Temperature‐Driven Reversible p‐to‐n Switching in Ionic Liquid‐Modified Single‐Walled Carbon Nanotube Thermoelectric Films for Flexible Waste Heat Recovery

ABSTRACT Temperature‐driven reversible p ‐to‐ n Seebeck coefficient switching has not been demonstrated in binary single‐walled carbon nanotube (SWCNT)‐ionic liquid systems. Autonomous polarity switching is achieved in SWCNT films modified with imidazolium‐based ionic liquids without polymer matrices, gelators, or external voltage. Ionic liquid treatment enhances electrical conductivity from 667 to 2391 S cm ^{− 1} while enabling temperature‐controlled polarity modulation. The Seebeck coefficient reversibly transitions from +43 to −45 µV/K at elevated temperatures (∼130°C–195°C) through anion thermodiffusion via the Soret effect, achieving a power factor of 260 µW m ^{− 1} K ^{− 2} . Switching temperature correlates quantitatively with anion molecular mass (130°C for 66 g/mol N(CN) ₂ ⁻ vs. 195°C for 137 g/mol [BF ₃ CF ₃ ] ⁻ ). Hall effect measurements confirm p ‐type electronic character at room temperature with mobility‐driven conductivity enhancement, while temperature‐dependent switching reflects thermally‐activated ionic contributions. A flexible seven‐leg prototype generates 53 mV and 0.5 µW at Δ T = 170°C, maintaining operational stability over 26 days with complete thermal reversibility. This binary architecture enables adaptive thermoelectric operation without external control for wearable electronics, automotive waste heat recovery, and industrial thermal management.