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

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 (S-e) 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 S-e) 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 S-e and ionic conductivity. UV-Vis and dielectric relaxation analysis revealed that the redox reaction of the hexa aquo complexes (Fe(H2O)(6)(3+/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, p-type TECs can be combined in-series with p-type (negative S-e) TECs to provide output powers high enough for practical application. (C) 2020 Elsevier Ltd. All rights reserved.