Phase-Transitional Ionic Thermocells for the Enhanced Thermoelectric Efficiency

With advancement in soft and wearable electronics, the demand for efficient and adaptable power sources has increased significantly. Ionic thermoelectrics, which converts wasted low-grade heat (<100 °C) into electrical energy, is a promising alternative due to its low cost, eco-friendliness, and conformability. To achieve practical implementation of ionic thermocells, innovative strategies that enhance voltage and power generation must be developed. Here, we present an ionic thermogalvanic cell using an ionic liquid, 1-allyl-3-methylimidazolium chloride. The ionic liquid undergoes a phase transition from solid to liquid in cold and hot regions, significantly improving performance without requiring additional components or complex processing. Compared to a homogeneous liquid-state cell, the phase-transitional system exhibits a 2.2-fold increase in the Seebeck coefficient (from 1.79 to 3.89 mV/K), a 3.66-fold enhancement in the power output (from 4.3 to 15.9 nW), and 3.47-fold improvement in the figure of merit (from 0.15 to 0.52) under the same temperature difference of 20 °C. Furthermore, the improvement was consistently observed across various electrolytes that undergo phase transitions, confirming the broad applicability of this strategy. This approach provides valuable direction into the advancement of ionic thermoelectric cells for efficient energy harvesting from low-grade heat, paving the way for practical and scalable applications in sustainable energy technologies.