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.