*3.1. Thermoelectric Materials*

It is estimated that between 50% and 80% of the studies related to thermoelectric generation systems (TEG) focus on the TE materials themselves. Therefore, the development of materials that have a low cost and high efficiency is fundamental to achieving commercial profitability in these systems [17,18].

In the systematic search carried out, 10 articles were found in which the TE material was studied in depth in order to increase its efficiency, decrease costs or develop viable materials in both environmental and sustainability aspects by analyzing factors such as the material's abundance and its degree of toxicity among others. Table 4 presents a classification of these items according to the cost, efficiency and sustainability of the TEG modules. In this table, the cost column expresses the materials cost reduction with a minus sign (−), and the materials cost increase with a plus sign (+). In this aspect, the elements that compose in some cases the type of processing are taken into account. Note that in most of the articles reviewed, the cost of the material was not stated explicitly. For efficiency, the minus sign refers to materials with low efficiencies (ZT < 1) and the plus sign (+) for materials with significant efficiencies (ZT > 1). Finally, the sustainability of the materials is assessed using measures such as the availability, toxicity and useful life; therefore, a minus sign indicates that the material is unsustainable and a plus sign that it is sustainable.

**Table 4.** Classification of the articles found according to the results obtained in the development of thermoelectric materials.


Cost: −, Low cost; +, high cost. Efficiency: +, high efficiency; −, low efficiency. Environmental aspects and sustainability: −, unsustainable; +, more sustainable.

Among the articles shown in Table 3, three case studies were found [19,58,59] that examined cost of TE materials, including their cost effectiveness and the processing technique used to develop them.

Among the cost-effective materials, the use of oxides as raw materials stands out. Hung et al. [43] studied different oxides (Na2CoO4, Ca3Co4O9, ZnO, SrTiO<sup>3</sup> and CaMnO3) in order to decrease costs in the manufacture of TEG modules since the cost of manufacturing the oxides is approximately 1.1 \$/kg, which is equivalent to only a quarter of composite materials composed of metals and rare earths. On the other hand, Lee et al. [39] studied the potential of TiO2-x for TE materials manufactured by plasma deposition. Ozturk et al. [30] studied two types of oxides: Ca2.5Ag0.3X0.2Co4O<sup>9</sup> type n and Zn0.96Al0.02Y0.02O type n, where X and Y are different dopants manufactured using the sol-gel method. In these works, the benefits of using oxides as raw materials are highlighted. Among these benefits are low cost, abundance, resistance to high temperatures, as well as simplified manufacturing processes not requiring controlled atmospheres. However, it can be seen that the purpose of these studies is to improve the efficiency of materials. In Table 5 it can be seen that the oxides present the least merit (efficiency). According to the review, the modules' oxidebased TEGs can increase their efficiency through the use of special processing [37] or doping techniques [28], which makes their use more viable.


**Table 5.** Thermoelectric materials type, merit rating, and temperature.

On the other hand, the use of cheap and abundant materials such as lead-based materials or silicides has also been the subject of recent studies. Han et al. [33] studied the feasibility of PbTe-SrTe base materials doped with 2% Te. They concluded there was a cost reduction through a low-cost processing method such as stable screen printing, although one of the base materials and the tellurium are high cost and low abundance elements. Jiang et al. [27] proposed PbS as an alternative to the base material PbTe, arguing that by doping with Sb and Se, efficiency can be considerably improved, in addition to them being abundant and low-cost materials.

Fu et al. [44] and Salvador et al. [52] developed materials such as FeNbSb (Half-Heuslers) and Yb0.09Ba0.09La0.05Co4Sb<sup>12</sup> (skutterudite), respectively, which are composed of low-cost and abundant elements, and by doping techniques are able to increase their efficiency. However, Ouyang et al. [36] evaluated some of the latest generation materials and recommended that materials such as skutterudites and half-heuslers could only be used in applications where cost is not of concern, due to the high manufacturing costs of these materials. On the other hand, Skomedal et al. [40] suggested the use of magnesium silicides as a favorable TE material, due to their low cost, abundance and low toxicity, despite their low efficiency when doping with elements such as Sn and Sb. They concluded that materials based on magnesium silicides are recommended for applications where low cost or low weight are more important than efficiency.

In addition, Homm et al. [56] analyzed some TE materials such as SiGe, PbTe, Bi2Te3, FeSi<sup>2</sup> and ZnO. The authors classified them according to selection criteria for different applications that required certain specifications for temperature, efficiency and cost, but taking into account the environmental aspects that each one presented.

According to the present review, it is observed that there is a conflict between the aspects of cost, efficiency and sustainability. Figure 4 presents a classification based on these aspects of recent studies addressed in this analysis. Three articles were found involving costs and efficiency in zone A, [27,29,44]; three articles between costs and sustainability in zone B [30,39,46]; one article involving efficiency and sustainability in zone C [36]; and tree articles involving all aspects, costs, efficiency and sustainability in zone U [40,52,56]. From this classification it is concluded that the oxides are inexpensive TE materials with important advantages. In particular, they are abundant, do not require high-cost processing, and resist high temperatures, which prevents premature degradation of the TE material. Moreover, they enable the formation of robust materials with a longer useful life, and have a good cost-sustainability ratio. However, their efficiency is reduced with respect to the commercially used TE materials, which prompts us to think about the different research approaches to improve them, such as nano-structuring, electronic band engineering, quantum confinement, as well as strategies such as crystal electron glass phonon, doping, and introduction of defects, among others. However, the use of any of these techniques requires specialized and complex processes, which would be reflected in the final cost of the product and would probably mean that the cost-efficiency ratio is not viable for developments in a commercial environment. Therefore, the development of this research is of utmost importance for providing not only a better future perspective of TE materials, but also because there are few investigations that specifically address the economic component of these materials.

**Figure 4.** Classification of the articles found according to the results obtained for TE materials.

Furthermore, from a sustainability perspective, little information is available on commonly used TE materials. An example of this is the use of toxic materials such as lead, tellurium and bismuth in their fabrication. Therefore, an important aim of research is to explore is the environmental risks that these materials can present at different stages of the useful life of TEG modules and to search for abundant and low-cost elements.

Interest in certain thermoelectric materials is based on a combination of their characteristics and performance. Figure 5 shows the trends in the number of publications in recent years in relation to some representative thermoelectric materials, according to a survey carried out in the Scopus database. The figure shows the growing research interest in these thermoelectric materials.
