Efficient Low-Grade Heat Conversion and Storage with an Activity-Regulated Redox Flow Cell via a Thermally Regenerative Electrochemical Cycle.

Efficient and cost-effective technologies are highly desired to convert the tremendous amount of low-grade waste heat to electricity. Although the thermally regenerative electrochemical cycle (TREC) has attracted increasing attention recently, the unsatisfactory thermal-to-electrical conversion efficiency and low power density limit its practical applications. In this work, a thermosensitive Nernstian-potential-driven strategy in the TREC system is demonstrated to boost its temperature coefficient, power density, and thermoelectric conversion efficiency by rationally regulating the activities of redox couples at different temperatures. With a Zn anode and [Fe(CN) 6 ] 4-/3- -guanidinium as the catholyte, the TREC flow cell presents an unprecedented average temperature coefficient of -3.28 mV K -1 , and achieves an absolute thermoelectric efficiency of 25.1% and apparent thermoelectric efficiency of 14.9% relative to the Carnot efficiency in the temperature range of 25-50 °C at 1 mA cm -2 . In addition, a thermoelectric power density of 1.98 mW m -2 K -2 is demonstrated, which is more than 7 times the highest power density of reported TREC systems. This activity regulation strategy can inspire research into high-efficiency and high-power TREC devices for practical low-grade heat harnessing.