Engineering asymmetric solvation structures for synergistically boosted quasi-solid thermocells

Abstract

Quasi solid-state thermocells (QTECs) based on the thermogalvanic effect offer a promising route for directly converting abundant low-grade heat into electricity. Introducing a single solvent into hydrogel electrolytes, a common strategy to enhance thermopower, often yields a marginal solvation entropy difference between redox ions and provides limited gains in ion transport. To break this longstanding trade-off, we present a simple yet highly effective co-solvent strategy that employs trimethyl phosphate and ethylene glycol to construct hybrid hydrogel electrolytes. This approach synergistically enlarges solvation entropy differences of redox ions, amplifies the concentration gradient across the thermocell, and enhances redox ion transport through the hydrogel network. The resulting hydrogel electrolyte achieves superior thermoelectrochemical performance and demonstrates efficient harvesting of low-grade heat even at sub-zero temperatures. Advanced characterization techniques, integrated with molecular simulations, elucidate that the enhanced thermoelectrochemical performance originates from co-solvent engineered asymmetric solvation structures. This work demonstrates targeted, additive-free modulation of the solvation environment in thermogalvanic hydrogels as a practical strategy to significantly enhance thermoelectrochemical performance.

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1 Introduction

The pursuit of effective and sustainable waste heat utilization techniques is crucial for global energy conservation and achieving carbon neutrality goals.^1–5 At present, approximately 60% of the world's energy is ultimately wasted as heat, most of which is low-grade heat (<100 °C).^6,7 Thermocells (TECs), which operate based on the thermogalvanic effect, have attracted growing attention due to their promising capacity for harvesting waste low-grade heat.^8–12 A typical TEC consists of two electrodes separated by an electrolyte containing redox couple ions. When a temperature gradient (ΔT) is applied between the electrodes, an electric potential difference develops through temperature-driven redox reactions, thus allowing nonintermittent heat-to-electricity conversion. Recently, quasi solid-state TECs (QTECs) based on hydrogel electrolytes (thermogalvanic hydrogels, THs) have attracted considerable interest as a practical and scalable platform to harvesting low-grade thermal energy, particularly for self-powered wearable electronics.^13,14 These systems are recognized for their considerable thermopower (known as the Seebeck coefficient, S_e), which often exceeds 1 mV K⁻¹, alongside advantages such as low costs.^15–19

The solvation structure of redox ions is a critical factor governing the thermoelectrochemical performance of THs, as it directly determines the entropy change (ΔS_rc) of the redox reaction.^{12,17,20–22} In the case of a widely used redox couple, Fe(CN)₆^4−/3−,^23–27 the high-valence ion experiences stronger charge–dipole interactions with the surrounding solvent molecules, which leads to a more ordered solvation shell than that of Fe(CN)₆³⁻. Such a difference in the local solvation structure generates disparity in the temperature-dependent response of the solvation free energy between two redox ions, essentially solvation entropy. This term contributes to the overall entropy change of a redox reaction, as captured in the expansion of Seebeck coefficient:²⁸

where ΔE is the potential difference developed across ΔT, n is the number of electrons transferred, and F is Faraday's constant. This expression decouples ΔS_rc into contributions from ion solvation (ΔS_solv) and concentration (ΔS_conc). In particular, ΔS_solv arises from the difference in the ratio of the redox ions' activity coefficients between the two electrodes (equivalently, the solvation entropy difference). On the other hand, ΔS_conc scales with the difference in the ratio of the redox ions' concentrations between the electrodes. Importantly, the ΔS_conc term itself is affected by ion solvation, as solvation dictates the solubility of redox ions.^18,29–31 Therefore, even minor changes in the ordering and dynamics of the local ion solvation environment can significantly influence the thermoelectrochemical performance.^17,32–34

A high S_e is desirable for practical applications, particularly for powering IoT sensors or wearable devices, where efficient utilization of low-grade heat under small ΔT is indispensable for sustainable operation. Recent research has demonstrated that S_e can be effectively enhanced by introducing organic solvents.^29,31,35 For example, a study on an ethylene glycol (EG)/polyacrylamide (PAAm)-based TH demonstrated through spectral characterization that EG modifies the solvation structures and solubility of Fe(CN)₆^4−/3− ions.³¹ Consequently, S_e was boosted from 1.30 to 2.04 mV K⁻¹ with this alteration. In another study with a bacterial cellulose-based TH, the introduction of propylene glycol induced selective reconstruction of the Fe(CN)₆⁴⁻ solvation shell, which contributed to a high S_e of 2.30 mV K⁻¹.²⁹ Despite these improvements, a fundamental challenge persists for practical deployment of single-solvent THs. The inverse relationship between key electrolyte properties, such as S_e and ionic conductivity σ, severely compromises the output power density, thereby limiting the energy-harvesting capability of QTECs.

Herein, we develop a co-solvent engineering strategy employing trimethyl phosphate (TMP) and EG within a polyacrylamide (PAAm) hydrogel to construct asymmetric solvation structures and enable efficient ion transport. Although EG is known to modulate solvation structures, its capacity to compete with water molecules in the primary solvation shell of Fe(CN)₆^4−/3− is mitigated by the resemblance of hydroxyl groups to water. To address this constraint, we introduced TMP, an unexplored solvent molecule in thermocells that features a highly polar phosphate ester group, to more effectively reconstruct the solvation structures of Fe(CN)₆^4−/3−. The combination of TMP and EG was found to synergistically enlarge the solvation entropy difference and amplify concentration gradients of the redox ions as captured by eqn (1). This effect, together with enhanced ion–polymer interaction and salt dissociation, delivers exceptional thermoelectrochemical performance in the co-solvent hydrogel electrolytes that persists even at sub-zero temperatures. Our work provides a practical strategy and new insights for the design of high-performance QTECs via the rational, additive-free manipulation of the solvation environment.

2 Results

2.1 Characteristics of co-solvent hydrogel electrolyte

The design strategy for enhancing the thermoelectrochemical performance of hydrogel electrolyte is illustrated in Fig. 1. A co-solvent (EG/TMP) hydrogel electrolyte (Fig. 1a) was fabricated via a standard two-step procedure as shown in Fig. S1. Initially, acrylamide, initiators, and crosslinkers were dissolved in ultrapure water and stirred to form a homogeneous solution. The solution was subsequently injected into a glass plat mold and heated to form a pure PAAm hydrogel. Next, EG solvent, TMP solvent, and K₃Fe(CN)₆–K₄Fe(CN)₆ salt were incorporated into the PAAm hydrogel via solution exchange. This yields a hydrogel electrolyte containing both aqueous and organic solvents. To investigate the relationship between intermolecular interactions and thermoelectrochemical performance, a series of parallel electrolytes were prepared with varying EG-to-TMP ratios. For clarity, the hydrogel electrolyte systems were denoted as “Hybrid-E_xT_y” (short for “Hybrid-(EG)_x(TMP)_y”), where x and y denote the volume percentages of EG and TMP in the solution phase, respectively.

Fig. 1 Design strategy of co-solvent hydrogel electrolyte. (a) Schematic of the Hybrid-E_xT_y assembly. (b) Comparison of the ion solvation structure and ion–polymer interactions between the PAAm and Hybrid-E₇T₂₃ electrolytes. (c) Co-solvent induced crystallization of Fe(CN)₆⁴⁻ and its role in enhancing the thermogalvanic effect. (d) Illustration of enhanced ion transport and anti-freeze properties of Hybrid-E₇T₂₃, along with its potential applications.

The continuous operation of the Hybrid-E_xT_y-based QTEC is sustained by the thermal voltage generated between the cold and hot sides under ΔT. The magnitude of this voltage, characterized by S_e, is determined by the solvation entropy difference between two redox ions and their concentration gradients between the hot and cold electrodes, as illustrated by eqn (1). The Hybrid-E_xT_y system demonstrates excellent thermoelectrochemical performance as compared to both pure PAAm electrolyte and single-solvent hydrogel electrolytes. The enhancement in the solvation entropy difference mainly originates from co-solvent (EG and TMP) induced restructuring of the solvation environment of redox ions. Due to the preferential interaction between Fe(CN)₆⁴⁻ and co-solvents, a greater number of solvent molecules are incorporated into the solvation shell as compared to Fe(CN)₆³⁻. This results in more asymmetric solvation structures to effectively enlarge their solvation entropy difference (Fig. 1b). Meanwhile, the accessibility of coordination sites along the PAAm backbone to Fe(CN)₆^4−/3− is modulated by the co-solvents. The induced differential polymer–ion interactions further amplify the solvation entropy difference between redox ions.

The restructuring of the solvation environment also leads to a marked alteration in the thermodynamic solubility of redox ions, as depicted in Fig. 1c. Fe(CN)₆⁴⁻ exhibits significantly lower solubility relative to Fe(CN)₆³⁻, primarily due to the different ion–solvent interactions that tend to stabilize one oxidation state rather than the other.³¹ Upon application of ΔT, an uneven concentration distribution of redox ions across the thermogalvanic cells will be developed. The resulting concentration gradient contributes directly to the S_e, a contribution that is further amplified by the reversible dissolution/crystallization of Fe(CN)₆⁴⁻ ions between the cold and hot electrodes. Apart from the substantial enhancement in S_e, the Hybrid-E₇T₂₃ system demonstrates improved ion transport, superior anti-freeze properties, and the capability to form a self-powered QTEC, as shown in Fig. 1d. Therefore, the synergistic interplay between the co-solvents enables Hybrid-E₇T₂₃ to achieve exceptional thermoelectrochemical performance.

2.2 Thermoelectrochemical performance

The Hybrid-E_xT_y-based QTEC operates on the thermogalvanic effect, wherein a temperature difference across the electrolyte drives reversible redox reactions that establish a potential difference between the electrodes. As illustrated in Fig. 2a, at the hot electrode, Fe(CN)₆⁴⁻ undergoes oxidation to Fe(CN)₆³⁻, which releases an electron that travels via the external circuit to the cold electrode. There, it facilitates the reduction of Fe(CN)₆³⁻ back to Fe(CN)₆⁴⁻. Consistent with thermogalvanic behavior, a linear relationship between the steady-state voltage and temperature gradient is observed. The pure PAAm hydrogel electrolyte exhibited a S_e of 1.32 mV K⁻¹, in agreement with previously reported values (Fig. S2).^15,31,36 The introduction of EG or TMP solvents significantly influenced the thermopower, with Hybrid-E₃₀T₀ and Hybrid-E₀T₃₀ achieving S_e values of 2.0 mV K⁻¹ and 2.38 mV K⁻¹, respectively (Fig. S3). With higher organic solvent content, however, only marginal gains in S_e are achieved and a detrimental effect on ionic conductivity occurs. Hence, an additional increase in co-solvent concentration was not pursued in this study.

Fig. 2 Thermoelectrochemical performance of hybrid electrolytes. (a) Schematic representation of the heat-to-electricity conversion mechanism of QTECs. (b) Ternary phase diagram of S_e in Hybrid-E_xT_y with varying volume ratios of EG, TMP, and water. (c) Output current–voltage–power curves of Hybrid-E₇T₂₃ at various ΔT, and (d) corresponding P_max and P_max/ΔT² values. (e) Comparison of S_e versus P_max/ΔT² for various QTECs. (f) Thermal energy conversion efficiency of Hybrid-E₇T₂₃. (g) Temporal variation of S_e of Hybrid-E₇T₂₃ under continuous operation for 10 days. (h) The S_e of the PAAm electrolyte and Hybrid-E₇T₂₃ at sub-zero temperatures.

The S_e of the hybrid electrolyte system was optimized through rational design of co-solvent composition to effectively modulate the solvation environment of redox ions. We systematically varied the volume ratios of H₂O, EG, and TMP while maintaining constant PAAm matrix content. Fig. 2b shows that S_e achieved a maximum of 2.52 mV K⁻¹ with a solvent ratio of V_Water : V_EG : V_TMP = 7 : 0.7 : 2.3, i.e., the Hybrid-E₇T₂₃ system. The ionic conductivity (σ) of the electrolytes was also significantly influenced by solvent composition. The pristine PAAm hydrogel exhibited σ = 5.48 S m⁻¹, while that in single-solvent systems decreased to 1.86 S m⁻¹ (Hybrid-E₃₀T₀) and 2.25 S m⁻¹ (Hybrid-E₀T₃₀), as shown in Fig. S4. However, the rationally designed co-solvent system Hybrid-E₇T₂₃ restored conductivity to 4.01 S m⁻¹. Our subsequent analysis suggests that single-solvent addition triggers selective precipitation of Fe(CN)₆⁴⁻ ions, which enhances thermopower but compromises ionic conductivity. In contrast, the co-solvent approach modifies the solvation structure to facilitate ion transport while maintaining high S_e.

The thermoelectrochemical performance of Hybrid-E₇T₂₃ was evaluated under varying ΔT. Fig. 2c depicts the measured output current–voltage–power curves when the cold electrode is maintained at 20 °C. As ΔT increased from 10 K to 40 K, both the open-circuit voltage (V_oc) and short-circuit current density (I_sc) increase steadily. The corresponding maximum output power (P_max) and normalized power density (P_max/ΔT²) are summarized in Fig. 2d. At ΔT = 40 K, P_max reached 821.4 mW m⁻², with a normalized power density P_max/ΔT² of 0.513 mW m⁻² K⁻², in close agreement with theoretical predictions (Note S1, SI). Notably, this value exceeds those of single-solvent QTECs (0.07 and 0.16) by factors of 7.3 and 3.2, respectively.^29,31 Further benchmarking against previous solvation-modulated QTECs without Gdm-based agent additives confirms significant improvements in both S_e and P_max/ΔT² (Fig. 2e and Table S2), thus confirming the efficacy of the co-solvent design strategy.

The thermal conductivity (κ) represents another parameter for quantifying heat-to-electricity conversion in QTECs.^37,38 In contrast to conventional liquid electrolytes, a reduced thermal conductivity of 0.58 W m⁻¹ K⁻¹ occurs in PAAm hydrogel electrolyte due to restricted convection within its polymer network. The co-solvents strengthen hydrogen-bond networks that restrict molecular collisions and increases viscosity to suppresses internal microconvection.³⁹ This contributes to further reduction of κ (0.443 W m⁻¹ K⁻¹) in the Hybrid-E₇T₂₃ electrolyte, a characteristic that favors the maintenance of a stable ΔT across the thermocell (Fig. S5). The thermoelectrochemical performance of Hybrid-E₇T₂₃ was further assessed based on its energy conversion efficiency (η) and Carnot-relative efficiency (η_r). Based on the formulation provided in Note S2 (SI), the maximum η and η_r respectively reached 0.0695% and 1.04% under a ΔT of 40 K. Moreover, the Hybrid-E₇T₂₃ electrolyte demonstrates excellent moisture retention (Fig. S6), indicating strong potential for long-term operation. Thus, the assembled QTEC preserves approximately 95.6% of its initial performance, characterized by a high S_e of 2.41 mV K⁻¹, even after ten days (Fig. 2g and S7).

As thermoelectrochemical conversion depends solely on ΔT rather than perceivable heat, we evaluated the electrolyte's heat-to-electricity conversion capability at sub-zero temperatures by fixing the hot electrode at 0 °C. When the cold side was cooled to −25 °C, the PAAm electrolyte showed severe performance degradation, yielding a S_e of 0.63 mV K⁻¹ due to impeded ion transport and sluggish redox kinetics (Fig. 2h). In contrast, differential scanning calorimetry (DSC) revealed that the Hybrid-E₇T₂₃ electrolyte exhibits a freezing point of −41.8 °C, attributable to the co-solvent's disruption of the hydrogen-bond network in water (Fig. S8). Consequently, the Hybrid-E₇T₂₃ electrolyte maintained virtually unaffected thermoelectrochemical performance at sub-zero temperatures. In particular, a S_e of 2.47 mV K⁻¹ was generated under the same conditions, a value that closely matches the room temperature data to exhibit a superb ∼97% retention (Fig. 2h). The excellent retention is corroborated by the robust ionic conductivity measured under the low-temperature conditions (Fig. S9). These results conclusively demonstrate the remarkable anti-freezing properties of our co-solvent QTEC, which allows for sustained thermal energy harvesting at sub-ambient temperatures.

2.3 Mechanism of enhanced thermoelectrochemical performance

The enhanced thermoelectrochemical performance of Hybrid-E₇T₂₃ can be elucidated via molecular-level interactions. At room temperature, Fe(CN)₆³⁻ remains fully dissolved in hybrid TMP/EG solutions, whereas Fe(CN)₆⁴⁻ readily precipitates (Fig. S10 and S11). The selective precipitation is also evident in hybrid solutions containing both Fe(CN)₆³⁻ and Fe(CN)₆⁴⁻ ions, as shown in Fig. S12. Ultraviolet-visible (UV-vis) spectra and optical photographs confirm that Fe(CN)₆⁴⁻ exhibits negligible intrinsic solubility in pure EG solvent (Fig. S13). X-ray diffraction (XRD) analysis identified the precipitate formed in both pure EG solvent (Fig. S14) and hybrid TMP/EG solutions (Fig. S15) as primarily Fe(CN)₆⁴⁻, with the characteristic peaks matching the standard powder diffraction file of K₄Fe(CN)₆. This difference in solubility, as documented in various solvent systems,^31,40 arises from differential ion–solvent interactions between redox ions that mediate long-range ionic correlations. Furthermore, Fe(CN)₆⁴⁻ exhibits pronounced thermosensitive solubility as shown in Fig. S16, which leads to spatial crystallization/dissolution under a temperature gradient. The total S_e in Hybrid-E_xT_y is thus governed by both the solvation entropy difference and the concentration gradient established by the redox couple (Note S3).

To validate the formation of a concentration gradient within THs, we characterize the morphologies of the hydrogels using scanning electron microscopy (SEM). As shown in Fig. 3a and S17, the pure PAAm hydrogel exhibits a porous 3D network structure, whereas the surface of Hybrid-E₇T₂₃ is covered with abundant micrometer-sized crystals, likely resulting from the precipitation of Fe(CN)₆⁴⁻ ions. This observation was further corroborated by Raman spectroscopy (Fig. S18). The crystallization is confined within the hydrogel network, which enables the establishment of a stable concentration gradient across Hybrid-E₇T₂₃. Furthermore, time-of-flight secondary ion mass spectrometry (TOF-SIMS) in positive ion mode revealed a complementary spatial distribution of CH₂N⁺ (representing PAAm chains) and K₂FeCN⁺ (derived from K₃Fe(CN)₆/K₄Fe(CN)₆) in Hybrid-E₇T₂₃ (Fig. 3b). The results indicate that the redox ions not only bind to the polymer matrix but also form crystalline phases within the hydrogel. Depth profiling (Fig. S20) was also applied to probe the distribution of components both on the surface and throughout the bulk of the hydrogel. The uniform distribution, as demonstrated from the resulting 3D reconstruction in Fig. 3c, ensures coherent ion transport pathways within the electrolyte.

Fig. 3 Characterization of electrolyte structures. (a) Morphological characterization of PAAm electrolyte and Hybrid-E₇T₂₃ electrolyte. (b) TOF-SIMS 2D spectra and (c) 3D reconstruction visual images of several secondary ion fragments in Hybrid-E₇T₂₃. (d) Schematic diagram of the in situ Raman measurement setup. (e) Real-time monitoring of the Fe(CN)₆^4−/3− redox couple via in situ Raman spectroscopy for the cold side of Hybrid-E₇T₂₃ at ΔT = 10 K. UV-vis spectra of (f) Fe(CN)₆³⁻, (g) Fe(CN)₆⁴⁻, and (h) Fe(CN)₆^4−/3− aqueous solutions with varying EG/TMP ratios.

During the operation of QTECs, the evolution of local ion concentrations under a ΔT was further examined using in situ Raman spectroscopy (Fig. 3d). Characteristic peaks at 2058 and 2093 cm⁻¹ can be assigned to Fe(CN)₆⁴⁻, while a peak at 2132 cm⁻¹ corresponds to Fe(CN)₆³⁻.^39,41 In the PAAm electrolyte, the Fe(CN)₆³⁻ peak intensity on the cold side decreased with prolonged heating, whereas the Fe(CN)₆⁴⁻ signal increased (Fig. S21), which is indicative of ΔT-driven ion transport and redox cycling at the electrodes. In contrast, Hybrid-E₇T₂₃ exhibited similar but more pronounced concentration fluctuations under ΔT, with different rates of spectral change (Fig. 3e). This behavior results from the thermosensitive solubility of Fe(CN)₆⁴⁻, where reversible crystallization/dissolution under ΔT sustains a stable concentration gradient that leads to additional entropy change of the redox reaction for thermopower enhancement.

To determine the relative contributions of solvation entropy differences versus concentration gradients to thermoelectrochemical performance, UV-vis spectroscopy was first employed to probe the ion–solvent interactions. Fig. 3f shows that as the EG/TMP ratio varied, the characteristic absorption peaks of Fe(CN)₆³⁻ remained unchanged in the UV-vis spectra. In contrast, the absorption peak of Fe(CN)₆⁴⁻ exhibited a noticeable shift from ∼218 nm as shown in Fig. 3g, indicating a stronger interaction between EG/TMP molecules and Fe(CN)₆⁴⁻ compared to Fe(CN)₆³⁻.^10,42,43 These differential ion–solvent interactions not only reduce the solubility of Fe(CN)₆⁴⁻ but are also believed to enlarge the difference in the solvation structure of redox ions.^21,31,44 Furthermore, the comparative UV-vis analysis in Fig. 3h revealed that Fe(CN)₆⁴⁻ concentration in three Hybrid-E_xT_y electrolytes differs significantly from that in aqueous electrolytes, while Fe(CN)₆³⁻ concentration remains largely unchanged. This signature correlates strongly with the observed thermosensitive solubility of Fe(CN)₆⁴⁻ ions in solutions containing solvent molecules. The enhanced S_e in hybrid electrolytes, particularly Hybrid-E₃₀T₀ and Hybrid-E₀T₃₀, relative to pure hydrogel electrolytes can be attributed to concentration gradients. However, the similar concentration differences of Fe(CN)₆^4−/3− ions are observed across various Hybrid-E_xT_y electrolytes. To isolate the solvation entropy contribution, control hydrogel electrolytes denoted as Hybrid-E_xT_y (sol) were prepared from the supernatant of equilibrated hybrid solutions. The S_e of Hybrid-E₇T₂₃ (sol) reaches 1.67 mV K⁻¹, far exceeding those of the single solvent electrolytes (Fig. S20). Therefore, the superior S_e in Hybrid-E₇T₂₃ relative to Hybrid-E₃₀T₀ and Hybrid-E₀T₃₀ originates primarily from ion solvation entropy differences rather than concentration gradient effects.

The above analysis demonstrates that the co-solvent strategy achieves synergistic enhancement through two mechanisms: (1) concentration gradients that enhance performance relative to pure hydrogels; (2) solvation entropy differences that provide an improvement in Hybrid-E₇T₂₃ relative to single-solvent hydrogels. Next, spectroscopic analyses were performed to investigate the role of solvent in modulating solvation structures in hybrid electrolytes. Raman spectra of the electrolytes show that the P–O–C vibrational modes in pure TMP, located at 737.6 and 752.8 cm⁻¹, shift noticeably in hybrid electrolytes (Fig. 4a). This is a consequence of interactions between TMP and other molecules or ions.^45–47 FTIR spectra further confirm these interactions (Fig. 4b), with characteristic TMP peaks shifting significantly upon hybrid electrolyte formation. Furthermore, TMP addition caused a shift in the O–H vibrational peak, an effect that is amplified by EG (Fig. S23). These co-solvent–water interactions disrupt the hydrogen-bond network, which is consistent with the observed freezing point depression.^48,49 The restructured hydrogen-bond network in Hybrid-E_xT_y is further supported by FTIR as shown in Fig. 4c. A blueshift in the C=O vibration (1600–1700 cm⁻¹) is also observed,⁵⁰ likely due to modified polymer–ion coordination and solvation structures. These findings collectively indicate that co-solvents significantly alter the microscopic solvation structure in Hybrid-E₇T₂₃.

Fig. 4 Effect of co-solvents on solvation of redox ions. (a) Raman spectra of various samples. (b) FTIR spectra of TMP solvent and various samples. (c) FTIR spectra of PAAm electrolyte and various samples. (d) Oxygen coordination number of Fe(CN)₆^4−/3− ions from EG, TMP, and polymer side chains (POL). (e) Probability distribution of oxygen coordination numbers from EG, TMP and POL around Fe(CN)₆^4−/3− ions. (f) Representative snapshots of the solvation shell structures of Fe(CN)₆³⁻ and Fe(CN)₆⁴⁻ ions in the Hybrid-E₇T₂₃ system. Color scheme: Fe(CN)₆^4−/3− (blue), solvation shell (large transparent sphere), EG molecules (yellow), TMP molecules (orange), and polymer chains (gray). Oxygen atoms (from the three species) within the solvation shells are shown in red spheres. Hydrogen atoms from the three species are omitted for clarity. Water molecules are shown as transparent bonds.

Molecular dynamics simulations were performed to elucidate the impact of the ion solvation structure on the solvation entropy difference. We quantified the composition of the solvation structure of redox ions via the number of coordinated oxygen atoms from different species in Hybrid-E_xT_y. Fig. 4d shows that more solvent molecules are in the solvation shell of Fe(CN)₆³⁻ than Fe(CN)₆⁴⁻, consistent with the stronger hydration of higher-valence ions. Owing to its higher dipole moment and Gutmann donor number relative to the EG,⁵¹ TMP exhibits stronger ion–solvent interactions toward Fe(CN)₆⁴⁻ ions. The introduction of co-solvents reduces solvent coordination numbers relative to their single-solvent counterparts and the magnitudes of reduction are more significant for EG solvents. Moreover, the number of oxygen atoms from the PAAm backbone coordinating to redox ions shows opposite trends for Fe(CN)₆⁴⁻ and Fe(CN)₆³⁻ in Hybrid-E_xT_y as compared to single-solvent electrolytes. The result indicates a synergistic co-solvent effect between EG and TMP that facilitates differential binding of PAAm sites to Fe(CN)₆^4−/3− ions, a key factor in amplifying the solvation entropy difference. Unlike conventional solvation-engineering strategies that rely solely on bulk liquid-phase regulation,^10,12 our system demonstrates the active contribution of the polymer matrix.

To obtain deeper molecular insights into the solvation environment, the distribution of oxygen atoms from EG, TMP, and polymer side chains of Hybrid-E_xT_y was examined as shown in Fig. 4e. The probability distributions of oxygen from the three species, particularly from side chains, around Fe(CN)₆³⁻ are consistently smoother and broader than Fe(CN)₆⁴⁻. This corresponds to a more disordered and structurally complex solvation shell with greater structural variability of low valence ions. Typical snapshots of solvation shell structures of Fe(CN)₆^4−/3− are depicted in Fig. 4f. Together with the oxygen coordination data, these observations highlight differential solvation structures between the redox ions that directly enhance the solvation entropy difference. Furthermore, ionic conductivity measurements reveal that Hybrid-E₇T₂₃ exhibits superior ion transport compared to single-solvent systems. To understand this improvement, we quantified the cation–anion pairing in the electrolyte (Fig. S27). The results point to a lower degree of K⁺ pairing to Fe(CN)₆^4−/3− in the co-solvent system, a trend that correlates with the observed increase in ionic conductivity. The co-solvents thus also promote salt dissociation for both K₃Fe(CN)₆ and K₄Fe(CN)₆, which leads to higher population of free ions available for enhanced ionic conductivity.

2.4 QTEC durability and device application

The QTEC operates continuously, as redox reactions proceed in opposite directions at the hot and cold electrodes while ionic transport replenishes reactants.⁵² To evaluate its operational stability, the lifespan of a Hybrid-E₇T₂₃-based QTEC was tested in the quasi-continuous discharge mode at room temperature with ΔT = 10 K. Fig. 5a shows the voltage over the first 11 cycles. Upon applying the temperature difference, the voltage increases and stabilizes at approximately 25 mV. The QTEC exhibits reproducible cycles of rapid discharge and recovery in the quasi-continuous discharge mode. The output performance in the 11th cycle (Fig. S28) shows a maximum power density of 40.2 mW m⁻², close to the initial value of 45.5 mW m⁻². To enable extended cycling, the thermocell was rested at room temperature (ΔT = 0 K) after every 11 charge–discharge cycles to allow recovery to its initial state. The excellent longevity was demonstrated by a stable thermovoltage of ∼24.1 mV even after 110 cycles (Fig. 5b). During this process, a minor voltage drop is also observed every 11 cycles due to polarization effects. The spontaneous voltage recovery arises from rapid redox ion diffusion that restores local concentration gradients at the electrode interfaces during the open circuit intervals. Furthermore, Hybrid-E₇T₂₃ maintains a stable voltage of ∼49.8 mV under a higher ΔT of 20 K as shown in Fig. 5c, demonstrating its potential for sustained heat-to-electricity conversion in a wide range of ΔT. Under a 2000 Ω load, the thermovoltage peaked and then decayed gradually, while the current density initially increased and then dropped (Fig. S29a). After 2 hours, the device still delivered ∼16.9 mV and ∼1.03 A m⁻², yielding an energy density of 234 J m⁻² (Fig. S29b).

Fig. 5 Potential application demonstration. (a) Voltage evolution along with the applied temperature difference in the quasi-continuous discharge process for the first 11 cycles. (b) Extended voltage life-span for 110 cycles at ΔT = 10 K. (c) Stability of the thermal voltage generated by Hybrid-E₇T₂₃ for 3 hours. (d) Schematic illustration of the integrated module with 20 coupled Hybrid-E₇T₂₃-based QTEC units. (e) Voltage–time curve and (f) the power electronic device for the module at ΔT = 30 K.

As a harvester of low-grade waste heat, a single QTEC module typically generates insufficient output voltage for practical applications due to the small temperature gradients involved. Device integration is often needed to meet application requirements. We connected 20 individual Hybrid-E₇T₂₃-based QTEC units in series to construct an energy-harvesting device capable of generating useful voltages. As illustrated in Fig. 5d and S30, the device was fabricated by segmenting the Hybrid-E₇T₂₃ hydrogel electrolyte, using graphite plates as electrodes, and connecting the units with copper tape. The assembly was encapsulated in an aluminum plastic film to ensure operational stability by shielding it from external interference. When operated at a moderate ΔT of 30 K, the integrated device generated a high output voltage of ∼1.26 V, corresponding to a notable S_e of ∼42 mV K⁻¹ (Fig. 5e). This performance demonstrates the device's ability to efficiently convert thermal energy into electricity. To assess its practical utility, the device was used to power electronic components. A light-emitting diode (LED) array was successfully illuminated under ΔT = 30 K (Fig. 5f). These results confirm the scalability of the Hybrid-E₇T₂₃ electrolyte and its practical potential for harvesting accessible low-grade waste heat in real-world applications.

3 Conclusion

In summary, we developed a co-solvent hydrogel electrolyte for QTECs that exhibits pronounced multiple synergetic effects for low-grade heat harvesting. The co-solvent hydrogel, composed of EG and TMP, leverages robust solvation ability to engineer an asymmetric solvation environment for the Fe(CN)₆^4−/3− redox couple. This environment simultaneously enhances the solvation entropy difference and amplifies the concentration gradient of redox ions. The co-solvent-induced restructuring of PAAm coordination sites also promotes differential binding of Fe(CN)₆^4−/3− ions. In addition, the co-solvents facilitate salt dissociation that favors continuum ion transport through the hydrogel. Beyond improving thermoelectrochemical properties, the reconfigured hydrogen-bond network endows the Hybrid-E₇T₂₃ electrolyte with anti-freezing capability and long-term stability. As a result, Hybrid-E₇T₂₃ delivers exceptional performance metrics: a S_e increased from 1.32 to 2.52 mV K⁻¹, a normalized power density (P_max/ΔT²) of 0.513 mW m⁻² K⁻², and sustained operation down to sub-zero conditions.

This study demonstrates that co-solvent interactions provide a viable strategy for tuning the solvation environment of redox ions in thermogalvanic hydrogels. The Hybrid-E₇T₂₃ system exemplifies this strategy, wherein maximizing the asymmetry between solvation structures resolves the trade-off between thermopower and ionic conductivity. Importantly, the performance enhancement is achieved through a simple, additive-free formulation. Our work presents fresh evidence for the principle that macroscopic properties can be engineered via molecular-level solvation control, thereby reinforcing its potential for the rational design of diverse high-performance, ion-conducting materials.

Author contributions

Wentao Lin: conceptulization, investigation, data curation, formal analysis, writing – original draft, visualization; Shuo Niu: computations, formal analysis, writing-simulation results, visualization; Shukai Wu: methodology, validation; Chao Fang: conceptulization, supervision, analysis, visulization, writing – original draft, review, editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6sc03942a.

Acknowledgements

This work was supported by the Guangzhou Basic and Applied Basic Research Scheme (No. 2025A04J3996) and Guangzhou-HKUST(GZ) Joint Funding Scheme (No. 2025A03J3872). Computations were performed at the HPC-AI platform of HKUST(GZ). Material characterization studies were performed at MCPF and BEST Lab of HKUST-GZ.

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