Electrolyte exhibiting a high positive Seebeck coefficient induced by semiclathrate hydrate formation for thermo-electrochemical conversion

Abstract

Various electrolyte designs have been explored to enhance the temperature dependence of the redox potential (Seebeck coefficient) as it determines the cell voltage of thermo-electrochemical devices such as thermally regenerative electrochemical cycles (TRECs). TRECs require redox couples with both high positive and negative Seebeck coefficients to achieve high performance. In our previous study, ferrocyanide/ferricyanide in a mixture of water and tetrabutylammonium fluoride (TBAF) exhibited a high negative Seebeck coefficient owing to the formation and dissociation of semiclathrate hydrate (SCH) induced by temperature variations. In this study, we found that the formation and dissociation of SCH can also provide a high positive Seebeck coefficient (+16 mV K⁻¹) by increasing the weight ratio of TBAF in the electrolyte. The key factor influencing the increase in the Seebeck coefficient is the change in TBAF concentration in the liquid phase, which significantly affects the redox potential of ferrocyanide/ferricyanide. When the TBAF weight ratio in the electrolyte exceeds that of SCH, the effect of SCH formation on the TBAF concentration in the liquid phase is reversed. Therefore, incorporating SCH can enhance the Seebeck coefficient in both positive and negative directions by tailoring the mixing ratio of TBAF. Additionally, we demonstrated a proof-of-concept TREC using the two electrolytes with high positive and negative Seebeck coefficients. The cell demonstrated a significant temperature dependence of the open-circuit voltage, allowing for a much higher average discharge voltage (271 mV) than charge voltage (145 mV), with a small temperature difference between the charge (299 K) and discharge (294 K) processes.