Enthalpy-Entropy Compensation Governs the Solvent-Mixing Effect in Electrochemical Thermoelectric Conversion.

Thermocells convert temperature gradients into electrical energy using the entropy change of a redox reaction. The performance of thermocells is governed by the temperature coefficient of electrochemical potential (α), and various strategies are reported to increase α. Solvent mixing has been recognized as one of the effective methods; however, the underlying mechanism remains elusive due to the absence of either systematic experimental or reliable theoretical validation. Herein, we demonstrate that enthalpy-entropy compensation provides a general framework for modeling solvent-mixing effects in thermocells. Variable-temperature electrochemistry reveals the linear relationship between hydrogen-bond entropy (ΔSHB) and enthalpy (ΔHHB) arising from the interactions between methanol and quinone dianions in acetonitrile. This enthalpy-entropy compensation principle enables the prediction of quinone derivatives with a large entropy change─and consequently a large α value─through DFT-based screening of ΔHHB. Notably, tetramethyl-para-benzoquinone exhibits an α value of -3.1 mV K-1, which is the highest absolute value in liquid-based all-organic thermocells. These findings show that enthalpy-entropy compensation is a general molecular design strategy for creating high-performance thermocells.