Adaptable and Wearable Thermocell Based on Stretchable Hydrogel for Body Heat Harvesting

Abstract The application of traditional inorganic thermoelectric materials to wearable energy harvesters has been hindered by the rigid bulkiness, while flexible organic conductive polymers have been long troubled by their intrinsic low thermopower. An ideal solution that possesses the merits of the two thermoelectric materials is desperately needed for practical application. Here it is demonstrated that a polyacrylamide (PAAm)‐based ultra‐stretchable hydrogel can be selected as a superior candidate matrix for flexible and stretchable thermocells to adapt to the curved surface of the human body and deformation of the ankle. Fe(ClO 4 ) 3 /Fe(ClO 4 ) 2 is selected as n‐type ion pair for their comparable thermoelectric performance to K 3 [Fe(CN) 6 ]/K 4 [Fe(CN) 6 ]. The p‐n pair hydrogel electrolytes show a voltage output of 29 mV and current output of 8.5 Am −2 with an average maximum power density of 0.66 mW K −2 m −2 for each p‐n cell (Δ T = 10 K). By integrating with the graphite paper electrode, a body‐conformal and portable thermocell device, employing the hydrogel electrolytes, is fabricated and reached a voltage output of 0.16 V with 14 pairs of p‐n cells (Δ T = 4.1 K). This commercially‐effective blueprint demonstrates the bright future of hydrogel‐based ionic thermocell in daily wearable scenarios.