Electron-ion coupling enables ionic hydrogel with high thermopower for low-grade heat harvest and sensitive fire warning

Abstract

Conventional ionic thermoelectric materials suffer from inherent intermittent power generation due to their reliance on ionic thermal diffusion. To overcome this limitation, we developed a flame-retardant thermoelectric hydrogel leveraging electron-ion thermoelectric synergy. The hydrogel was prepared through thermal polymerization using sodium alginate (SA), poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), polyacrylamide (PAAm), and sodium chloride (NaCl). Subsequent immersion in calcium chloride (CaCl₂) solution enabled strong SA-Ca²⁺ crosslinking, enhancing the flame retardancy and mechanical strength to yield a high-performance ionic thermoelectric hydrogel (APSG). The synergy between the electronic Seebeck effect (PEDOT:PSS) and ionic thermal diffusion (NaCl) achieved a high Seebeck coefficient of 4.25 mV K⁻¹. Under a 20 K temperature difference with voltage amplification, APSG generated a sustained output voltage of 2.5 V for over 10 min, demonstrating robust continuous power generation. APSG also exhibited sensitive fire-warning capability and outstanding flame retardancy. When exposed to a flame, it triggered the fire alarm within 1.7 s. Its limiting oxygen index (LOI) was as high as 42.3%, and achieved a UL-94 V-0 rating. This work presents an effective strategy for developing next-generation safe and efficient thermoelectric materials for energy harvesting.