Remarkable power factor improvement in a porous, nanostructured thermoelectric oxide functionalized with viologen molecules†

Abstract

Thermoelectric (TE) materials are attractive as a technology able to directly convert heat into electricity. Most of the successful strategies to improve TE performance are based on decreasing the thermal conductivity, while approaches aiming at increasing the power factor (PF = σS², where σ is the electrical conductivity and S the Seebeck coefficient) have been limited. Here, we introduce a new strategy to significantly improve this parameter by using a porous, nanostructured TE solid (Sb-doped SnO₂) functionalized with a redox molecule: bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride. We found that, after functionalization, a 50% average reduction in the electrical resistivity, with a small increase of 9% in the absolute value of the Seebeck coefficient, takes place, leading to a remarkable 2.5 times PF improvement. In order to explain the effects observed, impedance spectroscopy measurements were performed, concluding that the electrical resistivity decrease is produced by the donation of electrons from the redox molecules into the oxide material. This new strategy remarkably achieves a substantial decrease in electrical resistivity without a Seebeck coefficient reduction (there is even a small increase), which is highly beneficial and not usually common, demonstrating a high potential to increase the PF.