*3.8. Sustainability (Circular Economy)*

As noted above, commercial TE materials are not yet sufficiently cheap, and high efficiency materials are not yet mass produced. Until now, the most commonly used commercial TE materials are Bi2Te3-based alloys because they have advantages such as easy bulk processing. However, although they are precursors, high energy expenditure and expensive techniques are used in their processing both for power generation and for cooling at temperatures close to ambient levels. On the other hand, TE materials use elements such as bismuth, tellurium, antimony, selenium, and lead, among others, which are expensive, scarce, and sometimes toxic. As mentioned earlier, from the point of view of the circular economy, the recycling of TEG modules could generate great economic benefits since it would allow obtaining raw materials for the manufacture of new TEG modules or other electronic devices, generating a reduction in the consumption of scarce elements. Moreover, they also generate environmental advantages because the improper disposal of these materials is avoided, which can benefit both the environment and human health.

At the time of writing this paper, the scientific publications on recycling TEG modules are still quite sparse. However, currently there are different ways to recycle TEG modules based on tellurium bismuth, from which three approaches can be differentiated based on the separation techniques: (i) chemical, (ii) thermal, and (iii) bacterial methods. On the other hand, in some cases only some parts of the TEG modules are recycled or only the elements of the semiconductors are recovered. According to the bibliographic review carried out, approaches have been proposed for the recycling of commercial TEG modules based on bismuth tellurium, by taking advantage of the differences in melting temperature of the constituent materials. In this way, the separation of the different constituents of commercial modules (plastics, Cu, Bi2Te<sup>3</sup> and Al2O3) in an efficient way might be achieved by mechanical processing which relies on the entropy changes of these materials [27].

The TE materials have been separated by thermal processes followed by chemical separation processes, in which the characterization of the materials of the TE modules was conducted by techniques such as differential scanning calorimetry (DSC), X-ray diffraction (DRX) and field emission scanning electron microscopy (FESEM), which give information on the material types, melting temperatures and the distribution of the materials. This allows their separation based on the differences in thermal and chemical properties. This type of separation is initially carried out by means of thermal treatments such as hot oil baths at 250 ◦C for the removal of solder from the -n (Bi2Te3) and the p-type (Bi0.5Sb1.5Te3) semiconductors. Later they are subjected to a mixed acid solution (HCl and HNO3 in a 3:1 ratio) at room temperature. At this point, the Sb of the semiconductors precipitates. Then, the solution is then filtered, washed and sintered in order to obtain nano-powders of Bi2Te<sup>3</sup> -n type with a particle size of ~15 nm purity [25,36].

None of the previous works reports a characterization of the thermoelectric properties of the recovered TE materials. The characterization would be of great importance in order to know if the processes used for their separation in any way affect the properties of the recovered products and their possible use in future applications. Table 7 lists some recent works in relation to the final disposal of thermoelectric modules.


**Table 7.** Bibliographic review of the final disposal of thermoelectric modules in recent years.

It is clear then that there is a global need for sustainable technologies [75–77], and the circularity of materials and processes are areas where TE can have a significant impact as the main, partner, or complementary technology since it is a particularly adaptable technology [78].
