Large Power Factor Improvement in a Novel Solid-Liquid Thermoelectric Hybrid Device

The performance of thermoelectric materials has been improved in the past few years by a significant reduction of the thermal conductivity, but this parameter has now reached its (amorphous) limit, which makes the enlargement of the power factor (PF) critical. Here we present a new concept, based on a hybrid system formed by a porous thermoelectric solid permeated by a liquid with an inert salt dissolved (electrolyte), which can be designed to significantly enhance the PF. The concept is demonstrated in an Sb-doped SnO2 porous film permeated with different inert salts (X+(BF4)(-), X+ = Li+, Na+, K+) dissolved in 3-methoxypropionitrile (3-MPN). A 61.9% average decrease in the electrical resistivity without a significant variation of the Seebeck coefficient was found in the case of 1 M LiBF4 in 3-MPN. As a result, the PF was remarkably increased 3.4 times. On the other hand, 1-butyl-3-methylimidazolium (BMI+Y-, Y- = I-, BF4-) ionic liquids were also employed as electrolytes. Using the BMII ionic liquid, the electrical resistivity showed a more significant average decrease of 82.5%; however, the absolute value of the Seebeck coefficient was reduced by 35%, finally resulting in an average enhancement of 2.4 times of the PF. The large enhancements achieved are attributed to the modification of the electrostatic environment of the porous solid by the ions in the electrolyte at the solid-liquid interface. These results establish a new strategy for the significant improvement of the PF which is not restricted to certain materials and can be potentially applied widely.