Flexible and Robust Bacterial Cellulose‐Based Ionogels with High Thermoelectric Properties for Low‐Grade Heat Harvesting

Abstract The utilization of low‐grade and abundant thermal sources based on thermoelectric (TE) materials is crucial for the development of a sustainable society. However, high‐performance thermoelectric materials with biodegradable, mass‐productive, and low‐cost features are rarely reported. Here, from the perspective of sustainable development, natural polymer (bacterial cellulose, BC), and “green” solvent (ionic liquids, ILs) are combined to achieve a transparent, flexible, and robust ionogel (BCIGs) by using a facile and versatile modified co‐solvent evaporation method. The proposed BCIGs with 95 wt% 1‐ethyl‐3‐methylimidazolium dicyanamide ([EMIm][DCA]) can have high tensile strength (3.05 MPa), skin‐like mechanical stretchability (40.99%), and obvious adhesivity. The BCIGs are thermally stable up to 250 °C. They also exhibit a high ionic conductivity (2.88 × 10 −2 S cm −1 ), high ionic thermovoltage (18.04 mV K −1 ), and low thermal conductivity (0.21 W m −1 K −1 ), resulting in the great ionic figure of merit (ZT i ) of 1.33 at room temperature. Through the model of mesoscopic confined ion transportation under a thermal gradient, it is attributed the great thermoelectric properties to the synergistic effect between ion–cellulose interaction and ion–ion interaction. Moreover, a flexible ionic thermoelectric capacitor (ITEC) device is also demonstrated, showing the potential of the BCIGs in wearable energy supply.