Page 70 - Read Online

Liu et al. Soft Sci 2024;4:44 https://dx.doi.org/10.20517/ss.2024.59 Page 13 of 21

thermogalvanic hydrogels have gained widespread attention due to their rapid response and high precision.
Wang et al. engineered a hydrogel-based electronic skin with dual-mode temperature and strain-sensing
[131]
capabilities that harvest thermal energy from human body heat [Figure 8A] . They selected the iodine/
-
-
triiodide (I /I ) redox couple. By combining the thermogalvanic and piezoresistive effects, self-powered
3
temperature and strain sensing are successfully demonstrated through encapsulation and integration of the
hydrogels into human tissue. Wu et al. have made a significant breakthrough. By connecting ten pairs of p-
n thermoelectric modules in series, they constructed a system capable of real-time fire monitoring
[132]
[Figure 8B] . The thermogalvanic hydrogel was a self-powered electrical signal converter, transforming
the temperature signal into a voltage signal. The signal acquisition module connected to the transmitter was
responsible for capturing and relaying the temperature change information. The electrochemical
workstation then recorded voltage changes in real-time. When the signal exceeded the threshold of the
alarm circuit, the system lit up the red light-emitting diode and activated the speaker to sound the alarm.

Passive wearable sensors
Wearable sensors are another promising application area for thermogalvanic hydrogels [71,110,133-135] . Shen et al.
2-
reported for the first time a novel non-toxic redox couple of SO /SO as a high-performance, p-type
2-
4
3
thermogalvanic electrolyte ion that delivered a high Seebeck coefficient of 1.63 mV·K at the redox couple
-1
concentration of 0.1 M . Moreover, this PVA-SO hydrogel-based quasi-solid-state device showed the
[77]
2-
4/3
capacity for grabbing body heat to operate small electronics. Liu et al. have developed a tough and
[63]
stretchable thermogalvanic hydrogel with superior thermoelectric properties [Figure 9A] . This hydrogel
utilized the stretching-induced crystallization process and the thermogalvanic effect. With the help of
freeze-thaw cycles and stretching steps, polymer chains crystallize and arrange themselves in the stretching
direction, creating a layered anisotropic network. The researchers added guanidinium chloride to the
[Fe(CN) ] /[Fe(CN) ] system to change the solvent shell around the [Fe(CN) ] . The chaotropic cations
3-
4-
4-
6
6
6
promote the crystallization of [Fe(CN) ] and enhance the reversibility of the redox reaction, thereby
4-
6
increasing the thermopower. The thermogalvanic hydrogel got a high thermopower of 6.5 mV·K and a
-1
-2
specific output power density of 1,969 µW·m ·K . This represented a nearly fivefold increase compared to
-2
the thermopower of liquid [Fe(CN) ] /[Fe(CN) ] (1.4 mV·K ) and a fourfold increase in the output power.
3-
4-
-1
6
6
The stretching caused the polymer chains to densify, which increased the mechanical properties. The
hydrogel had a breaking strength of 19 MPa and a toughness of 163.4 MJ·m . The thermogalvanic hydrogels
-3
were also integrated as an array to harvest low-grade thermal energy from the environment and conduct
strain sensing and health monitoring. This device generated enough electrical power to drive small medical
devices, contributing to the promotion of green, sustainable, and wearable electronics in the Internet of
Things era. Yang et al. constructed a solar thermogalvanic hydrogel . The innovative system used external
[136]
sunlight-induced by swinging arms to dynamically change the temperature difference of the gel during the
thermoelectric conversion process [Figure 9B]. Han et al. developed a smart glove that integrates multiple
thermal sensor arrays . By monitoring all sensing nodes, the smart glove can feel the temperature and
[121]
touch the position of an object, showcasing the potential application of flexible thermal sensor arrays in an
intelligent environment [Figure 9C].
Coupling with catalysis
In addition to applications in energy, temperature sensing, and passive wearable sensors, thermoelectric
materials can also be used in catalysis . Wang et al. reported an in situ photocatalytically enhanced redox
[137]
reaction that generates hydrogen and oxygen to realize a continuous concentration gradient of redox ions in
thermogalvanic devices . They used polyacrylic acid (PAA) as a hydrogel matrix, FeCN as redox ions,
[65]
4-/3-
and WO (O -WO ) and ZnIn S as the photocatalysts (S -ZIS) for O production and H production. An O 2
2
3
2 4
3
v
v
2
-evolution photocatalyst aided the forward reaction from FeCN to FeCN and facilitated H O to O
4-
3-
2
2
production, resulting in a high FeCN concentration on the hot side . The H -evolution photocatalyst
4-
[138]
2