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Martin-Gonzalez et al. Energy Mater. 2025, 5, 500121 https://dx.doi.org/10.20517/energymater.2025.32 Page 9 of 35

[79]
electrical conductivity, resulting in an approximately five-fold enhancement in the zT . Similarly, a 3D
arrangement of stoichiometric and highly oriented [110] bismuth telluride was created by electrodeposition,
consisting of 55 nm diameter longitudinal nanowires joined by 20 nm diameter transversal nanowires. In
the longitudinal direction, it was shown that the electrical conductivity of the 3D nanowire network is
comparable to the high conductivity typical of Bi Te films produced by the same technique . On the other
[80]
3
2
hand, the Seebeck coefficient increases to values up to ~130 µV·K -1[55] , when they are measured as 3D
nanowires networks. Along the transverse nanowire interconnection direction, due to nanostructuring, the
-1
-1
thermal conductivity at room temperature reaches values as low as 0.5 W·m ·K , lower than 1D nanowires
of similar diameter . This value suggests that the phonon part of the thermal conductivity along the
[81]
direction of the transverse interconnection is negligible. The electronic component is thus the only one
contributing to thermal conductivity in that direction, and as a result the Seebeck coefficient and the Lorenz
number are the only variables that affect the zT in these 3D nano-networks (in the direction along the
transverse interconnections), opening a new research direction for performance enhancement.

Developments in 2D materials
Monolayers of two-dimensional (2D) materials are emerging as promising candidates for TE materials,
offering a novel platform where high electrical conductivity coexists with a high Seebeck coefficient [82,83] .
This unique combination is attributed to their low dimensionality, atomically clean surfaces, absence of
dangling bonds, and, when prepared with precision, minimal roughness of the 2D surface, which results in
reduced quantum well thickness variation. Furthermore, recent studies have uncovered novel phenomena
such as metal-insulator transitions [84,85] and electronic correlations that generate charge density waves [86,87] ,
which can enhance TE performance. The ability to create heterostructures also allows for bandgap tuning
[88]
[89]
and renormalization , expanding the functional capabilities of these materials.
Recent advancements have indicated the potential for large Berry curvature in stacked 2D materials, which
can lead to significant anomalous Nernst coefficients (in the absence of a magnetic field). This property
opens avenues for realizing TE spin-Nernst currents, which are particularly relevant for spintronic
applications [90-93] . Additionally, 2D materials typically exhibit moderate to low thermal conductivities,
especially in the cross-plane direction when monolayers are stacked [82,94,95] , which is advantageous for TE
efficiency.

The experimental techniques employed to fabricate and investigate the TE characteristics of 2D materials
include mechanical exfoliation, which produces single flakes but often results in structural or chemical
defects. This straightforward method allows for rapid assessment of TE properties, with promising PFs
reported for various 2D materials, such as WSe , which has demonstrated a PF of approximately
2
-1
3.7 mW·m ·K -2[96] . Continuous 2D films have also been produced through methods such as screen or inkjet
printing and vacuum filtration; however, these techniques typically rely on 2D flakes generated via
exfoliation, which can compromise film quality [97-99] . While selected 2D materials in film form such as WSe
2
achieve PFs rivaling bulk Bi Te , most monolayer 2D materials (e.g., MoS , WS ) underperform due to their
3
2
2
2
simplified band structures and interfacial scattering (see Table 2); thus, more research is required in terms
of materials development and integration to improve performance.
Alternative approaches, such as intercalation techniques previously applied to non-oxidized graphene, have
-1
shown promise in enhancing PFs to over 0.6 mW·m ·K -2[100-104] . Flexible 2D thermoelectric generators
(TEGs) are beginning to emerge in the literature, with examples based on NbSe (p-type, PF ~
2
[105]
-2
-1
0.026 mW·m ·K ) and WS (n-type, PF ~ 0.005 mW·m ·K ) at room temperature . Other 2D
-2
-1
2
semiconductors, including MoS and MoTe , have exhibited PFs around 0.03 and 0.8 mW·m ·K ,
-2
-1
2
2