Nonfaradaic Ionic Thermoelectric Conversion: The Soret Effect or Asymmetric Interfacial Ion Rearrangement?

The utilization of low-grade waste heat and the demand for detecting physical signals with the presence of heat as their manifestation, have driven the refinement and advancement of thermoelectric concepts in theory, materials, devices, and applications. Ionic thermoelectric materials, characterized by low cost, flexibility, and high ionic Seebeck coefficients, are emerging as the next-generation medium for the utilization and conversion of thermal energy. The Soret effect, also known as ion thermal diffusion is broadly recognized as the primary driving force for energy conversion in ionic thermoelectric materials. Significant efforts have been dedicated to material chemistry design, theoretical modeling, and thermoelectric voltage enhancement within the theoretical framework of the Soret effect. However, tracing the evolution of ionic thermoelectric concepts, another theoretical perspective has persisted throughout. Although widely overlooked, the equally critical electrode-electrolyte interface has increasingly been proven to be central to the generation of thermoelectric voltage, and the derived asymmetric interfacial ion rearrangement effect stands as a hidden gem in this field. Given the lack of consensus in the ionic thermoelectric theory, the organization of this review is both timely and necessary. This article will comprehensively review the historical development and relevant theories of nonfaradaic ionic thermoelectric conversion, and thoroughly analyze the mechanisms and relative contributions of the Soret effect and asymmetric interfacial ion rearrangement. The new opportunities presented by the theory of asymmetric interfacial ion rearrangement for the structure and performance of thermoelectric devices will be highlighted. It will conclude by outlining the key challenges and research priorities facing this field in its future development.