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Page 6 of 21 Liu et al. Soft Sci 2024;4:44 https://dx.doi.org/10.20517/ss.2024.59

electrolyte thermocells exhibiting high thermopower, with a maximum value of 2.65 mV·K -1[99-101] . Li et al.
significantly enhanced the performance of the K Fe(CN) /K Fe(CN) redox couple within an organic
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[102]
hydrogel thermocell [Figure 3] . This change also influenced the structure of the solvent shell around the
redox couples during the reaction, which led to a difference in the concentration of the redox anions.
Consequently, the thermopower was substantially increased from 1.27 to 2.30 mV·K . Owing to the high
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compressibility and inherent stretchability, these thermocells were subsequently assembled into self-
powered strain sensors. These sensors could monitor the movement of the human body under various
stretches and pressures in real-time with high sensitivity.
To enhance the thermopower beyond improving ∆S, an alternative approach involves increasing ∆C.
However, the redox couple cannot indefinitely sustain the concentration gradient between the hot and cold
ends, as this state is thermodynamically unstable and will eventually revert to a uniform state. When the
electrolyte reaches a steady state, ∆C is zero. Han et al. reported a new concept using methylcellulose to
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capture I at the hot side and then release I at the cold side, creating a concentration gradient of free I in
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the thermocell, leading to an increase and reversal of the thermopower . Zhou et al. successfully increased
[31]
the concentration difference between the hot and cold ends by introducing α-cyclodextrin (α-CD) and
potassium chloride (KCl) . Due to the host-guest interaction, α-CD bound to I at the cold end, thereby
[72]
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reducing the relative concentration of I . Consequently, the equilibrium spontaneously shifted to the right
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at the cold end, where oxidation of 3I to I occurred. Concurrently, with the temperature elevation, the
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α-CD-I complex spontaneously dissociated, resulting in an augmented concentration of I at the hot end.
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This resulted in the equilibrium shifting towards the reduction of I to 3I , thereby producing more I at the
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hot end. Compared to the original I /I system, introducing host-guest interactions created a larger
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concentration difference between the hot and cold ends, causing the internal cyclic reactions within the
thermal thermocell to proceed more efficiently. Consequently, the thermopower increased from 0.86 to
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1.97 mV·K [Figure 4].
In addition to the methods above, introducing electrolyte additives is another advantageous strategy [2,31,33,40] .
Cations with strong chaotropic properties can combine with redox ions, crystallizing redox substances. This
raises the ∆S of the redox electrolyte and amplifies the ∆C, significantly improving the thermopower of the
+
thermocell. Yu et al. added guanidinium ions (Gdm ) to the electrolyte. These ions selectively caused
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[Fe(CN) ] to crystallize at the cold end, which led to the formation of thermosensitive crystals [Figure 5A
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and B] . Owing to the concentration gradient, these thermosensitive crystals moved toward the hot end
[30]
and dissolved there. This phenomenon generated a substantial concentration disparity between the two
electrodes, resulting in higher thermopower . This was attributed to the fact that [Fe(CN) ] had a higher
[103]
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charge density compared to [Fe(CN) ] , and it interacted more strongly with Gdm . Yu et al. corroborated
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this hypothesis through comprehensive experimental and simulation analyses . At the low-temperature
[30]
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electrode (293 K), [Fe(CN) ] crystallized almost completely, with a concentration ratio of [Fe(CN) ] /
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[Fe(CN) ] of approximately 0.02. Conversely, near the high-temperature electrode (343 K), the [Fe(CN) ]
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crystals dissolved rapidly, bringing the [Fe(CN) ] /[Fe(CN) ] concentration ratio to about 0.94. More
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importantly, the large concentration gradient of the redox couple significantly amplified the thermogalvanic
effect. Adding Gdm caused the thermopower to rise from 1.4 to 3.73 mV·K , nearly 2.5 times than before.
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+
In addition to increasing ∆C, the crystals generated at the cold end could also inhibit thermal convection in
the system, decreasing thermal conductivity [Figure 5C]. Due to the synergistic optimization of these
parameters, its Carnot-relative efficiency reached 11.1%. Liu et al. combined stretch-induced crystallization
with thermoelectric effect and proposed a high-strength quasi-solid stretchable polyvinyl alcohol
[63]
thermogalvanic thermocell (SPTC) [Figure 5D] . Thus, the SPTC system had a large thermopower of 6.5
mV·K , a high specific output power density of 1969 μW m K . The extraordinary thermoelectric and
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