Abstract The E-I relations in a thin-layer thermocell with a soluble redox couple, potassium ferrocyanide and potassium ferricyanide in aqueous solution, were calculated numerically. Double diffusive convection in the cell placed horizontally and vertically was considered using the Navier-Stokes equation and the Onsager flux equation. The calculated results were in good agreement with the experimental results except for cases under strong convection. From the calculation with various concentration ratios of the redox couple, the optimum condition of the concentration ratio of the redox couple to obtain high electric power was ca. 1:1 with the vertical electrodes. The greatest electric power was 0.32 mW cm −2 with a temperature difference of 50 K. This was also supported by the experimental results. The calculated result also showed that the thermocell can be worked as a heat pump to give 0.1 K in temperature difference.