Pushing ionic thermoelectrics toward power supply applications: origins, advances, challenges, and future directions

The emerging ionic thermoelectrics (i-TEs) have been positioned as compelling candidates for low-grade heat harvesting owing to their ultrahigh thermopower, mechanical flexibility, environmental compatibility, and low cost. The past five years have witnessed the rapid advancement of i-TEs, yet their inherent inability to transport ions through an external circuit fundamentally limits sustained and stable power output, presenting the key bottleneck for practical applications. Figuring out the road traveled and the road forward for pushing i-TEs toward continuous power generation is urgent. This review aims to systematically sort out the advancement of i-TEs along the central theme of "sustainable power generation", across four representative paradigms-thermodiffusion, thermogalvanic, coupled thermodiffusion-thermogalvanic, and ion-electron hybrid systems. Their basic theories, operating mechanisms, material compositions, and advances in material design strategies and device engineering approaches, which collectively govern energy conversion performance and output stability/sustainability, are comprehensively and deeply discussed. Particular emphasis is placed on the development of i-TEs capable of generating continuous and reliable power output, tackling key scientific and technological challenges toward sustainable operation. Furthermore, this review highlights recent advances and emerging opportunities in the applications of i-TE-generated electrical energy, including power supply, self-powered sensing, biomedical monitoring, and therapy. Finally, this review outlines critical challenges, design principles, and interdisciplinary directions for guiding the rational development of durable, efficient, and practical ionic thermoelectric technologies.