Concentration dependence of resistance components in solutions containing dissolved Fe²⁺/Fe³

Electrolyte solutions containing Fe²⁺/Fe³⁺ are suitable for liquid thermoelectric conversion devices (LTEs), because they are inexpensive materials and exhibit a high electrochemical Seebeck coefficient α. Here, we investigated the concentration (c) dependence of resistance components, i.e., solvent (R_s), charge-transfer (R_ct), and diffusion (R_dif) resistances, of dissolved-Fe²⁺/Fe³⁺-containing aqueous, methanol (MeOH), acetone, and propylene carbonate (PC) solutions. We found that the c dependence of R_s and R_dif are well reproduced by empirical formulas, and , where η(c) is viscosity at c. We further found that the magnitudes of C_s and C_dif are nearly independent of solvent, suggesting that η is one of the significant solution parameters that determine R_s and R_dif.