Abstract
The generally low energy density from most heat sources—the Sun, Earth as well as most human activities—implies that solid-state thermoelectric devices are the most versatile heat harvesters since, unlike steam engines, they can be used on a small scale and at small temperature differences. In this opinion piece, we first discuss the materials requirements for the widespread use of thermoelectrics. We argue that carbon-based materials, such as conducting polymers and carbon nanotubes, are particularly suited for large area and low-temperature operation applications, as they are abundant, low-toxicity and easy to process. We combine experimentally observed macro-trends and basic thermoelectric relations to evaluate the major performance limitations of this technology thus far and propose a number of avenues to take the thermoelectric efficiency of organic materials beyond the state of the art. First, we emphasize how charge carrier mobility, rather than charge density, is currently limiting performance, and discuss how to improve mobility by exploiting anisotropy, high persistence length materials and composites with long and well-dispersed carbon nanotubes. We also show that reducing thermal conductivity could double efficiency while reducing doping requirements. Finally, we discuss several ways in which composites could further boost performance, introducing the concept of interface engineering to produce phonon stack-electron tunnel composites.This article is part of a discussion meeting issue ‘Energy materials for a low carbon future'.
Highlights
The generally low energy density from most heat sources—the Sun, Earth as well as most human activities—implies that solid-state thermoelectric devices are the most versatile heat harvesters since, unlike steam engines, they can be used on a small scale and at small temperature differences
We argue that carbon-based materials, such as conducting polymers and carbon nanotubes, are suited for large area and low-temperature operation applications, as they are abundant, low-toxicity and easy to process
We show that reducing thermal conductivity could double efficiency while reducing doping requirements
Summary
The transformation of heat into electricity has been established for many decades. Conversion efficiencies range from 10% to 50% for a heat source temperature varying from ca 400 K to 1100 K. Geothermal heat can be divided between hot spots (like Iceland) and an average location The former can be found at specific locations and geothermal power stations have been optimized to very efficiently harvest that source of energy. While domestic energy consumption typically results in low-temperature waste heat, industrial waste spans a wider range, from low to medium and even high temperatures depending on the specific industry [3] It is quite localized, often in accessible locations, which makes this an attractive energy source. Assuming that all of the chemical energy consumed by humans on average (food intake) is transformed into heat, each person can be considered as a 100 W–120 W heat source spread over 1.5 to 2 m2 of body surface (i.e. about 50 W m−2, resulting in usable temperature differences below 10 degrees). Given the solid potential of organic-based materials for sustainable thermoelectric applications, it would be desirable to understand what is currently limiting their performance and devise strategies to move forwards
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