A review on the recent progress in thermoelectric aerogels: Fabrications, generators, marine harvesting, sensors, and thermocells applications

Thermoelectric (TE) aerogels, as new class of TE materials, and their devices have recently captured the attention of the scientific community, as they have the features of both TE materials and aerogels in harvesting heat to electricity, low density, high porosity, super-low thermal conductivity, abundance, affordability and being environmentally friendly compared to the traditional TE materials. Therefore, the progress in fabrication processes of organic, inorganic and their hybrid aerogels has been extensively grown nowadays to obtain desirable TE aerogels and overcome their limitations in terms of low figure of merit (zT), scalability, mechanical durability, long-term stability, and reliability of flexible and stretchable thermoelectric aerogels are challenges today. This drives the researchers, in the last two decades, to increase the development of TE aerogels and reach the unobtainable TE modules, with new and desirable modular architectures (thicknesses/surfaces) and bendable features at the same time. Therefore, this study does not only introduce a summary of recent works in terms of their (organic, inorganic, and/or hybrid) structures, fabrication methods, TE functions, and key factors impacting TE functions and their related mechanisms, but it answers some important questions such as: Why is the aerogel considered as an important TE material?. And another question is: Is it possible to achieve a polymeric (PEDOT:PSS and/or others) aerogel with high electrical conductivity σ > 100 Scm⁻¹ and low thermal conductivity k < 100 mWmK⁻¹ without any additives?. And also another pivotal question: To what extent can aerogels replace the traditional TE materials?. Then, the wide range/variety of TE aerogel applications such as generators, single, dual and/or multifunction sensors, internet of things (IoT), thermocells, and ocean (marine) energy harvesting have been briefly described. In conclusions, the highlights on the current challenges and some preliminary proposals for future work perspectives are offered.