Self-healable polymer complex with a giant ionic thermoelectric effect

Thermoelectric (TE) devices are gaining traction for energy harvesting from waste heat due to their advantages: no moving parts, no pollution, light weight, and ease of production. Conventional TE generators utilize electrons/holes, while the emerging ionic thermoelectric capacitor (ITEC) leverages ion diffusion in ionic TE (ITE) materials. ITE materials offer higher output voltage (tens of mV·K⁻¹) than electronic TE materials (tens of µV·K⁻¹), making them suitable for self-powering wearable devices. ITE material performance hinges on the Soret effect (thermophoresis), described by the fourth law of thermodynamics (Onsager reciprocal relations). The ionic figure-of-merit (ZTᵢ) characterizes ITE performance: ZTᵢ = (Sᵢ²σᵢ/κ)T, where Sᵢ is the ionic Seebeck coefficient, σᵢ is ionic conductivity, κ is thermal conductivity, and T is temperature. High ZTᵢ requires simultaneous improvement in Sᵢ and σᵢ. Eastman's theory attributes thermophoresis to the entropy difference in the ion solvation shell. Sᵢ for a single ion (Sₓ) is Sₓ = Sₑqᵢ, where Sₑ is Eastman entropy, and qᵢ is the ion's charge. For multiple carriers, Sᵢ is a function of carrier concentration (nᵢ) and diffusion coefficient (Dᵢ). Improving Sᵢ involves optimizing the net ionic thermocurrent (Iₙₑₜ) and resistance (Rₙₑₜ). The relationship between Sᵢ and structural entropy is still under investigation, but studies suggest that hydration entropy (ΔSₕᵧd) plays a crucial role. Kosmotropic ions create structured solvation shells, while chaotropic ions have the opposite effect. The proton, a kosmotropic ion, exhibits a negative ΔSₕᵧd, further decreasing at higher temperatures, inducing proton thermodiffusion from hot to cold (positive Sᵢ). Existing ITE materials achieve ZTᵢ of 0.75–6.1, lacking stretchability and self-healing. This study aims to develop high-ZTᵢ ITE materials with enhanced mechanical properties for wearable applications.

Previous research has explored various ITE materials, achieving varying degrees of success. Fang et al. demonstrated a polyurethane (PU)-based stretchable ITE material with a Young's modulus (Y) of 0.63 MPa, maximum strain of 156%, Sᵢ of 34.5 mV K⁻¹, and ZTᵢ of 1.3. Xu et al. improved the maximum strain to 300% using a PU-based ITE material with a borate crosslinker (Y of 0.79 MPa, Sᵢ of 34.5 mV K⁻¹, and ZTᵢ of 0.99). Jia et al. presented a stretchable and self-healable ITE material with an ionic liquid and an amide-based self-healing polymer, but Sᵢ was only −1.4 mV K⁻¹. Previous work by the authors reported stretchable and self-healable ITE materials with Sᵢ of 8.1–38.3 mV K⁻¹ and ZTᵢ of 1.04–2.34 at 90% RH. Cho et al. reported a PVA-based self-healable ITE material with ZTᵢ of 7.2 at 80% RH. This study builds upon these advancements to create a highly performant material.

The self-healable ITE material PEDOT:PAAMPSA:PA was synthesized by adding 3,4-ethylenedioxythiophene (EDOT) monomer to an aqueous solution of poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA) and phytic acid (PA) as a physical cross-linker. Ammonium persulfate (APS) in deionized water initiated polymerization, indicated by a color change from light brown to dark blue. PEDOT:PAAMPSA:PA complexes formed, resembling PEDOT:PSS complexes. The PEDOT:PAAMPSA:PA structure comprises aligned PEDOT (positive) and PAAMPSA (negative) chains, with multivalent PA physically crosslinking through hydrogen bonds and electrostatic interactions. The PEDOT/PAAMPSA ratio was optimized to control proton concentration (primary charge carrier). Two proton sources exist: EDOT polymerization and PAAMPSA dissociation. The PEDOT⁺ complexes facilitate PAAMPSA dissociation, and the acidic environment forms HSO₄⁻ anions from APS. pH measurements and X-ray photoelectron spectroscopy (XPS) confirmed the influence of the PEDOT/PAAMPSA ratio on proton concentration. PEDOT:PAAMPSA:PA films were spin-coated onto Au electrodes. Sᵢ and σᵢ increased with PEDOT content up to 6.2 wt%, then decreased. The ionic power factor (PFᵢ = Sᵢσᵢ) was optimized at 6.2 wt% PEDOT (14.8 mW m⁻¹ K⁻² at 70% RH). Electronic conductivity was negligible. PA acted solely as a crosslinker. Net nᵢ and Dᵢ were determined by fitting dielectric constant-frequency curves. Higher PEDOT content led to PEDOT aggregates, hindering proton dissociation and reducing Dᵢ. ITE properties improved at higher RH due to water molecule formation of ion transport channels and enhanced charge dissociation. Thermal conductivity (κ) was determined using laser flash analysis (LFA) and differential scanning calorimetry (DSC). ZTᵢ reached 12.3 at 70% RH and 44.9 at 90% RH, the highest reported. PEDOT:PAAMPSA:PA films exhibited >1000% stretchability and self-healing, attributed to dynamic interactions between components. Self-healing was faster at higher RH. ITE properties remained stable after repeated self-healing and stretching cycles. ITEC devices were fabricated using PEDOT:PAAMPSA:PA films. Maximum power output and energy density were 4.59 µW·m⁻² and 1.95 mJ·m⁻², respectively, at 10 kΩ load resistance. A 9-pair ITEC module using PEDOT:PAAMPSA:PA and NPC40 (negative Sᵢ material) achieved a thermovoltage of 0.37 V K⁻¹.

The study successfully synthesized a novel self-healable and stretchable ITE material, PEDOT:PAAMPSA:PA, achieving a record-high ZTᵢ of 12.3 at 70% RH and 44.9 at 90% RH. The optimization of ITE properties was achieved by precise control of the PEDOT/PAAMPSA ratio, which directly impacted the proton concentration (primary charge carrier) and its diffusion coefficient. The optimal ratio of 6.2 wt% PEDOT resulted in the highest ZTᵢ due to the balance between the carrier concentration and diffusion coefficient. Higher RH values further enhanced the ITE performance by improving the ion transport. The material exhibited exceptional mechanical properties, including >1000% stretchability and rapid self-healing capabilities (within 60 seconds at 70% RH). Importantly, the ITE performance was remarkably stable even after repeated cycles of self-healing and stretching, suggesting robust functionality. The fabricated ITEC device and module demonstrated promising energy generation capabilities, achieving a maximum power output of 4.59 µW·m⁻² and an energy density of 1.95 mJ·m⁻² at an optimal load resistance. The 9-pair ITEC module produced a notable thermovoltage of 0.37 V·K⁻¹, showcasing the potential for practical applications in self-powered devices.

The exceptionally high ZTᵢ value achieved in this study significantly advances the field of ITE materials. The combination of high ITE performance, stretchability, and self-healing properties makes PEDOT:PAAMPSA:PA a promising candidate for flexible and wearable thermoelectric devices. The results demonstrate the effectiveness of controlling the ion carrier concentration and diffusion coefficient to enhance ITE performance. The remarkable stability of the material under mechanical stress highlights its robustness for real-world applications. The successful demonstration of an ITEC device and module further validates the material's potential for energy harvesting and self-powering applications. These findings open avenues for the development of advanced self-powered wearable sensors, actuators, and other electronic devices.

This research successfully developed a high-performance self-healable and stretchable ITE material, PEDOT:PAAMPSA:PA, with a record-high ZTᵢ. The material's exceptional properties and demonstrated ITEC device performance pave the way for flexible and durable self-powered electronics. Future research could explore diverse applications in wearable technology and investigate the material's long-term stability and scalability for mass production.

While the study demonstrates remarkable results, the high relative humidity requirement for optimal performance might limit certain applications. Further research is needed to explore methods to improve performance under lower humidity conditions. The long-term stability of the material under continuous operation and varying environmental conditions also warrants further investigation. Finally, a comprehensive cost-benefit analysis is needed to assess the economic viability of large-scale production and implementation.