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

Abstract

Low-temperature heat is abundant and considered potential renewable energy. A thermally regenerative electrochemical cycle (TREC) is one of the low-temperature thermos-electrochemical systems with the highest thermoelectric conversion efficiency. A charging-free TREC is more convenient for practical applications, but its power density and efficiency are lower than those of traditional electrically assisted TRECs. Herein, a series of charging-free TRECs based on the composite Prussian blue analogues CuNiCoHCF and Fe(ClO₄)_2/3 are developed. By adjusting the metal ratio in CuNiCoHCF, electrolyte concentration and intercalation ions, these systems can match different ranges of heat source with high temperature coefficients of −2.2 to −2.52 mV K⁻¹. In addition, a low-polarization operation strategy suitable for a charging-free TREC is proposed, where 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 the Cu₃Ni₁HCF/Na⁺//Fe(ClO₄)_2/3 system reached 2.42% and 4.29% between 20 °C and 60 °C when heat recuperation efficiencies were 0% and 50%, respectively.