Antisolvent engineering enhanced high-entropy liquid flow thermocells with giant power density for low-grade heat harvesting

Abstract

Liquid-flow ionic thermocells (LiTCs) with high thermoelectric performance are promising candidates for low-grade waste heat harvesting. However, it is significantly hindered by the low normalized maximum power density (Pmax/∆T2) due to the limited solvation entropy increase of redox ions. Herein, a cost-effective and facile strategy for enhancing the solvation entropy increase of redox ions was proposed through antisolvent engineering, which significantly improved ionic thermoelectric performance of LiTCs. An antisolvent (ethanol) of redox ions was introduced into the equimolar aqueous solutions of reductant and oxidant (K4Fe(CN)6/K3Fe(CN)6) flowed separately within the Y-shaped channel of LiTCs. The presence of the antisolvent would induce the agglomeration of Fe(CN)64- ions due to the competitive effect of antisolvent molecules with low polarity parameters, thereby reducing the interfacial free energy with water molecules and increasing the disorder within its solvation shell. This process would enhance the solvation entropy increase of redox ions through the intercalation of an ethanol molecule into the first solvation shell of Fe(CN)64- at certain ethanol content, thus significantly promoting the thermogalvanic effect. As a result, the antisolvent engineering can reach the giant ionic thermopower of -58.8 mV K-1 and Pmax/∆T2 of 64.5 mW m-2 K-2, which is ca. 100 times that of conventional ionic thermocells. Consequently, the Carnot-relative efficiency was the highest value of 11.8% until now, and significantly exceeded the predicted commercialization threshold (~5%). The high-entropy LiTCs with competitive cost-performance metric (1.01 $ W-1) and low temperature resistance (<-10 ºC) would have enormous potential for commercial application.