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Page 16 of 35 Martin-Gonzalez et al. Energy Mater. 2025, 5, 500121 https://dx.doi.org/10.20517/energymater.2025.32

eliminate S, on the contrary, very large S can be achieved upon SOC induced bandgap opening, further
assisted by the low-dimensionality and band linearity [159,160] . This, in addition to the ballistic topological
transport (protected from backscattering) which improves σ, could allow for unprecedented PFs.
Furthermore, the presence of a non-zero Berry curvature under certain conditions allows the realization of a
very large anomalous Nernst effect (ANE), which can also be modulated by gating [90,91,161-163] . These are novel
phenomena, but at this point, it is not yet clear if they can provide larger performance compared to the
conventional operation. A study on such a Heusler material heterostructure has speculated an incredible
-1
zT ~ 5, importantly as a result of its extremely high PF which reaches values as high as ~50 mW·m ·K -2[34] .
Although not yet reproduced successfully, it could be that topological effects, or operating in the presence of
those, could have a lot to offer to thermoelectricity. We mentioned a few exploratory directions, but we
emphasize that this is not an exhaustive list; other novel physics-based effects are investigated, ranging from
Anderson-type transport, atomic ordering , to spin-Seebeck, to magnon-based effects, to techniques that
[14]
[164]
soften the lattice and reduce thermal conductivity , to utilizing 4f electrons and their effect on the
DOS , to utilizing scattering of s-orbital electrons by d-orbitals in metals , and many more. It yet
[166]
[165]
remains to be seen if some of these will result in high PF performance, especially at room and elevated
temperatures.
Finally, we need to stress that there are many important developments in materials with important
performance features, but not high enough zT to make it into Figure 1, and because of this they sometimes
receive less attention. Other than the half-Heuslers mentioned earlier with very high PFs, but zT only
around 1-1.5 [167,168] , we would like to mention the case of Mg Sb and its derivatives [22,169-172] . This material has
2
3
a zT of ~0.7, but importantly this is at room temperature, which is a range where not many candidates can
challenge Bi Te . Such a non-Te base material composed of abundant elements can have a much larger
3
2
impact on room-temperature applications.
Beyond energy conversion and cooling applications, significant interest for TE effects is also encountered in
the field of spintronics, where spin currents or voltages need to be created. In particular, the spin-Seebeck
effect-SSE (the spin analog of the Seebeck effect), discovered in 2008, has been explored for generation of
“spin voltages” driven by thermal gradients (having the ability for spin injection into attached conductors).
This gave birth to the field of “spin-caloritronics”. Unlike typical thermoelectrics, which are optimized for
highly doped semiconductors, the SSE can be realized in a variety of magnets from - metals to
semiconductors and insulators; thus, it could enable insulator-based thermoelectrics [173,174] . SSE was observed
in both the transverse configuration, in which a spin current perpendicular to the temperature gradient can
be measured, and the longitudinal configuration, in which a spin current flowing in parallel to the
temperature gradient is created in the longitudinal direction; thus, this is referred to as the Longitudinal
Spin Seebeck Effect (LSSE). The latter has been observed in paramagnetic metal/ferromagnetic insulator
junctions, and due to its simplified structure is the mainstream SSE direction [175,176] . However, in this
configuration, proximity effects from the ferromagnetic material can result in the creation of the ANE in the
metal, in the same direction as the LSSE. Thus, when studying the SSE, separation of the two signals is an
[177]
important task that is undertaken . Only in magnetic insulators which do not develop the ANE, has the
LSSE been measured unequivocally . In general, from the application perspective, the output voltage of
[177]
LSSE is much smaller than that of conventional devices, but it can be used for spintronic applications, and
sensors.

Utilizing the diverse array of materials production and manipulation methods discussed above, a wide range
of materials has been explored for TE applications, including Chalcogenides [178,179] , Skutterudites [180,181] , Zintl
phases [182-184] , Clathrates [185-187] , full-Heusler [139,188-190] , half-Heusler [191-193] , Silicides [194,195] , Oxides [196-198] ,