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Choi et al. Energy Mater. 2025, 5, 500106 https://dx.doi.org/10.20517/energymater.2025.50 Page 17 of 28

The effect of cation size and charge distribution on IL-based thermoelectrics has also been extensively
studied. Horike et al. investigated imidazolium-based ILs with varying alkyl chain lengths and found that
+
-1
shorter alkyl chains (e.g., [EMIM] ) exhibited a higher Seebeck coefficient of 10.1 mV K , while longer alkyl
+
-1
chains (e.g., [DMIM] and [HMIM] ) led to lower values of 7.2 and 5.9 mV K , respectively
+
[Figure 9A and B] . This is attributed to steric effects and changes in cation diffusivity, as bulkier cations
[80]
experience greater frictional interactions, thus limiting thermodiffusion efficiency. Similarly, Zhao et al.
reported that ILs with smaller, highly mobile anions (e.g., [BF ] ) enhanced ionic conductivity, whereas
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4
larger anions (e.g., [TFSI] ) increased the Seebeck coefficient by promoting charge separation and
-
asymmetric ion transport [Figure 9C-E] . Notably, [EMIM][DCA] achieved a peak Seebeck coefficient of
[87]
26 mV K , demonstrating that precise tuning of ion mobility and polymer interactions is critical for
-1
optimizing IL-based i-TE performance. Beyond ion diffusivity, IL-polymer interactions are also important
in determining overall TE efficiency. The incorporation of ILs into hydrogel matrices not only enhances
mechanical stability but also modifies ion transport pathways, leading to greater asymmetry in cation and
anion migration rates. For example, IL-polymer coordination in PVDF-HFP-based IL gels improved ZT
values by enhancing charge carrier asymmetry, stabilizing the IL network, and preventing unwanted phase
separation. By strategically pairing ILs with specific polymer backbones, p-type and n-type behavior in i-TE
materials can be effectively manipulated. Given that IL-based i-TE materials are highly tunable, the selection
of cation-anion pairs, hydration levels, and polymer compatibility must be carefully optimized to achieve
the desired ionic Seebeck coefficient and ionic conductivity. Controlling hydration effects and charge
mobility asymmetry allows for the development of high-performance IL-based hydrogels with enhanced TE
properties. IL selection should be tailored to specific hydrogel matrices to maximize charge separation
efficiency, ion diffusivity, and mechanical durability, ensuring the stability and effectiveness of next-
generation soft TE materials.

APPLICATION OF IONIC THERMOELECTRIC HYDROGEL
i-TE hydrogels operating in thermo-diffusive mode offer a promising approach for harvesting low-grade
heat from the surrounding environment, making them particularly well-suited for wearable applications.
Notably, the three-dimensional polymer framework of hydrogels ensures both mechanical flexibility and
lightweight properties, enhancing their adaptability for thermal energy conversion in wearable and ambient
applications. In this section, we discuss the potential of i-TE hydrogels functioning in thermo-diffusive
mode in three key areas: capacitive energy storage systems, sensing platforms, and TE generators.

Hydrogel-based i-TE capacitors
A distinguishing feature of i-TE hydrogels driven by Soret effect is that the diffused ions do not directly
migrate into the metal electrodes. Instead, these ions accumulate on the electrode surface, forming an
electric double layer within the hydrogel, which induces a transient current. Consequently, i-TE hydrogels
relying on the Soret effect are unsuitable for conventional TE generators that require a stable current output.
i-TE hydrogels utilize temperature gradients to induce selective ion transport, enabling efficient charge
separation and enhancing energy density. This unique mechanism has led to growing interest in developing
self-charging ionic supercapacitors capable of efficiently converting low-grade heat into electrochemical
energy. Hydrogel-based i-TE capacitors operate in four-stage cyclic process: (i) thermo-ionic charging; (ii)
forward electronic working; (iii) thermo-ionic discharging by ion redistribution; and (iv) reverse electronic
working. Ion carriers migrate to the electrode surface under a temperature gradient (∆T), forming an
electric potential difference that enables the system to function as a capacitor. When the temperature
gradient is removed, ions redistribute, and reconnecting the circuit completes the discharge cycle. This
cyclic process enables continuous thermal energy harvesting and charge storage, making hydrogel-based i-
TE capacitors highly suitable for soft electronic devices.