Enhancing the Thermoelectric Performance of Sustainable Cellulose-Based Ionogels Through Water Content Regulation

Ionogels are widely studied as promising ionic thermoelectric (i-TE) materials to harvest low-grade waste heat into electrical energy due to their huge thermopower and good ionic conductivity, providing a feasible way to sustainable development. Herein, a p-type i-TE cellulose ionogel (CIG) based on Soret effect is prepared by dissolving cellulose in an ionic liquid (IL) and subsequent water-absorbing induced gelation. Its morphological structure and IL distribution are intuitively investigated through cryo-focused ion beam-scanning electron microscope. Experimental characterizations and molecular dynamic simulation studies elucidate that the regulation of water content induces the hydration of 1-butyl-3-methylimidazolium cation and the swelling of CIG, which greatly promotes the ions diffusion and expands the difference in mobility between anions and cations. The proposed CIG exhibits superior thermoelectric properties: an ionic conductivity of 51.2 mS cm^-1, an ionic Seebeck coefficient of 20.7 mV K^-1, and an ionic figure of merit zT_i of 2.36 at 30 °C, respectively. A CIG-based i-TE device is designed and assembled to demonstrate its great potential for wearable body heat-to-electricity conversion. The cellulose skeleton in CIG is completely biodegradable in nature and the used IL is recyclable and reusable, providing a green and sustainable strategy for energy harvesting.