Advances in Ionic Thermoelectrics: From Materials to Devices

As an extended member of the thermoelectric family, ionic thermoelectrics (i-TEs) exhibit exceptional Seebeck coefficients and applicable power factors, and as a result have triggered intensive interest as a promising energy conversion technique to harvest and exploit low-grade waste heat (<130 degrees C). The last decade has witnessed great progress in i-TE materials and devices; however, there are ongoing disputes about the inherent fundamentals and working mechanisms of i-TEs, and a comprehensive overview of this field is required urgently. In this review, the prominent i-TE effects, which set the ground for i-TE materials, or more precisely, thermo-electrochemical systems, are first elaborated. Then, TE performance, capacitance capability, and mechanical properties of such system-based i-TE materials, followed by a critical discussion on how to manipulate these factors toward a higher figure-of-merit, are examined. After that, the prevalent molding methods for assembling i-TE materials into applicable devices are summarized. To conclude, several evaluation criteria for i-TE devices are proposed to quantitatively illustrate the promise of practical applications. It is therefore clarified that, if the recent trend of developing i-TEs can continue, the waste heat recycling landscape will be significantly altered.