High entropy metal oxides for thermoelectric applications

Abstract

The demand for sustainable and alternative energy sources is growing because of fossil fuels on global warming. In this context, thermoelectric materials have gained significant interest for their effective energy conversion from waste heat to electricity. These materials have various applications in different sectors including automotive, military and more recently as selective absorber for solar-thermal plants. However, conventional thermoelectric materials have demonstrated low efficiency at high temperatures. To address this issue, researchers have explored nano structuring and the development of novel materials with low band gap energies. High entropy oxides (HEOs) have emerged as a promising approach, offering the ability to adjust their physical characteristics to suit specific thermoelectric application needs. HEOs have a multi-cation approach that stabilizes oxide materials, creating novel compounds with distinctive structure-property relationships, making them ideal candidates for high temperature thermoelectric applications. In this study, a Spark Plasma Sintering (SPS) synthesis route at high temperatures (850 to 1100 °C) was adopted for the fabrication of eight high entropy Co-Cr-Fe-Mn-Ni-O metal oxides. The developed heterostructures named H1 to H8 were examined to study their thermoelectric properties along with evaluating the material as selective solar absorbers for water desalination applications. The samples were characterized using various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM) and Energy dispersive spectroscopy (EDS). While optical properties and thermoelectric properties were determined using UV-Vis spectroscopy, thermocouple and four probe technique respectively. Finally, solar water evaporation technique was implemented to assess the functional properties of all the materials. The samples H1 to H3 were employed to the sintering temperature till 1050 °C along with 35 MPa pressure, while H4 till H8 were treated till 1100 °C but with different pressures of 39 MPa and 30 MPa respectively. The samples revealed two primary structures: rock salt Fm-3m and spinel Fd-3m, with R-3c also being present in minute amount. It was determined that the sintering temperature and pressure changes had a significant impact on the stability of primary structures, with the maximum concentration of the Fm-3m often being recorded in the samples treated close to 1100 °C. The presence of multiple phases had a profound affect on the optical, photothermal and thermoelectric properties. All the samples H1 till H8 exhibited direct band gap characteristics, whereas the H2 sample showed the lowest bandgap energy of 1.14 eV while also reporting the outstanding evaporation rate of 2.72 kg·m-2·h-1 under 1 sun illumination for 60 minutes. Moreover, H1 also displayed a considerably higher evaporation rate of 2.43 kg·m-2·h-1 in comparison to samples from H3 till H8. Furthermore, electrical conductivity and Seebeck coefficient values for H2 were 0.092 S·cm-1 and 117.4 μV·K-1 at 730 K respectively which showed a good performance at high temperature. While the thermoelectric values for other samples were in between 0.085 to 0.176 S·cm-1 for electrical conductivity and -90 to -130 μV·K-1 for Seebeck coefficients. The H7 reported the highest value for electrical conductivity of 0.16 S·cm-1 at 730 K, while H8 produced highest Seebeck coefficient with 128 μV·K-1 at 730 K. This study demonstrated the impact of high temperature and pressure on high entropy oxides (HEOs) materials, leading to the development of cost-effective and highly efficient selective solar absorbers. The results have practical applications in solar water heaters, solar dryers, and solar thermal power plants. By optimizing the temperature, pressure, and sintering time, the performance of HEOs can be further improved, allowing for their implementation in next-generation solar absorbers.