Ionic thermoelectric gels and devices: Progress, opportunities, and challenges

Thermoelectric materials are promising in relieving the energy crisis concerning harvesting waste heat and providing a new environment-friendly self-power source for Internet of Things (IoT) sensors. This has attracted significant interest from both the industry and scientific research communities. Fundamentally, general thermoelectric materials are defined as condensed matter that directly converts heat into electricity using electrons or ions as carriers. This review focuses on the emerging ionic thermoelectric (i-TE) gels characterized by distinguished advantages of high voltage output, flexibility, stretchability, and solution processing. Firstly, we systematically review the progress of both p-type and n-type i-TE gels from natural to synthesized gel materials. Secondly, we summarize several strategies for enhancing thermopower, such as entropy engineering, diffusion suppression of counter ions, and several synergistic effects. Thirdly, we briefly review three common modes in which i-TE gels can operate: generator, supercapacitor, and cycle mode. Fourthly, we discussed the effect of electrode structure and gel structure on the energy output. We also highlight the opportunity for i-TE gels to explore new applications based on their unique advantages. Finally, the challenges and perspectives are presented, suggesting a challenging technique road and a bright future in this emerging field.