Tuning the Hydration Entropy of Cations during Electrochemical Intercalation for High Thermopower

A large portion of heat is wasted in the form of low-grade heat less than 100 degrees C. There have been various efforts to utilize such energy, such as organic ranking cycles, thermoelectrics, and thermogalvanic cells. The thermally regenerative electrochemical cycle has achieved a noticeable energy conversion efficiency of low-grade heat harvesting by the thermodynamic cycle of the electrochemical cell that the charging voltage is lower than the discharging voltage. For higher efficiency, this system requires a larger temperature coefficient (alpha) affected by the change of entropy of electrochemical reaction. Herein, the tuning of the hydration status of cation by mixing acetonitrile in the aqueous electrolyte to improve both negative and positive alpha of PBAs for highly efficient low-grade heat harvesting is presented. It largely improves alpha of cobalt hexacyanoferrate, which are 0.93 mV K-1 at 10-30% state of charge (SOC) in 90% acetonitrile and -0.96 mV K-1 at 70-90% SOC in 10% acetonitrile. A clue to understand the origin of alpha and to improve it further with various solvents is provided. Eventually, tuning of alpha using mixing solvent allows building an electrochemical cell with a large positive and negative alpha of electrode for highly efficient low-grade heat harvesting.