Poly(diallyldimethylammonium)-based solid electrolytes to significantly enhance the power factor of a thermoelectric oxide film (Sb-doped SnO2)

Abstract

Thermoelectric (TE) materials are able to convert heat into electricity. Suitable TE materials should have high power factors (PFs) and low thermal conductivities, where PF = S²σ, with S being the Seebeck coefficient and σ the electrical conductivity. Most recent improvements in TE materials have been achieved by the reduction of the thermal conductivity, and strategies to improve the PF have been minor. Recently, our group reported a new concept to significantly increase the PF, based on the combination of a porous TE solid with an electrolyte. Herein, we made use of this new approach but using polyelectrolytes, rather than the liquid electrolytes previously employed. Poly(diallyldimethylammonium X) polyelectrolytes were tested, where X = Cl⁻ (C) or tosylate (Tos). An average PF improvement of 2.6 times was obtained when PDADMAC was used, similar to the enhancement with liquid electrolytes. This was due to average decreases of 13% and 71% in the absolute value of the Seebeck coefficient and the electrical resistance of the system, respectively. An electrochemical study by impedance spectroscopy and cyclic voltammetry revealed the better capability of PDADMAC to screen the charge introduced in the oxide compared with that of PDADMATos. The resistance reduction for PDADMAC was attributed to variations in the carrier concentration in the oxide after its equilibration with the polyelectrolyte. The notable PF improvement obtained paves the way for the use of polyelectrolytes to fabricate all-solid-state solid-electrolyte systems with enhanced PFs.