3− redox pairs, achieving a huge thermopower of 24.7 mV K⁻¹ and a specific output power density of 9.6 mW m⁻² K⁻². A wearable device was designed by utilizing a highly flexible, low-thermal-conductivity gelatin/GTA polymer as the connector to ensure mechanical stability and thermal insulation. Sixteen pairs of n-type and p-type i-TE cells were connected in series in a Z-shaped configuration.¹²⁷ By harvesting body heat, the device was able to generate a high voltage of 3.6 V and a remarkable output power of 115 mW. This work provides a feasible strategy to enhance the thermal stability and expand the operation temperature range of thermogalvanic hydrogels.

4. Device integration and applications

Due to excellent flexibility, low cost, simple operation, and adjustable mechanical and thermoelectric properties, thermogalvanic hydrogels have been widely used in low-grade thermal energy harvesting, wearable electronics, and other fields. The electricity generated by thermogalvanic hydrogels can not only directly drive low-power electronic devices, but also provide the possibility for developing self-powered sensing systems.

For the same type of thermogalvanic hydrogels, the device integration can be divided into two types (series and parallel integration). The series integration arranges multiple thermoelectric units in a Z-shape, with the hot side of each unit alternately connected to the cold side of its neighboring unit, creating a continuous temperature gradient. This design significantly enhances the overall open-circuit voltage, making it suitable for applications requiring high voltage output, such as driving low-power sensors with high working voltage. However, parallel integration allows them to share the same hot and cold ends. This setup increases the total output current, making it ideal for applications demanding high working current. For the p-type and n-type units, they are typically connected in series in a Π-shaped integration, which is like that of the same type of units in series in Z-shape configuration. For thermogalvanic hydrogels, the polymer matrix plays a crucial role in ensuring mechanical stability and effective thermal insulation, leveraging its high flexibility and low thermal conductivity. Furthermore, electrode modifications such as incorporating conductive fillers, three-dimensional layered electrodes, or nanostructured porous structures can enhance the interface contact area and reduce the charge transfer resistance, thereby improving both output voltage and current. By strategically combining Z-shaped (Π-shaped) and parallel connection integration methods, multiple thermogalvanic hydrogel units can be assembled to maximize the energy output. This approach provides valuable insights into the scalable and practical implementation strategies for future device development, paving the way for high-performance, self-powered electronic systems. In this section, we will focus on discussing the potential applications of thermogalvanic hydrogels and devices in everyday life.

4.1. Low-grade heat harvesting

As a constant temperature source, the human body is rich in heat energy. Converting human heat into electricity could help develop self-powered wearable electronics. Yang et al. developed a wearable p-/n-type PVA thermogalvanic hydrogel thermocell array based on Fe(CN)₆^4−/3− and Fe^3+/2+ redox ion pairs.⁷⁸ The p–n units were interconnected in Z-shape and then covered with flexible polyimide films to form wearable thermoelectric devices. At a working temperature of 5 °C, this thermoelectric device generated an open-circuit voltage of 0.7 V and a maximum output power of 0.3 mW by harvesting human body heat. The produced electricity can be directly utilized to drive low-power electronic devices or stored in capacitors. Zhang et al. designed a 5 × 5 flexible thermocell array with a Z-shaped arrangement of multiple thermosensitive crystal-enhanced thermogalvanic hydrogels with a high thermopower of 4.4 mV K⁻¹.⁸⁰ By fixing this thermocell array to human skin in a room-temperature environment, a stable output voltage of 0.42 V can be obtained. Furthermore, Zhang et al. developed wearable thermoelectric shoes based on anti-freezing thermogalvanic hydrogels for human body heat harvesting in a simulated cold environment (Fig. 9a).¹³⁰ The prepared thermogalvanic hydrogel can maintain excellent flexibility and thermoelectric properties (1.43 mV K⁻¹) at an ultralow temperature of −80 °C. When stepping on a cold source (−30 °C) as a simulated winter environment, the wearable thermoelectric shoe delivers a relatively high thermoelectric voltage of about 1.3 V, and the produced electricity can directly light a small-power LED through a voltage amplification system, showing remarkable potential for wearable and self-powered applications in an extremely cold environment.

Fig. 9 (a) Design of wearable thermoelectric shoes based on anti-freezing thermogalvanic hydrogels, showing a high output voltage when the shoes step on a cold source (−30 °C). Reproduced with permission.¹³⁰ Copyright 2023, the American Chemical Society. (b) Thermogalvanic hydrogels for battery cooling and power generation; the produced electricity can be stored in capacitors. Reproduced with permission.¹³¹ Copyright 2020, the American Chemical Society. (c) A thermogalvanic organogel placed on a CPU surface of device cooling and thermoelectric power generation, with excellent cooling ability. Reproduced with permission.⁹³ Copyright 2023, the American Chemical Society.

As electronic devices continue to miniaturize and integrate, substantial waste heat is generated during operation, and rising temperatures significantly degrade their performance. Studies indicate that every 2 °C increase in device temperature results in a 10% reduction in performance, which highlights the importance and necessity for efficient thermal management.^132–136 Research has shown that the evaporation of water molecules from hydrogels can effectively dissipate heat, lowering the device temperature.^80,83,93 Thus, thermogalvanic hydrogels present a promising solution for both thermal management and thermoelectric power generation.

As shown in Fig. 9b, Pu et al. designed a thermogalvanic hydrogel based on LiBr and Fe(CN)₆^4−/3− ions for waste heat recovery and device cooling.¹³¹ When the hydrogel film was attached to a phone battery at high temperatures, there was a temperature gradient across the hydrogel film, in which the redox ions generate reversible oxidation and reduction reactions for power generation, and water molecules undergo a water-to-vapor phase transition for efficient evaporative cooling. In addition, due to the addition of LiBr, the hydrogel can absorb water from the surrounding air to achieve recycling. Taking a 5000 mA h phone battery as an example, the thermogalvanic hydrogel can efficiently reduce the temperature of the phone battery by about 20 °C while converting waste heat into electricity. The produced electricity can be stored in capacitors with different capacities (0.6, 1, and 1.4 F), ensuring the efficient operation of energy systems and electric devices. Fang et al. also fabricated high-performance PVA thermogalvanic hydrogels with a DMSO/EG solvent, showing good anti-freezing ability and heat resistance, which greatly reduced the chip temperature by 17 °C and generated a high output voltage of ∼23 mV (Fig. 9c).⁹³ Additionally, this voltage suggests its potential as a self-powered status monitor for real-time device monitoring. In addition, it was found that the output voltage shows a linear relationship with the chip temperature by simulating different operating conditions, therefore, the produced thermoelectric voltage can be used for self-powered chip temperature monitoring.

4.2. Solar photothermal energy harvesting

In addition to directly collecting low-grade thermal energy from the human body and electronic devices, thermogalvanic hydrogels can efficiently collect solar energy (as a sustainable and green energy) through photothermal conversion technology. As shown in Fig. 10a, Shen et al. reported a solar-driven photo-thermo-electric thermogalvanic hydrogel with an interlocking structure between the hydrogel electrolyte and the corresponding photothermal layer.¹³⁷ The hydrogel electrolyte was fabricated by a dual-network structure of PAAm and carboxymethylcellulose (CMC), with Fe(CN)₆^3−/4− ions as redox pairs. The photothermal layer was formed by in situ crosslinking between pyrogallic acid (PA) and polyethyleneimine (PEI) based on the oxidation of Fe(CN)₆³⁻ ions. A large number of phenolic hydroxyl groups are in PA chains, which can promote the formation of interlocking structures between the photothermal film and the hydrogel electrolyte and then rapidly convert solar energy into electrical energy. Under the illumination of solar light, the fabricated thermogalvanic hydrogels can directly power a motor and a lamp with a voltage amplifier, showing excellent application potential in smart buildings.

Fig. 10 (a) Photo-thermo-electric thermogalvanic hydrogel with an interlocking photothermal layer for potential application in smart buildings. Reproduced with permission.¹³⁷ Copyright 2024, Wiley-VCH. (b)–(d) Thermogalvanic hydrogels for solar light energy harvesting and thermal-electric window. Reproduced with permission.⁹⁵ Copyright 2022, Elsevier.

Due to good transparency, thermogalvanic hydrogels can be designed as smart thermal–electric windows for electricity generation by harvesting solar energy. As illustrated in Fig. 10b, when the sun shines on the window, there is a ΔT between the inside and outside of the thermogalvanic hydrogel, which can be used to generate electricity.⁹⁵ To obtain large ΔT for a high thermoelectric output, Bai et al. used a carbon black foam as the hot side electrode for effectively converting solar light energy into heat energy, thus significantly increasing the temperature at the hot side and ΔT between the inside and outside of the thermogalvanic hydrogel (Fig. 10c). Although this thermoelectric structure can obtain relatively large ΔT and electrical signal, the utilization of a large-area carbon black foam greatly affects the light transmission of the window and reduces the lighting effect of the room. To address this, the authors further used a carbon black absorbing layer and a metal cooling layer, respectively, at both sides of the thermogalvanic hydrogel, thus obtaining a transverse temperature difference structure. Under solar illumination with a light intensity of about 600 W m⁻², the thermal-electric window can output a relatively high voltage of about 100 mV (Fig. 10d), and more important is that the whole thermal-electric window shows excellent daylighting properties.

4.3. Self-powered health monitoring

With the continuous development of wearable health medical devices, thermoelectric materials are gradually showing great advantages due to their good thermoelectric and mechanical properties.^92,138–143 Using the temperature difference between the human body and the external environment, thermogalvanic hydrogels could continuously and steadily generate thermoelectric signals.

As we know, improper pen-holding posture not only affects the speed and quality of writing but also causes some diseases such as myopia and cervical spondylosis. Therefore, maintaining the correct pen-holding posture and habits for writing is crucial.

Based on Fe(CN)₆^3−/4− ions redox pair, Zhang et al. have recently developed a self-powered PVA/gelatin thermogalvanic hydrogel smart pen for body posture recognition and adjustment by harvesting finger heat (Fig. 11a).¹⁴⁴ The smart pen has a high thermopower of about 2.05 mV K⁻¹ and a fast response time of 130 ms. By coupling thermoelectric and piezoresistive effects induced by finger temperature and pressure when holding a pen, the smart pen can realize precise assessment for pen-holding posture, stroke analysis, and identity recognition with intelligent deep machine learning. Research shows that the handwriting recognition accuracy for eight Chinese strokes is as high as 98.1%, and the recognition accuracy for nine different pen-holding postures reaches 99.1%. This thermogalvanic hydrogel smart pen exhibits significant application potential in AI-driven bioelectronic interfaces and human–machine interfaces.

Fig. 11 (a) A thermogalvanic hydrogel smart pen for self-powered handwriting monitoring and identity recognition. Reproduced with permission.¹⁴⁴ Copyright 2024, Elsevier. (b) Wearable thermogalvanic hydrogel patch for real-time body temperature monitoring. Reproduced with permission.⁹² Copyright 2021, the American Chemical Society. (c) Concept and demonstration of on-mask respiratory monitoring by integrating thermogalvanic hydrogels into facemask. Reproduced with permission.⁹⁷ Copyright 2022, the American Chemical Society.

Additionally, the thermogalvanic hydrogels can be used for monitoring body heat and vital signs. To demonstrate their potential applications in harvesting body heat and monitoring physiological conditions, a gel-based thermoelectric patch for real-time temperature sensing was developed. Owing to the excellent quasi-solid-state properties of gels, 25 hydrogel components were integrated in a Π-type parallel configuration, ensuring both high thermoelectric performance and superior wearability. As shown in Fig. 11b, the hydrogel patch exhibited a strong correlation between current output and temperature variations, as the generated electrical signal was highly dependent on the temperature difference between its two ends. The patch provided stable signals across three typical temperature regions (normal, vigilant, and dangerous temperatures for the human body).⁹² When applied to the forehead of the volunteers, the hydrogel patch can achieve real-time temperature monitoring, and also demonstrate good cooling capabilities for fevered patients due to the high specific heat capacity.

When the thermogalvanic hydrogel was integrated into a mask, a wearable thermoelectric facemask was formed. The thermoelectric facemask can realize self-powered respiratory monitoring by converting the thermal energy of the exhaled hot air into electrical signals. With deep machine learning and wireless communication technology (Fig. 11c),⁹⁷ the wearable thermoelectric facemask can monitor different breathing frequencies and also identify the respiratory characteristics of different groups of people, such as the elderly and children. These studies promote the potential of thermogalvanic hydrogels in advancing active and autonomous wearable healthcare devices. The thermogalvanic hydrogel is encapsulated and designed in the shape of a wearable wristband. By controlling the duration of finger pressure on the thermogalvanic hydrogel, voltage signals resembling “dots” and “dashes” can be generated for simulating Morse code (Fig. 12a–d).⁴⁷ This innovative thermogalvanic hydrogel wristband could assist individuals with speech impairments in communication through Morse code. As shown in Fig. 12e, using this wristband, a speech-impaired person can convey messages such as “I AM OK” or send distress signals, which indicates significant potential in smart healthcare and emergency rescue situations. To make it easier for readers to compare and assess the practical viability of thermogalvanic hydrogels in both energy harvesting and health monitoring, we summarized the performance metrics of thermogalvanic hydrogels in different use cases (materials, thermopower, output performance, mechanical strain, and real-world stability), as detailed in Table 1.

Fig. 12 (a) Thermogalvanic hydrogels for self-powered information conversion for healthcare based on the Morse Code. (b) International Morse Code table. Demonstration of the “dots” and “dashes” in Morse Code through finger press (c) and representative letters (d). (e) Signal waveform of the Morse Code of “I AM OK” by the thermogalvanic hydrogel wristband. Reproduced with permission.⁴⁷ Copyright 2023, Elsevier.

Table 1 Performances and applications of thermogalvanic hydrogels

Matrix-redox couple	S c (mV K^−1)	Strain (%)	Output performance	Real-world stability	Application
^78PVA/Fe^2+/3+-PVA/[Fe(CN)6]^4−/3−	1.02/−1.21	200–400	90 nW, ΔT = 20 K	Stable cyclic performance	Low-grade heat harvesting
^130PAAM-[Fe(CN)6]^4−/3−	−1.43 (−80 °C)	400	15.4 mW m^−2, ΔT = 30 K	95% (500 cycles)	Low-grade heat harvesting
^131PAAM-[Fe(CN)6]^4−/3−	−1.2	200–300	5 μW (2.2 C)	Stable cyclic performance
^93PVA-Fe^2+/3+	1.09 (−80 to 80 °C)	266 (−80 °C)	4.22 mW m^−2, ΔT = 20 K	85% (100 cycles)
^137PAAm/CMC-[Fe(CN)6]^4−/3−	−1.40	400	17.42 mW m^−2, ΔT = 30 K	15 simulated sunlight without attenuation	Solar photothermal energy harvesting
^95PVA/gelation-Fe^2+/3+	1.63	350	1.2 mW m^−2, ΔT = 40 K	Continuous operation for 12 hours without attenuation
^92P@PVA-Fe^2+/3+	0.79	200–300	50 nW, ΔT = 20 K	After 3 days the current decays but remains relatively stable	Self-powered health monitoring
^97PVA/gelation-[Fe(CN)6]^4−/3−	−1.29	320	5.7 mW m^−2, ΔT = 40 K	Stable cyclic performance
^144PVA/gelation-[Fe(CN)6]^4−/3−	−2.05	650	0.82 mW m^−2, ΔT = 20 K	Stable cyclic performance

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5. Conclusions and prospects

As a type of emerging quasi-solid-state thermoelectric material, thermogalvanic hydrogels have been widely used in low-grade heat harvesting and wearable self-powered electronic devices, owing to their ultrahigh thermopower and excellent mechanical properties. This review provided a comprehensive discussion and conclusions about thermogalvanic hydrogels, with a particular emphasis on strategies for enhancing their properties and exploring their potential applications in daily life activities.

Thermogalvanic hydrogels hold significant promise for thermal energy conversion, health monitoring, and other applications, but there are still some challenges. First, ensuring safety and biocompatibility is essential for wearable and self-powered devices. Biocompatible polymers such as proteins and polysaccharides can address these concerns, but these polymers often show weak thermoelectric performance due to limited solubility for redox pairs. Second, the narrow operating temperature range is another issue, though anti-freezing agents (e.g. alcohol organic solvent) can help extend it. Still, these organogels exhibit low ionic conductivity and poor solute solubility. Moreover, to obtain the best thermoelectric performance, it is necessary to find the influencing factors and matching relationship among the thermopower, electrical conductivity, and thermal conductivity of thermogalvanic hydrogels. However, these three parameters interact with each other, making the study complicated. Finally, an excellent thermoelectric performance match between n-type and p-type thermogalvanic hydrogels is essential for a higher power output. Current research mostly focuses on the p-type, and the lack of efficient n-type thermogalvanic hydrogels limits the overall performance, which is also something to consider in future studies.

Despite some shortcomings and many challenges, thermogalvanic hydrogels still have many opportunities for future development and applications. Emerging fabrication techniques such as 3D printing present exciting opportunities and strategies for the customizable and scalable fabrication of thermogalvanic hydrogels with various shapes and structures, especially in medical, wearable electronics, and artificial intelligence fields. These methods exhibit the characteristics of a straightforward fabrication process, enabling large-scale production with swift speed, high precision, and resolution. Moreover, they facilitate the effortless creation of highly integrated thermogalvanic hydrogel array devices with small volumes, thereby maximizing the utilization rate of heat energy. There is no denying that thermogalvanic hydrogels have already stood out for their outstanding comprehensive properties, such as high thermopower, excellent flexibility, low cost, and simple fabrication, which make them ideal candidates for human body heat harvesting, wearable self-powered devices for health monitoring, and smart thermal management. With the continuous development of thermogalvanic cells and hydrogel fabrication technology, it is believed that thermogalvanic hydrogels will make rapid development and great progress, thus promoting their application in efficient low-grade heat harvesting, portable flexible power supply, self-powered wearable electronics, smart thermal management, etc.

Data availability

All the data and images in the review were obtained from published articles, and we have obtained permission to use the images mentioned in this review. No primary data were generated or analyzed specifically for the purpose of this review.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Key R&D Program of China (Grant No. 2020YFA0711500), the National Natural Science Foundation of China (52473215, 52273248, and 52303238), and the Key Project of Natural Science Foundation of Tianjin City (S24JQU021 and QN20230102).

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