Effect of solvation shell structure on thermopower of liquid redox pairs

Abstract

Recent advancements in thermogalvanic batteries offer a promising route to efficient harvesting of low-grade heat with temperatures below 100 {\deg}C. The thermogalvanic temperature coefficient {\alpha}, usually referred to as effective thermopower, is the key parameter determining the power density and efficiency of thermogalvanic batteries. However, the current understanding of improving {\alpha} of redox pairs remains at the phenomenological level without microscopic insights, and the development of electrolytes with high {\alpha} largely relies on experimental trial and error. This work applies the free energy perturbation method based on molecular dynamics simulations to predict the {\alpha} of the {Fe^{3+}/Fe^{2+}} redox pair in aqueous and acetone solutions. We showed that {\alpha} of the {Fe^{3+}/Fe^{2+}} redox pair can be increased from 1.5{\pm}0.3 mV/K to 4.1{\pm}0.4 mV/K with the increased acetone to water fraction. The predicted {\alpha} of {Fe^{3+}/Fe^{2+}} both in pure water and acetone show excellent agreement with experimental values. By monitoring the fluctuation of dipole orientations in the first solvation shell, we discovered a significant change in the variance of solvent dipole orientation between Fe^{3+} and Fe^{2+}, which can be a microscopic indicator for large magnitudes of {\alpha}. The effect of acetone weight fraction in the mixed acetone-water solvent on the {\alpha} of {Fe^{3+}/Fe^{2+}} is also studied. Acetone molecules are found to intercalate into the first solvation shell of the {Fe^{2+}} ion at high acetone fractions, while this phenomenon is not observed in the solvation shell of the Fe^{3+} ion. Such solvation shell structure change of {Fe^{2+}} ions contributes to the enhanced {\alpha} at high acetone fractions. Our discovery provides atomistic insights into how solvation shell order can be leveraged to develop electrolytes with high thermopower.