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Page 6 of 21 Tang et al. Soft Sci. 2025, 5, 11 https://dx.doi.org/10.20517/ss.2024.62

an electromotive force (EMF). This potential difference is what drives the flow of electrons through an
external circuit, enabling electrical energy generation for sensing.

The conversion efficiency of thermocells is determined by three interconnected factors: the thermopower
(Se), the effective electrical conductivity (σ ), and the effective thermal conductivity (κ ). The thermal
eff
eff
power for materials utilizing the thermogalvanic effect is determined by [61,63] :
Eq (3)

Where ΔV represents the operating voltage of the ionic TE (iTE) material; ΔS denotes the difference in
partial molar entropy of the redox couple; n signifies the number of electrons involved in the redox process;
and F is the Faraday constant. The equation indicates that redox couples with a high absolute charge and
complex structures exhibit greater differences in the partial molar entropy of ions, which correlates with a
higher Seebeck coefficient.

Based on the ability of thermogalvanic hydrogel (TGH) to convert thermal energy into electrical signals, it is
an ideal candidate for self-powered sensing, eliminating the need for external power sources and extending
the operational life of sensors in remote or hard-to-reach locations. For instance, a TGH-based electronic
skin (E-skin) can harness temperature differences to generate electricity for self-powered on-body dual-
modal temperature and strain sensing . Meanwhile, TGCs can also be utilized to monitor environmental
[64]
conditions such as temperature gradients, which are crucial in various applications, including climate
change studies and industrial processes. For example, a TGH sensor can be used for temperature
monitoring of edibles, providing a safe and non-toxic method for self-powered sensing in food temperature
[65]
detection .

Soret effect
Recent studies have reported that redox-free electrolytes can exhibit substantial Seebeck coefficients due to
the thermo-diffusion of ions, a phenomenon driven by the Soret effect, also known as the ionic Seebeck
effect [66-69] . The Soret effect, or thermodiffusion, occurs when a temperature gradient applied to a fluid
containing multiple atomic or molecular species results in a nonuniform composition within the fluid
[70]
[Figure 2D] . The difference between Seebeck effect is that the carriers are cations/anions and the
materials are ionic liquids or polymers. This effect was first observed in the 19th century by Ludwig and
Soret, who noted that in an electrolyte solution within a tube, the concentration of salt was higher on the
cold side . The Soret effect is characterized by the accumulation of solute particles, such as ions, towards
[71]
the colder end of a temperature gradient, leading to a separation of components within a mixture. This
generates a thermovoltage that is determined by the thermal gradient across the electrolyte. The ionic
Seebeck coefficient describes the magnitude of the TE voltage produced by iTE materials under a certain
temperature gradient. [72-74] . Consequently, the Soret effect plays a crucial role in sensing applications by
enabling the conversion of thermal gradients into electrical signals, which can be harnessed for energy
storage and sensing purposes .
[75]

ADVANCED SENSING APPLICATIONS BASED ON TES
The integration of TE materials into advanced sensing applications has transcended its traditional energy
harvesting applications, opening new avenues for self-powered wearable biosensing, environmental
monitoring, and health diagnostics. This section explores the cutting-edge applications of TE materials in
creating sensors that can broaden our interaction with the environment and our bodies.