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.

Redox Targeting-Based Thermally Regenerative Electrochemical Cycle Flow Cell for Enhanced Low-Grade Heat Harnessing.

A large amount of low-grade heat (<100 °C) is produced in electrical devices and mostly wasted. This type of heat without effective dissipation also causes compromised device performance, reliability, and lifespan. To tackle these issues, a redox targeting (RT)-based flow cell with judiciously designed thermoelectrically active redox materials is demonstrated for the first time for efficient heat-to-electricity conversion through a thermally regenerative electrochemical cycle (TREC). Compared with the conventional TREC systems, the RT-based flow cell not only reveals considerably enhanced thermoelectric efficiency, but the flow of redox fluids also provides a cooling function to the system. In this work, solid material Ni0.2 Co0.8 (OH)2 and redox mediator [Fe(CN)6 ]4-/3- , both of which have negative temperature coefficient and share identical redox potential, are paired via RT-reactions to boost the capacity and meanwhile thermoelectric efficiency of a [Fe(CN)6 ]4-/3- /Zn0/2+ -based flow cell. Upon operating over the TREC cycle, the RT-based flow cell converts heat to electricity at an unprecedented absolute thermoelectric efficiency of 3.61% in the temperature range of 25-55 °C.