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 ( S e ) due to the large change of solvation entropy associated with redox reactions. Here, the S e of p -chloranil radicals/dianions (CA&z.rad; −/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 CA 2− forms a 4 : 1 solvent-ion pair, while the ethanol molecules binding to CA 2− dissociate upon its oxidation to CA&z.rad; − . The local solvation structures of CA 2− are in good agreement with density functional theory. This order-disorder transition of the local solvation structure around the CA&z.rad; −/2− ions produces a large entropy change and results in a large S e 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. One-electron oxidation of a chloranil dianion (CA 2− ) to its radical form (CA&z.rad; − ) disrupts the local solvation structure formed by ethanol in acetonitrile, resulting in a large entropy change and a large Seebeck coefficient of CA 2−/ &z.rad; − thermocells.