Mixed Ionic-Electronic Conducting PEDOT/PAMPS Polymer Composite Films for Transparent and Self-Healable Thermoelectric Harvesters

In these days, a lot of effort is put on energy harvesting technologies. As one of them, thermoelectric (TE) technology which harvests energy through conversion from heat to electricity is a future-oriented technology that contributes to energy reuse and environmental friendliness. However, the existing TE materials have low heat-to-electricity efficiency, so it is difficult to use their harvested energy for operating other devices. As an alternative, ionic polymer could be a promising candidate because it has an excellent energy harvesting property based on the Soret effect. In addition to excellent thermoelectric property, ionic thermoelectric materials (iTE) have the advantages of low-cost production, light weight, scalability, and environmental friendliness. However, the lack of electrical properties in the iTE devices is critical because there is little actual energy transfer. In addition, researches on n-type iTE materials have not been progressed much, and they showed lower energy harvesting performance than p-type iTE materials due to the difficulty of the transport of large-sized negative ion carriers.<br/>In this study, we developed mixed ionic-electronic conductor (MIEC) based TE material for better energy harvesting property. Poly(3,4-ethylenedioxythiophene) (PEDOT), an electrical conductive polymer, was co-polymerized in poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS), an ionic polymer-based media using ammonium persulfate (APS) as an initiator. Through transmission electron microscopy (TEM) analysis, the core-shell structure of PEDOT-PAMPS and the pi-pi interaction of the PEDOT domain acting as an electrical channel were characterized. We focused on finding an optimized PEDOT concentration that maintains MIEC characteristics while maintaining transparency in a thin film state. APS was additionally doped to the synthesized PEDOT-PAMPS polymer composite, resulting in n-type PEDOT-PAMPS (NPP) composite where bisulfate acts as anionic carrier. A novel n-type ion transport mechanism was designed in consideration of the deprotonation of the sulfonate group of PAMPS and polymer-polymer, polymer-ion interaction. The mechanism was scientifically elucidated through Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis, and bisulfate generation and ion transport were characterized using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and nuclear magnetic resonance (NMR) analysis. In addition, bisulfate-detecting fluorescent material was synthesized to visually show the bisulfate transport in real time when a temperature gradient (<i>Δ</i><i>T</i>) was given.<br/>The TE properties of NPP films were measured considering humidity, different <i>Δ</i><i>T</i>, and concentration of components. The optimized NPP film showed a negative ionic Seebeck coefficient of -25.0 mV K^-1, ionic power factor of 9.94 mW m^-1 K^-2, and an ionic figure of merit of 7.2 at 80% relative humidity and room temperature, which exhibited the highest TE performance in organic materials at room temperature. In addition to excellent TE properties, NPP film with Young's modulus similar to human skin and self-healing property of hydrogel-based composite could be applied to a flexible organic TE module. The TE module with 20 legs generated -2.75 V at a <i>Δ</i><i>T</i> of 5.5 K, and the flexible band-type TE module attached to the human wrist generated -1.88 V. To emphasize energy harvesting property and transparency, we succeeded in turning on the LED using the capacitor charged by the TE module for the first time using an organic-based TE device, and fabricated a TE module composed of legs dyed with water-soluble ink. In addition, by fabricating a phothermal-thermoelectric combined device, photothermal-to-thermoelectric voltage according to laser power was observed using a light source instead of heat. Various applications using NPP films have demonstrated the practicality and multifunctionality of this novel TE material, and its potential as a promising energy harvester.