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Page 20 of 28 Choi et al. Energy Mater. 2025, 5, 500106 https://dx.doi.org/10.20517/energymater.2025.50

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(PDK, 27.2 mW m ), the PVA-H O/DMSO-PSS-KCl (PDSK) hydrogel exhibited superior performance,
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primarily due to enhanced anion diffusion and stronger electrostatic interactions within the polymer
network [Figure 10H and I]. As research continues to address existing challenges in TE efficiency,
mechanical durability, and environmental adaptability, the future of self-charging ionic supercapacitors
appears increasingly promising for sustainable energy applications.

Hydrogel-based i-TE sensors
Hydrogels, known for their exceptional flexibility, tissue-like softness, adhesiveness, and ionic conductivity,
have been widely used in wearable electronics, biomedical monitoring, and human-machine interfaces.
Significantly, the main advantages of i-TE sensors for human monitoring applications include high
flexibility, sensitivity, and durability under long-term cyclic mechanical loads, ensuring practical usability
through a high Seebeck coefficient.

Chen et al. introduced a super-stretchable, high-performance i-TE hydrogel based on carboxylated BC
[37]
(TOBC), designed for self-powered sensing applications . The hydrogel incorporates LiTFSI as an ion
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provider, enhancing selective ion transport and yielding a high Seebeck coefficient of 11.53 mV K . This
material offered exceptional mechanical properties, with tensile deformation up to 3,100% and a stress value
of 0.85 MPa, facilitated by hydrogen bonding within the PAM network and borate ester interactions in
TOBC. The TOBC-based i-TE hydrogel demonstrated excellent mechanical durability and real-time
applicability for wearable sensing. When integrated with a boost converter and an Arduino-based circuit, it
enabled wireless signal transmission for real-time monitoring of physiological movements. Applied to the
human palm, the device accurately distinguished between bending and stretching motions, with signals
successfully transmitted to a mobile interface [Figure 11A-H].

Han et al. developed ultrasensitive flexible thermal sensor arrays utilizing high-thermopower i-TE hydrogels
for enhanced thermal sensing . A smart glove integrated with PQ-10/NaOH-based i-TE hydrogel sensor
[40]
arrays demonstrated real-time thermal sensing capabilities for wearable electronics. Each finger was
equipped with flexible sensors patterned onto a flexible printed circuit board, allowing the device to detect
and distinguish temperature variations upon contact with diverse objects. The hydrogel generated distinct
TE responses - negative voltages for warm surfaces and positive voltages for cold - while maintaining
stability under mechanical deformation. This responsive and flexible configuration highlights the potential
of i-TE materials for tactile thermal sensing in intelligent wearables [Figure 12A-H].

Hydrogel-based i-TE generators
In contrast to e-TE generators, the i-TE generators (ITEG) based on Soret effect exhibit capacitive behavior,
as neither ions nor electrons can transport across the electrode-electrolyte interface. Due to the unique
[52]
characteristics of ITEGs, they are inherently required to generate current through an indirect method .
The ITEG required a prior voltage build-up step (charging). When the external circuit is connected after
voltage build-up step, a transient current is generated in the external circuit (discharging). This power
supply gradually decays, depending on the external load resistance and the capacitive voltage of ITEG. This
behavior is principally analogous to the charging process of i-TE capacitors.

Fu et al. introduced a thermosensitive ionic hydrogel with an integrated TE energy harvesting capability,
expanding its applications to adaptive thermal management and energy-efficient building technologies
[Figure 13A] . The hydrogel is composed of physically cross-linked PAA and polyethylene oxide (PEO)
[89]
network blending with sodium dihydrogen citrate (SDC) as the electrolyte. This hydrogel featured a lower
critical solution temperature effect, transitioning from transparent to opaque at approximately 26 °C,
effectively regulating light transmittance while enhancing TE properties. Notably, the thermotropic phase