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) of the redox pairs Fe(CN)63−/Fe(CN)64− and Fe3+/Fe2+, showing excellent agreement with experimental values. We showed that of the Fe3+/Fe2+ redox pair can be increased from 1.70.4 mV/K to 3.80.5 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 . In mixed acetone‐water solvent, 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 .image

Thermodynamics of Ionic Thermoelectrics for Low-Grade Heat Harvesting

“…(i) Methods of predicting the Soret and thermogalvanic thermopower are in urgent demand for the development of high-thermopower i-TE materials but remain highly challenging due to the complicated and long-range features of ion−ion and ion−solvent interactions. Although recent MD simulations have been applied to predict S tg of Fe 3+ /Fe 2+ and Fe(CN) 6 3− / Fe(CN) 6 4− redox pairs, 98 it is only applicable to a dilute solution of simple redox pairs and cannot predict thermopower of redox reactions involving chemical species from the electrodes. A computational framework for studying S td in complex electrolytes (e.g., polyelectrolytes, ionic liquids, and gels) is still lacking.…”

Anion Effects on Thermopower of Electrochemical Systems for Low-Grade Heat Harvesting

“…Such a property affects the dynamics of water molecules and thereby the hydrogen bond network in the electrolyte . Recent atomistic simulations revealed that the hydration entropy change is highly related to the solvent dipole orientations, which provides microscopic insights into electrolyte design with high α . Overall, it is considered that there is a specific anion effect on α associated with the solvation structure variation and ion–water interactions …”

“…Considering the combined effects of Δ S e,1 , Δ S s , and Δ S e,2 , the total entropy change follows the order of SO 4 2– < Cl – < NO 3 – < ClO 4 – , the reverse order of the Hofmeister series, resulting in the observed variation trend of α. The mechanism can be further investigated from a microscopic perspective with the help of atomistic simulations on the hydration shell structures at the molecular level. , …”

“…16 Recent atomistic simulations revealed that the hydration entropy change is highly related to the solvent dipole orientations, which provides microscopic insights into electrolyte design with high α. 17 Overall, it is considered that there is a specific anion effect on α associated with the solvation structure variation and ion−water interactions. 18 Here, we present the impact of anions on the solvation entropy change during the intercalation of a cation into the ion hosting material.…”

“…The mechanism can be further investigated from a microscopic perspective with the help of atomistic simulations on the hydration shell structures at the molecular level. 17,21 Based on the above analysis, CuHCFe with NaClO 4 electrolyte was selected for its promising large α to demonstrate the energy harvesting performance. The anode material was the Fe 2+ /Fe 3+ redox pair thanks to its high positive α.…”