Abstract
Heat is an abundant but often wasted source of energy. Thus, harvesting just a portion of this tremendous amount of energy holds significant promise for a more sustainable society. While traditional solid-state inorganic semiconductors have dominated the research stage on thermal-to-electrical energy conversion, carbon-based semiconductors have recently attracted a great deal of attention as potential thermoelectric materials for low-temperature energy harvesting, primarily driven by the high abundance of their atomic elements, ease of processing/manufacturing, and intrinsically low thermal conductivity. This quest for new materials has resulted in the discovery of several new kinds of thermoelectric materials and concepts capable of converting a heat flux into an electrical current by means of various types of particles transporting the electric charge: (i) electrons, (ii) ions, and (iii) redox molecules. This has contributed to expanding the applications envisaged for thermoelectric materials far beyond simple conversion of heat into electricity. This is the motivation behind this review. This work is divided in three sections. In the first section, we present the basic principle of the thermoelectric effects when the particles transporting the electric charge are electrons, ions, and redox molecules and describe the conceptual differences between the three thermodiffusion phenomena. In the second section, we review the efforts made on developing devices exploiting these three effects and give a thorough understanding of what limits their performance. In the third section, we review the state-of-the-art thermoelectric materials investigated so far and provide a comprehensive understanding of what limits charge and energy transport in each of these classes of materials.
Highlights
Heat is an abundant but often wasted source of energy
We review three major classes of thermoelectric materials and devices working in the low-temperature range (T < 250 °C) and based on highly abundant elements: (i) thermoelectric generators based on conducting polymers, carbon nanostructures, and their composites, (ii) thermogalvanic cells, and (iii) ionic thermoelectric supercapacitors/ batteries
Because the main effort in OTEGs is to optimize the thermoelectric properties of materials, we review the milestones with regard to material development and understanding
Summary
Global electricity production largely relies on fossil fuels (67%), water (16%), and nuclear energy (11%), while a small but rising fraction of electricity is produced by wind (>4%) and solar energy (>2%).[1,2] the conversion of primary energy sources into electricity is not always efficient TE sensors may potentially impact fire detection, homeland security, and many other aspects of human life.[21,22] TE-based sensors can be employed to monitor the environmental temperature along sections of a domestic or industrial plant for evaluating efficiency For all of these applications, mechanical flexibility is a strict requirement, and organic semiconductors, thermoelectric fluids, and gels exploiting the thermogalvanic effect offer new opportunities in the field of stretchable and flexible electronics.[23] the exploration of the ionic Seebeck effect through the Soret effect in polymer electrolytes leads to a surprisingly high Seebeck coefficient (∼10 mV K−1), promoting a new research direction that uses this effect to charge supercapacitors with heat.[24]. In the third section, we provide an overview of the key materials investigated so far by the research community, and summarize the recent efforts to produce devices employing such materials
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