Highly Stable Quasi‐Solid Thermocells for Continuous Power Generation Across a Broad Humidity Range

ABSTRACT Ionic hydrogel‐based quasi‐solid (IH‐QS) thermocells are promising for low‐grade heat harvesting, but suffer from humidity‐induced instability (i.e., severe dehydration at low humidity and excessive swelling at high humidity), degrading their mechanical and thermoelectric performances. Herein, we develop humidity‐stable IH‐QS thermocells via the incorporation of amphiphilic networks and hygroscopic electrolytes. This design ensures operational stability across a wide relative humidity (RH) by synergistically combining hydrophilic ─SO ₃ ⁻ groups and hygroscopic electrolytes for high water retention at low humidity and hydrophobic phenyl moieties that form physical crosslinks to suppress swelling at high humidity. Furthermore, the ─SO ₃ ⁻ and phenyl moieties of amphiphilic networks enhance the entropy difference of the [Fe(CN) ₆ ⁴⁻ /Fe(CN) ₆ ³⁻ ] redox couple via electrostatic and ion‐π interactions, boosting thermopower. The resulting IH‐QS thermocell achieves high thermopower and normalized power density across 30%–99% RH (e.g., 2.17 mV K ⁻¹ and 0.19 mW m ⁻² K ⁻² at 30% RH; 1.63 mV K ⁻¹ and 0.55 mW m ⁻² K ⁻² at 99% RH), overcoming a critical barrier to their practical deployment.