Electrolytes design for improving the performance of continuous thermally-regenerative electrochemical flow cycles.

With its wide distribution and abundant reserves, low-temperature thermal energy is of great significance for enhancing energy efficiency when efficiently converted into electrical energy. It is a promising technology for low-temperature thermal energy harvesting and utilization by coupling the thermally regenerative electrochemical cycle with flow battery (TREC-FB), but the duration and power density of continuous operation need to be further improved. In this work, three electrolyte optimization methods are proposed for the Fe(CN)63-/4-//I-/I3--TREC-FB system, including changing the electrolyte concentration ratio, adding the organic solvent of ethanol or metal ion of Ca2+ to the I-/I3- electrolyte. With the improved electrolytes, the temperature coefficient of Fe(CN)63-/4-//I-/I3--TREC-FB can be elevated from -1.57 mV/K to a maximum of -2.13 mV/K, accompanied by an increase in peak power density to the highest 2.11 W/m2 from the initial value of 0.84 W/m2. Furthermore, continuous operation performance is enhanced markedly, and the systems with improved electrolytes by changing the electrolyte concentration ratio and adding the organic solvent ethanol, operated continuously for 113 h and 86 h with average power densities of 0.79 W/m2 and 1.15 W/m2, respectively. Moreover, thermoelectric conversion efficiencies of 0.12% and 0.26% (without heat recuperation) or 1.1% and 2.1% (with 90% heat recuperation) are received with the two improved electrolytes mentioned above, respectively.