The amplifying effect of natural convection on power generation of thermogalvanic cells

Recent development of new electrolytes and electrode materials has shifted the research focus away from other routes to improved thermogalvanic cells, also known as thermocells or thermoelectrochemical cells. A thermogalvanic cell can be designed to achieve a relatively high maximum power output (Pmax) if convection is incorporated within the cell. Here we show through experimentation and modeling that natural convection is not a hindrance, but a plausible aid to improve power generation of thermogalvanic generators. The natural convection conditions are varied via orientation of the cell and electrode spacing. Experimental testing revealed an optimum concentration of 0.7M CuSO4 aqueous electrolyte, with the addition of 0.1M H2SO4 as background electrolyte, results in Pmax improvements of up to 100%, from ∼0.8 to ∼1.6μWcm−2, in a horizontal cell operating between the vertical cold and hot copper (Cu) electrode temperatures of 5 and 65°C, respectively. The experimental data reported here compare well with those from the few previously reported studies of Cu/Cu2+ cells. However, comparison with the conventional Fe(CN)64−/Fe(CN)63− cells revealed a completely different dependence of the electrode spacing on the power generation, that the Pmax of Cu/Cu2+ cell increases, instead of decreases, with the electrode spacing due to the difference in ionic transfer mechanism. Herein, we also develop a simple expression which predicts the ratio between the Pmax of a thermogalvanic generator with and without convection in terms of Sherwood, Nusselt, and Lewis numbers. The predictions are in reasonable agreement with our experimental data; both show that convection is primarily responsible for the enhancement in Pmax of thermogalvanic generators.