Modulating thermo-diffusion/galvanic coupling via ion speciation engineering enables high-performance ionic thermoelectric cells

Domain: neuroscience
Published: 2026-01-31
Editorial outcome: major_revision

Abstract
Ionic thermoelectric (i-TE) materials have demonstrated a high thermopower in harvesting low-grade heat, emerging as superior candidates for self-powered electronics. However, coupling two i-TE effects in n-type materials is scarce, which restricts the development of high-performance systems. Herein, we uncover an overlooked thermogalvanic redox reaction between Cu2+ and Cu+ stabilized by Cl⁻ and quantitatively track the progressive reaction process by operando characterization. In binary polyvinyl alcohol (PVA)-CuCl2 gels, an interactive i-TE coupling effect driven by ion speciation is validated, which exhibits an enhanced thermogalvanic redox as CuCl2 concentrations increase while suppressing the thermodiffusion contribution. By distinguishing and quantifying coordination species, we reveal the impact of [Cu-Cl] speciation on the i-TE effect contributions. Correspondingly, a giant thermopower of −30.6 mVK−1 and a remarkable power density of 0.6 mW m-2 K-2 are achieved, respectively, by tuning ion coordination speciation. The long-term power generation exhibits a reversible and sustainable heat-to-electricity conversion. High output voltage of 3.5 V and power of 22 µW are produced in 16-cell i-TE modules when harvesting 15 K. Our findings reveal an interactive thermo-diffusion/galvanic coupling effect based on coordination chemistry, offering a potential design principle for high-performance i-TE materials.

Introduction
Low-grade waste heat, the ubiquitous byproduct of natural or industrial processes, can be generated from solar energy, geothermal energy, manufacturing, and even the human body, representing a massive and untapped resource1–3. Most of this heat exists below 100°C, making it difficult for conventional power generation. Existing technology, such as electronic-thermoelectric generators (e-TEGs), can convert such heat into electricity; however, their high costs (e.g., rare metal elements) and low thermopower (or Seebeck coefficient) (e.g., ~100–200 µVK-1) limit their broad applications4–6. Ionic thermoelectric (i-TE) gels represent an emerging class of soft and flexible materials that generate electricity from low-grade heat sources using ions instead of electrons7,8. These gels are inexpensive to produce, and recent advances have shown they can achieve an exceptionally high thermopower of dozens of mVK-1 (refs. 9,10). However, a bottleneck remains. Some i-TE gels produce high voltages by thermodiffusion of charge carriers, but the output lasts only in short bursts11, while others provide steady power through temperature-dependent redox reaction (thermogalvanic effects), but with disappointingly low voltages12,13. Recent efforts exemplified in the pioneering work by Liu et al., i.e., gelatin-KCl-Fe(CN)6 4-/3- p-type gels, which demonstrate the potential of hybrid i-TE mechanisms14. Their ternary system synergized thermodiffusion and thermogalvanic effects, significantly improving both thermopower and output power.

On the other hand, n-type analogs, essential for constructing practical thermoelectric devices, also undergo similar limitations. There remain underexplored synergy mechanisms or material systems, despite the rapid development over the past years. Thermodiffusion-based gels always employed metal salts or ionic liquids, showing a remarkable negative thermopower, such as polyethylene (PEO)-LiTFSI-EmimBF4 with −15 mVK−1 (ref. 15.), polyvinyl alcohol (PVA)/cellulose-CuCl2 with −26.3 mVK−1 (ref. 16), and bacterial cellulose-CaCl2 with −27.2 mVK−1 (ref. 17). Gels with typical n-type redox couples, including Fe3+/2+ (ref. 18), [Co(bpy)3]3+/2+ (ref. 19), and I-/I3- (refs. 20,21) are widely investigated, showing a sustainable output performance. The above research optimized i-TE properties either in thermopower or output power, but still cannot realize them simultaneously. A key unexplored opportunity lies in coupling thermodiffusion and thermogalvanic effects in n-type systems. As a model system, we uncover a hidden thermogalvanic mechanism in CuCl2-based gels, in which Cl− ions regulate the interaction of interfacial redox reactions and bulk thermodiffusion, enabling the achievement of continuous output current from electrodes to an external circuit and high built-in thermal voltage, respectively. Inspired by the early studies on copper-based redox flow batteries22,23, Cu+ ions can be stabilized through coordination chemistry via halogen (e.g., Cl⁻). Temperature gradient induces reversible Cu2+ + e↔Cu+ redox reaction, stabilized by Cl⁻ ligands to form [Cu-Cly-(H2O)6-y]2-y/[Cu-Cl2]⁻ (y=0-2, which represents the coordination number of Cl⁻) redox couples. Crucially, Cl⁻ plays dual roles: one is as the thermodiffusion carriers under the Soret effect, generating negative thermopower; the other is as coordination ligands for Cu2+/Cu+ redox couples, producing a positive temperature coefficient (equivalent to negative thermopower). These two i-TE processes exhibit an interactive coupling due to the competition of Cl⁻, and their relative contributions can be regulated by adjusting the concentration of CuCl2. This is relatively different from the reported p-type gelatin-KCl-FeCN4-/3- synergetic i-TE system, in which the KCl and FeCN4-/3- were employed as thermodiffusion ion providers and thermogalvanic redox couple, respectively, showing a “synergistic-coupling” effect in parallel. Particularly, increasing CuCl2 concentration promotes the output performance (more thermogalvanic redox species) while suppressing the thermal voltage. Modulating the thermo-diffusion/galvanic coupling via speciation engineering offers a rational strategy to achieve both high voltage and continuous power output within a single system, addressing the long-standing trade-off between the intermittent behavior of i-TE capacitors and the low voltage of i-TE generators.

Unlike prior hybrid designs that require complex multi-component gels24,25, our work reports a simple binary system guided by coordination chemistry, specifically PVA-CuCl2, which can surpass state-of-the-art n-type materials. For the first time, operando surface-enhanced Raman scattering (SERS) was employed to quantify the coordination evolution, in combination with density functional theory (DFT) calculations, to reveal thermogalvanic redox pathways. In addition, the coordination [Cu-Cl] complexes were identified using UV–vis spectroscopy combined with molecular dynamics (MD) simulations to determine their quantitative distribution, revealing a correlation with the i-TE performance. By modulating ion speciation, we achieved a giant thermopower of −30.6 mVK−1 and a remarkable output power performance of 0.60 mW m−2 K−2 in the PVA-CuCl2 i-TE system. In 16-cell modules, ultra high output voltage of 3.5 V and output power of 22 µW were generated when harvesting a low-grade heat of 15 K. This approach not only circumvents the limitations of the solid-state thermoelectric but also provides a promising strategy to achieve both high thermal voltage and power output in flexible i-TE systems.

Results
Uncovering the overlooked thermogalvanic effect
Copper-based systems have been widely investigated in both i-TE solutions and gels. Copper electrodes were consumed unsustainably in CuSO4 solutions to construct the Cu2+/Cu0 redox couple26. The CuCl2 has been widely employed as a common active salt for thermodiffusion in the previous i-TE gels16,27–30. However, the impact of the thermogalvanic effect, associated with the redox reaction of Cu2+/Cu+ stabilized by the Cl⁻, has been overshadowed in the literature so far. Electrochemical characterization of PVA-xCuCl2 (x from 0.05 to 1.7 M) i-TE cells firstly revealed a pronounced enhancement in thermogalvanic redox reaction activity. The detailed fabrication process of as-prepared i-TE gels and cells was demonstrated in Supplementary Fig. 1. The morphology and mechanical properties of the PVA-1.0M CuCl2 i-TE gels are shown in Supplementary Figs 2 and 3, exhibiting a 3D interconnected and porous structure and high break elongation of ~250%. Cyclic voltammetry (CV) showed a near two-order-of-magnitude increase in faradaic current density (jD in Fig. 1a), while redox reaction peak potentials (ED) shifted cathodically with increasing x (Supplementary Fig. 4). Electrochemical impedance spectroscopy (EIS) further corroborated accelerated kinetics, showing a four-order-of-magnitude reduction in charge transfer resistance (Rct) (Fig. 1b and c, and Supplementary Fig. 5). These trends align with the growing population of redox reaction couples, which facilitate faster electron transfer. The interplay between thermodiffusion and thermogalvanic contributions was quantified through thermal voltage (Voc), short-circuit current density (Isc), and instantaneous maximum power density (Pmax) (Fig. 1d and Supplementary Fig. 6). An opposite trend of i-TE performance was demonstrated: decreasing Voc and increasing Isc with increasing x. This concentration-dependent trade-off underscores the crucial role of speciation in optimizing i-TE performance, specifically the highest thermal voltage at x=0.1 M and the highest output power at x=1.5 M. According to the linear V-I curves, we calculated the internal resistance of PVA-x CuCl2 i-TE cells and discussed feasible strategies of further improving output power performance (Supplementary Fig. 7 and Supplementary Note 1). As a result, normalized output power (Pmax/(ΔT)2) values ranging from 0.09 to 0.43 mW m−2 K−2 were achieved in binary PVA-xCuCl2 gels, as x increased from 0.1 to 1.5 M, showing a great advantage compared with other intrinsic n-type thermogalvanic redox couples, including Fe2+/3+, [Co(bpy)3]2+/3+, and I-/I3- (Fig. 1e and Supplementary Table 1). Furthermore, the thermopower of PVA-0.1M CuCl2 showed a giant value among the reported i-TE gels based on thermodiffusion or thermogalvanic effect (Fig. 1f and Supplementary Table 2). Binary PVA-CuCl2 gels exhibit both promising advantages in thermopower and output performance due to the coupling effect of thermodiffusion and thermogalvanic.

Thermo-diffusion/galvanic coupling mechanism
To investigate the speciation-dependent coupling of thermodiffusion and thermogalvanic effects, we systematically varied the CuCl2 concentrations (x) from 0.01 to 2 M in the PVA-x CuCl2 gels. Density functional theory (DFT) calculations revealed the free energies of possible coordination species and predicted the three prominent redox couples (Fig. 2a, Supplementary Figs. 8 and 9, as detailed in Supplementary Note 2). Three dominant oxidized species with the lowest negative free energy were demonstrated: fully hydrated [Cu-(H2O)6]2+ and chlorinated complexes [Cu-Cl-(H2O)5]⁺ and [Cu-Cl2-(H2O)4] (collectively denoted as [Cu-Cly-(H2O)6-y]2-y, y=0-2). We calculated three possible reduced species (e.g., [Cu-Cl], [Cu-Cl-H2O], and [Cu-Cl2]⁻) and finally identified [Cu-Cl2]⁻ as the most likely formed due to its lowest energy of 4.6 eV31. The operando SERS also helps to verify the reduced species, as discussed below.

Figure 2b shows the detailed ion diffusion process and redox reaction under the temperature gradient (ΔT) in PVA-xCuCl2 systems. When giving a ΔT, the oxidized species of [Cu-Cly-(H2O)6-y]2-y could be reduced to [Cu-Cl2]⁻ at the hot electrode, then the ΔT drives the diffusion of the [Cu-Cl2]⁻ and promotes the oxidation at the cold electrode, thereby forming the reversible redox couple. [Cu-Cl2]⁻ showed a higher solvation entropy than [Cu-Cly-(H2O)6-y]2-y, because of more freedom in the arrangement of water molecule shells due to its simple coordination configuration. For the redox reaction of [Cu-Cly-(H2O)6-y]2-y + (2-y) Cl⁻ + e- ↔ [Cu-Cl2]⁻ + (6-y) H2O, the temperature coefficient (αR) could be defined as αR = (S[Cu-Cl2]⁻ - S[Cu-Cly-(H2O)6-y]2-y)/F > 0, where F is the Faraday constant. On the other hand, the chelation interactions between Cu2+ with the hydroxyls of PVA restricted its translational migration freedom16,28,32, leading to a higher thermal mobility of Cl⁻ (D- Ŝ- /kBT, D-: diffusion coefficient and Ŝ-: entropy)14, which determines a negative thermodiffusive thermopower. Generally, a positive temperature coefficient (negative thermopower attributed to thermogalvanic effect) and a negative thermodiffusive thermopower could be synergistically combined (as detailed in Supplementary Note 3).

Unlike the single synergetic effect14,33,34, the thermo-diffusion/galvanic coupling in PVA-xCuCl2 gels exhibits a trade-off relationship due to the competition of Cl⁻. This mechanism leads to an opposite trend of i-TE performance as x increases: achieving high output power driven by [Cu-Cly-(H2O)6-y]2-y/[Cu-Cl2]⁻ redox couples while decreasing the thermal voltage mainly from the thermodiffusion contribution, which could be attributed to coordinated species evolution. We found that the proportion of [Cu-(H2O)6]2+ decreased from 96.7% to 36.4%, while the [Cu-Cl] species (a joint name of species with Cl⁻ coordination) increased from 2.8% to 49.9% (from MD results), as the x increased from 0.01 to 2 M. Compared with the [Cu-(H2O)6]2+, [Cu-Cl] species showed a weaker hydration structure, suggesting a faster diffusion rate16,35. The suppression of the thermodiffusion effect from two sides: one is the increased cation diffusion coefficient (D+) driven by [Cu-Cl] clusters; the other is that the competition of Cl⁻ in the thermogalvanic redox reaction leads to a decrease of thermal mobility of D- Ŝ- /kBT. The interaction relationship between two i-TE effects could be modulated just by manipulating the ion concentrations, endowing dual i-TE performance in a single system, i.e., high thermopower (at low x of 0.1 M) and output performance (at high x of 1.5 M).

Revealing the thermogalvanic redox process by operando SHINERS
Raman spectroscopy has been used to observe the ion thermodiffusion process at electrode surfaces36 or to reveal the redox reaction in Eastman aqueous thermogalvanic cells2. However, Raman investigations in hydrogel systems are primarily challenged by the weak signals, which also lack quantitative analysis. Here, we employed an advanced SERS technique called shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)37, quantitatively revealing the coordination evolution within the thermogalvanic redox reaction. The synthesis of shell-isolated nanoparticles (SHINs), Au@SiO2 core-shell nanoparticles, is reported in refs. 37,38. The inert silica shell prevented direct interaction between the plasmonic Au cores (Raman signal amplifier) and the samples to be probed, ensuring unperturbed vibrational data. This setup enabled sensitive, non-destructive detection across diverse substrates, leveraging strong enhanced electromagnetic fields at nanoparticle-substrate junctions to amplify signals by millions of times.

In our experiments, SHINs were uniformly deposited onto the surface of i-TE gels (Supplementary Fig. 10). The operando SHINERS test was conducted in a PVA-0.1M CuCl2 gel under a ΔT of 10 K, collecting signals from the top surface of the gel (near cold electrodes TC) (Fig. 3a). As thermal charging time (t) increased from 0 to 30 mins, the Raman peak of [Cu-Cl2]⁻ approximately at 261 cm−1 significantly increased, attributed to migration of reduction product from the TH to the TC (Fig. 3b). While the Raman peak of [Cu-(H2O)6]2+ approximately at 420 cm-1 (refs. 16,30.) gradually decreases, indicating the coordination environment change from [Cu-H2O] to [Cu-Cl]. We also conducted an isotopic substitution (D2O) experiment to verify the [Cu-H2O] Raman peak (Supplementary Fig. 11). Then, without ΔT, the redox reaction stopped, and the species gradually distributed in equilibrium, decreasing the intensity of [Cu-Cl2]⁻. The test was also conducted in the cross-section of the gel to further observe the migration of redox species inside the gel (Supplementary Fig. 12). To correlate coordination species changes with valence states, we complemented SHINERS with XPS mapping of Cu2+ and Cu+ distribution near the surface of the gel. The Cu2+ (Cu 2p3/2 and Cu 2p1/2 peaks at 935.1 and 955 eV, respectively) and Cu+ (Cu 2p3/2 and Cu 2p1/2 peaks at 932.3 and 952 eV, respectively) were detected at both sides after thermal charging, showing a larger intensity of Cu2+ than Cu+ at the gel bottom while reversing at the gel top27,28,30 (Fig. 3c). Not only in the gel surface, but we also combined the X-ray absorption near-edge structure (XANES) characterization to further confirm the existence of Cu2+/Cu+ redox couple ions in the i-TE gel bulk (Supplementary Fig. 13). Furthermore, we proposed a methodology to quantify a normalized [Cu-Cl] factor defined by integrating the [Cu-Cl] Raman peak area. The [Cu-Cl] factors increased from 1 to 3.58 as t increased from 0 to 30 mins and then decreased to 1.80 without ΔT, corresponding to the thermal voltage decreased from -2 to -234 mV, and then recovered to -9 mV (Fig. 3d). The higher [Cu-Cl] factors were observed at higher x, suggesting promoted thermogalvanic redox reactions (Supplementary Fig. 14, Tables 3 and 4). Our experimental characterization, combining SHINERS with surface analytical XPS characterization, is readily adaptable to other redox reaction-active hydrogels or soft-material systems, enabling precise mapping of both coordination and electronic states under operando conditions.

We further verified the redox reaction by electrochemical characterization. One PVA-0.1M CuCl2 i-TE cell was operated with four thermal charging-reactivation cycles (Supplementary Fig. 15). After the first cycle, the jsc and Rct showed a significant increase and decrease, respectively, indicating that the induced redox species facilitated the kinetics (Supplementary Fig. 16). We also compared the redox behaviors of i-TE systems with CuSO4 and Cu(NO3)2, in which only [Cu-(H2O)6]2+ species existed from UV-vis spectra (Supplementary Fig. 17). In the three-electrode CV test, two obvious anodic peaks were discovered in CuCl2 solution due to the intermediate of Cu+, while only one anodic peak corresponding to Cu0 to Cu2+ was observed in CuSO4/Cu(NO3)2 solutions (Fig. 3e). Correspondingly, a pair of redox reaction peaks were observed only in the PVA-0.1M CuCl2 i-TE cell (Fig. 3f). The additional CaCl2 doping could further increased redox species from CV results (Supplementary Fig. 18). The UV-vis spectra of PVA-0.1/2M CuCl2/CaCl2 gels showed a broad absorption range from 250 to 350 nm, indicating the multiple [Cu-Cl] species (Supplementary Fig. 19). The increased Isc and Pmax by doping CaCl2 revealed the enhanced thermogalvanic effect, also indicating an effective output performance optimization strategy (Supplementary Fig. 20).

Impact of [Cu-Cl] speciation on thermopower
Understanding the chemical speciation of cupric ions in chloride solutions is critical for optimizing thermoelectric energy conversion, in addition to its relevance in geochemical39 and environmental40. However, identifications of multiple chemical speciation in the CuCl2 system have yet to emerge, especially at high concentrations41. Here, we combine molecular dynamics (MD) simulations (as detailed in Supplementary Note 4) with UV-vis spectroscopy (Integrating Sphere device) to resolve [Cu-Cl] speciation across a concentration range (x=0.01 to 2 M). A representative snapshot of the equilibrated molecular configuration at x=2.0 M is shown in Fig. 4a, where multiple coordination species are observed (as listed in the right-side legend). Figure 4b quantitatively maps the distribution of these [Cu-Cl] coordination complexes, revealing a concentration-dependent shift from fully hydration-dominated to chloride-dominated species, including polynuclear clusters. The UV-vis spectra showed a series of absorption peaks due to the ligand-to-metal charge transfer (LMCT) effect41, suggesting the complexity of the species. As the x increased from 0.01 to 2 M, the absorption peaks progressively redshifted from 220 to 400 nm (Fig. 4c), reflecting the growing contribution of higher-order [Cux-Cly]2x-y clusters42,43, which is attributed to entropy-driven solvation restructuring. This is the key factor when correlating the decreased thermopower trend with the increased x, which will be discussed below. We conducted principal component analysis (PCA) on UV-vis spectra to estimate the number of primary species in the gels44,45, showing an increased number from 2 to 5 as x increased from 0.05 to 2 M (Fig. 4d and Supplementary Fig. 21, as detailed in Supplementary Note 5). Correspondingly, the proportion of [Cu-Cl] species increased from 11.9% to 49.9% (Supplementary Fig. 22). These quantitative resolutions of [Cu-Cl] speciation provide a foundation for tailoring coordination species in thermoelectric systems.

Next, we correlated the output voltage performance with x. The thermal voltages showed a significant decrease with the increased x (Fig. 4e and Supplementary Figs. 23, 24, and 25). We attributed such trend to an interactive balance between two i-TE processes: thermodiffusion of [Cu-H2O]2+ and Cl⁻ (at low x), in which the high migration entropy difference generates a large ΔVtd; and thermogalvanic reaction (at high x) driven by redox couple, leading to a smaller ΔE0 (Supplementary Fig. 26) (ref. 14.). This interactive balance explains the non-monotonic trend in thermopower Si (Fig. 4f): At low concentration (0.01 M ≤ x ≤ 0.07 M), the thermopower increased from −19.5 to −28.7 mVK−1; at intermediate concentration (0.1 to 0.7 M), the thermopower achieved a maximum value of −30.6 mVK−1 at x=0.1 M and then gradually decreased; and at high concentration (x ≥ 1 M), enhanced thermogalvanic effects collapse the thermopower to −5.9 mVK−1 (x=2.0 M). We also use an isothermal three-electrode system to determine the temperature coefficient (αR) of PVA-xCuCl2 gels (Supplementary Fig. 27). The values were finally determined as 0.58, 0.64, 1.32, 1.54, 1.12, 1.05, and 0.98 mVK−1 as x = 0.01, 0.05, 0.1, 0.5, 1.0, 1.5, and 2.0 M (Supplementary Figs. 28 and 29). The calculated thermopower contribution of two i-TE mechanisms exhibited an opposite trend, which is consistent with the coupling relationship (Supplementary Fig. 30). Combining the thermopower performance with MD results, we found that the dominant existing of [Cu-(H2O)6]2+, [Cu-Cl-(H2O)5]⁺, and [Cu-Cl2-(H2O)4] (0.1 ≤ x ≤ 0.7 M) showed benefits on regulating both thermodiffusive and thermogalvanic thermopower (−α), while the formation of higher-order [Cux-Cly]2x-y (e.g., [Cu2-Cl2-(H2O)8]2+ and [Cu2-Cl3-(H2O)8]+ etc., at x ≥ 1 M) declined thermopower of both parts. Critically, we identify an optimal species distribution for maximizing thermopower: approximately 84.6% [Cu-(H2O)6]2+, 12.6% [Cu-Cl-(H2O)5]⁺, and 2.8% [Cu-Cl2-(H2O)4], achieved in PVA-0.1M CuCl2 gels (Supplementary Fig. 22).

Except for [Cu-Cl] coordination species, other divalent metal chloride gels only showed water coordination environments (Fig. 4g). Compared with other divalent metal chlorides, only PVA-0.1M CuCl2 exhibited the thermogalvanic property and achieved the highest thermopower of −30.6 mVK−2 (Fig. 4h and Supplementary Fig. 31), showing a unique advantage of [Cu-Cl] coordination species on regulating thermopower. We also found that the increased weight ratio of PVA to water (rw) can further enhance thermal voltages, probably by enhancing the chelation of PVA and Cu2+ (Supplementary Fig. 32). As a result, the thermal voltages of −73, −132, −156, −171, and −181 mV were optimized in PVA (15 wt%)-0.03M CuCl2 gel, also showing giant values within ΔT of 0–5 K (Fig. 4i, Supplementary Fig. 33 and Table 5).

Optimization of output performance and long-term power generation
A chloride-rich environment has a direct impact on boosting the output power performance in our i-TE cells by stabilizing more Cu2+/Cu+ redox couples in the thermogalvanic process. In this part, we further increased the amount of the redox couple by enriching the content of Cl⁻. Different metal chlorides with the same Cl⁻ content (0.5 M) were doped into the PVA-0.5M CuCl2 i-TE cells, showing enhanced redox reaction kinetics but leading to declines in Voc (Supplementary Fig. 34A to C). As a result, the optimized Pmax was achieved by introducing the 0.25M CaCl2 (Fig. 5a and Supplementary Fig. 34D). Then, the concentrations of CuCl2 were optimized, and the highest Pmax value of 37 mW m-2 was obtained in the PVA-1.5/0.25M CuCl2/CaCl2 (Supplementary Fig. 35). Finally, the concentrations of CaCl2 were further adjusted in the PVA-1.5/xM CuCl2/CaCl2 (x = 0.125, 0.25, 0.5, and 1.0 M) i-TE cells. The MD result shows little regulation in [Cu-Cl] coordination species upon adding 0.25M CaCl2, indicating that the increased Cl⁻ shows a dominant role in stabilizing Cu+ during the redox reaction (Supplementary Fig. 36). A 40-nm-thick gold layer was deposited on carbon paper electrodes (Au@CP) to further accelerate the electron transfer process46. The highest Pmax value of 60 mW m-2 was achieved in Au@CP|PVA-1.5/0.25M CuCl2/CaCl2|Au@CP i-TE cell (Fig. 5b). The opposite trend between Voc and Isc also showed the trade-off relationship as increasing Cl⁻ content. The EIS plots showed a continuous decrease in Rct with increasing CaCl2 concentrations and modifying carbon paper electrodes (Fig. 5c), indicating the facilitation of redox reaction near the electrode interface. Except for Pmax, the continuous energy harvesting and output were illustrated using the i-TE generator and capacitor working modes (Supplementary Fig. 37). The i-TE generator of the CP|PVA-1.5/0.25M CuCl2/CaCl2|CP cell showed a continuous voltage and current output at a ΔT of 10 K (Fig. 5d). The highest energy density of 2h (E2h) of 1.5 J m−2 was achieved when loading an external resistance of 30 kΩ (Supplementary Fig. 38). The heat-to-energy efficiency of the i-TE generator was calculated at η=0.02% (as detailed in Supplementary Note 6 and Supplementary Fig. 39). The i-TE capacitor of the CP|PVA-0.1M CuCl2|CP cell showed a charge storage property at a ΔT of 10 K and capacitor discharging without ΔT (Fig. 5e). The highest energy density of 1h (E1h) of 0.004 J m-2 was achieved when loading a 50 kΩ resistance (Supplementary Fig. 40). The i-TE generator showed a significant advantage in continuous power output, compared with the energy storage property by using the i-TE capacitor mode (Fig. 5f). We also tested the long-time (4h) output current and calculated normalized output electric charge factors, showing a significantly enhanced thermogalvanic redox contribution with increased CuCl2 concentrations (Supplementary Fig. 41). Under near-short-circuit conditions, the CP|PVA-1.0M CuCl2|CP i-TE cell exhibited a high short-circuit current (Isc) peak value of 181 µA and continuous Isc (approximately 0.8 µA) output for 1 hour, showing the sustained redox-cycled self-current in high-concentration composition (Supplementary Fig. 42). Furthermore, twenty cycles were performed to show the sustainable heat-to-energy conversion of the i-TE generator. In the long-term power generation, the reactivation stage is modified from self-recovery to drive a reverse ΔT and directly connected with the voltage build-up stage of the next cycle without rest (Fig. 5g, as detailed in Supplementary Note 7). The curve of long-term output voltage and current versus time is shown in Supplementary Fig. 43. The thermal voltage and maximum output current showed a retention of 91% and 90%, respectively, after a 20-cycle power generation. The total harvesting output energy density of the long-time duration test (nearly 30h) is 15.9 J m−2, suggesting the potential application in the self-powered wearable sensors (Fig. 5h). The good cycle performance showed the reversibility of the [Cu-Cly-(H2O)6-y]2-y/[Cu-Cl2]⁻ redox couple and sustainable long-term service of the i-TE cells.

16-cell i-TE module
Finally, a 16-cell i-TE module was assembled in series, as shown in (Fig. 6a). Here, PVA-1.5/0.25M CuCl2/CaCl2 gel with Au@CP electrode and PVA (15 wt%)-0.03M gel with CP electrode were used as i-TE units to fabricate two types of modules, called the generator and capacitor module, respectively. The Voc, Isc, and Pmax produced by the generator module were 0.9 V, 60 µA, and 22 µW, respectively, with a ΔT of 15 K (Fig. 6b, c). The module could directly drive a thermohydrometer when harvesting a ΔT of 20 K (Fig. 6d), owing to its high output power. Figure 6e showed the ultra high thermal voltage of 3.5 V generated with a ΔT of 15 K using a capacitor module (Fig. 6e). The generated voltage is enough to drive different sensors without additional DC-DC voltage boosters, such as pressure sensors (1.3 to 3.6 V), humidity sensors (1.6 to 3.6 V), and gas sensors (1.8 to 3.6 V)14. We also demonstrated a showcase for heat-to-energy conversion by attaching a 9-cell capacitor module to a cup containing warm water, showing a sustainable output voltage of approximately 320 mV (Fig. 6f). Furthermore, the quick thermal charging time of 30 s exhibits a quick response of the i-TE module to temperature fluctuation. The high-performance n-type i-TE gels reveal great potential for applications in low-grade heat harvesting, temperature monitoring, and sensing.

Methods
Materials
The raw materials are shown as follows: poly (vinyl alcohol) (MW= 89000~98000, 95%), CuSO4 (MW=159.61, 99%), Cu(NO3)2.xH2O (MW=187.56, 99.0%), Na2SO4 (MW=142.04, 99.5%), KCl (MW=74.55, 99.5%), CaCl2 (MW=110.98, 99.99%), MgCl2 (MW=95.21, 99.99%), CuCl2 (MW=134.45, 98%), ZnCl2 (MW=136.3, 99.95%), CoCl2.6H2O (MW=237.93, 99.99%), NiCl2 (MW=129.6, 99%) were provided by Macklin Biochemical CO., Ltd. NaCl (MW=58.44, 99.5%), AlCl3 (MW=133.34, 99%) were purchased from Aladdin Industrial Corporation. Carbon paper (TGP-H-060) was purchased from Toray Industries. Inc. (Japan).

Preparation of PVA hydrogel
The PVA hydrogel was prepared by using the freeze-thaw method. In this work, we regulated the weight ratio of PVA to water (rw = 7.5%, 10%, 12.5%, and 15%). As an example, for preparing 10 wt% PVA hydrogel, 6 g of PVA powder was first dissolved and stirred in 54 mL DI water for 30 min. Then, the PVA solution was heated and stirred in a water bath at 95°C for 6 h. Finally, the as-prepared solution was defoamed and poured into a mold (size: 15mm×15mm×3mm (length×width×height)), frozen at −20°C for 6 h and thawed at 25°C for 1 h, repeating three times, to obtain the PVA hydrogel. The mold was prepared by using the Bambu Lab X1E 3D printer.

Preparation of PVA-based i-TE gels
The general formula of the i-TE gels was PVA(rw)-xMX (x: the molar concentration of MX salts; MX = NaCl, KCl, CuSO4, Cu(NO3)2, CuCl2, CaCl2, ZnCl2, MgCl2, CoCl2, NiCl2, and AlCl3). All the i-TE gels in this work were prepared via immersing the PVA hydrogels in the corresponding salt solutions. As an example, for preparing the PVA (10 wt.%)- 1.5/0.25M CuCl2/CaCl2 i-TE gel: firstly, the 10 wt.% PVA hydrogel was prepared as the above method; then 4.034 g CuCl2 and 0.555 g CaCl2 were dissolved into 20 mL DI water to obtain the mixture salt solution; finally, the PVA hydrogel was immersed in the solution for 12 h and obtained the i-TE gel.

Fabrication of i-TE cells
Carbon paper was used as the electrodes for assembling the i-TE cells with a 3 mm-thick sandwich structure CP|i-TE gel|CP. The size of the as-prepared i-TE gel is 15mm×15mm×3mm, and the size of the carbon paper is 25×25mm. The cell molds are identical to those used in the PVA preparation process. To optimize the output power, gold-coated carbon paper was also employed as electrodes46. Gold (Au) sputtering on the carbon paper was conducted using a Denton Vacuum Desktop Pro system at an argon pressure of 20 mTorr and an operating power of 120W for 40s, resulting in a film thickness of approximately 40 nm. The as-fabricated i-TE cell was encapsulated in a polyethylene film using a hot press machine to prevent the gel from losing water.

Fabrication of the i-TE module
The 16-cell module was assembled in series using titanium (Ti) taps. The size of each i-TE unit was 10mm×10mm×3mm, which can be uniformed prepared by the 3D-printed molds. The size of Au@CP and the CP electrode was 11×11mm. The as-fabricated i-TE module was encapsulated in a polyethylene film using a hot press machine. The temperature difference was generated by the heat plate and the semiconductor cooling platform. Other performance tests are the same as those of the i-TE cell.

Ultraviolet-visible (UV-vis) spectroscopy (Integrating Sphere) characterization
The Ultraviolet-visible (UV-vis) spectroscopy was conducted on an Agilent Cary 5000 spectrometer using an integral ball accessory. The absorption signals of UV-vis are always unlimited when the CuCl2 concentrations are higher than 0.1M. Herein, an integrating sphere was employed to select the diffuse reflection signals of i-TE gels and then to achieve the corresponding absorption intensity. The size of the gels is 15mm×15mm×0.5mm. In this work, all the UV-vis spectra of the gels were obtained by using this method.

X-ray photoelectron spectroscopy (XPS) characterization
The XPS test was conducted on a Thermo-Scientific Escalab QXi microprobe. The gels were immediately taken out from the i-TE cells after thermal charging with a ΔT of 10 K for 30 min and freeze-dried for 48 h to obtain the dried samples. A ΔT of 10 K was constructed from the bottom to the top across the i-TE gel. The gel top is near the cold side of the electrode (TC) while the gel bottom is near the hot side of the electrode (TH).

X-ray absorption near-edge structure (XANES) characterization
The XANES spectra at Cu K-edge were performed at BL11B beamline of Shanghai Synchrotron Radiation Facility. The data were obtained under ambient conditions in fluorescence mode for the Cu K-edge. The gel sample is a freeze-dried PVA-0.1M CuCl2 gel that has been thermally charged for 30 min. The CuO and Cu2O were used as the standard samples to confirm the copper electron states in the i-TE gel.

Operando Raman spectra characterization
The Xplora confocal microprobe Raman system (HORIBA) was used to record the Raman spectra. All measurements were performed using a He-Ne laser with an excitation wavelength of 637.8 nm and a 50x long working distance (8mm) microscope objective with a numerical aperture of 0.55. The Raman frequency was calibrated using a silicon wafer in each experiment. In this study, PVA-CuCl2 gels containing different concentrations of CuCl2 (0.01, 0.05, 0.1, 0.5, and 1 M) were placed in a specially designed temperature-controlled Raman cell for operando testing of the gel surface and cross-sections. The temperature difference was controlled at 10 K, crossing from the bottom to the top in the gel top surface test, and from left to right in the cross-section test, respectively. The humidity was maintained at 60%. In addition, PVA-0.01M CuCl2 gel was soaked in heavy water (D2O) for 12 h, and isotopes were used to confirm that the peak at 420 cm−1 is the Cu-O stretching vibration of [Cu-H2O]. Here, we defined a normalized [Cu-Cl] factor to quantify the coordination species change. For the original state of PVA-0.1M CuCl2 i-TE gels, the Raman peak from 215.2 to 296.5 cm−1 of [Cu-Cl] species, and the corresponding integration area was 11717. We defined the [Cu-Cl] factor of this state as 1. The factors at different reaction times could be calculated by making a ratio between the integrated [Cu-Cl] peak area with 11717. Taking PVA-0.1M CuCl2 gel as an example, as the thermal charging time increased from 5 to 10, 15, 20, 25, and 30 mins, the integrated [Cu-Cl] Raman peak area was 13357, 16404, 23786, 31402, 36557, and 41947, respectively, corresponding to the normalized [Cu-Cl] factor increased from 1 to 1.14, 1.40, 2.03, 2.68, 3.12, and 3.58. The normalized [Cu-Cl] factors of others samples with different CuCl2 concentrations were achieved by using the same method.

Electrochemical characterization
The electrochemical characterization, including Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) test, was measured on an electrochemical workstation (Admiral Instruments, America). For aqueous salt solutions, CV test scanning from −0.6 V to +0.8 V was conducted with a three-electrode configuration, carbon paper (size: 1.5cm×1.5cm) was used as the working electrode and platinum (size: 1cm×1cm) and Ag/AgCl electrodes were used as the counter and reference electrodes, respectively. 2M Na2SO4 was employed as the supporting electrolyte in aqueous media to reduce the ohmic overpotential. For symmetrical i-TE cells, CV test scanning from −0.6 and +0.6V was conducted with a two-electrode configuration, in which one carbon paper electrode served as the working electrode while another was employed as the counter and reference electrodes simultaneously. The EIS was adopted to analyze the resistance properties of the cells between 0.05 Hz to 10000 Hz at the AC amplitude of 10 mV. The ionic conductivities of gels were calculated using Eq. 1

σ = l / RS (1)

where l, S, and R are the thickness, the cross-sectional area, and the resistance of the i-TE cells, respectively. The R is the first interception of the high-frequency semicircle (fitted Rs) in the results of the EIS measurement.

I-TE performance measurement. The ionic thermoelectric performance was measured using an Ionic Thermoelectric Testing System (ITTS, PURI Materials). The thermal voltage (ΔV) and temperature difference (ΔT) were recorded using a built-in test program. Thermopower was the slope of ΔV versus ΔT. The thermopower (Si) was calculated according to Eq. 2:

Si = (V(TH) - V(TC)) / ΔT (2)

Where Si is the thermopower, V(TH) and V(TC) are the potential at the hot side and cold sides of the i-TE cells, respectively. TH and TC are the temperatures at the hot and cold sides of the i-TE cells, respectively. The ΔT was set to 1, 2, 3, 4, and 5K, and the cold side (TC) was maintained at 298.15K.

The instantaneous output power was measured using a Keithley-2400 at a voltage-current (V-I) discharge at a ΔT of 10 K. TC was fixed at 298.15K. For normalized maximum power density, the distance between the two electrodes was 0.3 cm, and the cross-sectional area of the electrode was 2.25 cm−2. The normalized instantaneous output power density (Pmax) was calculated according to Eq. 3:

Pmax = Voc * Isc / 4 (3)

Where Voc and Isc are the open-circuit voltage and short-circuit density, respectively.

The continuous output voltage and current were measured using the ITTS at a ΔT of 10 K. TC was fixed at 298.15K. The energy density (E) was calculated according to Eq. 4:

E = ∫0t V(t)I(t)dt (4)

Where the V(t) and I(t) are the output voltage and current dependent on t, t is the output power time.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This study reports that the competitive-coupling effect, driven by speciation, facilitates the development of high-performance and sustainable n-type ionic thermoelectric (i-TE) cells. The authors identified a novel n-type redox couple, Cu²⁺/Cu⁺, stabilized by Cl⁻ ions, and demonstrated a competitive-coupling i-TE mechanism, driven by speciation, in polyvinyl alcohol (PVA)-CuCl₂ gels. This mechanism enhances the thermogalvanic effect while suppressing the thermodiffusion of Cl⁻ ions as CuCl₂ concentration increases. By modulating ion-coordination speciation, the system achieves a substantial thermopower of -30.6 mV K⁻¹ and an outstanding power density of 0.6 mW m⁻² K⁻². Furthermore, a 16-cell module generates a thermal voltage of 3.5 V and an output power of 22 µW under a temperature difference (ΔT) of 15 K. While the research topic is intriguing, the evidence supporting the competitive-coupling i-TE mechanism is insufficient. Several critical issues require major revision:\n(1) The \"competitive-coupling\" mechanism proposed by the authors in the article is essentially a linear superposition of the Soret and thermogalvanic effects, weighted by ionic concentration. It may be more accurate to characterize this as a synergistic effect modulated by ion concentration.\n(2) To establish a correlation between coordination species changes and valence states, the authors employed SHINERS alongside XPS mapping of Cu²⁺ and Cu⁺ distribution near the gel surface. However, given that XPS sampling is limited to a few nanometers, while ion migration in gels occurs on the micrometer scale, it is advisable to supplement the analysis with Cu K-edge XANES. This would confirm whether the valence state distribution observed at the surface is representative of the bulk gel.\n(3) In the main text on page 8, Fig. 3c and Fig. 3d are cited out of sequence and should be swapped to align with their actual order in Fig. 3.\n(4) The thermoelectric conversion efficiency should be included to provide a comprehensive evaluation of i-TE cell performance.\n(5) The reported energy density and operating time (2 h) are not particularly remarkable. A longer-duration test is essential for practical applications, particularly in self-powered wearable sensors.\n(6) In this section, the authors increased the redox couple concentration by enriching Cl⁻ content, achieving an optimized Pmax with 0.25 M CaCl₂. However, it remains unclear whether Ca²⁺ participates in coordination or merely exerts an ionic-strength effect.", "reviewer_2": "This manuscript reports the development of a high-performance n-type i-TE material by coupling two synergistic effects. The i-TE material is the hot research spot of the field, especially the high-performance n-type ones. This work is well prepared by exploring new way to improve the i-TE performance with both experimental tests and theoretical simulation. I would recommend its publication. Below are some comments to improve the quality.\n(1) Photos of fabricated samples, and corresponding structure, composition and morphology characterization as well as mechanical properties upon varying key component contents are needed, to prove the achievement of the material. This also applies to the i-TE device with close look at the details the device such as the size and the thickness.\n(2) Drying of hydrogel is a key issue hindering the practical long-term use. How does the developed one dry over time, and is there any anti-drying treatment used?\n(3) Cycling charge-discharge test of the assembled i-TE cells is only done with 10 cycles, which is too small. More cycles are needed, to verify the repeatability and consistence.", "reviewer_3": "The authors report a CuCl₂-based binary ionic thermoelectric (i-TE) system in which Cl⁻ ions are proposed to regulate the coupling between interfacial redox reactions and bulk thermodiffusion, leading to a reported thermopower of −30.6 mV K⁻¹. The work combines experimental thermopower characterization with molecular dynamics (MD) simulations to rationalize the distribution of Cu–Cl–H₂O complexes. The topic is timely and relevant to the expanding field of ionic thermoelectrics, and the data appear internally consistent.\nHowever, the mechanistic interpretation and novelty require substantial clarification. The reported transient current profiles, limited thermogalvanic (TG) quantification, and incomplete benchmarking against the state of the art raise significant questions about the claimed TG-dominated mechanism. Substantial revision, including direct quantification of the TG contribution, clarified novelty, and improved benchmarking, would be necessary for to justify publication in this journal.\n1. Novelty and Relation to Previous Work: The claimed TG-in-TD mechanism is conceptually similar to prior reports: eg. Han et al., Science 2020, 368, 1091–1098 and Zeng et al., Adv. Energy Mater. 2023, 13, 2204357. Both studies describe synergistic thermodiffusion and thermogalvanic contributions producing large thermopower. Although the current materials differ in composition (n-type), the mechanistic framework and observed behavior appear analogous. The authors should clearly delineate what is genuinely new in this work—whether in chemical design, redox kinetics, or thermodynamic coupling—and avoid implying a fundamentally new mechanism without quantitative differentiation.\n2. Transient versus Steady-State Current Behavior: The long-term current–voltage data (Supporting Fig. 34) display sharp transient spikes that rapidly decay to near zero, rather than steady currents under ΔT. This response is typical of thermodiffusion-dominated systems, not TG-driven ones. If a true thermogalvanic process were dominant, a sustained redox-driven current would persist as long as the temperature gradient remains, as in the TG + TD system of Han et al. (Science 2020, 368, 1091–1098). The combination of transient decay and low open-circuit voltage relative to Cu²⁺/Cu⁺ redox potentials weakens the mechanistic conclusion. The authors should provide quantitative TG validation e.g., steady-state current monitoring, or equivalent circuit separation of ionic vs. redox contributions.\n3. Interpretation of “Simultaneous High Current and Voltage”: The claim of achieving both large current output and high voltage is misleading. The CuCl₂-based gel cannot transport electrons internally; current appears only when an external load provides an electronic pathway. The authors should correct this statement to specify that current flow arises from external-circuit conduction, not from intrinsic electronic transport in the gel.\n4. Quantitative Estimation of TG Contribution: While the XPS results suggest Cu²⁺/Cu⁺ redox involvement, peak-area ratios cannot be directly correlated with current density or Seebeck enhancement. Without quantitative estimation of the TG fraction, the mechanistic claim remains speculative. Given the non-sustained current profile, the system appears thermodiffusion-dominated with a limited or transient TG component. The authors should revisit the data interpretation and possibly reframe the paper around TD-enhanced ionic thermopower rather than TG dominance.\n5. Comparison with the State of the Art: Figure 1f and Supporting Table 1 and 2 omit key recent references. For example, a recent report in Energy & Environmental Science (Energy Environ. Sci., 2025,18, 1294-1309) achieved thermopower exceeding −40 mV K⁻¹ at ΔT = 5.3 K. Accordingly, the statement “Binary PVA–CuCl₂ gels exhibit remarkable advantages in thermopower and output performance compared to other reported multicomponent n-type i-TE systems” is overstated and should be revised. Comprehensive benchmarking with the latest literature is required for an accurate contextual comparison.\n6. Power Density Discrepancy: The reported power density of this work (binary PVA–CuCl₂ gel) is orders of magnitude lower than comparable systems: For eg. Adv. Energy Mater. 2023 , 13, 2204357. The authors should discuss the physical origin of this low performance—e.g., internal resistance, poor interfacial kinetics, or limited ionic–electronic coupling—and provide quantitative reasoning for why the binary PVA–CuCl₂ gel performs so modestly despite the large thermopower."}, "author_response": "Revision Report for Manuscript NCOMMS-25-68639\nCompetitive-coupling effect driven by speciation enables high-performance and\nsustainable n-type ionic thermoelectric cells\nYuchen Li1†, Ying-Ru Qiu2†, Jinsha Liao3, Helios Y. Li1, Zhouyuntong Liu1,\nYuxiang Chen1, Mao Yu5, Lizhi Xu1, Chang Liu4, Jefferson Zhe Liu3, Jian-Feng Li2,\nWeishu Liu5, Yao-Hui Wang2*, Nicholas X. Fang1*\nWe are very grateful for all three reviewers’ comments on the manuscript which\nmotivated us to carry out new experiments to clarify the concerns of the reviewers. The\nnew explanations and experiments further consolidate our original conclusion. In the\nfollowing, we provide a detailed, point-by-point response to the reviewers' comments.\nWe highlight our critical points in bold type, and figures and cited references in\nlight blue. Revisions made in the main text and supporting information are\nmarked in red.\nReviewer #1:\nGeneral comments:\nThis study reports that the competitive-coupling effect, driven by speciation, facilitates\nthe development of high-performance and sustainable n-type ionic thermoelectric (i-\nTE) cells. The authors identified a novel n-type redox couple, Cu²⁺/Cu⁺, stabilized by\nCl⁻ ions, and demonstrated a competitive-coupling i-TE mechanism, driven by\nspeciation, in polyvinyl alcohol (PVA)-CuCl₂ gels. This mechanism enhances the\nthermogalvanic effect while suppressing the thermodiffusion of Cl⁻ ions as CuCl₂\nconcentration increases. By modulating ion-coordination speciation, the system\nachieves a substantial thermopower of -30.6 mV K⁻¹ and an outstanding power density\nof 0.6 mW m⁻² K⁻². Furthermore, a 16-cell module generates a thermal voltage of 3.5 V\nand an output power of 22 µW under a temperature difference (ΔT) of 15 K. While the\nresearch topic is intriguing, the evidence supporting the competitive-coupling i-TE\nmechanism is insufficient. Several critical issues require major revision:\nResponse:\nThe authors are grateful to the reviewer for the valuable comments. These\ncomments help to improve the quality of our manuscript. We revised our paper carefully\nand performed some necessary experiments according to these comments.\nComment 1:\nThe \"competitive-coupling\" mechanism proposed by the authors in the article is\nessentially a linear superposition of the Soret and thermogalvanic effects, weighted by\nionic concentration. It may be more accurate to characterize this as a synergistic effect\nmodulated by ion concentration.\nResponse:\nWe appreciate the reviewer’s valuable comment. While we agree with the reviewer\nthat concentration is the tuning parameter, we need to emphasize that the underlying\nphysics is a competition between two effects mediated by speciation, not a synergy.\nTherefore, we proposed this “competitive-coupling” i-TE mechanism to\ndistinguish from the reported “synergistic-coupling” i-TE effect [Science, 368,\n1091-1908, (2020)]. In this work, we did lots of verifications on the competitive-\ncoupling relationship between the Soret effect and the thermogalvanic redox reaction\nof Cu2+/Cu+ stabilized by Cl-, by combining operando-SHINERS, DFT calculations,\nand MD simulations.\nWe first explain the competitive relationship by comparing with the p-type\ngelatin-KCl-FeCN4-/3- systems based on a synergistic mechanism [Science, 368,\n1091-1908, (2020)]. In gelatin-KCl-FeCN4-/3- i-TE gels, the KCl and FeCN4-/3- were\nemployed as thermodiffusion ions and thermogalvanic redox couples, respectively.\nTherefore, these two i-TE processes show a synergistic effect in parallel, i.e., one plus\none equals two (Figure R1a). However, in our work based on PVA-CuCl gels, Cl⁻\nacts both as the primary charge carrier for the Soret effect and as an essential\nligand for the Cu2+/Cu+ redox couple in the thermogalvanic effect. Increasing the\nconcentration does not merely \"weight\" two independent processes; it actively\nshifts the system's speciation, diverting Cl⁻ from its role in thermodiffusion to its\nrole in coordination and redox chemistry (Figure R1b). Correspondingly, the i-TE\nperformance data clearly demonstrate an antagonistic relationship: as the CuCl₂\nconcentration increases, the thermogalvanic effect is enhanced (leading to higher output\nshort-circuit current, I ), but this occurs at the direct expense of the thermodiffusion\nsc\neffect (leading to a decrease in open-circuit thermal voltage, V ) (Figure R1c). This\noc\nopposite trend between the thermal voltage and output current is unusual,\ncompared with the reported p-type gelatin-KCl-FeCN4-/3- systems. In gelatin-KCl-\nFeCN4-/3- cells based on “synergistic-coupling” i-TE effect, higher thermal voltage\nresults in a higher output current and power.\nNext, we want to illustrate the understanding of “coupling”. Although the\ncompetitive relationship, thermopower values of the two i-TE effects could also be\na numerical superposition. We used an isothermal three-electrode system to determine\nthe temperature coefficient (α ) of PVA-x CuCl gels, which showed a positive value,\nR 2\nindicating the n-type thermogalvanic property of Cu2+/Cu+ redox couple. Therefore, a\nnegative thermodiffusive thermopower and a positive temperature coefficient\n(equal to negative thermogalvanic thermopower) could be numerically added. The\ncompetitive-coupling” effect leads to a non-monotonic trend in thermopower S: at low\ni\nconcentration (0.01 M ≤ x ≤ 0.07 M), the thermopower increased from -19.5 to -28.7\nmV K-1; at intermediate concentration (0.1 to 0.7 M), the thermopower achieved a\nmaximum value of -30.6 mV K-1 at x = 0.1 M and then gradually decreased; and at high\nconcentration (x ≥ 1M), suppressed thermodiffusion and dominant thermogalvanic\neffects collapse the thermopower to -5.9 mV K-1 (x = 2.0 M) (Figure R1d). These two\nparts' contributions show a competitive relationship but could be combined.\nAs a result, we called this mechanism “competitive-coupling”, in which the\ntwo i-TE effects show a competitive relationship, but the values of each part could\nbe numerically superposition. This new coupling effect indicates a more complex\ncompetitive relationship between thermodissusion and thermogalvanic effects\nmodulated by ion concentration, not only the simple synergy of “one plus one\nequals two”. In summary, what is genuinely new is the discovery and validation of\na coupling mechanism where the two effects are not independent partners but are in\ncompetition for a shared reactant (Cl⁻), a situation governed by coordination\nchemistry (speciation). This is a distinct thermodynamic and kinetic framework\nfrom the synergistic model.\nFor clarification, we revised this part in the main text to further highlight the\nnecessity of proposing a “competitive-coupling” i-TE mechanism, compared with\nthe reported “synergistic-coupling” effect.\n“These two i-TE processes show a competitive relationship, and their relative\ncontributions can be precisely tuned by adjusting the concentration of CuCl . This is\n2\nrelatively different from the reported p-type gelatin-KCl-FeCN4-/3- synergetic i-TE\nsystem, in which the KCl and FeCN4-/3- were employed as thermodiffusion ion\nproviders and thermogalvanic redox couple, respectively, showing a “synergistic-\ncoupling” effect in parallel. Here, we proposed the mechanism of this work as a\n“competitive-coupling” effect.” was revised on page 4 in the main text.\nComment 2:\nTo establish a correlation between coordination species changes and valence states, the\nauthors employed SHINERS alongside XPS mapping of Cu²⁺ and Cu⁺ distribution near\nthe gel surface. However, given that XPS sampling is limited to a few nanometers, while\nion migration in gels occurs on the micrometer scale, it is advisable to supplement the\nanalysis with Cu K-edge XANES. This would confirm whether the valence state\ndistribution observed at the surface is representative of the bulk gel.\nResponse:\nWe thank the reviewer for this insightful comment for confirming the electron state\nof copper in the i-TE gel bulks. In direct response to this comment, we have\nconducted the Cu K-edge X-ray absorption near-edge structure (XANES) test at\nBL11B beamline of Shanghai Synchrotron Radiation Facility. The sample is the\nrapidly frozen-drying i-TE gel (PVA-0.1 M CuCl ) after thermal charging at a\n2\ntemperature difference of 10 K for 30 minutes. The CuO, Cu O, and Cu foil were first\n2\ntested as the standard samples, as shown in Figure R2. The data confirms the presence\nof both Cu²⁺ and Cu⁺ states within the bulk of the gel after thermal charging. The\nXANES edge position of PVA-0.1 M CuCl is between that of Cu O and CuO,\n2 2\nindicating the Cu2+/Cu+ state mixture in the i-TE gels bulk after the\nthermogalvanic redox reaction. The results are consistent with the XPS results,\nsuggesting the existence and transfer of Cu2+/Cu+ redox couple from the gel bulk\nto the gel surface. This XANES characterization further confirms the migration of the\nredox couple ions in the gel bulk, which is induced by the thermogalvanic redox\nreaction driven concentration difference, as shown in Figure 3a in the main text.\nFor your concern, we have supplemented this important result in the revised\nmain text and supporting information.\n“Not only in the gel surface, but we also combined the X-ray absorption near-edge\nstructure (XANES) characterization to further confirm the existence of Cu2+/Cu+ redox\ncouple ions in the i-TE gel bulk (Supplementary Fig. 13).” was revised on page 8 in\nthe main text. Figure 3a and Supplementary Fig.13 were revised on page 26 of the main\ntext and page 28 in the supporting information, respectively.\n“X-ray absorption near-edge structure (XANES) characterization. The XANES\nspectra at Cu K-edge were performed at BL11B beamline of Shanghai Synchrotron\nRadiation Facility. The data were obtained under ambient conditions in fluorescence\nmode for the Cu K-edge. The gel sample is a freeze-dried PVA-0.1 M CuCl gel that\n2\nhas been thermally charged for 30 minutes. The CuO and Cu O were used as the\n2\nstandard samples to confirm the copper electron states in the i-TE gel.” was revised on\npage 15 of the main text.\nComment 3:\nIn the main text on page 8, Fig. 3c and Fig. 3d are cited out of sequence and should be\nswapped to align with their actual order in Fig. 3.\nResponse:\nThanks for pointing this out. We have adjusted the explanation order in the\nmain text to align with their order in Figure 3.\n“To correlate coordination species changes with valence states, we complemented\nSHINERS with XPS mapping of Cu2+ and Cu+ distribution near the surface of the gel.\nThe Cu2+ and Cu+ were detected at both sides after thermal charging, showing a larger\nintensity of Cu2+ than Cu+ at the gel bottom while reversing at the gel top27,28,30 (Fig.\n3c). Not only in the gel surface, but we also combined the X-ray absorption near-edge\nstructure (XANES) characterization to further confirm the existence of Cu2+/Cu+ redox\ncouple ions in the i-TE gel bulk (Supplementary Fig. 13). Furthermore, we proposed\na methodology to quantify a normalized [Cu-Cl] factor defined by integrating the [Cu-\nCl] Raman peak area. The [Cu-Cl] factors increased from 1 to 3.58 as t increased from\n0 to 30 mins and then decreased to 1.80 without ΔT, corresponding to the thermal\nvoltage decreased from -2 to -234 mV, and then recovered to -9 mV (Fig. 3d). ” was\nrevised in page 8 in the main text.\nComment 4:\nThe thermoelectric conversion efficiency should be included to provide a\ncomprehensive evaluation of i-TE cell performance.\nResponse:\nThanks very much for your critical comment. We have calculated the conversion\nefficiency of the i-TE cell. In 2020, Liu et al. derived an efficiency formula for the\ngelatin-KCl-FeCN4-/3 [Science, 368, 1091-1908, (2020)], which could be expressed by\nthe following equation:\n𝜂 = 𝜂 / 𝜂 = ∫ 0 𝑡𝑖𝑖𝑣(𝑡)∙𝐼(𝑡)𝑑𝑡 ÷( 𝛥𝑇 / 𝑇 ) = 𝑡𝑖𝑖 𝑃𝑎𝑣𝑒 𝑇 𝑑 𝑟 𝜂𝐶𝑎𝑟𝑛𝑜𝑡 𝜆 𝛥 𝑑 𝑇 ∙𝐴∙(𝑡𝑖+𝑡𝑖𝑖 ) 𝑇𝐻 𝑡𝑖+𝑡𝑖𝑖 (𝛥𝑇)2 𝐻 𝜆\nwhere 𝑣(𝑡) and 𝐼(𝑡) are the time-dependent voltage and current during the power\noutput stage. 𝐴 , 𝑑 , 𝜆 , 𝑃 and 𝑇 are the area and thickness of the i-TE cell,\n𝑎𝑣𝑒 𝐻\nthermal conductivity, average power density, and high-side temperature, respectively.\nMore important parameters based on the i-TE devices, such as the thermal-charging\nand discharging time, output power, and service temperature range, were considered in\nthis equation. Therefore, we used this equation to evaluate the conversion efficiency in\nthis work.\nThe steady-state heat flow method was employed to measure the thermal\nconductivity (𝜆) of the prepared ionic gel samples (Figure R3). The tested 𝜆 of PVA-\n1.5/0.25 M CuCl /CaCl gel is 0.66 W m-1 K-1. The i-TE cell discharged to a constant\n2 2\nresistance of R = 30 kΩ at ΔT = 10 K, as shown in Figure R4. The t and t are 19.4 and\ni ii\n120 mins, respectively. The calculated 𝑃 is 0.2 mW m-2 (Figure R5). The T and\n𝑎𝑣𝑒 H\nΔT are 308.15 and 10 K, respectively. The d of the i-TE device is 0.003 m. Based on\nsuch parameters, the calculated efficiency of CP| PVA-1.5/0.25 M CuCl /CaCl is\n2 2\n0.02%. This value shows a similar level to the reported work using the same\nefficiency equation, such as 0.01% in gelatin-KCl-FeCN4-/3- i-TE cell [Science, 368,\n1091-1908, (2020)], 0.04% in PDMAA-[EMIM][DCA]-FeCN4-/3- i-TE cell\n[Advanced Materials, 2312249, (2024)]. This value is much higher than 0.002%\nreported in the CaBC-NaCl i-TE capacitor (based on single thermodiffusion effect)\n[Nano letters, 22, 8152-8160, (2022)].\nFor your concern, we have supplemented the calculation of i-TE conversion\nefficiency to the revised main text and supporting information.\n“The heat-to-energy efficiency of the i-TE generator was calculated at η = 0.02% (as\ndetailed in Supplementary Note 5 and Supplementary Fig. 39).” was revised on\nPage 11 of the main text. Supplementary Fig. 39 was revised on page 58 of the\nsupporting information.\n“Supplementary Note 4: The calculation of heat-to-energy efficiency.\nThe heat-to-electrical conversion efficiency (η) relative to Carnot efficiency (η ) for\nCarnot\nthe i-TE cell could be expressed by the following equation in previously reported work:\n𝜂 = 𝜂 / 𝜂 = ∫ 0 𝑡𝑖𝑖𝑣(𝑡)∙𝐼(𝑡)𝑑𝑡 ÷( 𝛥𝑇 / 𝑇 ) = 𝑡𝑖𝑖 𝑃𝑎𝑣𝑒 𝑇 𝑑 𝑟 𝜂𝐶𝑎𝑟𝑛𝑜𝑡 𝜆 𝛥 𝑑 𝑇 ∙𝐴∙(𝑡𝑖+𝑡𝑖𝑖 ) 𝑇𝐻 𝑡𝑖+𝑡𝑖𝑖 (𝛥𝑇)2 𝐻 𝜆\nwhere 𝑣(𝑡) and 𝐼(𝑡) are the time-dependent voltage and current during the power\noutput stage. 𝐴 , 𝑑 , 𝜆 , 𝑃 and 𝑇 are the area and thickness of the i-TE cell,\n𝑎𝑣𝑒 𝐻\nthermal conductivity, average power density, and high-side temperature, respectively.\nThe steady-state heat flow method was employed to measure the thermal conductivity\n(𝜆) of the prepared ionic gel samples (Supplementary Fig. 33). The tested 𝜆 of PVA-\n1.5/0.25 M CuCl /CaCl gel at three different temperature intervals are 0.63, 0.68, and\n2 2\n0.66 W m-1 K-1, respectively, with an average value of 0.66 W m-1 K-1. The i-TE cell\ndischarged to a constant resistance of R = 30 kΩ at ΔT = 10 K. The t and t are 19.4 and\ni ii\n120 mins, respectively. The calculated 𝑃 is 0.2 mW m-2. The T and ΔT are 308.15\n𝑎𝑣𝑒 H\nand 10 K, respectively. The d of the i-TE device is 0.003 m. Based on such parameters,\nthe calculated heat-to-energy efficiency of CP| PVA-1.5/0.25 M CuCl /CaCl i-TE cell\n2 2\nis 0.02%.” was revised on page 13 of the supporting information.\nComment 5:\nThe reported energy density and operating time (2 h) are not particularly remarkable.\nA longer-duration test is essential for practical applications, particularly in self-\npowered wearable sensors.\nResponse:\nThanks for the insightful comment on the longer-duration test. We have\nsupplemented the longer continuous power output (20 cycles) of the i-TE cells with\na duration time of nearly 30 h, as shown in Figure R6. Compared to the other\nreported works ([Nat. Commun., 15, 1172, (2024)], [Adv. Energy Mater., 12,\n2103666, [2022], and [Energy Environ. Sci., 15, 5379–5390 (2022)]), the cycle\nnumber without a reactivation stage is achieved to 20 in this work, more than other\nreported 7 cycles. The working mode is the continuous-working mode, in which the\nreactivation stage is modified from self-recovery without ΔT to convert the ΔT direction.\nIt is directly connected with the voltage build-up stage of the next cycle without rest\n(Figure R6a). The thermal voltage and maximum output current showed a\nretention of 91% and 90%, respectively, after 30 h duration test (Figure R6b). The\ntotal harvesting energy density is about 15.9 J m-2. The long-time heat-to-energy\nconversion shows the potential application in the self-powered wearable sensors.\nFor your concern, we have further increased the duration time of the i-TE cell\nto 30 h, and revised this part in the main text and supporting information.\n“The curve of long-term output voltage and current versus time is as shown in\nSupplementary Fig. 42. The thermal voltage and maximum output current showed a\nretention of 91% and 90%, respectively, after a 20-cycle power generation. The total\nharvesting output energy density of the long-time duration test (nearly 30 h) is 15.9 J\nm-2, suggesting the potential application in the self-powered wearable sensors (Fig. 5h).\nThe good cycle performance showed the reversibility of the [Cu-Cl -(H O) ]2-y/[Cu-\ny 2 6-y\nCl ]⁻ redox couple and sustainable long-term service of the i-TE cells.” was revised on\n2\npage 12 of the main text. Fig. 5h and Supplementary Fig.42 were revised on page 28 of\nthe main text and page 61 of the supporting information, respectively.\nComment 6:\nIn this section, the authors increased the redox couple concentration by enriching Cl⁻\ncontent, achieving an optimized Pmax with 0.25 M CaCl₂. However, it remains unclear\nwhether Ca²⁺ participates in coordination or merely exerts an ionic-strength effect.\nResponse:\nThanks very much for the valuable comment. We have constructed further MD\nsimulations and UV-vis experimental tests to show the impact of the introduced\nCaCl on the [Cu-Cl] coordination species. As shown in Figure R7a and b, the\n2\nspecie type and quantity of principal [Cu-Cl] complexes show no significant change\nafter adding 0.25 M CaCl , from the MD simulation results. As shown in Figure R7c,\n2\nthe peak of the UV-vis spectrum exhibits a slight shift beyond 400 nm, due to a few\nnovel [Cu -Cl ] species (such as [Cu Cl ]2+ and [Cu Cl ]3+ from the MD results). The\nx y 4 6 3 3\nmain species shows no significant change (such as [Cu-(H O) ])2+, [Cu-Cl-(H O) ]+,\n2 6 2 5\nand [Cu-Cl -(H O) ], also indicating that the introduction of 0.25 M CaCl shows\n2 2 4 2\nno significant impact in tuning [Cu-Cl] diffusion species. The main role is to increase\nthe total Cl- concentration, thereby stabilizing Cu+ and increasing the number of redox\ncouples in the thermogalvanic process.\nFor your concern, we have supplemented this part to the revised main text\nand supporting information.\n“A chloride-rich environment has a direct impact on boosting the output power\nperformance in our i-TE cells by stabilizing more Cu2+/Cu+ redox couples in the\nthermogalvanic process.” “The MD result shows little regulation in [Cu-Cl]\ncoordination species upon adding 0.25 M CaCl , indicating that the increased Cl⁻ shows\n2\na dominant role in stabilizing Cu+ in the redox reaction (Supplementary Fig.36).” was\nrevised on page 11 of the main text. Supplementary Fig.36 was revised on page 55 of\nthe supporting information.\nReviewer #2:\nGeneral comments:\nThis manuscript reports the development of a high-performance n-type i-TE\nmaterial by coupling two synergistic effects. The i-TE material is the hot research spot\nof the field, especially the high-performance n-type ones. This work is well prepared by\nexploring new way to improve the i-TE performance with both experimental tests and\ntheoretical simulation. I would recommend its publication. Below are some comments\nto improve the quality.\nResponse:\nThe authors are grateful to the reviewer for the positive and encouraging comments.\nWe revised our paper carefully and performed some necessary experiments according\nto these comments.\nComment 1:\nPhotos of fabricated samples, and corresponding structure, composition and\nmorphology characterization as well as mechanical properties upon varying key\ncomponent contents are needed, to prove the achievement of the material. This also\napplies to the i-TE device with close look at the details the device such as the size and\nthe thickness.\nResponse:\nWe appreciate the important comment. Your kind reminder is necessary for proving\nthe achievement of the gels. We have supplemented the photos of the pure PVA gels,\nPVA-x CuCl gels (x = 0.01, 0.05, 0.1, 0.5, 1.0, 1.5, and 2.0 M), and the fabricated i-TE\n2\ncell, as shown in Figure R1. The morphology of PVA-1.0 M CuCl is shown in Figure\nof pure PVA and PVA-1.0 M CuCl gels are shown in Figure R3. The tensile stress and\n2\nelongation at the break of the PVA-1.0 M CuCl2 gels are 1000 kPa and 250%, which\naccommodate the natural deformation of the human body, usually <200%.\nFor your concern, we have supplemented this important part to the main text\nand supporting information.\n“The detailed fabrication process of as-prepared i-TE gels and cells was demonstrated\nin Supplementary Fig. 1. The morphology and mechanical property of the PVA-1.0 M\nCuCl i-TE gels are as shown in Supplementary Figs 2 and 3, exhibiting a 3D\n2\ninterconnected porous structure and high break elongation of ~250%.” was revised in\npage 5 of the main text. Supplementary Figs 1, 2, and 3 were revised on pages 15,\n16,\nand 17 of the supporting information.\nComment 2:\nDrying of hydrogel is a key issue hindering the practical long-term use. How does the\ndeveloped one dry over time, and is there any anti-drying treatment used?\nResponse:\nThanks for the valuable comment. The anti-drying treatment is critical in long-term\npower generation. Here, we introduce the fabrication process of the i-TE cell, which\ncontains the anti-drying consideration. Firstly, the 3M very high bond (VHB) tape based\non polyimide was used as the i-TE mould to attach a carbon paper electrode(Figure\nR4a). Then we put the as-prepared i-TE gels inside the mould and attach two carbon\npaper electrodes tightly, to prevent the evaporation of the i-TE gel in the long-term\npower generation (Figure R4b and R4c). The whole view of the i-TE cell was\ndemonstrated in Figure R4c. Finally, we used a Deli sellotape to seal the entire i-TE\ncell (Figure R4d). This seal fabrication process using two tapes could be suitable for\nmost kinds of i-TE bulk cells to prevent the drying of hydrogels.\nComment 3:\nCycling charge-discharge test of the assembled i-TE cells is only done with 10 cycles,\nwhich is too small. More cycles are needed, to verify the repeatability and consistence.\nResponse:\nWe appreciate this valuable and critical comment. We have added more repeat\ncycles from 10 to 20 in the long-term power generation, exhibiting a duration time\nof nearly 30 h without a reactivation stage, as shown in Figure R5. The thermal\nvoltage and maximum output current showed a retention of 91% and 90%,\nrespectively, after 20 cycles of long-term power generation. The total harvesting\nenergy density is about 15.9 J m-2. We attributed the decline of output performance to\nthe limited thermal-induced Cu+ reduced species. The further optimization of the cycle\nperformance in our future work is to improve the redox couple ions content and ensure\nthe electrochemical stability of the redox couples. Compared to other reported works\n([Nat. Commun., 15, 1172, (2024)], [Adv. Energy Mater., 12, 2103666, [2022], and\n[Energy Environ. Sci., 15, 5379–5390 (2022)]), the cycle number without a\nreactivation stage is achieved to 20 in this work, more than 7 cycles. The long-term\npower generation exhibits a reversible and sustainable heat-to-electricity conversion.\nFor your concern, we have further increased the duration time and cycle\nnumber of the i-TE cell to nearly 30 h and 20, and revised this part in the main\ntext and supporting information.\n“The curve of long-term output voltage and current versus time is as shown in\nSupplementary Fig. 42. The thermal voltage and maximum output current showed a\nretention of 91% and 90%, respectively, after a 20-cycle power generation. The total\nharvesting output energy density of the long-time duration test (nearly 30 h) is 15.9 J\nm-2, suggesting the potential application in the self-powered wearable sensors (Fig. 5h).\nThe good cycle performance showed the reversibility of the [Cu-Cl -(H O) ]2-y/[Cu-\ny 2 6-y\nCl ]⁻ redox couple and sustainable long-term service of the i-TE cells.” was revised on\n2\npage 12 of the main text. Fig. 5h and Supplementary Fig.42 were revised on page 28 of\nthe main text and page 61 of the supporting information, respectively.\nReviewer #3:\nGeneral comments:\nThe authors report a CuCl₂-based binary ionic thermoelectric (i-TE) system in which\nCl⁻ ions are proposed to regulate the coupling between interfacial redox reactions and\nbulk thermodiffusion, leading to a reported thermopower of −30.6 mV K⁻¹. The work\ncombines experimental thermopower characterization with molecular dynamics (MD)\nsimulations to rationalize the distribution of Cu–Cl–H₂O complexes. The topic is timely\nand relevant to the expanding field of ionic thermoelectrics, and the data appear\ninternally consistent.\nHowever, the mechanistic interpretation and novelty require substantial clarification.\nThe reported transient current profiles, limited thermogalvanic (TG) quantification,\nand incomplete benchmarking against the state of the art raise significant questions\nabout the claimed TG-dominated mechanism. Substantial revision, including direct\nquantification of the TG contribution, clarified novelty, and improved benchmarking,\nwould be necessary for to justify publication in this journal.\nResponse:\nWe appreciate the reviewer's insightful and critical comments on improving the\nquality of our manuscript and the fact that the reviewer acknowledges the topic and data\nof our work. We have worked diligently to make thorough revisions that address all of\nthe reviewer’s comments.\nComment 1:\nNovelty and Relation to Previous Work: The claimed TG-in-TD mechanism is\nconceptually similar to prior reports: eg. Han et al., Science 2020, 368, 1091–1098 and\nZeng et al., Adv. Energy Mater. 2023, 13, 2204357. Both studies describe synergistic\nthermodiffusion and thermogalvanic contributions producing large thermopower.\nAlthough the current materials differ in composition (n-type), the mechanistic\nframework and observed behavior appear analogous. The authors should clearly\ndelineate what is genuinely new in this work—whether in chemical design, redox\nkinetics, or thermodynamic coupling—and avoid implying a fundamentally new\nmechanism without quantitative differentiation.\nResponse:\nWe thank the reviewer for the opportunity to clarify the fundamental conceptual\nadvance of our work relative to the seminal studies mentioned. As you mentioned, one\nprevious work demonstrated the synergistic effect of thermodiffusion (TD) and\nthermogalvanic (TG) effects in gelatin-KCl-FeCN4-/3- gels [Science, 368, 1091-1908,\n(2020)]. The other one is designing the redox reaction on the polymer redox polyaniline\n(PANI)/carbon woven fabric (CWF) electrode surface, but not synergizing two i-TE\neffects in the gels, thereby showing a poor long-term stability (38.2% energy retention\nafter five thermal-charging/discharging cycles) [Adv. Energy Mater., 13, 2204357,\n(2023)]. Therefore, we explain the competitive relationship by comparing with the\np-type gelatin-KCl-FeCN4-/3- systems based on a synergistic mechanism [Science,\n368, 1091-1908, (2020)], which shows the coupling effect design in the i-TE gel\nbulks without consuming electrodes. In gelatin-KCl-FeCN4-/3- i-TE gels, the KCl and\nFeCN4-/3- were employed as thermodiffusion ions and thermogalvanic redox couples,\nrespectively. Therefore, these two i-TE processes show a synergistic effect in parallel,\ni.e., one plus one equals two (Figure R1a).\nHowever, in our work based on PVA-CuCl gels, Cl⁻ acts both as the primary\n2\ncharge carrier for the Soret effect and as an essential ligand for the Cu2+/Cu+ redox\ncouple in the thermogalvanic effect (Figure R1b). This is like a single engine where\nthe same fuel (Cl⁻) can be allocated to two different, competing processes. Diverting\nmore fuel to the TG process inherently starves the TD process. This creates an\nintrinsic trade-off relationship. As we increase CuCl₂ concentration (x), we enrich the\nsystem with [Cu-Cl] complexes, thereby enhancing the TG effect. However, this\nsimultaneously diverts Cl⁻ from its mobile, free state and increases cation mobility,\nboth of which act to suppress the anionic TD contribution. This is quantitatively\ndemonstrated by the opposing trends in decreased thermal voltage (dominated by\nTD) and increased output short-circuit current (dominated by TG), as increasing\nCuCl concentrations (Figure R1c). This opposite trend between the thermal voltage\n2\nand output current is unusual, compared with the reported p-type gelatin-KCl-FeCN4-\n/3- systems. In gelatin-KCl-FeCN4-/3- cells based on “synergistic-coupling” i-TE effect,\nhigher thermal voltage results in a higher output current and power.\nAlthough the competitive relationship, thermopower values of the two i-TE\neffects could also be a numerical superposition. We used an isothermal three-\nelectrode system to determine the temperature coefficient (α ) of PVA-x CuCl gels,\nR 2\nwhich showed a positive value, indicating the n-type thermogalvanic property of\nCu2+/Cu+ redox couple. Therefore, a negative thermodiffusive thermopower and a\npositive temperature coefficient (equal to negative thermogalvanic thermopower)\ncould be numerically added. We further found a non-monotonic trend in thermopower\nS due to the competitive-coupling” effect, At low concentration (0.01 M ≤ x ≤ 0.07 M),\ni\nthe thermopower increased from -19.5 to -28.7 mV K-1; at intermediate concentration\n(0.1 to 0.7 M), the thermopowerachieved a maximum value of -30.6 mV K-1 at x = 0.1\nM and then gradually decreased; and at high concentration (x ≥ 1M), suppressed\nthermodiffusion and dominant thermogalvanic effects collapse the thermopower to -5.9\nmV K-1 (x = 2.0 M) (Figure R1d).\nIn summary, what is genuinely new is the discovery and validation of a coupling\nmechanism where the two effects are not independent partners but are in\ncompetition for a shared reactant (Cl⁻), a situation governed by coordination\nchemistry (speciation). This is a distinct thermodynamic and kinetic framework\nfrom the synergistic model.\nFor clarification, we revised this part in the main text to further highlight the\nnecessity of proposing a “competitive-coupling” i-TE mechanism, compared with\nthe reported “synergistic-coupling” effect.\n“These two i-TE processes show a competitive relationship, and their relative\ncontributions can be precisely tuned by adjusting the concentration of CuCl . This is\n2\nrelatively different from the reported p-type gelatin-KCl-FeCN4-/3- synergetic i-TE\nsystem, in which the KCl and FeCN4-/3- were employed as thermodiffusion ion\nproviders and thermogalvanic redox couple, showing a “synergistic-coupling” effect in\nparallel. Therefore, we proposed the mechanism of this work as a “competitive-\ncoupling” effect.” was revised on page 4 of the main text.\nExcept for the new “competitive-coupling” i-TE mechanism, there are also three\nother novelties listed below.\n1. Discovering novel n-type thermogalvanic redox couple, i.e., Cu2+/Cu+ stabilized\nby Cl-.\nThe CuCl has been widely employed as a common active salt for thermodiffusion\n2\nin the previous i-TE gels, such as [npj Flex Eletron, 6, 79, (2022)], [Energy Environ.\nSci., 13, 859-867, (2020)], [Adv. Funct. Mater., 35, 2419762 (2025).], and [Adv.\nMater., e10199 (2025)]. However, all these works were focused on the\nthermodiffusion process of Cu2+ and Cl-, and missed the critical thermogalvanic\nredox reaction between the Cu2+/Cu+.\nIn this work, we discovered the Cu2+/Cu+ redox reaction stabilized by Cl- in\nCuCl -based i-TE materials. The surface analytical XPS and XANES\n2\ncharacterization gave evidence of existing Cu2+/Cu+ electronic states after thermal\ncharging (Figure R2a). The Cu2+ and Cu+ were detected at both sides after thermal\ncharging, showing a larger intensity of Cu2+ than Cu+ at the gel bottom, while reversing\nat the gel top (Figure R2b). Not only in the gel surface, but we also combined the X-\nray absorption near-edge structure (XANES) characterization to further confirm the\nexistence of Cu2+/Cu+ redox couple ions in the i-TE gel bulk (Figure R2c), indicating\ntheir potential migration process under the thermogalvanic reaction. The redox reaction\nwas verified from the CV test and could be enhanced by increasing CuCl\n2\nconcentrations, as shown in Figure R2d. We uncovered the critical role of Cl- in the\nredox process by comparing it with other copper-based salts such as CuSO and\n3 2\ncouple with other widely used n-type thermogalvanic redox couples (polymer-\nredox couples with stable electrodes), such as Fe2+/3+, [Co(bpy) ]2+/3+, and I-/I -. This\n3 3\nnovel binary n-type thermogalvanic gel shows an obvious advantage with 0.43 mW\n2\nm-2 K-2, and other ternary thermogalvanic systems show a P /(ΔT)2 of below 0.20\nmax\nmW m-2 K-2, as shown in Figure R2e. Furthermore, due to the “competitive-coupling”\nmechanism, high thermopower from -30.6 to -10 mV K-1 is achieved by regulating the\nCuCl concentrations from 0.1 to 1.5 M. This discovery broadened the n-type\n2\nthermogalvanic i-TE system and offers potential designs of electron transfer based\non the CuCl -based i-TE materials, but not only based on the thermodiffusion\n2\neffect.\n2. New operando characterization methodologies to quantify [Cu-Cl] evolution\nunder thermogalvanic process, i.e., Operando SHINERS.\nDue to the first time uncovering the thermogalvanic redox reaction in CuCl -based\n2\ni-TE system, we employed lots of methods to verify the redox process. The most novel\nand important technology is shell-isolated nanoparticle-enhanced Raman\nspectroscopy (SHINERS), which was cooperated with Prof. Jianfeng Li and Prof.\nYaohui Wang from Xiamen University. The inert silica shell prevented direct\ninteraction between the plasmonic Au cores (Raman signal amplifier) and the samples\nto be probed, ensuring unperturbed vibrational data. This setup enabled sensitive, non-\ndestructive detection across diverse substrates, leveraging strong enhanced\nelectromagnetic fields at nanoparticle-substrate junctions to amplify signals by millions\nof times. The intensity of reduced species [Cu-Cl ]⁻ (261 cm-1) showed a significant\n2\nenhancement as thermal charging time (t) increased (Figure R3a). This change is\nattributed to the migration of the reduction product from the T to T , as shown in\nH C\nFigure R3b. We also conducted an isotopic substitution (D O) experiment to verify the\n2\n[Cu-H O] Raman peak (420 cm-1). Furthermore, we propose a methodology to\n2\nquantify a normalized [Cu-Cl] factor defined by integrating the [Cu-Cl] Raman\npeak area. The [Cu-Cl] factors increased in the thermal charging process and then\ndecreased without ΔT (Figure R3c), indicating the redox reaction process of Cu2+/Cu+\nstabilized by Cl-. We also demonstrated the evaluation of [Cu-Cl] factors with different\nCuCl concentrations (Figure R3d). The increased quantitative [Cu-Cl] factors show a\n2\nwell consistency with the increased faradaic current density (Figure R2d), indicating\nthe enhanced thermogalvanic redox as increasing the CuCl concentrations. Our\n2\nexperimental SHINERS characterization is readily adaptable to other redox\nreaction-active hydrogels or soft-material systems, enabling precise mapping of\ncoordination complexes under operando conditions.\n3. Quantification of [Cu-Cl] species by combining the UV-vis and MD simulations.\nIdentifications of multiple chemical speciation in the CuCl system have yet to\n2\nemerge, especially at high concentrations. The complex [Cu-Cl] coordination species\nresult in distinct i-TE performance. However, in other reported works ([npj Flex\nEletron, 6, 79, (2022)], [Energy Environ. Sci., 13, 859-867, (2020)], [Adv. Funct.\nMater., 35, 2419762 (2025).], and [Adv. Mater., e10199 (2025)]), the distinction and\nquantification of [Cu-Cl] complex coordination structures is lake. Therefore, the\nmechanism of the impact of how coordination species affect i-TE performance is\nunclear. Here, we made lots of effort to identify and quantify the [Cu-Cl] species at\n2\ndifferent CuCl concentrations, aiming to illustrate the critical role of coordination\n2\nspecies on thermopower. In this work, we combine molecular dynamics (MD)\nsimulations with UV-vis spectroscopy (Integrating Sphere device) to resolve [Cu-\nCl] speciation across a concentration range (from 0.01 to 2 M) (Figure R4a and b).\nWe also conducted principal component analysis (PCA) on UV-vis spectra to\nestimate the number of primary species in the gels (Figure R4c). Correspondingly,\nthe proportion of [Cu-Cl] species was quantified, increasing from 11.9% to 49.9% as\nCuCl concentrations increased from 0.05 to 2 M. Combining the thermopower\n2\nperformance with [Cu-Cl] species (Figure R1d and R4b), we found that the dominant\nexisting of [Cu-(H O) ]2+, [Cu-Cl-(H O) ]⁺, and [Cu-Cl -(H O) ] (0.1 ≤ x ≤ 0.7 M)\n2 6 2 5 2 2 4\nshowed benefits on regulating both thermodiffusive and thermogalvanic thermopower\n(-α), while the formation of higher-order [Cu -Cl ]2x-y (e.g., [Cu -Cl -(H O) ]2+ and\nx y 2 2 2 8\n[Cu -Cl -(H O) ]+ etc., at x ≥ 1 M) declined thermopower of both parts. We identify\n2 3 2 8\nan optimal species distribution for maximizing thermopower: approximately 84.6%\n[Cu-(H O) ]2+, 12.6% [Cu-Cl-(H O) ]⁺, and 2.8% [Cu-Cl -(H O) ], achieved in\n2 6 2 5 2 2 4\nPVA-0.1 M CuCl\n2\ngels.\nComment 2:\nTransient versus Steady-State Current Behavior: The long-term current–voltage data\n(Supporting Fig. 34) display sharp transient spikes that rapidly decay to near zero,\nrather than steady currents under ΔT. This response is typical of thermodiffusion-dominated systems, not TG-driven ones. If a true thermogalvanic process were dominant, a sustained redox-driven current would persist as long as the temperature gradient remains, as in the TG + TD system of Han et al. (Science 2020, 368, 1091–1098). The combination of transient decay and low open-circuit voltage relative to Cu²⁺/Cu⁺ redox potentials weakens the mechanistic conclusion. The authors should provide quantitative TG validation e.g., steady-state current monitoring, or equivalent circuit separation of ionic vs. redox contributions.\nResponse:\nThanks for the useful and critical comment. We appreciate your pointing out this\nimportant point. We think maybe the extremely long horizontal time scale and\ninsufficient explanation misled the judge. For your convenience, we put the long-\nterm voltage-current curve (Supporting Figure 34) with detailed labels in Figure\nR5. The section with the black dotted line is the thermal charging process, and the\nsection with the red dotted line is the output stage. Therefore, even in the last cycle,\nthe voltage and current gradually decreased during the power output process.\nThese sharp transient spikes of thermal voltage are the thermal charging process\ndriven by directly reversing the temperature difference reaction, but not the power\noutput process.\nHere, we want to first introduce the reason for the thermal voltage and output current\ndecline. The reason for the declining voltage is that the generated electrons by redox\nreaction can not be supplied to the consumed electrons that flow to the external circuit\nduring the power output process, mainly due to the slow mass dynamic process of the\nredox couple ions in gels. The decline of thermal voltage and current in i-TE gels is\na common trend when powering an external circuit, such as [Nat. Mater., 18, 608-\n613, (2019)], [Energy Environ. Sci., 17, 1664–1676, (2024)], [Energy Environ. Sci.,\n17, 1559–1569, (2024)], [Adv. Energy Mater., 13, 2204357, (2023)], and [Energy\nEnviron. Sci., 15, 5379–5390 (2022)]. Therefore, the output power time of the i-TE\ncells is always set to nearly 2 hours. Not only for the i-TE gel system, but also the i-\nTE liquid cell of H O/formamide-FeCN4-/3--GdmCl based on a single thermogalvanic\n2\neffect, showed a similar decline without steady-state voltage and output current (Figure\nR6a), due to limited mass transfer dynamics [Nat. Commun., 15, 1172, (2024)]. Due\nto the different ionic migration media, the decline of the thermal voltage or output\ncurrent may be slightly less in the liquid-state i-TE cell than in the i-TE gel cells.\nAs you mentioned in this reported work [Science, 368, 1091-1908, (2020)], the\nvoltage declined about 66.4% after nearly 1.7 h of power output time (Figure R6b),\nindicating a thermogalvanic (TG) and thermodiffusion (TD) synergistic-coupling effect.\nHere, we put the output voltage curves of PVA-1.5/0.25 M CuCl /CaCl i-TE cell\n2 2\n(Figure 5d in the main text) in Figure R6c. By comparing Figure R6b, the thermal\nvoltage declined 64.3% after 2 h of power output time; this result is very similar to the\nreported work. Therefore, it also indicates a similar coupling effect of TG and TD. We\nalso compare the output voltage curve of PVA-0.1 M CuCl i-TE cells (Figure 5e in\n2\nthe main text), which shows a rapid decline of 90% only in 3 mins (Figure R6d).\nThe quick deterioration indicates the capacitor mode of the PVA-0.1 M CuCl i-TE cell,\n2\nwhich is caused by the TD-dominant effect. Therefore, by comparing Figure R6b\nand Figure R6c, we can conclude that the PVA-1.5/0.25 M CuCl /CaCl i-TE\n2 2\nmaterial shows the i-TE coupling effect, which is similar to the gelatin-KCl-FeCN4-\n/3- p-type material. By combining Figures R6c and R6d, we can conclude that the\nTG effect is enhanced significantly as CuCl concentrations increase, showing an\n2\noutput mode conversion from the i-TE capacitor to the i-TE generator.\nAs you suggested, we have added the long-time output current monitoring (for\n4h) to verify that the TD effect is dominant at low CuCl concentrations and then\n2\nthe TG effect is enhanced at high CuCl concentrations (Figure R7a). Here, we test\n2\nfour i-TE cells using the i-TE generator mode with the same external resistance of 30\nkΩ, i.e., PVA-0.1 M CuCl , PVA-0.5 CuCl , PVA-1.0 M CuCl , and PVA-1.5 M CuCl\n2 2 2 2\ni-TE cells. The decline in output current becomes gradually smoother as the CuCl\n2\nconcentration increases to 1.5 M, indicating a transition from the i-TE capacitor\n(TD-dominant effect) to the i-TE generator mode (TG effect enhancement). We\nt\ncalculated the cumulative released charge quantity using the equation of 𝑄 = ∫ 𝐼𝑡 𝑑𝑡,\n0\nwhere I and t are continuous output current and corresponding output time, respectively.\nThe calculated Q values are 0.0011, 0.0014, 0.0020, 0.0036, and 0.0051 C, respectively,\ncorresponding to the CuCl concentrations are 0.05, 0.1, 0.5, 1.0, and 1.5 M (Figure\nto evaluate the thermogalvanic redox reaction contribution. We defined the\nnormalized output electric charge intensity (attributed to TG contribution) of the PVA-\n0.05 M CuCl i-TE cell as 1, and intensities of other concentrations could be calculated\n2\nby making a ratio between the corresponding charge quantity. As a result, the intensity\nincreased nearly 4.6 times when increasing the CuCl concentrations from 0.05 to\n2\n1.5 M, indicating the TG contribution of Cu2+/Cu+ enhanced significantly at high\nconcentrations of CuCl .\nand output electric charge intensity (Figure R8). This also indicates the competitive-\ncoupling effect, in which the TD-dominant effect is observed in the low CuCl\n2\nconcentrations with high thermal voltage but less output current, while the TG redox\nreaction plays a critical role at high concentrations, showing higher current although a\nlow thermal voltage. Critically, the thermogalvainc output performance should be\noptimized in our future works, such as designing electrode structures or further\nincreasing the content of redox species. In this work, we mainly want to uncover\nthe promising output performance of this novel redox couple and show a new\n31\n“competitive-coupling” effect between the TG and TD effects. In this new coupling\neffect, the TG effect is enhanced while suppressing the TD contribution, with\nincreasing CuCl concentrations.\nthermogalvanic effect in the main text and supporting information, to show the\nincreased TG redox reaction with increased CuCl concentrations.\n2\n“We also tested the long-time (4 h) output current and calculated normalized output\nelectric charge factors, showing a significantly enhanced thermogalvanic redox\ncontribution with increased CuCl concentrations (Supplementary Fig. 41).” was\n2\nrevised on page 12 in the main text. Supplementary Fig. 41 was revised on page 60 of\nthe supporting information.\nComment 3:\nInterpretation of “Simultaneous High Current and Voltage”: The claim of achieving\nboth large current output and high voltage is misleading. The CuCl₂-based gel cannot\ntransport electrons internally; current appears only when an external load provides an\nelectronic pathway. The authors should correct this statement to specify that current\nflow arises from external-circuit conduction, not from intrinsic electronic transport in\nthe gel.\nResponse:\nThanks for the valuable comment on the statement. We have corrected the statement\nin the abstract. I fully agree with your point: the electrons are actually produced in\nthe interface between the gel and electrode by redox reactions, and flow to the\nexternal circuit, not transferring in the gel bulk.\nWe have revised the statement and corrected the current generation and flow.\n“As a model system, we uncover a hidden thermogalvanic mechanism in CuCl -based\n2\ngels, in which Cl ions regulate the interaction of interfacial redox reactions and bulk\nthermodiffusion, enabling the achievement of continuous output current from\nelectrodes to an external circuit and high built-in thermal voltage, respectively.” was\nrevised on page 3 of the main text.\nComment 4:\nQuantitative Estimation of TG Contribution: While the XPS results suggest Cu²⁺/Cu⁺\nredox involvement, peak-area ratios cannot be directly correlated with current density\nor Seebeck enhancement. Without quantitative estimation of the TG fraction, the\nmechanistic claim remains speculative. Given the non-sustained current profile, the\nsystem appears thermodiffusion-dominated with a limited or transient TG component.\nThe authors should revisit the data interpretation and possibly reframe the paper\naround TD-enhanced ionic thermopower rather than TG dominance.\nResponse:\nThanks very much for the valuable comment. As shown in comment 3, we have\ncalculated the output electric charge intensity to quantitatively evaluate the\nincreased contribution of the TG effect at high CuCl concentrations.\nother quantitative parameters to show the increased TD contribution with the\nincreased CuCl concentrations (x), as shown in Figure R9. Opposing trends in\n2\ndecreasing V and increasing I with increasing x underscore a competitive coupling\noc sc\nmechanism: enhanced thermogalvanic activity suppresses thermodiffusion\ncontributions. The distinct trend indicates the i-TE mechanism converts from TD-\ndominant to enhanced TG effect. This concentration-dependent trade-off underscores\nthe crucial role of speciation in optimizing i-TE performance, specifically the highest\nthermal voltage at x = 0.1 M and the highest output power at x = 1.5 M.\nAs you mentioned, we put a lot of effort and work into framing our paper. We think\nthe most important novelties in this work are discovering the [Cu2+/Cu+-Cl] redox\ncouples and uncovering the new “competitive-coupling” mechanism. The main line\nof this paper is focused on this discovery. Here, we want to introduce our writing\napproach for this paper.\n1. In the first part, we uncover the opposite trend of the thermal voltage and output\nperformance in PVA-x CuCl i-TE cells from the electrochemistry test and the\n2\ni-TE performance. This means there will be a new n-type redox couple and i-\nTE mechanism, compared with other reported works.\n2. In the second part, we propose a “competitive-coupling” i-TE effect by\ncombining the DFT calculations, where the thermogalvanic effect is enhanced\nwhile suppressing the thermodiffusion of Cl⁻ with increasing CuCl\n2\nconcentrations. This mechanism can give a mechanism explanation of the\nperformance phenomenon in the first part.\n3. In the third part, we did lots of characterization experiments to confirm the new\nthermogalvanic process, due to its first discovery in i-TE fields. The operando\nSHINERS, XPS, and XANES were employed to verify the existence of\nCu2+/Cu+ electron state and migration process of the [Cu-Cl] species in the i-TE\ngel bulks.\n4. In the fourth part, we reveal the impact of [Cu-Cl] species on the thermopower\nand thermal voltage, which is attributed to the thermodiffusion effect. We\nemployed the MD simulation and UV-vis spectrum to identify and quantify the\n[Cu-Cl] species.\n5. In the fifth part, we optimized the output performance of the i-TE cell by further\nincreasing Cl- content and showed the continuous power output property using\nthe i-TE generator mode.\n6. The last part is to demonstrate the applications of the i-TE module in low-grade\nheat harvesting, temperature monitoring, and sensing.\nIn summary, we did lots of paragraphs to uncover a novel Cu2+/Cu+\nthermogalvanic redox couple and the new “competitive-coupling” i-TE effect. We\nproposed that the enhanced TG suppresses the TD effect based on this coupling\neffect, which shows opposing trends in decreasing V and increasing I in the\noc sc\nelectrochemistry test. In this work, our main line is to discover this new redox\ncouple, verify the thermogalvanic process, and illustrate this “competitive-\ncoupling” mechanism. We just offer an effective strategy for optimizing the\nperformance of this new [Cu2+/Cu+-Cl] redox couple in the fifth part, while more\nefforts to further increase the output performance should be done in our future\nwork.\nFor your concern, we have further introduced our writing approach and\nsupplemented the quantitative evaluation results of thermogalvanic effect in the\nmain text and supporting information, to show the enhanced TG contribution with\nincreased CuCl concentrations.\n2\n“We also tested the long-time (4 h) output current and calculated normalized output\nelectric charge factors, showing a significantly enhanced thermogalvanic redox\ncontribution with increased CuCl concentrations (Supplementary Fig. 41).” was\n2\nrevised on page 12 in the main text. Supplementary Fig. 41 was revised on page 60 of\nthe supporting information.\nComment 5:\nComparison with the State of the Art: Figure 1f and Supporting Table 1 and 2 omit key\nrecent references. For example, a recent report in Energy & Environmental Science\n(Energy Environ. Sci., 2025,18, 1294-1309) achieved thermopower exceeding −40 mV\nK⁻¹ at ΔT = 5.3 K. Accordingly, the statement “Binary PVA–CuCl₂ gels exhibit\nremarkable advantages in thermopower and output performance compared to other\nreported multicomponent n-type i-TE systems” is overstated and should be revised.\nComprehensive benchmarking with the latest literature is required for an accurate\ncontextual comparison.\nResponse:\nThanks for pointing this out. We have added this reported work [Energy Environ.\nSci., 2025,18, 1294-1309] to our comparison list in Figure 1f and Table S2. We\nrevised the statement of “Binary PVA–CuCl₂ gels exhibit remarkable advantages in\nthermopower and output performance compared to other reported multicomponent n-\ntype i-TE systems” as “Binary PVA-CuCl gels exhibit both promising advantages in\n2\nthermopower and output performance due to the coupling effect of thermodiffusion and\nthermogalvanic.”\nFor your concern, we have supplemented the mentioned reference in our\ncomparison list and revised the corresponding statements.\n“Binary PVA-CuCl gels exhibit both promising advantages in thermopower and output\n2\nperformance due to the coupling effect of thermodiffusion and thermogalvanic.” was\nrevised on page 5 of the main text.\nFigure 1f and Table S2 were revised on page 24 of the main text and page 64 of the\nsupporting information, respectively.\nComment 6:\nPower Density Discrepancy: The reported power density of this work (binary PVA–\nCuCl₂ gel) is orders of magnitude lower than comparable systems: For eg. Adv. Energy\nMater. 2023 , 13, 2204357. The authors should discuss the physical origin of this low\nperformance—e.g., internal resistance, poor interfacial kinetics, or limited ionic–\nelectronic coupling—and provide quantitative reasoning for why the binary PVA–CuCl₂\ngel performs so modestly despite the large thermopower.\nResponse:\nThanks for the valuable comment. This suggestion is very valuable for us to consider\nour future work. As you mentioned, compared with the reported work focus on\nimproving the output power of the i-TE cell [Adv. Energy Mater., 13, 2204357,\n(2023)], the output performance in our work is truly low. However, these two works\nare relatively different in the i-TE mechanism. In the reported work, the redox\nreaction was designed in the polyaniline (PANI)/carbon woven fabric (CWF)\nelectrodes, but not in the gel bulk; the reaction is not reversible. In other words, the\nelectrodes are involved in redox reactions; therefore, they can achieve a relatively high\npower density. However, there is also a limitation of the cycle property, showing\nonly 38.2% energy retention after five thermal-charging/discharging cycles. In our\nwork, the carbon paper electrode is just a current collector without other side reactions;\nthe redox reaction of Cu2+/Cu+ occurred in the gel top and bottom, and the electrons\nflowed to the external circuit through the carbon paper electrodes. The energy\nretention showed 80% after 10 thermal-charging/power output cycles in our work.\nThe long-term stable service of the i-TE device is critical in the real application.\nIn this work, we mainly optimized the output power density of the [Cu2+/Cu+-Cl]\nthermogalvanic output performance to show its potential application as a new n-type\nthermogalvanic redox couple. Compared to other p-type/n-type thermogalvanic\nredox couples (polymer-oxidized/reduced species) with stable electrodes (such as\nPt, Au, and carbon-based materials), this binary PVA-CuCl /CaCl gel showed a\n2 2\nnormalized power density value of 0.60 mW m-2 K-2, as shown in Tables R1 and\nR2. It indicates that the Cu2+/Cu+ is a promising i-TE thermogalvanic redox couple,\ncompared with the widely used p-type/n-type redox couple, including Fe2+/3+,\n[Co(bpy) ]2+/3+, I-/I -, and FeCN4-/3-.\n3 3\nAs you suggested, we also calculated the internal resistance of the PVA-\nCuCl /CaCl i-TE cells, using the linear V-I curves. The value decreased from 7926\n2 2\nto 171 Ω by increasing the CuCl concentration and modifying the carbon paper\n2\nelectrode with Au (Figure R10a). The internal resistance is actually higher than the\nreported work (41 Ω) [Adv. Energy Mater., 12, 2103666, (2022)], which focuses on\n3D electrode designing. The high internal resistance results from two aspects: one is\nthe poor ionic conductivity of the gel (high Rs and Z ), and the other is limited interface\nw\nreaction kinetics (R ), as shown in Figure R10b. Three potential reasons limit the\nct\noutput power: 1. the limited gel conductivity, 2. the limited content of redox couples,\nand 3. the limited design of the electrode.\nHere, we want to discuss our future work plans that focus on increasing the ionic\nconductivity and facilitating the interfacial reaction kinetics.\n1. To enhance the gel conductivity, adding some high-conductivity\nintermediate material into the PVA hydrogel may be efficient, such as\noxidized graphene or MXene nanosheets.\n2. Furthermore, the Cu+ reduced species are mainly induced by the\ntemperature gradient in the thermogalvanic redox reaction, which limits\ntheir content. We aim to find the precursor species of Cu+, but not only the\nthermally induced product. One efficient way is to select a more stable\nligand than Cl-, such as SCN-, S O 2-, Br-, or I-, thereby achieving much\n2 3\nmore stable Cu+ coordination species in the previous sample preparation\nsteps.\n3. To promote the reaction kinetics, the electrode optimization may be\nanother efficient way besides increasing the content of Cu+ reduced species.\nThe carbon-based electrodes with a high specific surface area or ordered\nmicro-structure design of ionic-electron transport channels may be the\npriority.\nFor your concern, we supplemented the quantitative internal resistance of the i-\nTE cells and a discussion of improving the power density in the main text and\nsupporting information.\n“According to the linear V-I curves, we calculated the internal resistance of PVA-x\nCuCl i-TE cells and discussed feasible strategies for further improving output power\n2\ndensity (Supplementary Fig. 7 and Supplementary Note 1). ” was revised on page 5\nof the main text. Supplementary Fig. 7 was revised on page 22 of the supporting\ninformation.\n“Supplementary Note 1: Strategies for further improving the power density of\nPVA-CuCl i-TE cells.\nfocusing on increasing the ionic conductivity and facilitating the interfacial reaction\nkinetics. To enhance gel conductivity, adding some high-conductivity intermediate\nmaterial into the PVA hydrogel may be efficient, such as oxidized graphene or MXene\nnanosheets. Furthermore, the Cu+ reduced species are mainly induced by the\ntemperature gradient in the thermogalvanic redox reaction, which limits their content.\nPreparing the precursor species of Cu+ is necessary, but not only using the thermally\ninduced product. One efficient way is to select a more stable ligand than Cl-, such as\nSCN-, S O 2-, or I-, thereby achieving stable Cu+ coordination species in the previous\n2 3\nsample preparation steps. To promote the reaction kinetics, electrode optimization may\nbe another efficient way besides increasing the content of Cu+ reduced species. The\ncarbon-based electrodes with a high specific surface area or ordered micro-structure\ndesign of ionic-electron transport channels may be the priority.” was revised on page 2\nof the supporting information.\nResponse to Reviewers: Manuscript NCOMMS-25-68639A\nModulating thermo-diffusion/galvanic coupling via ion speciation engineering\nenables high-performance ionic thermoelectric cells\nYuchen Li1†, Ying-Ru Qiu2†, Jinsha Liao3, Helios Y. Li1, Zhouyuntong Liu1, Yuxiang Chen1, Mao Yu5, Lizhi Xu1, Chang Liu4, Jefferson Zhe Liu3, Jian-Feng Li2, Weishu Liu5, Yao-Hui Wang2*, Nicholas X. Fang1*\nWe are very grateful for all three reviewers’ comments on the manuscript. We are pleased that\nReviewers #1 and #2 have accepted the revised version. We also deeply appreciate the constructive\nand detailed suggestions from Reviewer #3, which have prompted us to perform additional\nexperiments and refine our interpretations. In the following, we provide a point-by-point response\nto the reviewers' comments. We highlight our critical points in bold type, and figures and cited\nreferences in light blue. Revisions made in the main text and supporting information are\nmarked in red.\nReviewer #3:\nGeneral comments:\nOverall, the added data strengthen the manuscript, but the mechanistic evidence and\nbenchmarking are not yet sufficiently rigorous to support the claimed TD→TG transition and\n“competitive-coupling” as framed for a high-impact journal. Specifically, although the\nconcentration-dependent trends (decreasing Voc, increasing current/charge output) and the added\nCV/XPS–XANES results are consistent with enhanced redox activity at higher CuCl2, the key\nreadouts used (decay shapes and integrated charge remain intrinsically mixed signals that can\ninclude TD, TG, and transport effects, so they do not uniquely establish TG dominance or a true\nmechanistic “transition.” This issue is compounded by the strong concentration-dependent\nreduction in internal resistance (Fig. R10), which can plausibly explain higher current and power\nwithout requiring a TG-dominated regime, and therefore the authors should quantitatively\ndecouple “TG enhancement” from simple conductivity/kinetic improvements. While the long-\nduration outputs under finite loads are helpful, their decays are still compatible with charge\nrelaxation and mass-transport limitations typical of i-TE gels, and the study still lacks the most\ndirect validation expected for a TG-dominated generator mode: near–short-circuit measurements\nshowing a sustained, redox-cycled self-current in the proposed TG-rich compositions. Without a\nrigorous TD/TG decomposition (e.g., equivalent-circuit/kinetic modeling extracting distinct TD vs\nTG amplitudes/time constants and correlating output with TG-specific descriptors rather than only\nCuCl2 concentration), the manuscript should moderate claims of a definitive TD→TG transition\nand a fundamentally new “competitive-coupling” framework, and instead frame the results more\ncautiously as a speciation/transport-tuned balance between TD-like and redox contributions.\nResponse:\nWe thank the Reviewer for their thoughtful and challenging critique, which has pushed us to\nstrengthen our evidence and clarify our claims. We have addressed each of the four key concerns\nraised, as detailed below. In response, we have also moderated the language regarding\ndefinitive \"mechanism-transition\" and \"competitive-coupling\" throughout the text, framing\nour findings instead as a “speciation-tuned, interactive balance between thermodiffusion and\nthermogalvanic contributions”.\nSpecifically, we have moderated several key terms as suggested:\n• “Thermogalvanic-dominant” has been revised to “enhanced thermogalvanic redox.”\n• “Competitive-coupling effect” has been reframed as “interactive thermo-\ndiffusion/galvanic coupling.”\n• “TG to TD mechanism transition” is now described as an “i-TE mechanism with a\ntrade-off relationship.”\nThe manuscript title has also been updated as “Modulating thermo-diffusion/galvanic\ncoupling via ion speciation engineering enables high-performance ionic thermoelectric cells”\nto more accurately reflect this refined narrative.\nFurthermore, we have supplemented the near-short-circuit current measurement of the\nhigh-concentration i-TE device, providing the direct validation the reviewer sought.\nThe first comment：\n“Specifically, although the concentration-dependent trends (decreasing Voc, increasing\ncurrent/charge output) and the added CV/XPS–XANES results are consistent with enhanced redox\nactivity at higher CuCl2, the key readouts used (decay shapes and integrated charge remain\nintrinsically mixed signals that can include TD, TG, and transport effects, so they do not uniquely\nestablish TG dominance or a true mechanistic “transition.”\nResponse:\nThanks for this valuable comment. Firstly, we want to explain the main i-TE performance\nparameters of the thermosdiffusion and thermogalvanic effects. Based on the single\nthermodiffusion effect, the carrier ions (cations or anions) could show a large transport entropy\ndifference, thereby generating a large thermal voltage but with less leak current. For the single\nthermogalvainic effect, the redox reaction occurred at the hot and cold electrodes separately,\ntherefore generating continuous and self-recycled electron flow. But their thermopower (equal to\nthe temperature coefficient of redox couples in electrochemistry) is relatively low compared with\nthe themodiffusive i-TE system. Therefore, in this work, using thermal voltage and output\nperformance as critical parameters to evaluate the strength of the two i-TE effects is\nreasonable.\nWe agree with your point that “the decay shapes and integrated charge remain intrinsically\nmixed signals that can include TD, TG”. In this work, we first found that the CuCl -based i-TE\n2\ngels are based on a thermo-diffusion/galvanic coupling, in which these two effects are interactive\nand trade off depending on ion concentrations, not only additive-synergistic-coupling. Lots of\nresults in this work confirmed the i-TE coupling effect.\nThe thermal voltage, continuous output voltage/current retention (decay shapes), and the\nintegrated quantity of electric charge are critical performances to evaluate relative\ncontribution in the thermo-diffusion/galvanic coupling process. Based on the basic\nthermogalvanic mechanisms, the continuous electron flow is generated by thermogalvanic\nredox reaction, which can achieve smoother voltage/current decay and a high integral\nelectricity charge. As shown in Figure R1, we compared the continuous voltage/current curves\nof PVA-0.1 M CuCl and PVA-1.5/0.25 M CuCl /CaCl i-TE cells, showing a rapid decline of 90%\n2 2 2\nonly in 3 mins and a smoother decline of 64.3% after 2 h, respectively. The sharp decline of the\nPVA-0.1 M CuCl device is due to its i-TE capacitor property based on the strong\n2\nthermodiffusion effect. However, the strong thermogalvanic redox reaction is beneficial for\nkeeping a more stable output current in the PVA-1.5/0.25 M CuCl /CaCl device. Also, the\n2 2\ncontinuous energy density (E ) of the PVA-1.5/0.25 M CuCl /CaCl i-TE device is 1.5 J m-2,\n2h 2 2\nwhich is 375 times higher than that of the PVA-0.1 M CuCl device (0.004 J m-2).\nmore reasonable to reflect the trade-off relationship between thermo-diffusion/galvanic effect. The\ni-TE cells with low CuCl concentrations exhibit high thermal voltages but low output electric\n2\ncharge quantity. This is because the low-concentration samples are mainly based on the\nthermodiffusion effect with less thermogalvanic redox reaction, showing an intermittent\nbehavior of i-TE capacitors. On the contrary, high-concentration i-TE devices exhibited low\nthermal voltages but large integral electricity charges. It indicates that the thermogalvanic redox\nreaction is enhanced as the CuCl concentration increases, offering sustainable electrons and\n2\ncurrent output. This result is consistent with the trend of V and I (Instantaneous discharge\noc sc\nindex), indicating the trade-off relationship between thermodiffusion and thermogalvanic\neffects. Furthermore, the CV curves and V-I-P curves also verify the enhanced thermogalvanic\nredox reaction, as shown in Figure R3.\nTherefore, one viewpoint can be confirmed: the redox output performance (strengthened\nthermogalvanic redox reaction) is enhanced with increased CuCl concentrations, but the thermal\n2\nvoltage performance is declined (weakened thermodiffusion contribution).\nWe agree with the reviewer that the term \"TG dominance\" in such a coupled system can\nbe ambiguous. Therefore, we have moderated our language in the revised manuscript,\nfocusing on the enhancement of TG redox activity and the trade-off relationship between the\ntwo effects, rather than implying a complete binary switch.\nThe second comment：\nThis issue is compounded by the strong concentration-dependent reduction in internal resistance\n(Fig. R10), which can plausibly explain higher current and power without requiring a TG-\ndominated regime, and therefore the authors should quantitatively decouple “TG enhancement”\nfrom simple conductivity/kinetic improvements.\nResponse:\nThis is a critical point. The internal resistance shown in Fig. R10 is calculated by V-I curves,\nmaking it hard to separate each resistance contribution of the electrochemistry process.\nTo address it, we analysed the distinct components of the overall resistance present in the\nsystem via electrochemical impedance spectroscopy (EIS). As shown in Figure R4, we\ndifferentiate between:\n• Ohmic/solution resistance (𝑅 ): Related to bulk ionic conductivity.\n𝑠\n• Charge-transfer resistance (𝑅 ): Directly related to the kinetics of the interfacial redox\n𝑐𝑡\n(TG) reaction.\nOur data shows that as CuCl₂ concentration increases from 0.05 M to 1.5 M:\n• 𝑅 decreases by a factor of ~3.2 (from 132 Ω to 41.2 Ω).\n𝑠\n• 𝑅 decreases by four orders of magnitude (from ~10⁵ Ω to 40.9 Ω).\n𝑐𝑡\nWhile the modest decrease in 𝑅 explains some improvement in current, it cannot account for\n𝑠\nthe 7.4-fold increase in output power P , and the 375-fold increase in sustained energy output\nmax\nE . We can conclude that the significantly increased output performance at high ion\n2h\nconcentrations is mainly caused by the enhanced TG redox reaction, but not the ionic\nconductivity improvement. The remarkable decrease in R directly indicates a drastic\nct\nenhancement in interfacial redox kinetics. This provides strong, quantitative evidence that the\nboosted output at high concentrations is primarily due to enhanced TG activity, not merely\nimproved ionic conductivity.\nThe third comment：\nWhile the long-duration outputs under finite loads are helpful, their decays are still compatible\nwith charge relaxation and mass-transport limitations typical of i-TE gels, and the study still lacks\nthe most direct validation expected for a TG-dominated generator mode: near–short-circuit\nmeasurements showing a sustained, redox-cycled self-current in the proposed TG-rich\ncompositions.\nResponse:\nWe thank the reviewer for this excellent suggestion. In direct response, we performed the\nrequested experiment on a TG-rich composition (PVA-1.0 M CuCl ). After reaching a saturated\n2\nstate under a ΔT of 10 K, the cells were placed under near-short-circuit conditions. The short-\ncircuit current (I ) was tested by using a Keithley 2400 meter. As shown in Figure R5, the output\nsc\nI of PVA-1.0 M CuCl i-TE cell showed a peak value of 181 µA, and then declined, mainly\nsc 2\nbecause of the slow mass dynamic process of the redox couple ions in gels. Finally, the I remained\nsc\nmore stable at 0.8 µA for 3600 s (sustained and redox-cycled self-current). This trend of I is\nsc\nconsistent with the long-duration current output to external resistance. We also conducted the same\nexperiment for TG-poor composition (PVA-0.05 M CuCl i-TE cell) (Figure R6). Although the\n2\nopen-circuit voltage (V = 210 mV) is much higher than that of the PVA-1.0 M CuCl sample (V\noc 2 oc\n= 105 mV), the I peak is only 26 µA and quickly declines to 0 µA for just 40 s. By comparing the\nsc\nI peak value and long-time sustained I , we can conclude that the TG effect is enhanced\nsc sc\nsignificantly as CuCl concentrations increase from 0.05 to 1.0 M. This sustained redox-cycled\n2\nself-current test is a definitive signature of a continuous TG generator mode and provides\nthe direct validation the reviewer sought.\nFurthermore, to account for the decline of I , we also test the short-circuit current output\nsc\nof PVA-0.2/0.2 M K [Fe(CN) ]/K [Fe(CN) ] (FeCN4-/3-) i-TE device based on a single TG\n3 6 4 6\neffect for a comparison experiment. The FeCN4-/3- is a widely reported p-type redox couple,\nexhibiting a single TG effect and TG output performance. The concentration (0.2 M) of FeCN4-/3-\nis the optimal content for output performance, which is referred to the reported PVA-based FeCN4-\n/3- TG i-TE cells [Nature Communications, 16:1963 (2025)]. According to the CV curves (Figure\nR7a), the faradaic current density peak was extremely high with a value of 27 A m-2 (180 times\nhigher than that of PVA-1.0 M CuCl ), indicating an ultra-high TG performance. Although there\n2\nwas a giant TG redox reaction, the I\nsc\noutput also showed a significant decline in the long-time test,\nfrom 1.05 to 0.005 mA within 3600s (Figure R7b). Therefore, the drop of I is an unavoidable\nsc\ntrend in the i-TE field due to mass-transport limitations, even though in a full TG-based i-\nTE system with ultra-high output performance. The challenge to improve the continuous\ncurrent output is critical and ongoing in the i-TE field.\nWe thank the reviewer for their valuable comment. The continuous monitor on the I output\nsc\ncan directly prove the sustained TG redox reaction in the i-TE generator. We have supplemented\nthis critical result in the revised main text and supporting information.\n-On page 12 of the main text and page 60 of the supporting information\n“Under near-short-circuit conditions, the CP |PVA-1.0 M CuCl\n2\n| CP i-TE cell exhibited a high\nshort-circuit current (I )peak value of 181 µA and continuous I (approximately 0.8 µA) output\nsc sc\nfor 1 hour, showing the sustained redox-cycled self-current in high-concentration composition\n(Supplementary Fig. 42).” was revised on page 12 of the main text and page 60 of the supporting\ninformation.\nThe fourth comment：\nWithout a rigorous TD/TG decomposition (e.g., equivalent-circuit/kinetic modeling extracting\ndistinct TD vs TG amplitudes/time constants and correlating output with TG-specific descriptors\nrather than only CuCl2 concentration), the manuscript should moderate claims of a definitive\nTD→TG transition and a fundamentally new “competitive-coupling” framework, and instead\nframe the results more cautiously as a speciation/transport-tuned balance between TD-like and\nredox contributions.\nResponse:\nWe agree with the reviewer's overarching suggestion. The intertwined roles of Cl⁻ (as both a\ndiffusing ion and a redox-stabilizing ligand) make a complete mathematical decoupling of TD and\nTG contributions exceptionally challenging within the scope of this study. While developing a full\nequivalent-circuit/kinetic model is a valuable goal for future work, we acknowledge that our\ncurrent evidence is most consistent with a tunable balance rather than a sharp transition.\nTherefore, we have carefully revised the manuscript to moderate our claims. We believe\nthese revisions, supported by the new near-short-circuit current data and clarified\nimpedance analysis, now provide a rigorous and compelling narrative that meets the high\nstandards of Nature Communications. We are grateful to the reviewer for their guidance in\nstrengthening our work.\nDetailed revised contents are shown below.\n- The title of this paper was modified to “Modulating thermo-diffusion/galvanic coupling via ion speciation engineering enables high-performance ionic thermoelectric cells” on page 1 in the main\ntext.\n- In the abstract of the main text (page 2)\n“In binary polyvinyl alcohol (PVA)-CuCl gels, an interactive i-TE coupling effect driven by ion\n2\nspeciation is validated, which exhibits an enhanced thermogalvanic redox as CuCl concentrations\n2\nincrease while suppressing the thermodiffusion contribution.” and “Our findings reveal an\ninteractive thermo-diffusion/galvanic coupling effect based on coordination chemistry, offering a\npotential design principle for high-performance i-TE materials.” were revised.\n- In the introduction of the main text (page 4)\n“These two i-TE processes exhibit an interactive coupling due to the competition of Cl⁻, and their\nrelative contributions can be regulated by adjusting the concentration of CuCl .”\n2\n“Particularly, increasing CuCl concentration promotes the output performance (more\n2\nthermogalvanic redox species) while suppressing the thermal voltage. Modulating the thermo-\ndiffusion/galvanic coupling via speciation engineering offers a rational strategy to achieve both\nhigh voltage and continuous power output within a single system, addressing the long-standing\ntrade-off between the intermittent behavior of i-TE capacitors and the low voltage of i-TE\ngenerators.”\n“By modulating ion speciation, we achieved a giant thermopower of -30.6 mV K-1 and a\nremarkable output power performance of 0.60 mW m-2 K-2 in the PVA-CuCl i-TE system.” These\n2\nthree paragraphs were revised.\n-On page 5 of the main text\n“An opposite trend of i-TE performance was demonstrated: decreasing V and increasing I with\noc sc\nincreasing x.” was revised.\n-On page 6 of the main text\n“Thermo-diffusion/galvanic coupling mechanism” was revised.\n-On page 7 of the main text\n“Unlike the single synergetic effect14,34,35, the thermo-diffusion/galvanic coupling in PVA-x CuCl\n2\ngels exhibits a trade-off relationship due to the competition of Cl⁻. This mechanism leads to an\nopposite trend of i-TE performance as x increases: achieving high output power driven by [Cu-\nCl -(H O) ]2-y/[Cu-Cl ]⁻ redox couples while decreasing the thermal voltage mainly from the\ny 2 6-y 2\nthermodiffusion contribution, which could be attributed to coordinated species evolution.”\n“The interaction relationship between two i-TE effects could be modulated just by manipulating\nthe ion concentrations, endowing dual i-TE performance in a single system, i.e., high thermopower\n(at low x of 0.1 M) and output performance (at high x of 1.5 M).” These two paragraphs were\nrevised.\n-On page 10 of the main text\n“We attributed such trend to an interactive balance between two i-TE processes: thermodiffusion\nof [Cu-H O]2+ and Cl⁻ (at low x), in which the high migration entropy difference generates a large\n2\nΔV ; and thermogalvanic reaction (at high x) driven by redox couple, leading to a smaller\ntd\nΔE0 (Supplementary Fig. 26) (ref.14). This interactive balance explains the non-monotonic trend\nin thermopower S (Fig. 4f): At low concentration (0.01 M ≤ x ≤ 0.07 M), the thermopower\ni\nincreased from -19.5 to -28.7 mV K-1; at intermediate concentration (0.1 to 0.7 M), the\nthermopower achieved a maximum value of -30.6 mV K-1 at x = 0.1 M and then gradually\ndecreased; and at high concentration (x ≥ 1M), enhanced thermogalvanic effects collapse the\nthermopower to -5.9 mV K-1 (x = 2.0 M).” was revised.\n-On page 11 of the main text\n“The opposite trend between V and I also showed the trade-off relationship as increasing Cl⁻ content.” was\noc sc\nrevised.\n-On page 13 of the main text\n“A thermo-diffusion/galvanic i-TE coupling driven by [Cu-Cl] speciation is validated in PVA-x\nCuCl₂ gels (x: CuCl concentrations).” was revised.\n2\n-On page 25 of the main text\n“Fig. 2: The thermo-diffusion/galvanic coupling mechanism.” and “b, Schematic figure of the\nthermo-diffusion/galvanic coupling mechanism, including ion diffusion process and redox\nreaction under the temperature gradient.” were revised.\nResponse to Reviewers: Manuscript NCOMMS-25-68639B\nModulating thermo-diffusion/galvanic coupling via ion speciation engineering\nenables high-performance ionic thermoelectric cells\nYuchen Li1†, Ying-Ru Qiu2†, Jinsha Liao3, Helios Y. Li1, Zhouyuntong Liu1, Yuxiang Chen1, Mao Yu5, Lizhi Xu1, Chang Liu4, Jefferson Zhe Liu3, Jian-Feng Li2, Weishu Liu5, Yao-Hui Wang2*, Nicholas X. Fang1*\nWe are pleased that all the Reviewers have accepted the revised version. We are grateful to the\nreviewers for their insightful and constructive comments, which have significantly improved the\nquality of our work."}]

Decision letter