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Martin-Gonzalez et al. Energy Mater. 2025, 5, 500121 https://dx.doi.org/10.20517/energymater.2025.32 Page 23 of 35

Table 4. State-of-the-art thermoelectric module efficiencies (η) across various temperature gradients (ΔT) and their associated
limitations
η
Material ΔT (K) Key limitations Ref
(%)
Bi Te 8 230 Bi Te modules suffer from Te scarcity (€1100/kg) and [80,303-305]
2 3 2 3
degradation above 250°C
PbTe (Apollo mission) 6.3 328 High-cost; Both elements are toxic; limited temperature [306,307]
stability; Interfacial reactions
GeTe-Mg SbBi 10 350 High materials cost (Ge/Te); toxicity (Te/Sb); Other [308]
3
considerations for MgSb- and GeTe-based systems as
discussed below
Mg Sb -single leg 10 350 Stability issue at higher temperatures; Mg sublimates above [309]
3 2
450°C, requiring encapsulation
GeTe-Cu Te-PbSe- single leg 14 440 High Cost of Ge (€3500/Kg according to Aldrich webpage) [310]
2
and toxicity concerns (Ge/Pb/Te); Phase transitions at
elevated temperatures
GeTe-based 13.3 506 Expensive; Te-toxicity; [311]
Diffusion of materials and electrodes; Service reliability
concerns, as the secondary phases are observed
Ge 0.89 Cu 0.06 Sb 0.08 Te-Yb Co Sb 12 12 545 Aging ingresses the interface resistivity. Phase transition in [312]
4
0.3
GeTe at elevated temperature
Mg Sb -MgAgSb 7.3 593 Brittleness; Oxidization and moisture sensitivity; Bipolar effect [313,314]
3
2
(hot
side)
PbTe-Bi Te cascaded module 12 590 Deterioration of the interfaces between the legs and [315]
2 3
electrodes; toxic and high-cost material
(Nb Ta ) Ti FeSb -Hf Zr NiSn 0.98 Sb 0.02 8.3 655 Challenges in scaling up homogeneous materials for large- [316,317]
0.2 0.8
0.5
0.5
0.8
0.2
scale applications; dopant Ta hard to melt and diffuse.
(Nb, Ta, Ti, V) FeSb 15.2 670 Significant efficiency drops at lower temperature; Though [318,319]
stable contacts long term stability concerns.
SiGe (Voyager mission) 6.3 700 Low efficiency; Increasing cost of Ge; Prone to microcracks [306,320-322]
and mechanical failures; Interdiffusion of electrodes
Therefore, the broader adoption of thermoelectric modules depends on a holistic approach that advances
materials development and improves device integration, as discussed in detail above, while simultaneously
addressing both cost-effectiveness, environmental sustainability and recyclability.
CONCLUSION AND OUTLOOK
In this paper, we have reviewed the progress of the thermoelectric research field and discussed the most
promising future directions. We focused mainly on novel concepts that have been introduced over the
years, which have allowed for the remarkable progress experienced. Over the last 20 years, the resurgence in
thermoelectric research and the advances in the zT are attributed significantly to nanostructuring and the
large reduction in the thermal conductivity that accompanies it. Advanced nanostructuring methods have
been developed, in which defects are introduced in a pristine material at the atomic-, nano-, and macro-
scale, scattering phonons of different mean-free paths and reducing the thermal conductivity across the
phonon spectrum. zTs have more than doubled to values of zT ~ 2, not only for the traditional
thermoelectric materials, but for many more material families, which entered the field due to this drastic
reduction in thermal conductivity. Some of the best results were reached when care was taken not to reduce
the electronic conductivity significantly, for example, by using iso-electronic doping/alloying and nano
inclusions whose band edges are aligned with the matrix material. Nanostructuring can also offer significant
advantages to the power factor as well, utilizing the concept of energy filtering, which allowed
unprecedented PF values in some cases that followed specific design guidelines. An even higher boost in the
zT is on its way by utilizing band structure engineering techniques to improve the PF further. The most