Hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin–I5− complex in aqueous I−/I3− thermocells and enhancement in the Seebeck coefficient

Abstract

A large Seebeck coefficient (S_e) of 1.9 mV K⁻¹ was recorded for the I⁻/I₃⁻ thermocell by utilizing the host-guest complexation of hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin (Me₁₈-α-CD) with the oxidized iodide species. The thermocell measurement and UV-vis spectroscopy unveiled the formation of an Me₁₈-α-CD–pentaiodide (I₅⁻) complex, which is in remarkable contrast to the triiodide complex α-CD–I₃⁻ previously reported. Although the precipitation of the α-CD–I₃⁻ complex in the presence of an electrolyte such as potassium chloride is a problem in thermocells, this issue was solved by using Me₁₈-α-CD as a host compound. The absence of precipitation in the Me₁₈-α-CD and I⁻/I₃⁻ system containing potassium chloride not only improved the S_e of the I⁻/I₃⁻ thermocell, but also significantly enhanced the temporal stability of its power output. This is the first observation that I₅⁻ species is formed in aqueous solution in a thermocell. Furthermore, the solution equilibrium of the redox couples was controlled by tuning the chemical structure of the host compounds. Thus, the integration of host-guest chemistry with redox couples extends the application of thermocells.