Figure 4 (a) Stress−strain curve of the hydrogel. The tests were carried out with a stretching rate of 20 mm min−1. Insets show pictures of the TG hydrogel in its original and stretched state. The scale bar is 1 cm. (b) Thermogravimetric characterization of thermogalvanic hydrogel. Reprinted with permission from Pu et al. Copyright 2020, American Chemical Society. (c) The double-network thermocell with a thickness of 3 mm and a width of 5 mm lifts a bag of oranges with a weight of 1.5 kg. The red circle indicates the location of the thermocell. (d) The nominal stress-strain curve of the double-network thermocell (un-notched). The yellow dashed line indicates the strain-at-break of a notched sample. Reproduced with permission from Lei et al. Copyright 2021, Elsevier. (e) Stress-strain curves of the PAAm and PAAm/Alg hydrogels. Inset shows the stretching process of the PAAm/Alg hydrogel, which can be easily stretched over 700%. (f) The elastic modulus, breaking stress, and elongation data of the PAAm and PAAm/Alg hydrogels.[22]Copyright 2022, Springer Nature.
Meanwhile, the thermoelectric properties of these hydrogels are the biggest concern. Hu’s TG hydrogel exhibits favorable physical and electrochemical properties.Figure 5a depicts the thermoelectric properties of the hydrogel, including the\(S\) is 1.2 mV K-1, theeffective conductivity of approximately 100 mS/cm, and the thermal conductivity ranging from 0.30 to 0.39 W (mK)-1 , and the power generation factor of 6.5-12 μW(m K2)-1. Chen et al. observed a remarkably significant \(S\)value of up to 1.43 mV K-1for the PAMM/ALg hydrogel, while simultaneously retaining high stretchability. Previously, Wu et al. achieved a remarkably high\(S\)value of 1.5 mV K-1 for their double-network thermocell, influenced by the concentration of NaCl. Despite the satisfactory thermoelectric performance of hydrogels, there is ample opportunity for improvement.