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Page 8 of 28 Choi et al. Energy Mater. 2025, 5, 500106 https://dx.doi.org/10.20517/energymater.2025.50

COMPONENTS OF IONIC THERMOELECTRIC HYDROGEL
Hydrogels, a class of soft materials, consist of a water-filled polymer network that provides both mechanical
flexibility and processability. These characteristics make hydrogels particularly attractive as i-TE materials,
which exhibit a quasi-solid-state nature combining both solid-like and fluid-like properties. A crucial
component of hydrogels for i-TE applications is the ionic conductor, which facilitates ion migration and
enhances TE performance [21,33,41] . Intrinsically, hydrogels contain water that undergoes autoionization to
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+
[66]
produce H O and OH ions, enabling some degree of TE activity . However, the resulting Seebeck
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coefficient is relatively low and insufficient for practical applications. To overcome these issues, researchers
have designed hydrogels to incorporate high-potential ionic conductors, such as salts and ILs, within the
polymer matrix to enhance ion motility and concentration. The overall TE performance of i-TE hydrogels is
governed by the interactions between the polymer networks and the ionic conductors. The polymer
network serves as a structural framework that ensures mechanical integrity and modulates ion transport
pathways. In parallel, the ionic conductor governs the Seebeck coefficient and charge carrier type (p-type or
n-type) through polymer-ion interactions and ion mobility within the hydrogel matrix. Since the Seebeck
coefficient is closely linked to thermally driven ion diffusion, understanding these interactions is crucial for
optimizing i-TE material design [64,67-71] . This section explores the key polymeric materials commonly used as
i-TE hydrogels, followed by an in-depth discussion on different ionic conductors, including salts and ILs, to
elucidate their roles in governing TE performance and charge transport mechanisms.

Polymer networks
The polymer network is a fundamental component of i-TE hydrogels, providing both mechanical integrity
and interconnected pathways for ion diffusion, which directly influences TE performance. Natural polymers
such as cellulose, gelatin, and chitosan offer biocompatibility, renewability, and environmental
sustainability, making them suitable for eco-friendly and biomedical wearable applications [39,65] . In
particular, cellulose is widely used due to its porous structure that facilitates ion conduction. In contrast,
synthetic polymers such as PVA and PAM provide high structural tunability and mechanical stability,
allowing for tailored material optimization. PVA forms strong hydrogen bonds with water through its
hydroxyl (-OH) groups, resulting in excellent water retention, flexibility, and chemical stability [46,50] .
Similarly, PAM possesses amide groups (-CONH ) that enhance water retention and ionic conductivity,
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serving as an effective platform for i-TE applications . This section focuses on the representative polymeric
[72]
materials used in i-TE hydrogels, specifically cellulose as a natural polymer and PVA and PAM as synthetic
polymers. Each material exhibits unique physical and chemical characteristics that influence their
interaction with ionic conductors through ion-matrix-water interactions. This section provides a detailed
analysis of their roles in optimizing ion mobility, charge selectivity, and overall TE efficiency, along with
strategies for engineering them to enhance i-TE performance.

Cellulose
Cellulose, a naturally derived polymer, is widely utilized in i-TE materials due to its renewability,
biocompatibility, and ionic conductivity. Consisting of glucose monomers with hydroxyl (-OH) groups,
cellulose forms a hydrogen-bonded network that provides mechanical stability and efficient ion
transport . Its hierarchical fibril structure promotes anisotropic ion diffusion, while surface-exposed
[73]
negatively charged oxygen species (O ) selectively facilitate cation hopping, favoring p-type behavior.
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Chemical modifications, such as the incorporation of carboxyl (-COO ) or sulfonate (-SO ) groups, further
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enhance ion selectivity and mobility. Additionally, the addition of salts or ILs optimizes thermally induced
ion transport, improving the Seebeck coefficient.