Charging-free thermally regenerative electrochemical cycles with high temperature coefficient for heat-to-electricity conversion

Abstract

Low-temperature heat is massive and considered as a potential renewable energy. Thermally regenerative electrochemical cycle (TREC) is one of the low-temperature thermos-electrochemical systems with the highest thermoelectric conversion efficiency. The charging-free TREC is more convenient for practical applications, but the power density and efficiency are lower than the traditional electrically-assisted TREC. Here, a series of charging-free TRECs based on composite Prussian blue analogues CuNiCoHCF and Fe(ClO4)2/3 are developed. By adjusting the metal ratio in CuNiCoHCF, electrolyte concentration and intercalation ions, the systems can match different ranges of heat source with high temperature coefficients of -2.2~-2.52 mV/K. In addition, a low-polarization operation strategy suitable for charging-free TREC is proposed, and the power density, energy density and absolute efficiency can be increased by 19.3-57.7%, 17-43.6% and 6.6-12%, respectively. The thermoelectric conversion efficiency of Cu3Ni1HCF/Na+//Fe(ClO4)2/3 system reached 2.42% and 4.29% between 20oC and 60oC when heat recuperation efficiencies are 0 and 50%.