Anti-freezing hydrogel thermocells with confined microcrystallization for enhanced low-grade heat harvest

Thermocells based on ionic Seebeck effect utilize waste low-grade heat for electrical energy storage/supply, offering promising solutions for global energy sustainability. Creation of mechanically flexible thermocells with enhanced ionic Seebeck effect in a wide temperature window is highly required for their wide application, but remains challenging. Here we report a general strategy based on selective confined microcrystallization to create hydrogel thermocells fulfilling the above requirement. The thermocell is created by solvent-induced selective [Fe(CN)6]4− crystallization from [Fe(CN)6]4−/[Fe(CN)6]3− couples in the hydrogel networks. The thermocell allows excellent anti-freezing and mechanically flexible properties, and enhanced ionic Seebeck effect in a wide temperature window (−33 ∼ 62 °C). Integrating 30 thermocells can efficiently harvest energy from a heat source and a frozen cold source. This system provides new insights into efficient low-grade heat harvest in wide scenarios.