Na/K mixed electrolyte for high power density and heat-to-electricity conversion efficiency low-grade heat harvesting system

Low-grade heat widely exists in nature and industry processes. Due to the lack of effective recovery technologies, a large amount of low-grade heat is wasted. Thermally regenerative electrochemical cycle (TREC) system can efficiently convert low-grade heat into electrical energy. However, the low power density limits the practical application of TREC system. To improve the power density and heat-to-electricity conversion efficiency, a TREC system with Na/K mixed electrolytes (4 M sodium nitrate [NaNO 3 ] + 1 M potassium nitrate [KNO 3 ]) is proposed. When the temperature difference of the system is 34°C (between 46 °C and 12 °C), the heat-to-electricity conversion efficiency can be as high as 3.9% (relative Carnot efficiency of 36.4%). At the current density of 40.16 mA/g , the power density can reach 23.29 mW/g, increasing to 6.5 times of conventional system, which is the highest level in TREC system. The comprehensive results of electrochemical test and elemental analysis show that K + has superior electrochemical reaction characteristics. The proposed sodium/potassium (Na/K) mixed electrolyte system improves the power density and heat-to-electricity conversion efficiency by reducing the voltage hysteresis and increasing the operating current and voltage.