Enhanced output of Fe²⁺/Fe³⁺ liquid energy conversion using a methanol-acetone mixed solvent

Energy-harvesting technologies that scavenge energy from ambient environments have attracted growing research interest to facilitate progress toward the sustainable development goals (SDGs). Liquid thermoelectric devices (LTEs) convert environmental heat into electricity using thermopower α, making them promising systems for achieving SDGs. LTE performance is evaluated using the maximum output, W _max. Therefore, increasing α and/or reducing device resistance R _cell is important for high W _max. Here, we focused on the solvent mixing effect and systematically investigated the dependence of α, R _cell, and W _max on the solvent mixing ratio in Fe²⁺/Fe³⁺ solutions using methanol (MeOH)-acetone mixed solvents. We found that α was maintained at a high value (about 2.9 mV K^-1) at a methanol molar ratio (x _MeOH) = 0.2, while R _cell was lower than that in either the MeOH solution or the acetone solution. As a result, W _max increased by about 38% compared with that of the acetone solution, and we found that the solvent mixing could potentially improve thermogalvanic performance. We interpreted the mixing-ratio dependence of α from macroscopic and microscopic perspectives based on viscosity η and the Fe-O bond length, respectively, and that of R _cell in terms of the resistance components.