Thermoelectric Conversion by Thin-Layer Thermogalvanic Cells with Soluble Redox Couples

Abstract

Thermogalvanic cells containing a redox couple of [Fe(CN)6]4−⁄3− or Fe2+⁄3+ as an electroactive species were built for thermoelectric conversion from low-grade heat with a temperature less than 100 °C. The distance between high- and low-temperature electrodes was set to 1 mm in order to produce a high power density. Thermoelectric powers, inner resistances, and heat fluxes of the cells, which were placed horizontally (i.e. in which the temperature gradient was vertical) were measured under steady state conditions. Natural convection, which was induced by a temperature difference and current flowing in appropriate directions, produced lower inner resistances than those with no convection by several times. It had only a small effect on thermoelectric powers and heat-transfer coefficients. Thus, both the power densities and the thermal efficiencies increased by nearly one order of magnitude due to the convection. A power density of 2.6 W m−2 was obtained during steady state using a cell containing an aqueous solution of 0.13 mol dm−3 K4[Fe(CN)6] and 0.85 mol dm−3 K3[Fe(CN)6] at a temperature difference of 72 K and a thermal efficiency of 6.7×10−5; the upper electrode was kept at a lower temperature.