Resistance components in organic electrolytes containing Fe2+/Fe3+ for liquid thermoelectric conversion devices

Abstract

Organic electrolytes containing Fe²⁺/Fe³⁺ exhibit a large electrochemical Seebeck coefficient α and are suitable for liquid thermoelectric conversion devices (LTEs). Here, we investigated the resistance components, i.e., solvent (R_s), charge-transfer (R_ct), and diffusion (R_dif) resistances, of organic electrolytes against solvent and solute at a fixed molar concentration (=0.1 M). We found that R_s and R_dif are the dominant resistance components in these electrolytes. In iron chloride and iron perchlorate solutions, each R_s increases in proportion to the viscosity η of the solvent. With the use of the slope of the R_s–η plots, the effective radius r was estimated to be 1.1 nm for [FeL₆]²⁺/[FeL₆]³⁺ (L is ligand molecule) and 1.3 nm for [FeCl₄]²⁻/[FeCl₄]⁻.