Refining the Isothermal Cell Technique to Measure the Temperature Coefficient of Redox Active, Aqueous, Organic Compounds

Thermo-electrochemical cells (thermocells) are devices used in the conversion of waste heat into sustainable energy. The performance of a thermocell is dictated, in part, by the temperature coefficient. The temperature coefficient (also referred to as Seebeck coefficient) is defined by the equation, where the change in voltage with temperature, is affected by a contribution via thermal gradient induced ion migration, S^*_M, and a contribution via entropy of reaction, S^*_E. Traditionally, the temperature coefficient is measured with a non-isothermal cell, which consists of two cells at different temperatures connected by a salt bridge. Recently, a new experiment called the isothermal cell, consisting of only one electrochemical cell, has been proposed as a viable technique to measure the temperature coefficient of redox compounds. Due to its ease of use and lack of a thermal gradient, the isothermal cell is an accessible alternative to a non-isothermal cell while also reporting a temperature coefficient more intrinsic to a compound. In this work, we verified the technique in multiple experimental conditions and environments by studying the effects of redox-active concentration, supporting electrolyte concentration, and CV scan rates on the reported temperature coefficient of the common aqueous ferri/ferrocyanide redox couple (Fe(CN)₆^3-/4-) with sodium sulfate (Na₂SO₄) as the supporting electrolyte. Next, we used the isothermal cell to report the temperature coefficient of 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl-4-yl (TEMPO^1+/0), ethyl viologen dibromide (EtVioBr^2+/1+), and 2,6-dihydroxyanthraquinone (DHAQ^2-/4-) to be -0.08, -0.11, and -1.25 mV/k respectively. This result aligns with the classical model that suggests the temperature coefficient increases in magnitude with the oxidative/reductive charge of the species. We further looked into effect of 2,6-DHAQ concentration and KOH concentration on the temperature coefficient and see trends similar to that of the Fe(CN)₆^3-/4- survey, implying that the effect of varying concentration on the temperature coefficient is similar for different chemical species. Finally, we show that the location of the hydroxide groups on anthraquinone also affect the temperature coefficient by reporting the temperature coefficient of 1,2-DHAQ and 1,8 DHAQ to be -0.6 mV/k and -0.8 mV/k respectively. This result is especially interesting because it suggests that the temperature coefficient can be controlled not only by redox center, but by the type and location of charged pendants connected to the organic molecule.