Improvement of the Thermoelectric Power Factor of ZnO Using Ionic Liquids

TL;DR

Researchers improved the thermoelectric performance of ZnO by using ionic liquids, which increased electrical conductivity without significantly affecting the Seebeck coefficient.

Contribution

A novel method to enhance the thermoelectric power factor of ZnO using ionic liquids, specifically 1-butyl-3-methylimidazolium iodide.

Findings

Combining Ag–ZnO with BMII ionic liquid increased electrical conductivity by 70.2% with minimal change in Seebeck coefficient.

This led to a 1.46 times improvement in the thermoelectric power factor.

Electron injection from iodide ions in BMII increased carrier concentration in ZnO.

Abstract

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.

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