Improvement of the Thermoelectric Power Factor of ZnO Using Ionic Liquids

Thermoelectric (TE) materials are able to convert heat into electricity. Since many heat sources are available in our sourroundings (e.g., body heat, sun, domestic heat), thermoelectricity have drawn significant interest for energy harvesting. The power factor (PF = S ²σ, being S and σ the Seebeck coefficient and the electrical conductivity, respectively) and the thermal conductivity are key parameters to assess materials' performance. Our group reported a few years ago a concept to significantly improve the PF. It was based on the combination of a porous TE solid with a liquid electrolyte. PF improvements above 3 times were achieved when Sb-doped SnO₂ was used as the solid. However, despite this substantially high PF enchancement, not very high PFs were reached due to the modest TE properties of Sb/SnO₂. Here, we aim at introducing similar PF improvements in a high-performance porous TE oxide, a Ag-ZnO composite, by means of its combination with different ionic liquids acting as electrolytes: 1-butyl-3-methylimidazolium iodide (BMII) and 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)-imide (BMITFSI). For the BMII ionic liquid, it was found a significant increase (70.2%) in σ with only a small variation in S. This uncommon and highly befenefitial disconnection of S and σ, which typically are adversely related, led to a PF improvement of 1.46 times. In contrast, samples contacted with BMITFSI showed almost no PF variation. Scanning electron microscopy, X-ray diffraction, and impedance spectroscopy experiments showed that the electrical conductivity increase is due to a rise in the carrier concentration in the oxide, produced by the injection of electrons from the iodide ions. Hence, our work demonstrates that the strategy of enhancing the PF using electrolytes can also be extended to ZnO, one of the best-performing TE oxides.