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Choi et al. Energy Mater. 2025, 5, 500106 https://dx.doi.org/10.20517/energymater.2025.50 Page 25 of 28

anions directly affects selective ion transport and the resulting thermo-voltage. Incorporating functional
groups that interact strongly with specific ions can enhance ion selectivity, allowing for tailored ion
migration pathways. Additionally, balancing hydration stability and ionic mobility is crucial for maintaining
consistent TE performance, especially under varying environmental conditions. Despite significant
advancements, challenges remain before hydrogel-based i-TE materials can be fully utilized in practical
applications. One major challenge is maintaining long-term water retention, as dehydration can
significantly degrade ionic conductivity and TE efficiency. Enhancing mechanical durability under repeated
stress is also essential to ensure reliability in daily life. Furthermore, optimizing the balance between ionic
conductivity and thermal stability remains critical to achieving high energy conversion efficiency. We hope
that this review establishes a comprehensive theoretical framework for i-TE materials, providing deeper
insights into their transport mechanisms and guiding the design of high-performance i-TE hydrogels. By
advancing our understanding of ion-matrix-solvent interactions and optimizing material compositions,
future developments can drive the development of practical and efficient energy harvesting systems.

DECLARATIONS
Authors’ contributions
Proposed the topic of this review: Kim, H.
Wrote the manuscript, designed and drew the Figures: Choi, Y.; Kang, B. C.
Discussed and revised the manuscript: Kim, J. Y.; Kang, B. C.; Kim, H.

Availability of data and materials
Not applicable.

Financial support and sponsorship
This work was supported by the National Research Foundation of Korea (NRF) (RS-2024-00452255 and RS-
2024000449743).

Conflicts of interest
All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate
Not applicable.

Consent for publication
Not applicable.

Copyright
© The Author(s) 2025.

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