Self-healable polymer complex with a giant ionic thermoelectric effect

The work addresses the challenge of developing ionic thermoelectric (ITE) materials capable of efficient thermal-to-electrical energy conversion with properties suitable for wearable electronics. Conventional thermoelectrics rely on electronic carriers, whereas ITEC devices depend on thermodiffusion of ions (Soret effect) and operate via charging/discharging like capacitors, yielding much higher Seebeck coefficients (tens of mV K−1). The performance of ITE materials is governed by the ionic Seebeck coefficient (Si), ionic conductivity (σi), and thermal conductivity (κ), summarized in the ionic figure of merit ZTi = Si²σiT/κ. Si is linked to the Eastman entropy of ions and, for multi-ion systems, also depends on the balance of carrier concentrations and diffusion coefficients. Previous ITE materials have achieved ZTi ≈ 0.75–6.1, but often lack stretchability and self-healing needed for wearables. This study aims to engineer a polymer complex (PEDOT:PAAMPSA:PA) that simultaneously optimizes thermophoresis of protons for high ZTi and delivers extreme stretchability and autonomous self-healing under ambient conditions, while retaining performance under mechanical stress.

The paper situates ITEC within broader thermoelectrics and energy harvesting for wearables, noting advantages in output voltage and flexibility. The Soret effect and Onsager relations underpin ITEC operation, with Eastman’s framework linking Seebeck response to ion solvation entropy. Prior reports on solid and quasi-solid ITE systems achieved ZTi values typically between 0.75 and 6.1. Mechanical compliance has been addressed with PU-based and hydrogel systems: Fang et al. realized Y ≈ 0.63 MPa, 156% strain, Si ≈ 34.5 mV K−1, ZTi ≈ 1.3; Xu et al. reached ≈300% strain with borate-crosslinked PU (Si ≈ 34.5 mV K−1, ZTi ≈ 0.99). A self-healable ionic-liquid/amidic polymer system showed limited Si (≈ −1.4 mV K−1). Authors’ prior work reported stretchable, self-healing ITE materials with Si ≈ 8.1–38.3 mV K−1 and ZTi ≈ 1.04–2.34 at high RH. Very recently, a PVA-based self-healable ITE reported ZTi ≈ 7.2 at 80% RH. Collectively, these studies highlight the trade-offs between thermoelectric performance and mechanical properties, and the need for higher ZTi with robust mechanical resilience.

Synthesis: PEDOT:PAAMPSA:PA was synthesized via oxidative polymerization of EDOT in an aqueous PAAMPSA solution with phytic acid (PA) as a physical crosslinker. An APS oxidant solution was prepared separately. Under ice-bath and ambient air, APS was added to the EDOT:PAAMPSA:PA mixture (EDOT:APS molar ratio 0.85), leading to solution color change indicating polymerization. Complexes of cationic PEDOT and anionic PAAMPSA formed, with PA providing hydrogen-bonding and electrostatic physical crosslinking. A PEDOT:PAAMPSA control (without PA) was also prepared. Device fabrication: Glass substrates with thermally evaporated Au electrodes (80 nm) were UV-O3 treated. Films were spin-coated (2000 rpm, 30 s) and baked (5 min, 50 °C), yielding ~5 µm thickness. For the ITEC module, a flexible PP substrate with patterned Au was UV-O3 treated; PEDOT:PAAMPSA:PA (positive Si) and NPC40 (negative Si; PEDOT:PSS doped with CuCl2 at 0.4 wt. ratio) solutions were alternately drop-cast and dried (60% RH, 23–25 °C, 3 h). Characterization and optimization: Ionic Seebeck coefficient (Si) and ionic conductivity (σi) were measured on films with varying PEDOT/PAAMPSA ratios across RH conditions. Electronic conductivity was confirmed negligible. Dielectric spectroscopy was used to fit dielectric constant–frequency curves to extract net ion carrier concentration (ni) and ion diffusion coefficient (Di). XPS probed sulfur chemical shifts; pH measurements of solutions assessed proton concentration changes with composition. Morphology was studied via optical microscopy and SEM. Thermal conductivity (κ) was calculated from κ = Cp ρ α, with thermal diffusivity (α) from laser flash analysis and specific heat capacity (Cp) from DSC; density (ρ) from sample properties. Mechanical properties were evaluated via stress–strain tests on free-standing films; stretchability and self-healing were assessed under various RH, with self-healing tracked on thin films and free-standing films (healing times at 50–70% RH). Stability under mechanical stress was tested by repeated cut-heal cycles and cyclic stretching (100% strain). ITE device testing: ITEC operation was characterized through staged charging/discharging with a load resistor under a temperature gradient. Power output and energy density were measured versus load resistance, and time-resolved thermovoltage profiles recorded. A 9-pair module (PEDOT:PAAMPSA:PA // NPC40) was tested for thermovoltage and power density at 80% RH.

• PEDOT:PAAMPSA:PA achieved record-high ionic thermoelectric performance: at 70% RH, Si = 21.9 mV K−1, σi = 0.309 S cm−1, κ = 0.358 W m−1 K−1, yielding ZTi = 12.3; at 90% RH, κ ≈ 0.403 W m−1 K−1 and ZTi rose to 44.9.
• The ionic power factor was optimized at a PEDOT content of 6.2 wt.%: PFi = Si·σi = 14.8 mW m−1 K−2 at 70% RH; further enhanced to 60.8 mW m−1 K−2 at 90% RH (Si = 27.3 mV K−1, σi = 0.816 S cm−1).
• Optimization mechanism: Increasing PEDOT/PAAMPSA ratio increased proton concentration (lower pH, XPS sulfur shifts) up to 6.2 wt.% PEDOT, improving Si via a more kosmotropic environment for protons; beyond this, PEDOT aggregates reduced Di and increased competition from HSO4− anions, lowering performance. Net ni increased while net Di decreased with PEDOT content; σi peaked at intermediate ni and Di (6.2 wt.% PEDOT).
• Water uptake (higher RH) improved both Di (water channels) and ni (facilitated dissociation), boosting PFi and ZTi.
• Mechanical properties: Free-standing films exhibited extreme stretchability (>1000% maximum strain reported; reproducibly >500%) and autonomous self-healing under ambient conditions; healing completed in 60 s at 70% RH for thin films and within 0.5–6 h for free-standing films depending on PEDOT content.
• Durability: After 20 self-healing cycles, Si retained ~95% and after 30 cycles σi retained ~90% of initial values; Si remained >95% after 50 stretch–release cycles at 100% strain.
• Device performance: Single ITEC device (PEDOT:PAAMPSA:PA) achieved maximum power output 4.59 µW m−2 and energy density 1.95 mJ m−2 at 10 kΩ load (70% RH). A 9-pair module (with NPC40) delivered 0.37 V K−1 thermovoltage, with 0.21 µW m−2 maximum power output and 0.35 mJ m−2 energy density at 80% RH.

The study demonstrates that precisely tuning the PEDOT/PAAMPSA ratio modulates proton concentration and mobility, thereby optimizing the balance of ionic Seebeck coefficient and ionic conductivity critical for high ZTi. Up to 6.2 wt.% PEDOT, increased proton availability (supported by pH and XPS) enhances Si via favorable ion–solvent structural entropy differences, while maintaining sufficient Di for σi. Above this threshold, PEDOT aggregation hinders ion diffusion and introduces competing anionic transport (HSO4−), degrading Si and σi. Humidity plays a dual role by forming aqueous transport pathways (increasing Di) and promoting acid group dissociation (raising ni), further elevating performance. The dynamic hydrogen-bonding and electrostatic crosslinking via PA enables extreme stretchability and rapid self-healing without sacrificing proton transport pathways, evidenced by near-complete retention of Si and σi after repeated cutting and stretching. Collectively, the materials design yields the highest reported ZTi for ITE materials to date and maintains functionality under mechanical stresses, directly addressing the needs of wearable self-powered systems.

This work introduces a PEDOT:PAAMPSA:PA polymer complex that unites record-high ionic thermoelectric performance (ZTi up to 12.3 at 70% RH and 44.9 at 90% RH) with exceptional mechanical compliance and autonomous self-healing. By optimizing proton thermophoresis through control of carrier concentration, diffusion, and ion–solvent entropy, and by employing PA-mediated dynamic crosslinking, the material attains high Si and σi while retaining performance after extensive mechanical cycling. ITEC devices and a 9-pair module validate practical energy harvesting with notable thermovoltage and energy density, underscoring the feasibility of self-powered wearable electronics. Future directions include enhancing performance in lower-humidity or variable environments, scaling modules for higher absolute power, long-term environmental stability studies, and integration with wearable sensors and energy storage for fully autonomous systems.

Performance is strongly humidity-dependent, with highest PFi and ZTi achieved at 70–90% RH; behavior at low RH or across broader temperature ranges is not detailed. Absolute power outputs remain modest, and while cyclic mechanical durability is shown (up to 30 healing and 50 stretch cycles), long-term operational stability, aging, and environmental robustness (e.g., dehydration, sweat, contaminants) are not extensively characterized. The role of competing anions (e.g., HSO4−) at higher PEDOT content suggests sensitivity to composition and processing, warranting further control for scalability.