Hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin–I(5)(–) complex in aqueous I(–)/I(3)(–) thermocells and enhancement in the Seebeck coefficient

A large Seebeck coefficient (S(e)) of 1.9 mV K(–1) was recorded for the I(–)/I(3)(–) thermocell by utilizing the host-guest complexation of hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin (Me(18)-α-CD) with the oxidized iodide species. The thermocell measurement and UV-vis spectroscopy unveiled the formation of an Me(18)-α-CD–pentaiodide (I(5)(–)) complex, which is in remarkable contrast to the triiodide complex α-CD–I(3)(–) previously reported. Although the precipitation of the α-CD–I(3)(–) complex in the presence of an electrolyte such as potassium chloride is a problem in thermocells, this issue was solved by using Me(18)-α-CD as a host compound. The absence of precipitation in the Me(18)-α-CD and I(–)/I(3)(–) system containing potassium chloride not only improved the S(e) of the I(–)/I(3)(–) thermocell, but also significantly enhanced the temporal stability of its power output. This is the first observation that I(5)(–) 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.