High thermopower of ferri/ferrocyanide redox couple in organic-water solutions

Thermogalvanic cell (thermocell) represents a promising technology for converting low grade waste heat to electricity. For the cell to be attractive, however, the voltage generated for a given temperature difference has to be high. We report that the electrochemical thermopower can be more than doubled to 2.9 mV K⁻¹ when an organic solvent with an appropriate solubility parameter is added to the aqueous electrolyte of ferri/ferrocyanide. This value is better than any reported for the redox couples in aqueous electrolyte as well as in organic solvents. The addition causes a significant rearrangement of the solvation shell, which leads to an increase in the entropy change of the overall redox system and thus an increase in the electrochemical thermopower. For an evaluation of thermoelectric conversion performance, a thermocell with a circulating electrolyte is fabricated as a candidate technology for harvesting thermal energy from liquid cooling systems. The maximum power density obtained, normalized to the square of the inter-electrode temperature difference (~4.1 °C), is 0.64 mW m⁻²K⁻², which is approximately 1.8 times higher compared with the thermocell based on pure aqueous electrolyte.