Thermodynamics of Ionic Thermoelectrics for Low-Grade Heat Harvesting

TL;DR

This review discusses the thermodynamics and electrochemistry of ionic thermoelectric devices, emphasizing ion transport and reaction dynamics to improve low-grade heat harvesting efficiency.

Contribution

It clarifies fundamental thermodynamics for ionic thermoelectrics and introduces a modified thermoelectric factor Z that accounts for ion transport and electrochemical reactions.

Findings

Analysis of methods to enhance ionic thermopower.

Derivation of a modified thermoelectric factor Z.

Review of hybrid devices with improved performance.

Abstract

More than half of the waste heat rejected into the environment has temperatures lower than 100 $^\circ C$, which accounts for nearly 85 PWh/year worldwide. Efficiently harvesting low-grade heat could be a promising step toward carbon neutrality. Recent developments of ionic thermoelectrics (i-TE) with giant thermopower have provoked intensive interest in using ions as energy and charge carriers for efficient thermal energy harvesting. However, current literature primarily focuses on improving thermopower only, while the ion transport and thermodynamics affecting the efficiencies have been largely neglected. This review article clarifies the fundamentals of electrochemistry and thermodynamics for developing highly efficient i-TE devices. Two major types of i-TE devices, thermo-ionic capacitors (TIC) and thermogalvanic cells (TGC), are discussed in detail. The article analyzes the methods of enhancing ionic thermopower in the literature by taking an entropic point of view. We also derived modified thermoelectric factor Z for both TICs and TGCs that fully incorporate the dynamics of ion transport and electrochemical reactions. Recent developments of hybrid devices showing improved efficiencies, power density, and multifunctionality are reviewed. Finally, we comment on the remaining challenges and provide an outlook on future directions.