Unravelling ionic speciation and hydration structure of Fe(III/II) redox couples for thermoelectrochemical cells

Abstract Thermoelectrochemical cells (TECs) are promising devices for harvesting heat waste, but their widespread use has been hindered by their low thermopower densities. High-power TECs require an electrolyte solution that exhibits both high Seebeck coefficient (Se) and high ionic conductivity; thus far, this has been a challenge. Recently, we demonstrated that proper selection of the counter anion of Fe(III)/(II) salts can resolve the aforementioned issue, that n-type (positive Se) TECs employing the Fe(III)/(II) perchlorate redox couple display unprecedented high areal power densities compared to TECs employing Fe(III)/(II) chloride or Fe(III)/(II) sulfate couple. Herein, we unravel that the excellent performance of the Fe(III)/(II) perchlorate is ascribed to the non-coordinating nature of its perchlorate anion, which suppresses the formation of the ion pairs that reduce the Se and ionic conductivity. UV–Vis and dielectric relaxation analysis revealed that the redox reaction of the hexa aquo complexes (Fe(H2O)63+/2+), formed Fe(III)/(II) perchlorate, is accompanied by a hydration-number change larger than those of anion-coordinated species, which are dominant in chloride or sulfate media. In addition, n-type TECs can be combined in-series with p-type (negative Se) TECs to provide output powers high enough for practical application.