An ultrathin ionic thermoelectric cell design utilizing near body heat for self-powered wearable electronics

Domain: biology
Published: 2026-04-01
Editorial outcome: minor_revision

Abstract
Harvesting low-grade heat is a sustainable way to power wearable electronics, and quasi-solid-state ionic thermoelectric cells offer a flexible, low-cost option. Their use, however, has been limited by a key trade-off: miniaturization reduces the internal thermal gradient and compromises performance. Here, we address this challenge with an ultrathin asymmetric architecture that separates thermal energy harvesting from the conventional reliance on a sustained through-plane temperature gradient. The design couples thermally driven ionic modulation at one interface with engineered pseudocapacitive charge storage at the other. Our 1-mm-thick device delivers an open-circuit voltage of 0.1 V, a power density of 1.6 W m-2, and an energy density of 1500 J m-2 using near-body heat. An array of 20 cells generates 1.9 V and a peak power of 23 W m-2, enabling continuous smartwatch operation. This strategy provides a practical route to ultrathin ionic thermoelectric cells for self-powered wearable systems.

Introduction
Low-grade thermal energy (< 100 oC), which accounts for more than 65% of global energy dissipation, is abundant in sources such as industrial waste heat, environmental fluctuations, and human body heat.1, 2 Efficiently converting this low-grade heat into electricity is of growing interest, especially for powering flexible and wearable electronics.3, 4, 5, 6 Conventional solid-state thermoelectric materials, typically based on rigid inorganic semiconductors, have been explored for this purpose, especially in ultrathin or flexible formats.7, 8 However, their practical deployment is constrained by inherently low Seebeck coefficients near body temperature (usually <100 μV K-1), mechanical rigidity that limits true conformability, and high manufacturing costs.9, 10 As a compelling alternative, ionic thermoelectric cells (ITCs) have emerged, utilizing the thermodiffusion and thermogalvanic effects of ions in an electrolyte to generate giant thermopower on the order of mV K-1 near room temperature, an order of magnitude higher than their solid-state counterparts.11, 12, 13, 14, 15, 16 In particular, quasi-solid-state ionic thermoelectric cells (QS-ITCs) based on gel electrolyte offer enhanced safety, mechanical softness, and integration compatibility for wearable systems.17, 18, 19, 20, 21

Despite these advantages, conventional ionic thermoelectric cells face their own critical and unresolved challenge, that is, a fundamental trade-off between device form factor and performance. For true wearability, devices must be ultrathin (<2 mm), but this miniaturization inevitably collapses the through-plane thermal gradient especially under mild heat sources such as human skin (ΔT<5 K). This leads to a sharp decline in voltage and energy output, creating a miniaturization–performance trade-off that remains the principal barrier to practical application. Significant efforts have been dedicated to enhancing their performance by engineering electrolyte composition,22, 23, 24, 25 tuning solvation structures,26, 27 exploiting thermally induced phase transitions,28, 29, 30 and optimizing device architectures31, 32, 33. These strategies have successfully pushed the energy output, with some systems reporting energy densities approaching 400 J m-2.31, 34, 35 However, these advances have not resolved the core trade-off, as top-performing devices are typically several millimeters thick to maintain a sufficient ΔT, fundamentally limiting their integration into truly conformable, low-profile systems. Furthermore, the challenge is compounded in ultrathin QS-ITCs by sluggish ion transport and interfacial charge-transfer kinetics, which further suppress both power density and overall energy conversion efficiency. As such, achieving a high-performance QS-ITC that is both ultrathin and effective under sub-5 K gradients remains a critical challenge.

Here, we resolve this long-standing trade-off by introducing an ultrathin, asymmetric QS-ITC architecture that decouples thermal energy harvesting from the conventional reliance on a sustained through-plane temperature gradient. This architecture is enabled by synergistic interfacial engineering, coupling thermally driven ionic modulation at one electrode with spatially gradient redox activity at an asymmetric counter-electrode, which allows for directional charge accumulation and release. This temporally and spatially decoupled operation permits our 1 mm-thick device to efficiently harvest energy near body temperature (ΔT≈3 K), storing it as a high-potential charge and subsequently delivering stable electrical output. As a result, the device achieves a record energy density of 1500 J m-2 over 2 hours and a steady-state output of 0.1 V under low ΔT conditions. Moreover, a 20-cell integrated array delivers 1.9 V and a peak power density of 23 W m-2, successfully powering a commercial smartwatch using only body heat. This work provides a foundational strategy for overcoming the miniaturization bottleneck in ionic thermoelectric cells, unlocking their potential for self-powered, autonomous wearable systems.

Results
Design of ultrathin QS-ITC
Integrating multiple ITCs into arrays via series and parallel connections represents an effective strategy for enhancing output voltage and power.30, 36, 37 Conventional designs, reliant on vertical heat flow between the human body and environment, necessitate a thick, block-sandwich-like configuration to establish a sufficient thermal gradient.7, 38, 39 This arrangement imposes a fundamental trade-off between device miniaturization and effective thermal gradient (Fig. 1a and Supplementary Table 1). To quantify this trade-off, the temperature-difference utilization (𝜑 ), defined as the ratio of the effective temperature difference (∆𝑇 ) to the available temperature difference (∆𝑇 ) is simulated using finite element analysis (Supplementary Note 1). The simulations are conducted for quasi-solid-state gel electrolytes of various thickness (1, 3, 5, 10, 15 and 20 mm) under conditions representative of wearable applications, encompassing environmental temperatures from 10 °C to 30 °C and airflow velocities from 0.1 m s-1 to 1.0 m s-1 (Fig. 1b and Supplementary Figs. 1, 2). The analysis reveals a positive correlation between ∆𝑇 ，gel thickness (H), ∆𝑇 and airflow velocity. For gels thinner than 5 mm, maintaining a significant thermal gradient becomes challenging. For instance, even under a 10 °C ambient condition (∆𝑇 = 26 °C) with an airflow velocity of 1.0 m s-1, the maximum achievable ∆𝑇 is limited to 3.6 °C, corresponding to a 𝜑 of only 12%. A further reduction in thickness to 1 mm results in a near-isothermal condition, which suppresses any substantive thermal gradient and thus precludes effective thermoelectric conversion. This trend is corroborated by experimental measurements (Fig. 1c). When subjected to a 36 °C heat source in a 12 °C ambient environment, a 10 mm-thick gel electrolyte attains a ∆𝑇 of merely 7 °C. This value diminishes to 3 °C and 1 °C as the thickness is reduced to 5 mm and 1 mm, respectively. Such minimal thermal gradients are insufficient to generate substantial power, even with state-of-the-art ITC materials possessing high thermopower (e.g., 6.5 mV K⁻¹).28

From a thermodynamic perspective, establishing a sufficiently large ∆𝑇 necessitates an adequate thermal diffusion length, which scales with the gel thickness (Fig. 1d). However, increasing the thickness of the QS-ITC introduces greater thermal resistance, resulting in a lower average temperature (𝑇 ) and extended ion migration paths. Both factors suppress ionic conductivity due to its Arrhenius temperature dependence and the elevated ionic resistance. In contrast, device miniaturization through thickness reduction enhances ion transport by increasing 𝑇 and shortening diffusion paths, yet simultaneously reduces ∆𝑇 because of the limited spatial temperature gradient, thereby lowering the output voltage (Fig. 1e and Supplementary Note 2). As a result, the output power is constrained by the competing effects of reduced voltage and moderately enhanced conductivity. This intrinsic trade-off, rooted in the coupled limitations of heat conduction and ionic diffusion, represents a critical barrier to improving the thermoelectric performance of thin QS-ITCs for wearable energy harvesting.

To address this limitation, an ultrathin QS-ITC that achieves directional-dependent decoupling between the thermal field and potential field is proposed (Fig. 1f). This device is constructed using asymmetric electrodes with distinct thermo-electrochemical characteristics in the ionic electrolyte during the heating process (Fig 2a). A dual-network copolymer hydrogel (PAM/DAC), which is endowed with elevated ionic conductivity, is synthesized via UV-initiated polymerization of acrylamide (AM) and acryloyloxyethyl-trimethyl ammonium chloride (DAC) monomers.40, 41 A capacitive cathode (carbon cloth, CC) and a battery-type anode (polyaniline, PANI) are positioned in close proximity to each other on the same surface of the ultrathin gel. This configuration ensures intimate engagement with the ionic electrolyte while eliminating any inadvertent thermal gradient between electrodes (Supplementary Fig. 3). The resultant assembly of asymmetric electrodes and a laminar gel electrolyte constitutes the ultrathin QS-ITC.

When the ultrathin QS-ITC resides at an initial 𝑇 , the disparity in Fermi levels between the asymmetric electrodes establishes an intrinsic electrochemical potential difference, which manifests as the electrochemical voltage (𝑉 ) (Fig. 2b and Supplementary Note 3).42 Upon heating from 𝑇 to 𝑇 , a thermo-voltage (𝑉 ) arises from two synergistic processes. First, the capacitive CC cathode undergoes rapid and highly reversible pseudocapacitive reactions, wherein the elevated temperature promotes the chemisorption and desorption of electrolyte ions at its oxygen-containing functional groups.43 This Faradaic process subsequently lowers the cathode potential, thereby increasing the open-circuit voltage (𝑉 ). Concurrently, the PANI anode exhibits a modest potential decrease owing to its intrinsic positive temperature coefficient.44 The thermodynamically favorable reduction of Fe3+ to Fe2+ (Fe3+ + e- = Fe2+), coupled with the oxidation of reduced PANI chains, supplies electrons for the redox process and further modulates the electrode potential.

Upon connection to an external load, PANI is further oxidized, and electrons traverse the external circuit to the CC cathode, sustaining power delivery at 𝑇 . The discharge capacity originates jointly from the valence-state transition of the PANI electrode and the Fe3+/Fe2+ redox couple. Although the accumulation of electrons at the CC causes its potential to rise and the discharge voltage (𝑉 ) to diminish, the continuous consumption of Fe3+ adsorbed at the CC mitigates the rate of this voltage decline, thereby reinforcing the current output. Following the removal of the load and subsequent cooling to 𝑇 , the accumulated Fe2+ is oxidized back to Fe3+ (Fe2+ = Fe3+ + e-), while the released electrons reduce the PANI anode, regenerating its active state under acidic conditions. Simultaneously, ions desorb from the CC surface as the temperature decreases. These collective processes restore both electrode potentials and 𝑉 to their initial states, enabling cyclical and reversible operation of the ultrathin QS-ITC. The thermoelectric conversion in the QS-ITC is governed by an ionic thermoelectric effect, a mechanism fundamentally distinct from the classical thermogalvanic effect. It operates not via a spatial ∆T but is instead activated by uniform heating. The thermal input induces a synergy of asymmetric, temperature-sensitive interfacial processes (the pseudocapacitive response of the CC cathode, the intrinsic positive temperature coefficient of PANI, and the Fe3+/Fe2+ redox equilibrium) that collectively modulate the electrode potentials to generate electricity.

The fabrication of asymmetric electrodes of ultrathin QS-ITC
Commercial carbon cloth, derived from the high-temperature carbonization of polyacrylonitrile fibers, is a standard current collector valued for its exceptional electrical conductivity, chemical resistance, and mechanical flexibility. A primary constraint of the pristine material is its inherently low specific surface area and limited porosity, which restrict its electrochemical performance.45 Furthermore, the smooth, non-polar nature of its surface leads to negligible intrinsic electrochemical activity. To overcome these limitations, a mixed-acid thermal activation strategy is employed to modify the carbon cloth surface (Supplementary Fig. 4), a procedure that substantially enhances its electrochemical properties.46 During activation, oxygen-containing functional groups (e.g., C=O and O–C=O) are introduced onto the fiber surface. These groups facilitate the reversible chemisorption and desorption of protons and small-radius cations from the electrolyte, thereby enabling temperature-induced pseudocapacitive responses.47 Concurrently, the incorporation of polar functional groups (–COOH and –OH), along with the formation of a roughened, porous surface morphology, endows the activated carbon cloth with superhydrophilic properties. This modification promotes intimate contact at the electrode–electrolyte interface, accelerating interfacial mass and charge transport. Additionally, the activated surface, with its high density of nucleation sites, serves as a conducive platform for the in situ growth of electroactive nanostructures.

The PANI/CC noncovalent composite electrode is fabricated via electrochemical deposition of polyaniline from an acidic aniline solution.48 This process utilizes the activated carbon cloth as the working electrode within a three-electrode system that includes a platinum counter electrode and a mercurous sulphate reference electrode (Supplementary Fig. 5). Compared with physical methods such as dip-coating, the in situ electrochemical growth of PANI on carbon fibers ensures intimate interfacial contact and strong adhesion. This strategy is highly controllable, providing a standardized and reproducible assembly principle: the loading mass can be precisely determined by the current density and deposition time, and the resulting layer shows exceptional distribution stability, thereby overcoming common interfacial limitations (Supplementary Figs. 6, 7 and 8). During electrodeposition, polyaniline assembles into coral-like nanofibers that interweave densely across the carbon surface, forming a continuous, intricately entangled network (Figs. 3a, b). Energy dispersive spectroscopy (EDS) mapping confirms the abundant and uniform deposition of PANI on the carbon cloth (Fig. 3c). This porous, interconnected network structure enhances electrical conductivity and facilitates ion diffusion from the electrolyte into the interior, effectively shortening ion transport pathways and promoting rapid Faradaic reactions. The uniform and stable deposition of PANI is enabled by strong π–π stacking interactions between the polyaniline chains and the carbon surface, as well as by the presence of the oxygen-containing functional groups that serve as anchoring sites. This noncovalent integration yields a robust and electrochemically active interface, well-suited for efficient energy conversion.

The structural characteristics of the activated CC, the PANI active material, and the PANI/CC composite electrode are investigated using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and Raman spectroscopy (Figs. 3d-f). The XRD pattern of activated CC exhibits broad diffraction peaks near 26° and 43°, corresponding to the (002) and (100) planes of graphite, respectively. The peak broadening suggests a low degree of crystallinity and a disordered graphitic structure. Both PANI and PANI/CC display consistent peaks at approximately 15°, 21°, and 26°, assigned to the (011), (020), and (200) planes, respectively, confirming the formation of the emeraldine salt form of PANI and its effective deposition onto the CC substrate.49 The graphite-related peak of CC at 26° likely overlaps with that of PANI, while the characteristic peak at 43° remains visible in the PANI/CC composite, indicating that some regions of the CC surface remain uncovered by the active material.

In the FTIR spectra, pristine CC exhibits negligible absorption due to its sp2-hybridized carbon framework and lack of polar functional groups, resulting in the PANI/CC spectrum closely mirroring that of pure PANI. The dominant vibrational bands of PANI appear near 1580, 1500, 1300, 1140 and 830 cm-1, corresponding to C=C stretching in the quinoid and benzenoid rings, C–N stretching, C–H in-plane bending, and out-of-plane C–H bending of para-disubstituted benzene, respectively.50

Raman spectral analysis of the acid-treated CC reveals two characteristic peaks at 1345 and 1590 cm-1, corresponding to the D and G bands of carbon materials, respectively. PANI presents multiple distinct bands at 812, 1160, 1210, 1480, and 1590 cm-1, assigned to benzenoid ring deformation, in-plane bending of C-H in semi-quinonoid structures, C-N stretching in benzenoid units, C=N and C=C stretching in quinonoid units. The peak at 1320 cm-1, attributed to C-N+ stretching vibrations of polarons, confirms the presence of doped PANI in the emeraldine salt (ES) state within the PANI/CC composite.51

X-ray photoelectron spectroscopy (XPS) analysis provides further substantiation of the surface composition and chemical bonding (Figs. 3g-i). The survey spectra of PANI and PANI/CC, relative to pristine CC, display additional signals for nitrogen (N1s, 399.5 eV) and chlorine (C12p, 532.5 eV), affirming the successful incorporation of HCl-doped PANI. The high-resolution C1s spectrum of PANI/CC is deconvoluted into four components, namely peaks at 284.8 eV (sp2-hybridized carbon, C–C/C=C), 286.3 eV (C–O/C=N/C-N), 287.4 eV (C=O/C-N+), and 288.6 eV (O–C=O).51 The N1s spectrum identifies four distinct nitrogen species: non-protonated imine (–N=, 398.3 eV), non-protonated amine (–NH–, 399.3 eV), protonated amine (–NH+–, 400.1 eV), and protonated imine (–NH+=, 401.7 eV). The concomitant presence of these oxidized and reduced nitrogen species corroborates the formation of a conductive, doped PANI framework and suggests effective π-π conjugation with the carbon substrate.

Working mechanism of ultrathin QS-ITC
The asymmetric electrode configuration is integrated into an ultrathin QS-ITC, coupled with a charge-neutral polyacrylamide (PAM) gel containing FeCl electrolyte (Supplementary Fig. 9). The device with Fe3+ electrolyte exhibits a noteworthy 𝑉 of approximately 0.1 V at 36 °C and a sustainable discharge behavior, with an initial current of 300 μA that stabilizes at ~100 μA after 600 s (Supplementary Fig. 10). In contrast, substituting FeCl with KCl leads to a two-order-of-magnitude suppression in the discharge current, which rapidly decays below 1 μA within 30 s, a behavior indicative of a purely capacitive process. Furthermore, the utilization of Fe2+ as the primary cation results in a reversed 𝑉 that counteracts the desired 𝑉 , thereby constraining overall performance.

The comparative analysis of thermoelectric performance across various electrolyte systems yields several key insights: (a) the 𝑉 is governed by the specific identity of the specific ionic species present; (b) the 𝑉 is the result of coupled thermo-electrochemical reactions between the asymmetric electrodes and the electrolytes during heating; (c) the sustained discharge capability originates predominantly from the thermogalvanic effect of the Fe3+/Fe2+ redox couple. To validate these insights, temperature-dependent variations of individual electrode potentials are recorded using an Ag/AgCl reference electrode in systems incorporating FeCl , FeCl , and KCl as electrolyte.

The selective interactions between electrode materials and ionic species result in different Fermi level alignments at the interfaces, thereby generating differing initial 𝑉 .42 Upon heating, the oxygen-containing functional groups (C=O/O-C=O) on the activated CC promote pseudocapacitive behavior by enhancing temperature-dependent ion adsorption at the electrode–electrolyte interface. The coordination of small-radius Fe3+ ions with these oxygen sites, along with proton interactions, yields the highest temperature coefficient in the FeCl electrolyte (~1.9 mV K-1) (Supplementary Fig. 11). The larger ionic radius and lower surface affinity of K+ and Fe2+ result in lower temperature coefficients of ~1.32 and ~1.0 mV K-1, respectively (Supplementary Figs. 12 and 13). Distinct from the Fe2+ and Fe3+ systems, K+ does not participate in redox reactions with the electrodes, and the resultant temperature coefficient of ~1.0 mV K-1 is attributed to the combined effects of the CC-side pseudocapacitive behavior and the intrinsic positive temperature coefficient of the PANI electrode (~0.3 mV K-1). Therefore, the substantial voltage and persistent high current in the FeCl -based system are a consequence of the synergistic effects of temperature-induced pseudo-capacitance at the CC side and redox interactions between the PANI electrode and the electrolyte.

To elucidate the underlying thermodynamic mechanism, a series of ex situ characterizations are performed. The generation of Fe2+ during heating is first confirmed using the colorimetric assay o-phenanthroline (o-Phen), which forms an orange-red [Fe(o-Phen) ]2+ complex (Fig. 4a and Supplementary Note 4). Asymmetric electrodes heated to 40 °C in the electrolyte produce a significant color change under both open-circuit and short-circuit conditions, confirming the redox reaction Fe3+ + e- = Fe2+ occurs within the device (Fig. 4b). Quantitative analysis via UV-Vis spectroscopy indicates that approximately 15–20% of the total iron is converted to the Fe2+ state, with a higher fraction observed under short-circuit conditions (Fig. 4c).

To determine the spatial distribution of this reaction, an H-type cell separated by a cation-exchange membrane (CEM) is employed. This experiment confirms that the reduction of Fe3+ to Fe2+occurs preferentially at the PANI/CC anode (Supplementary Figs. 14, 15 and Supplementary Note 5). This spatial bias is further investigated in the quasi-solid-state gel. Ex situ UV-Vis spectroscopy reveals that as the temperature increases, the concentration of the hydrated Fe3+ complex ([Fe(H O) ]3+) diminishes near both electrodes, indicating its consumption (Figs. 4d, e, h). Concurrently, the concentration of Fe2+ shows a substantial increase near the PANI/CC anode but only a marginal increase near the CC cathode (Figs. 4f, g, i). This spatial disparity is maintained by a dynamic equilibrium: the continuous, thermodynamically favorable reduction of Fe3+ at the PANI anode creates a local concentration gradient. This gradient drives the diffusion of Fe3+ from the CC side to replenish the reactants at the anode, while the accumulated Fe2+ product diffuses from the anode toward the cathode. In contrast to the thermal charging stage, Fe2+ ions appear and their concentration continuously increases near the CC electrode during external discharge (Supplementary Fig. 16). This observation confirms that a persistent reduction reaction occurs at the CC side. Upon circuit closure, the potential difference between PANI and CC drives electron transfer toward the CC electrode. At this stage, the further oxidation of PANI, coupled with the reduction of Fe3+ near CC electrode, jointly sustains the power output.

Moreover, ex situ XPS is employed to monitor the chemical state transitions of PANI across a full operational cycle (Figs. 4j, k and Supplementary Fig. 17). The N1s spectra are deconvoluted into oxidized (-NH+=, -NH+- and -N=) and reduced (-NH-) nitrogen species. During thermal charging (Stage I → Stage II), the fraction of reduced nitrogen (-NH-) decreases from 42.8% to 30.6%, while oxidized species increase, signifying the oxidation of PANI coupled with the reduction of Fe3+. Upon external discharge (Stage II → Stage III), the PANI is further oxidized, with the -NH- content declining to 26.9%. Finally, during self-recovery upon cooling (Stage III → Stage IV), the PANI electrode undergoes chemical self-reduction. The accumulated Fe2+ in the acidic environment donates electrons back to the PANI, restoring its reduced nitrogen components and returning the electrode to its initial state (Supplementary Notes 6, 7).

Overall, the thermo-voltage generation is a synergistic effect of the thermally responsive pseudocapacitive behavior of the CC electrode, the redox reaction between PANI and Fe3+, and the intrinsically positive temperature coefficient of PANI. During thermal charging, the reduction of Fe3+ occurs exclusively at the PANI/CC anode. Upon discharging, the high-capacity PANI electrode delivers electrons to the CC electrode through the external circuit. Simultaneously, Fe3+ near the CC electrode is reduced to Fe2+ by these incoming electrons. This reaction at the cathode not only mitigates the consumption of oxygen-containing functional groups on the CC surface, thereby enhancing reversibility, but also delays the voltage decay during discharging. The sustained oxidation of the PANI electrode, together with the continuous redox interactions between the asymmetric electrodes and the electrolyte, endows the ultrathin QS-ITC with a prolonged discharge duration.

Component optimization and thermoelectric performance
The influence of electrolyte composition and PANI electrode formulation on thermoelectric performance is systematically investigated using a custom-designed experimental setup (Supplementary Fig. 18). The distinct thermo-electrochemical behaviors of the asymmetric electrodes and the electrolyte establish a thermo- voltage that is positively correlated with temperature. This effect is attributed to the synergistic contribution of a pronounced positive temperature coefficient from the pseudocapacitive CC and the redox reaction between PANI and Fe3+ (Fig. 5a and Supplementary Fig.19). For instance, in a 0.1 M FeCl electrolyte, the 𝑉 of the ultrathin QS-ITC increases from 76 mV to 89 mV as the temperature rises from 278 K to 309 K.

The post-heating 𝑉 consists of the electrochemical voltage (𝑉 ) and the thermo-voltage (𝑉 ). The V represents the intrinsic electrochemical potential difference, which is determined by the selective interaction between electrode materials and ionic species and by the reconfiguration of interfacial thermodynamic equilibria. These factors are influenced by ion species, concentration, solvation structure, and interfacial chemistry. As the concentration of FeCl increases from 0.1 M to 1.5 M, the 𝑉 continuously declines from 89 mV to 48 mV, a trend attributed to the inverse correlation between ionic concentration and electrochemical potential (Fig. 5b and Supplementary Fig. 20). In contrast, the maximum power density (𝑃 ) exhibits a non-monotonic dependence, initially increasing and then decreasing, with a peak at 0.5 M. Despite the reduction in 𝑉 at higher concentrations, the decreased internal resistance due to enhanced carrier density improves the ionic conductivity of the gel electrolyte, yielding the highest 𝑃 of 0.34 W m-2 at the optimal 0.5 M FeCl concentration. To further enhance conductivity, DAC, which features quaternary ammonium groups, is introduced as a comonomer with PAM. This modification has a negligible effect on 𝑉 , but boosts 𝑃 to 0.42 W m-2 at an optimal DAC:AM mass ratio of 0.3, (Fig. 5c and Supplementary Fig. 21). Beyond this ratio, elevated crosslinking density constrains ion mobility and diminishes power output (Supplementary Fig. 22).

The PANI loading on the anode is also a pivotal parameter. Increasing the PANI loading enhances 𝑉 at 309 K due to a lower electrode potential of the PANI layer relative to the CC cathode, while the augmented electroactive surface area facilitates charge transfer (Fig. 5d and Supplementary Fig. 23). Consequently, 𝑃 reaches 1.0 W m-2 at a PANI loading of 2.8 mg cm-2. Loading beyond this optimum, conversely, diminishes performance, as the excessively thick PANI layer extends ion diffusion paths and hinders electron transport, leading to increased interfacial and migration resistances. A decline in 𝑃 is observed at a loading mass of 3.4 mg cm-2. Additionally, excessive loading could compromise electrode integrity, with the loosely bound PANI material being susceptible to detachment during operation (Supplementary Fig. 24). Moreover, reducing the inter-electrode distance from 10 mm to 5 mm significantly shortens the ion transport path, enhancing 𝑃 from 1.0 W m-2 to 1.6 W m-2 (Supplementary Fig. 25).

The complete operational cycle of the ultrathin QS-ITC comprises four stages: thermal charging, power output, equilibration, and thermal recovery (Fig. 5e). Upon heating from 278 K to 309 K, a prompt thermo-voltage emerges from the asymmetric Faradaic reactions and saturates within three minutes. When connected to an external load, electrons flow from the battery-type anode to the capacitor-type cathode, diminishing the internal electrostatic field and causing a gradual decrease in discharge voltage. After disconnection, partial voltage recovery occurs, as Fe3+ reduction on the CC side preserves surface functionalities, though incomplete reversal of the PANI redox state limits full restoration. Finally, during cooling under open-circuit conditions, oxidized PANI reacts with Fe2+to regenerate its reduced form, completing the self-regeneration cycle of both PANI and Fe3+.

The ultrathin QS-ITC demonstrates excellent rechargeability and stability under both quasi-continuous and continuous operation. In quasi-continuous mode, the ultrathin QS-ITC charges to saturated voltage at 309 K and undergoes 20 charge-discharge cycles using a 5000 Ω load (10s per discharge) (Fig. 5f). The voltage is capable of recovering within 3 minutes between cycles, retaining over 98% of its initial value and indicating high reversibility (Supplementary Figs. 26, 27). Moreover, the QS-ITC exhibits excellent thermo-response stability over 50 repeated heating and cooling cycles, maintaining its output voltage with only minor decay (Supplementary Fig. 28). This robust reversibility is primarily attributed to the highly reversible redox behavior of PANI in acidic conditions, which enables sustained interfacial reactions and stable ionic transport. In continuous operation, the ultrathin QS-ITC discharges over 2 hours under a 500 Ω load after saturation at 309 K (Fig. 5g). Despite the rapid attenuation of both output voltage and current attributable to ohmic losses and other parasitic effects, a state of quasi-steady-state output is eventually achieved. By integrating the voltage-current profiles over the 2-hour duration, the quantified energy density across a series of external resistances reveals a parabolic dependence (Figs. 5h, i and Supplementary Fig. 29). The device achieves a peak energy density of 1500 J m-2 for a 5mm electrode distance and 500 Ω load, significantly surpassing those recorded at 50, 1000, and 1500 Ω (250, 900, and 750 J m-2, respectively), and markedly exceeding the saturated energy density at a 10 mm electrode distance (600 J m-2). By integrating thermally induced interfacial ionic modulation with spatially engineered electrochemical activity, the ultrathin QS-ITC (1 mm) addresses the critical challenge of achieving high energy output in an ultrathin form factor under the small temperature gradients with thin/bulk QS-ITCs. This strategy demonstrates excellent performance of a 𝑉 of 100 mV, a 𝑃 of 1.6 W m-2, a relative Carnot efficiency of 4%, and an 𝐸 of 1500 J m-2 at 309 K, representing a comprehensive enhancement in thermoelectric performance that markedly outperforms existing ITCs (Figs. 5j, k and Supplementary Figs. 30-32, Note 8, Tables 2, 3). 12, 17, 32, 34, 35, 52, 53, 54, 55, 56, 57

Proof-of-concept demonstration and stability in wearable applications
To highlight the advantages of the ultrathin asymmetric architecture, a comparative evaluation of diurnal output stability is conducted between the ultrathin QS-ITC and a high-performance conventional thermogalvanic cell [Fe(CN) 3-/4- with Gdm+] under identical environmental conditions (Fig. 6a). With the 𝑇 fixed at 36 °C to simulate body temperature, the conventional thermogalvanic cell exhibits a substantial voltage fluctuation, ranging from 10 mV to 67 mV, as the ambient temperature varies between 11.7 °C and 28.1 °C over a 12-hour period. This instability is exacerbated by higher thermopower, which renders the device more sensitive to temperature perturbations. In stark contrast, the ultrathin QS-ITC maintains a highly stable output voltage within the narrow range of 85-100 mV under the same conditions. This exceptional thermal resilience is a direct result of its operation mechanism, which generates voltage from asymmetric interfacial reactions on an isothermal plane, rather than being dictated by a directional thermal gradient. The design strategically decouples the temperature field from potential generation, reconciling the competing demands of an ultrathin architecture and effective thermal utilization, thereby achieving stable, high-performance output in dynamic ambient environments.

Flexible, wearable ultrathin QS-ITC arrays composed of 5, 10, and 20 units are assembled via straightforward serial integration to demonstrate scalable low-grade heat harvesting (Fig. 6b and Supplementary Fig. 33). These serially connected devices exhibit pronounced thermoelectric responses, with the 𝑉 increasing linearly with the number of units, reaching 0.48 V, 0.94 V, and 1.9 V at 36 °C for arrays comprising 5, 10, and 20 units, respectively (Figs. 6c, d). Although the short-circuit current density shows a gradual decline due to the cumulative interconnection resistance, the 20-unit array nonetheless achieves a peak power density of 23 W m-2, underscoring the excellent scalability of the ultrathin QS-ITC architecture (Fig. 6e). When benchmarked against existing wearable devices for human body heat harvesting, this ultrathin QS-ITC configuration delivers not only stable output but also a more than a five-fold enhancement in power output at comparable voltage levels (Fig. 6f and Supplementary Table 4).17, 24, 25, 31, 34, 53, 54

Benefiting from its stable, high-performance output and the inherent mechanical flexibility of all components, the 20-unit array achieves full conformability with the human body and can effectively harvest body heat to directly power commercial electronics, such as a smartwatch (Fig. 6g and Supplementary Movie 1). Furthermore, the high 𝑉 of 1.9 V enables the device to directly operate a digital hygrometer/thermometer, a task that typically requires a voltage amplifier for other QS-ITCs (Fig. 6h and Supplementary Fig. 34). In addition, to evaluate the biocompatibility of the device for potential wearable applications, a skin irritation test is performed (Supplementary Fig. 35). It is demonstrated that no visible erythema, edema, or irritation is observed on the skin surface after direct contact with the device, confirming its excellent dermal compatibility. Moreover, the epithelial tissue remains intact with no inflammatory cell infiltration even after 72 hours of continuous contact with the device, indicating negligible cytotoxicity and excellent biocompatibility. These demonstrations establish the ultrathin QS-ITC as a high-performance, mechanically compliant platform for wearable thermal energy conversion, offering significant promise for powering next-generation electronics.

Methods
Materials
The following raw materials were used: Ferric chloride (FeCl ), Ferrous chloride (FeCl ), Dibasic Sodium Phosphate (Na HPO ), Acrylamide (AM), 2-hydroxy-2-methylpropiophenone (HMPP), Poly(ethylene glycol) diacrylate (PEGDA), Acryloyloxyethyl-trimethyl ammonium chlorid (DAC), Aniline were purchased from Aladdin industrial Corporation. HCl, H SO , HNO were purchased from Sinopharm Chemical Reagent Co., Ltd. 1,10-Phenanthroline Monohydrate (o-Phen) was purchased from TCI (Shanghai) Development Co., Ltd. The purity of all chemicals regents has reached analytical grade, and they can be directly employed in experiments without further purification. Carbon cloth (W0S1011) was purchased from CeTech CO., Ltd., China.

Fabrication of the PANI/CC electrode
Carbon cloth (CC) was sequentially cleaned with acetone, sulfuric acid, deionized water, and ethanol, and then vacuum-dried at 40°C for 3 hours. Subsequently, the dried CC was immersed in a mixed acid solution of sulfuric acid and nitric acid (volume ratio of 3:1) and heated in an 80 °C oil-bath for 12 hours to prepare the activated CC. PANI was electrodeposited onto active CC in a three-electrode configuration using a mercury sulfate electrode as reference and Pt foil as counter electrode. The electrodeposition electrolyte contained 0.2 M aniline in 1 M HCl. Galvanostatic polymerization was conducted at a 1 mA cm-2. The loading mass of PANI was controlled by adjusting the constant current and deposition time. Subsequently, the obtained PANI/CC electrode was rinsed with ultrapure water and ethanol under mild agitation, vacuum-dried at 40°C for 12 hours and immersed in a 1 M HCl solution for 48 hours to ensure full protonation.

Fabrication of the ultrathin QS-ITC
The quasi-solid-state electrolyte is denoted as PAM/x DAC/m FeCl , where x is the mass ratio of DAC to AM and m is the molar concentration of FeCl . For PAM/0.5 DAC/0.3M FeCl , 2.986 g AM monomer was dissolved in 7 mL of deionized water, followed by addition of PEGDA (10 μL) and HMPP (10 μL) as crosslinker and photoinitiator. After stirring in the dark for 10 min, 0.9 g DAC solution was added and stirred for another 10 min to obtain the precursor solution for the PAM/DAC gel. The precursor solution was then poured into a glass mold and exposed to UV light (365 nm) for 5 minutes, resulting in the formation of the PAM/0.3 DAC hydrogel substrate. An ionic electrolyte solution was prepared by adding 1 vol% of 3 M HCl to 0.3 M FeCl . Unless otherwise stated, the electrolyte solution is prepared with HCl at this concentration. Finally, the PAM/DAC gel was immersed in the electrolyte solution for 24 hours to facilitate ion exchange, thereby producing a quasi-solid-state electrolyte. For ultrathin QS-ITC assembly, activated CC (15 mm in length and 5 mm in width) and PANI/CC electrode of identical dimensions were fixed to same surface of the quasi-solid-state electrolyte (20 mm in length, 5 mm in width, and 1 mm in thickness), with a 10 mm gap between the electrodes. The assembled ultrathin QS-ITC was then encapsulated with 0.5 mm thick VHB tape.

Characterization
The morphology of the electrodes and the quasi-solid-state electrolyte were examined by scanning electron microscopy (SEM, Zeiss Ultra PLUS). The crystal structure and phase composition of PANI were analyzed by X-ray diffraction (XRD, Rigaku D/max-2550VB). The chemical composition and structural features of the electrodes were characterized using Fourier transform infrared (FTIR, Thermofisher Nicolet IS50) spectroscopy and Raman spectroscopy (HORIBA XploRA PLUS). The elemental states and redox reaction mechanisms on the surface of the PANI/CC were investigated via X-ray photoelectron spectroscopy (XPS, Escalab 250Xi). Variations in ion concentration within the electrolyte were measured using ultraviolet-visible (UV-vis, UV-2600) spectroscopy.

Data availability
All data supporting the findings of this study are available within this article and the Supplementary Information or from the corresponding author upon request. The data generated in this study are provided in the Supplementary Information/Source Data file. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General Comment: The manuscript presents an interesting concept of a temperature-responsive electrochemical cell based on a PANI anode, carbon cloth cathode, and the Fe3+/Fe2+ redox couple. While the experimental results are solid, I would like to raise a conceptual concern regarding the terminology used throughout the manuscript.\n\nTraditionally, a thermocell (thermogalvanic cell) operates on the principle of a temperature gradient (ΔT=TH−TC) between two electrodes, where the potential difference is determined by the entropy change of the redox reaction according to the Nernst equation. A key figure of merit is the Seebeck coefficient, and power generation directly relies on the existence of two electrodes maintained at different temperatures.\n\nIn contrast, in this work the entire cell is heated from Tc to TH and then cooled back, rather than being operated under a steady-state temperature gradient. The voltage variation originates from temperature-dependent electrode processes (pseudocapacitive reactions at the CC cathode, the positive temperature coefficient of PANI, and the Fe3+/Fe2+ redox equilibrium), rather than from a true thermogalvanic effect driven by ΔT.\n\nTherefore, the device demonstrated here is more accurately described as a temperature-triggered rechargeable electrochemical cell rather than a conventional thermocell. The authors should not highlight this work by comparing to conventional thermocells, they instead should make a comparison with other electrochemical cells. For this reason, I can not recommend to accept this work, and I strongly recommend that the authors reconsider the terminology and clearly state the distinction, to avoid confusion with established definitions. Below are some specific comments:\n\n1. The manuscript repeatedly refers to the device as a “thermocell,” yet no temperature gradient (ΔT) between electrodes is established. Instead, the whole cell is heated and cooled uniformly. This is conceptually inconsistent with the definition of a thermocell (thermogalvanic cell), where the output voltage arises from entropy-driven potential differences under a thermal gradient. The author should clearly state this in the manuscript to avoid confusion for the readers.\n\n2. No Seebeck coefficient (S=ΔV/ΔT) or related figure of merit is reported, which is standard in thermocell studies. Without this, the work cannot be compared to the conventional thermocell literature.\n\n3. The origin of the temperature-dependent potential changes is attributed to “pseudocapacitive reactions” and the “positive temperature coefficient of PANI,” but these explanations remain qualitative. What does positive temperature coefficient mean here?\n\n4. The device essentially functions as a temperature-sensitive rechargeable battery. Similar concepts of thermally responsive electrodes and temperature-modulated pseudocapacitive processes have been reported in electrochemical energy storage systems, e.g. https://doi.org/10.1073/pnas.1415097111. The claimed novelty as a “thermocell” is not convincing.\n\n5. While the authors claim “reversible operation,” only limited cycles are shown. Long-term cycling stability under repeated heating/cooling conditions is missing, which is essential for assessing practical applicability.\n\n6. The manuscript is often verbose and imprecise, which obscures the actual findings. For instance, the distinction between thermodynamic driving forces and kinetic effects is blurred, and several terms are used inconsistently (e.g., “thermally triggered voltage,” “intrinsic electrochemical potential difference”). Substantial rewriting would be required to meet the standards of clarity expected in a high-impact journal.", "reviewer_2": "In this manuscript, the authors report a planar asymmetric ionic thermocell architecture that synergistically integrates thermally-driven ionic modulation at one interface with spatially engineered pseudocapacitive charge storage at another, overcoming the trade-off between device miniaturization and performance. The 1-mm-thick thermocell delivers excellent output performance at body temperature, especially a high output voltage of 0.1V and an enhanced 2-hour energy density of 1500 J m-2. The study is interesting and meaningful. In my view, it can be considered for the publication after addressing following issues.\n\n1. The article employs an electrochemical deposition strategy to prepare the PANI/CC electrode. Does this strategy, compared to methods such as dip-coating, better address interfacial issues between functional species and the electrode? Potential practical applications require standardized assembly principles. Please supplement the explanation on how the loading ratio, microstructure, and distribution stability of the functional material are controlled.\n\n2. Although the manuscript verifies the thermoelectric conversion mechanism from multiple angles (e.g., XPS, UV-Vis, colorimetric assays), the characterization of ion concentration by UV-Vis is limited to the thermal charging stage. The characterization results regarding the interactions within the gel and between ions and electrodes during the discharge stage are lacking to support the description of the thermoelectric mechanism.\n\n3. To validate the role of the asymmetric electrode system under the small temperature gradients described in the manuscript, it is recommended to supplement the performance of corresponding symmetric electrode systems (e.g., CC/CC, PANI/PANI) under the same operating conditions. This would further clarify the distinct thermochemical responses of the described asymmetric electrodes and electrolyte ions at the same temperature.\n\n4. Please further elaborate on the trade-off between device miniaturization and the construction of an effective temperature gradient, based on existing research, and provide comparative data from prior studies. This should include details such as actual device thickness, ambient temperature, effective temperature difference, and temperature difference utilization rate.\n\n5. The article does not provide detailed data such as specific heat capacity and mass involved in the efficiency calculation. Please supplement these data and fully describe the efficiency calculation process.\n\n6. In the simulation conditions of the finite element analysis, the article only provides temperature distribution cloud images for gel thicknesses of 10, 15, and 20 mm, without corresponding data such as the effective temperature difference utilization rate similar to Supplementary Fig. 2. Please supplement this information.", "reviewer_3": "This paper presents a high-performance, wearable quasi-solid-state ionic thermocell which achieves record high energy density, proving to solve the long-standing challenge of decoupling miniaturization-performance. As the device is made thinner, the performance sinks since it is extremely challenging to create reasonable temperature gradients. However, thin enough (below 2mm) devices are required for true wearability technology. This study overcomes this obstacle, showing a very comprehensive study that includes fabrication, characterization together with the assembly and test of a wearable device, proving it as a promising route towards efficient energy harvesting.\n\nThe paper is well written, with a clear introduction, results presentation and discussion. I would recommend this paper for publication once the authors address the following comments:\n\n1. A reference should be added to support the first sentence claiming that 65% of global energy dissipation happens through low grade thermal energy.\n\n2. The results shown in this paper are really promising. As the main application is a wearable device, authors should comment about any potential toxicity, and if any, how to avoid/overcome it.\n\n3. In figure 5j,k and figure 6f, authors should add the corresponding reference to each of the symbols in the legend, not only in the figure caption, so the reader can directly look up the reference that (s)he is interested in. For example, reference 7 cited in the Figure caption, shows results on underdoped Bi2Sr2CaCu2O8+x. However, this material is not mentioned in the legend, is this a typo?"}, "author_response": "Point-by-point replies:\nNote that all the changes are highlighted using blue color in the revised marked-up manuscript and supplementary information, and the positions mentioned here are based on the marked-up manuscript and supplementary information.\n\nReviewer #1:\nGeneral Comment: The manuscript presents an interesting concept of a temperature-responsive electrochemical cell based on a PANI anode, carbon cloth cathode, and the Fe3+/Fe2+ redox couple. While the experimental results are solid, I would like to raise a conceptual concern regarding the terminology used throughout the manuscript.\n\nResponse: We sincerely thank the Reviewer for the time and effort in reviewing our manuscript. We are greatly encouraged by the positive assessment, particularly the recognition of our work as an “interesting concept” with “solid” experimental results. We also appreciate the Reviewer’s insightful feedback regarding the conceptual concern on the terminology used. We have taken this important point very seriously and have addressed it along with all other valuable suggestions in the revised manuscript. The following is our point-by-point response to all the comments. We hope that we have satisfactorily addressed all of the Reviewer’s concerns, and we would appreciate it if the Reviewer would recommend our revised manuscript for publication.\n\n[... (Full author response text provided in the prompt) ...]"}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have addressed all my mentioned concerns, and the current version can be accepted.", "reviewer_3": "The authors comprehensively addressed all the comments. I recommend this paper for publication as it is now."}, "author_response": ""}]

Decision letter