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.