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Page 2 of 28 Choi et al. Energy Mater. 2025, 5, 500106 https://dx.doi.org/10.20517/energymater.2025.50

ionic conductors. It also provides critical insights into practical applications such as wearable thermoelectric
generators and capacitive energy storage devices. Furthermore, we propose innovative strategies to overcome key
limitations, those related to long-term stability and mechanical durability. By consolidating current knowledge and
identifying future research opportunities, this review establishes a foundation for the development of next-
generation flexible and efficient hydrogel-based i-TE materials.

Keywords: Ionic thermoelectric materials, hydrogel, Soret effect, polymer, ion conductor

INTRODUCTION
Energy harvesting from waste heat has become an important strategy for improving energy efficiency, with
[1-3]
growing interest in its application to flexible and wearable electronics . Among various energy conversion
technologies, thermoelectric (TE) materials offer a direct means of converting thermal energy into electrical
energy through the Seebeck effect, which relies on the movement of charge carriers under a temperature
gradient . However, conventional TE materials face challenges in harvesting low-grade heat (< 100 °C),
[4]
which is typically dissipated under realistic environmental conditions, due to drawbacks such as rigidity,
[5,6]
toxicity, and relatively low Seebeck coefficients in the tens of μV K range . These challenges have driven
-1
the search for alternative TE materials capable of efficiently converting low-grade heat into electrical
energy [7-10] . Notably, waste heat released in daily life mainly originates from non-planar surfaces such as the
human body. To achieve effective heat-to-electricity conversion from such curved heat sources, next-
generation TE materials are required with (i) high energy conversion efficiency near room temperature [11-13] ;
(ii) adaptable mechanical flexibility to conform to curved geometries [14,15] ; and (iii) sustainable stretchability
to accommodate abrupt movements [16-18] .

Ionic thermoelectric (i-TE) materials have emerged as a promising alternative to traditional e-TE materials
due to their unique combination of high Seebeck coefficient, mechanical flexibility, biocompatibility, and
processability [19,20] . Unlike e-TE materials, i-TE materials convert thermal energy into electrical energy under
a temperature gradient through two primary mechanisms: the Soret effect and the thermo-galvanic
[21]
[22]
effects . Soret-effect-based i-TE materials have recently gained attention due to their high Seebeck
coefficient and ability to maintain a stable potential difference without involving redox reactions. The Soret
effect refers to the phenomenon where a thermal gradient induces the migration of cationic and anionic
ions with different mobilities, leading to charge separation and the generation of an electric potential
[23]
difference at the ends . Notably, i-TE materials can exhibit Seebeck coefficients in the millivolt-per-Kelvin
[24]
range owing to the entropy difference between cations and anions at different temperatures . This
characteristic makes them well-suited for harvesting low-grade heat, including body heat, in wearable
electronics [21,25,26] .

The i-TE materials can be categorized into three types based on their physical states: solid-state, liquid-state,
and quasi-state . Hydrogels, a promising class of quasi-state i-TE materials, consist of a three-dimensional
[27]
polymer network filled with water and ionic conductors. A key advantage of i-TE hydrogels is their intrinsic
tunability, allowing their structural and compositional components to be tailored for specific performance
requirements [28-30] . This tunability enables precise control over mechanical robustness and flexibility, while
[31]
facilitating synergistic interactions among the polymer, water, and ions . These features synergistically
enhance TE performance by promoting ionic mobility and selectivity, establishing hydrogel as a versatile
platform for diverse TE applications. Their diverse mechanical properties allow them to conform to
irregular surfaces, ensuring intimate thermal contact with the skin, which makes them highly suitable for
wearable applications [32-34] . Furthermore, their inherent biocompatibility enables integration into biomedical
devices, such as implantable energy harvesters and self-powered biosensors. The unique combination of