Page 68 - Read Online

Choi et al. Energy Mater. 2025, 5, 500106 https://dx.doi.org/10.20517/energymater.2025.50 Page 3 of 28

high stretchability, biocompatibility and excellent ionic conductivity make hydrogels highly attractive for
wearable applications.

Hydrogel-based i-TE materials primarily consist of two key components: the polymer matrix and the ionic
conductor, whose composition directly determines TE performance [Figure 1] [35-40] . The polymer matrix not
only provides mechanical support but also facilitates ion transport by influencing interactions between
polymers and ion conductors such as salts and ionic liquids, to regulate the diffusion ratio between both
ends [41,42] . The hydrogel matrices in i-TE materials can be broadly classified into natural and synthetic
polymers. Natural polymers, such as cellulose, offer sustainability, biocompatibility, and a hierarchical
structure that enhances ion transport [30,43] . In contrast, synthetic polymers, such as polyvinyl alcohol (PVA)
and polyacrylamide (PAM), provide tunable mechanical properties, improved durability, and chemical
stability, allowing optimization for specific applications [44-47] . The hydrogen-bonded networks of polymers
[48]
can be engineered to influence ion mobility and TE response . Increasing the density of negatively charged
functional groups within the polymer matrix enhances cation mobility, resulting in positively charged ions
serving as the dominant charge carriers, which favors p-type behavior. Conversely, introducing strongly
bound cations that suppress their mobility promotes anion transport, leading to the predominance of
negatively charged ions and, consequently, n-type behavior. Another key advantage of hydrogel-based i-TE
materials is their capability to facilitate selective ion transport, which determines whether the material
exhibits p-type or n-type behavior [29,49,50] . The transport of specific ions can be controlled by adjusting the
polymer composition, incorporating functional groups, or selecting specific ionic species. This tunability is
a defining feature of hydrogel-based i-TE materials, offering versatile design strategies for optimizing TE
performance [36,47] . Beyond polymer selection, the choice of ionic conductor also significantly impacts TE
efficiency. Salts such as LiCl, NaCl, and KCl provide high ionic conductivity and a substantial Seebeck
coefficient at low cost. In contrast, ionic liquids (ILs), which are low-volatile organic salts, offer superior
thermal and chemical stability, ensuring sustained TE performance over extended periods. The intricate
interactions among the polymer matrix, water content, and ionic species significantly impact energy
conversion efficiency by governing both the Seebeck coefficient and overall conductivity [27,51,52] . Therefore,
the rational design of hydrogel-based i-TE materials is essential to achieve structural stability, optimized ion
transport, and enhanced TE performance.

This review aims to provide a comprehensive understanding of hydrogel-based i-TE materials, covering
aspects ranging from their mechanical properties to wearable applications, with a particular focus on the
intermolecular interactions between polymers and ion conductors. By examining their fundamental
mechanisms, material compositions, and performance optimization strategies, this review provides
important factors influencing TE efficiency. Three types of polymers - cellulose, PVA, and PAM - are
discussed in terms of their roles in facilitating selective ion transport, along with the impact of salt-based
and IL-based electrolytes on TE performance. By elucidating these relationships, this review offers valuable
insights into rational material design and presents potential strategies for enhancing TE efficiency.
Additionally, it systematically analyzes polymer matrices, ionic conductors, and their interactions to
evaluate TE performance based on the type of salts and ILs. This chapter aims to establish design principles
for the development of next-generation i-TE materials to address critical energy challenges in wearable
electronics, biomedical devices, and sustainable power generation, as well as to overcome major challenges
for practical applications, including temperature and humidity stability and mechanical durability.

BASIC PRINCIPLE
Key parameters of TE performance in hydrogel-based thermoelectric systems
The performance of TE materials is commonly assessed using the dimensionless figure of merit (ZT), which