Figure 6 (a) Crack propagation per stretching cycle (dc/dN) of
the thermocell under increasing energy release rate. (b) A comparison of
the mechanical properties (toughness, fatigue threshold, and strength)
between the thermocell and existing quasi-solid thermocells with
mechanical records. The yellow, blue, and purple regions represent the
quasi-solid thermocells based on physically crosslinked, chemically
crosslinked, and double-networks, respectively. Reproduced with
permission from Lei et al. Copyright 2022
Wiley-VCH.[36] (c) Mechanical properties of
PVA-SO4/32– hydrogels. Stress-strain
curves of hydrogels with different concentrations of the redox couple,
the illustration shows a hot cell hydrogel with a thickness of 2 mm and
a length of 5 cm lifting a weight of 1 kg. (d) Tensile stress-strain
curves of the hydrogels with varying ratios of the redox couple.
Reproduced with permission from Tian et al. Copyright 2023,
Elsevier.[93] (e) Mechanical properties of the
organogel. The tensile stress−strain curves of organogels with varying
EG contents. (f) Stress and Young’s modulus values of organogels with
varying EG contents. Reprinted with permission from Fang et al.
Copyright 2023, American Chemical Society.[94]
In addition, to facilitate the
operation of thermal batteries at low temperatures, Chen et
al.[48] considered the effects of solubility and
used Fe(II/III) redox couples as thermocouples. They observed a
significant reduction in the entropy elasticity of the polymer chain,
along with limitations on the entropy difference of the redox ions. In
order to break the strong hydrogen bond within the aqueous quasi-solid
thermal cell, EG was introduced to lower the freezing point of the
electrolyte solution to ‒40 °C. Additionally, Ma et
al.[92] conducted comprehensive investigations on
gels designed for extreme temperature conditions and successfully
developed a stretchable thermogalvanic hydrogel thermocell, exhibiting
an unprecedented specific output power density through ion-induced
effects. Surprisingly, the gel retained its functionality at a high
temperature of 75 °C, highlighting its robustness. Moreover, to evaluate
its performance at low temperatures, the cold end was set at ‒35 °C, and
the gel continued to maintain its exceptional working state. This
ground-breaking research signifies the immense potential of the
developed gel for diverse applications, particularly in extreme
temperature conditions. Recent work by Feng et al. has resulted in the
development of cellulose-based thermogalvanic cells (TGC) with excellent
mechanical properties.[97] To further enhance its
capabilities, LiBr was added into the system, resulting in an expanded
operating temperature range for the cell, which functioned between ‒50
°C and 50 °C, as demonstrated in Figures 7d and 7e. This innovative
approach holds great potential and warrantsfurther exploration for its
use in various applications. Overall, these studies provide valuable
insights into the advancement of high-performance thermocell materials.