Figure 3 (a) is a p-type thermocouple and (b) is an n-type thermocouple. Mm+and Mn+ stands for redox couples.
2.2. Factors affecting thermocouples based on thermogalvanic effect
2.2.1. Impacts of hydrogel composition
Traditional semiconductor thermoelectric materials demonstrate a favorable S at high temperatures. However, when being utilized in absorbing and converting low-degree heat energy, their coefficients plummet to below 200 µV K–1, exacerbating the unfavorable issue of material flexibility. Quasi-solid electrolytes, such as hydrogels, address these issues by presents a safer, efficient, environmentally friendly, and flexible alternative.[26]
Ionic gels possess a broad operating temperature range, enhanced ionic conductivity, and higher S , making them ideal for energy conversion and utilization. At the same time, gels offer diverse applications, further highlighting their versatility. A notable example is the study conducted by Wang et al. in 2021, where they utilized gels and functional carbon nanomaterials to prepare a flexible supercapacitor.[27] The study discusses the ionotropic gelation properties when introduced into various matrices, with particular attention to its impacts.
In recent years, there has been a considerable focus on hydrogel electrolytes as flexible quasi-solid polymers. These hydrogels are developed through the chemical or physical crosslinking of hydrophilic natural or synthetic polymers. Polysaccharides and polypeptides are frequently used as natural hydrophilic polymers in the preparation of hydrogels. Meanwhile, synthetic hydrophilic polymers include but are not limited to, acrylic acids, alcohols, and their derivatives.[28] Among them, polyacrylamide (PAAm) and polyvinyl alcohol (PVA) are two of the most commonly employed hydrophilic polymer materials in research studies.
Acrylamide (AM) is an organic compound that readily polymerizes via ultraviolet irradiation or at high temperatures due to the presence of a carbon-carbon double bond and amide group. PAAm hydrogels exhibit remarkable resilience and excellent elasticity, as well as mechanical and chemical stability. [29–31] The chemical properties of these hydrogels can be adjusted by controlling the gel pore size via varying the concentrations of monomers and cross-linkers, thereby tailoring their usage conditions accordingly. In this regard, Hu et al.[32] prepared a smart thermocouple hydrogel film achieving efficient evaporative cooling and waste-heat recovery (Figure 4). The study investigated the recycling of\([{Fe(CN)}_{6}^{4-}\)/\({Fe(CN)}_{6}^{3-}]\) redox couple to drive redox thermoelectricity, harnessing K+, Li+,Br,and\([{Fe(CN)}_{6}^{4-}\)/\({Fe(CN)}_{6}^{3-}]\)plasma confined in either water or polymer to enhance thermoelectric conversion. Moreover, the moisture content of hydrogels was regulated through the controlled equilibrium of Li+ and Br ions. Meanwhile, any excess heat generated was dissipated by the evaporation of free water molecules present in the hydrogel. High mechanical strength and an impressive tensile strain of 0.24 MPa allow hydrogels to elongate 2–3 times beyond their original length. Further, Wu et al.[18]introduced a double network hydrogel, incorporating AM and AMPS with remarkable thermal and electrical capabilities. As depicted in Figure 4c, the hydrogel demonstrated exceptional tensile strength, enabling it to lift a 1.5 kg fruit without being ruptured. Additionally, this hydrogel displayed around 220% elongation under stress levels of nearly 1200 kPa (Figure 4d). In order to obtain a hydrogel with a combination of high tensile strength, superior elongation, and excellent thermal and electrical efficiency, Chen et al.[22]employed a double-network structure composed of PAAm/Alg. This as-obtained hydrogel displayed outstanding tensile strength (up to 700%), as shown in Figures 4e and 4f. Further, the elastic modulus, fracture stress, and elongation of the hydrogel were also recorded. Collectively, these findings suggest the promising potential of AM-based hydrogels in single/double network systems.