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Summary: I−/I3− thermocells are promising for low-grade heat harvesting due to their low cost, easily regulated ionic Seebeck coefficient (Se) and potential in integrated thermocells. However, the existing technologies to increase Se just apply to very low concentration of I−/I3− (mmol L−1), which restricts the current and power. Herein, we present a thermosensitive crystallization strategy to increase Se from 0.51 to 1.2 mV K−1 in 0.4 mol L−1 I−/I3− thermocells, leading a maximum power density of 0.74 W m−2. Specifically, the additive of Cs+ can selectively combine with I3− and form thermosensitive crystal CsI3 at the cold side while releasing I3− at the hot side, which induces a concentration gradient of free I3−, thereby improving Se. Besides, the CsI3-driven I−/I3− thermocell can assemble a high-performance device by integrating with a classic p-type cell. Therefore, the thermosensitive crystallization strategy paves a new way to build high-power thermocells for low-grade heat harvesting.

INSIGHTS Harvesting waste heat (e.g., solar irradiation, or from industrial processes or the human body) is crucial for carbon neutrality and sustainable development (1). One promising way to improve Se is to build a concentration gradient of redox ions (DC), which has been proven to boost efficiency of a single cell (6, 7). To this end, Wang et al. propose a strategy that employs photocatalysts to facilitate the conversion and accumulation of redox ions and hence the formation of their concentration gradients. [Extracted from the article]