Effect of solvation shell structure on thermopower of liquid redox pairs

Abstract

Developing redox electrolytes with high thermopower is the key to making efficient thermogalvanic batteries for harvesting low-grade heat. This work applies molecular dynamics simulations to predict the thermopower (i.e. thermogalvanic temperature coefficient) $\alpha$ of the redox pairs Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ and Fe³⁺/Fe²⁺, showing excellent agreement with experimental values. We showed that $\alpha$ of the Fe³⁺/Fe²⁺ redox pair can be increased from 1.7$\pm$0.4 mV/K to 3.8$\pm$0.5 mV/K with the increased acetone to water fraction. We discovered a significant change in the variance of solvent dipole orientation between Fe³⁺ and Fe²⁺, which can serve as a microscopic indicator for large $\alpha$. In mixed acetone-water solvent, $\alpha$ of Fe³⁺/Fe²⁺ showed a rapid increase at high acetone fractions, due to the intercalation of acetone molecules into the first solvation shell of the Fe²⁺ at high acetone fractions. Our discovery provides insights into how solvation shell order can be engineered to develop electrolytes with high $\alpha$.