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.

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