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Solid-liquid thermoelectric systems with uncorrelated properties

Periodic Reporting for period 1 - UncorrelaTEd (Solid-liquid thermoelectric systems with uncorrelated properties)

Reporting period: 2020-01-01 to 2020-12-31

More than 60% of the global power is lost as waste heat. Thermoelectric (TE) materials can convert vast amounts of this waste heat into electricity and significantly contribute to the current energy challenge. Despite large efforts to identify better TE materials, still, the TE technology is limited by low efficiency. One of the two performance improvement routes, thermal conductivity reduction, has already reached its limit, which makes the other route, power factor (PF) improvements, crucial.

Current strategies targeting PF enhancements have only reached modest improvements, mainly due to the adverse interdependence of the Seebeck coefficient (S) and the electrical conductivity (σ), which produces a decrease in one of these properties if the other is increased. This is a serious obstacle to achieve the widespread application of the TE technology, since PF=σS^2.

UncorrelaTEd will realize the dream of breaking the S-σ correlation by introducing a new paradigm in thermoelectricity that comes from the connection of TEs and electrochemistry, using a properly designed hybrid system, formed by a porous TE solid permeated by a liquid electrolyte (ions in a liquid), as shown in Fig. 1. The porous solid provides a low thermal conductivity, whereas the electrolyte tactically interacts with the solid to enlarge the PF. Unprecedented PF improvements (above 35 times) have already been observed by UncorrelaTEd members in this system using a material with modest TE properties (Sb-doped tin oxide, ATO). UncorrelaTEd aims at extending these improvements to different materials (bismuth telluride alloys, oxides, and polymers) with state-of-the-art TE properties, potentially leading to an extraordinarily powerful technology able to provide more than 4 times larger PF than state-of-the-art low-mid temperature (<150 ºC) materials and ZTs>3.
During the first year of the project, the main tasks developed by UncorrelaTEd have been: (1) to understand the phenomena that produced the observed PF improvements in our preliminary results, which were obtained using ATO as TE solid material, (2) to fabricate ATO films with different porosity levels in order to investigate how porosity affects the PF improvements, (3) to synthesize bismuth telluride based materials and fabricate films of different porosity and doping levels with TE properties close to the state of the art.

1. To understand the phenomena that produced the observed PF improvements obtained using ATO: In our preliminary results, we observed that using nanostructured and porous ATO as the TE solid and HI dissolved in water as electrolyte, the S of ATO increased from -38 μV/K (without electrolyte) to -213 μV/K (with electrolyte). On the other hand, the electrical resistance of the device varied from 19 kΩ (without electrolyte) to 17 kΩ (with electrolyte). That led to an unprecedented PF improvement of 35.8 times.

After investigating the reasons behind these improvements, it was found that the anion I- from the HI electrolyte can react with oxygen in the solution to produce I3-, leading to an electrolyte that contains both I- and I3-. These two anions form what is called a redox couple, which is the combination of two compounds that can exchange electrons with the metallic contacts (electrodes) of the device. These metallic contacts are attached to each side of the ATO film and are also in contact with the electrolyte (see Fig. 1).
We fabricated a device with the same configuration, but now without the presence of ATO, that is, two metallic contacts (Pt) separated by a I-/I3- electrolyte. When S and σ was measured in this device, we found the same values as in the device with ATO. This led to the conclusion that ATO was doing nothing in the device and all the TE properties come from the electrode/electrolyte/electrode configuration, which is known as the configuration of a thermocell [1].

After realising that the unprecedented preliminary results found came from a thermocell device, a new approach was investigated. In this new approach, it was proven that a metallic material, such as Pt, could exhibit a large S (above 1 mV/K) without a significant variation of its σ. This is remarkably interesting since metals show good electrical conduction, but their S values are very small (1-10 μV/K usually), and it can lead to extremely large PFs never achieved before.

2. To fabricate ATO films with different porosity levels in order to investigate how porosity affects the PF improvements: In order to prepare these films, a controlled nanoparticle aggregation approach was followed. A suspension of ATO nanoparticles in a solvent was formed that led to an interconnected network of these particles (gel) in the solvent after employing a destabilizer agent. Once the gel is formed, the solvent was removed (drying) under controlled conditions, leading to the network of interconnected nanoparticles which form porous films. The level of porosity was controlled by modifying the deposition and drying conditions. Film porosities covered a wide range, from 8% to 58% (see Fig. 2).

3. To fabricate bismuth telluride films of different porosity and doping levels with TE properties close to the state of the art: In order to fabricate these films, nanoparticles of different compositions based on Bi2-xSbxTe3 were prepared first using three different solution synthesis methods assisted by microwave heating.

The aggregation of the prepared nanoparticles in order to obtain porous films was performed by electrophoretic deposition (EPD) process. EPD is a process in which charged colloidal particles, dispersed or suspended in a liquid medium, are migrated by an applied electric field in order to be adsorbed on a substrate.

A thorough systematic testing of different parameters allowed to obtain good results for n-type and p-type materials (Bi2Te3 and Sb2Te3 , respectively), with quite high substrate coverage and the capability to reach thickness of 6 μm and above (see Fig. 3 and Fig. 4).

References:

[1] Dupont MF, MacFarlane DR, Pringle JM. Thermo-electrochemical cells for waste heat harvesting-progress and perspectives. Chem Commun 2017;53:6288.
The results achieved so far have demonstrated the possibility to have metals with S values above 1 mV/K without affecting their high σ. This will lead to unprecedented values of the power factor and the TE efficiency. In addition, it has been demonstrated the preparation of nanostructured and porous bismuth telluride films by electrophoresis deposition, a method not considered before for this common TE material, which has been shown to produce highly homogeneous films of high porosity levels.

These aforementioned results will be developed further in the next period of the project and are expected to lead to extraordinarily high TE efficiencies in the conversion of heat to electricity. At the end of the project, UncorrelaTEd results will allow the widespread application of TE energy conversion technologies. Thermoelectricity will become competitive for energy harvesting for µW to W applications (wearable devices, sensors, the internet of things, etc.), empowering the elimination of batteries and maintenance requirements. Moreover, it can achieve significant power generation from low grade (<150 ºC) heat, available e.g. in factories, the environment, biological entities, solar-thermal and geothermal energy.
Fig. 3. SEM images of the n-type Bi2Te3 (polyol) film fabricated using EPD at different magnitudes.
Fig. 4. SEM images of the p-type Sb2Te3 (polyol) film fabricated using EPD at different magnitudes.
Fig. 1. (a) Schematic of the hybrid solid-liquid UncorrelaTEd device and (b) a picture of the same.
Fig. 2. SEM images of selected Sb:SnO2 porous films covering the porosity range from 8% to 58%.