Exploring the local solvation structure of redox molecules in a mixed solvent for increasing the Seebeck coefficient of thermocells

A thermocell is an emerging alternative to thermoelectric devices and exhibits a high Seebeck coefficient (Se) due to the large change of solvation entropy associated with redox reactions. Here, the Se of p-chloranil radicals/dianions (CA˙−/2−) in acetonitrile was drastically increased from −1.3 to −2.6 mV K−1 by the addition of ethanol, and the increment surpassed the estimation of the classical Born model with continuum solvent media. UV-vis spectroscopy and electrochemical measurements at various mixing ratios of acetonitrile to ethanol revealed that the strong hydrogen bonding between ethanol and oxygen atoms of CA2− forms a 4 : 1 solvent–ion pair, while the ethanol molecules binding to CA2− dissociate upon its oxidation to CA˙−. The local solvation structures of CA2− are in good agreement with density functional theory. This order–disorder transition of the local solvation structure around the CA˙−/2− ions produces a large entropy change and results in a large Se value. The tailored solvation structure of redox ions by hydrogen bonding is a versatile method applicable to a variety of redox pairs and solvents, contributing to the development of electrolyte engineering for thermocells.