Large Temperature Dependence of Redox Potential Driven by Semiclathrate Hydrate Formation for Thermo-Electrochemical Conversion

The temperature dependence of the redox potential (temperature coefficient) is a critical parameter for redox couples employed in thermoelectrochemical conversion devices, such as thermogalvanic cells and thermally regenerative electrochemical cycles (TRECs). We developed a novel strategy for boosting the temperature coefficient of ferro/ferricyanide through the formation/dissociation of a semiclathrate hydrate (SCH). The aqueous solution with ferro/ferricyanide and tetrabutylammonium fluoride (TBAF) showed SCH formation/dissociation by small temperature variations, which contributed to a huge temperature coefficient (-13.8 mV K^-1) near ambient temperature. The large-temperature coefficient was attributed to a significant change in the TBAF concentration in the liquid phase caused by SCH formation/dissociation, resulting in the rearrangement of the ion pair of ferricyanide and cations. We introduced the electrolyte to a charging-free TREC device driven by a small temperature swing (9 K) and achieved the highest normalized power density (4.8 mW m^-2 K^-2). This electrolyte design strategy will pave the way for electrochemical energy harvesting from small temperature changes such as diurnal temperature variations. In addition, this study creates a new research field for semiclathrate hydrate chemistry.