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Martin-Gonzalez et al. Energy Mater. 2025, 5, 500121 https://dx.doi.org/10.20517/energymater.2025.32 Page 21 of 35

[284]
nanoscale paste forms, yet they do carry cost and environmental concerns . Recent developments of
[285]
transient liquid phase bonding offer low-temperature solutions while maintaining high-temperature
stability, alleviating the detrimental effects of thermal damage on the TE material and interface.

Operational degradation: The operational degradation of TE materials constitutes an additional challenge,
driven by mechanisms such as elemental diffusion, self-diffusion, and thermal stresses . Critical design
[286]
considerations, including encapsulation techniques, have emerged as effective strategies for mitigating these
degradation mechanisms. Encapsulation protects TE materials from environmental factors, such as
oxidation, which can undermine their stability at high temperatures. For example, encapsulation strategies
in Ag Se-based flexible devices have been demonstrated to prevent the propagation of microcracks,
2
significantly enhancing mechanical flexibility while maintaining performance integrity even after 2,000
[287]
bending cycles . Moreover, to mitigate surface-related degradation and ensure long-term stability - such
as oxidation and magnesium loss in Mg Bi -based systems - coating or encapsulation with chemically inert
3
2
[288]
materials such as BN, MgO, and SiC has proven to be an effective approach . Furthermore, advanced
techniques, such as Mg-vapor annealing, bolster thermal stability by preventing adverse phase
transitions [288, 289] . In similar vein, Sulfur infusion is reported to enhance both TE performance and stability
of sulfides . The application of Al O atomic-layer-deposited coatings of 50 nm for on semiconducting
[290]
2
3
single-walled carbon nanotube has demonstrated impressive retention of 95% of conductivity and PF after
[291]
300 h in air, underscoring the need of encapsulating materials to enhance TE resilience .
Material degradation: The degradation of TE materials during operation is another critical challenge.
Several mechanisms contribute to this degradation, including elemental diffusion and self-diffusion within
the TE materials and contacts, sublimation of dopants, moisture-induced degradation, structural defects
accumulation, thermal stresses, chemical interactions with other materials in the TE system and the
formation of intermetallic compounds. These can compromise the mechanical integrity and TE
performance of the modules over time [292,293] . Furthermore, the coefficients of thermal expansion (CTE) of
different materials in a TEG must be carefully matched to prevent mechanical stresses during thermal
cycling. Mismatches can lead to cracking and delamination at the interfaces, significantly affecting the
durability and performance of the device . The design of the module must account for these differences to
[294]
ensure long-term stability under operational conditions. Such considerations also hold true for high-
performing Ag/Cu/Sn-based selenides, which exhibit excellent TE performance, but undergo several phase
transitions at elevated temperatures. Sublimation of Se and the need to use an extremely pure environment
to prevent oxidation are critically important to maintaining the high performance of these materials .
[12]

Mechanical stability: The mechanical stability and reliability of TEGs under varying thermal and
mechanical loads is a significant concern. The design must ensure that the modules can withstand harsh
environments and repeated thermal cycling without failure. This requires a thorough understanding of the
mechanical properties of the materials and the effects of thermal stress on the interfaces. Operational
control mechanisms are essential for maintaining consistent performance in TEGs. Effective temperature
regulation requires advanced control systems that can monitor and adjust operational parameters, such as
temperature and electrical output. The integration of power management systems is also vital to ensure
compatibility with external loads, which adds complexity to the overall system design.

Integration with modern electronics: The solid-state nature and versatility in size and shape of TE make
them suitable candidates for integration into modern electronics and sensors [279,287] , as well as integration
with other systems such as photovoltaic , wearables [296,297] , and power electronics , etc. For integration
[295]
[298]
with power electronics and other applications, modern TEGs require co-design with wide-bandgap