Soret effect-driven ionic thermoelectrics

Harvesting ubiquitous low-grade waste heat has remained challenging, and ionic thermoelectrics (i-TEs) have shown greater promise than electronic thermoelectrics. The i-TEs are of two types: (1) redox reaction-based i-TEs and (2) Soret effect-driven i-TEs. This work summarizes recent advances in Soret effect-driven i-TEs, which generate a voltage due to differences in the thermophoretic mobilities of electrolyte ions when the i-TE is subjected to a temperature gradient, with the cold end at room temperature. Unlike electronic thermoelectrics, Soret effect-driven i-TEs exhibit a high thermopower of several mV/K. We correlate the thermopower generated by such Soret effect-driven i-TEs to (1) the various entropic contributions of the electrolyte, and (2) electrode porosity. We highlight the role of electrode porosity in driving novel electrochemical voltage oscillations, which can open new avenues for i-TE applications. Notably, we present proof-of-concept demonstrations of Soret effect-driven i-TEs and discuss various schemes employed by researchers to continuously generate power—a hot discussion topic in the field of i-TEs. Lastly, we compare the efficiencies of reported i-TEs and propose future directions to guide the research community in Soret effect-driven i-TEs. In this invited review article, we summarize the (1) developments in electrolyte discovery for Soret effect-driven i-TEs and link them to their entropic contributions, (2) role of electrode porosity in governing thermopower and voltage oscillations, (3) different continuous operation schemes, (4) proof-of-concept demonstrations reported to date, and (5) future research directions.