Wearable Device with High Thermoelectric Performance and Long‐Lasting Usability Based on Gel‐Thermocells for Body Heat Harvesting

Utilizing the thermogalvanic effect, flexible thermoelectric materials present a compelling avenue for converting heat into electricity, especially in the context of wearable electronics. However, prolonged usage is hampered by the limitation imposed on the thermoelectric device's operational time due to the evaporation of moisture. Deep eutectic solvents (DESs) offer a promising solution for low-moisture gel fabrication. In this study, a bacterial cellulose (BC)/polyacrylic acid (PAA)/guanidinium chloride (GdmCl) gel is synthesized by incorporating BC into the DES. High-performance n-type and p-type thermocells (TECs) are developed by introducing Fe(ClO₄)_2/3 and K_3/4Fe(CN)₆, respectively. BC enhances the mechanical properties through the construction of an interpenetrating network structure. The coordination of carboxyl groups on PAA with Fe³⁺ and the crystallization induced by Gdm⁺ with [Fe(CN)₆]^4- remarkably improve the thermoelectric performance, achieving a Seebeck coefficient (S) of 2.4 mV K^-1 and ion conductivity (σ) of 1.4 S m^-1 for the n-type TEC, and ‒2.8 mV K^-1 and 1.9 S m^-1 for the p-type TEC. A flexible wearable thermoelectric device is fabricated with a S of 82 mV K^-1 and it maintains a stable output over one month. This research broadens the application scope of DESs in the thermoelectric field and offers promising strategies for long-lasting wearable energy solutions.