Figure 5 (a) Dependence of the output voltage on applied temperature differences. Reprinted with permission from ref. 32. Copyright 2020 American Chemical Society. (b) Comparison of the maximumS of different PAAm/Fe-Alg hydrogels (the Se at 0.25 h for the 0.2, 0.1, 0.05, 0.025, 0.01, and 0.005 M samples, and the Se at 6 h for the 0.002 M sample).[22]Copyright 2022, Springer Nature.(c) \(S\)and \(\sigma\) of the thermocells with different NaCl concentrations. The [\({Fe(CN)}_{6}^{4-}\)/\({Fe(CN)}_{6}^{3-}\)] concentration is fixed at 0.4 M. Error bar represents standard deviation. Reproduced with permission from Lei et al.. Copyright 2021, Elsevier.
Furthermore, PVA possesses excellent characteristics that make it a potential matrix for the development of hydrogel electrolytes. It is cost-effective, easily processed, and can be formed into films with outstanding bending performance.[33–40] In the 1980s, Petty-Weix et al.[41] made significant progress by developing a proton-conductive polymer electrolyte from PVA/H3PO4, which showed high ionic conductivity. PVA servesas an ideal matrix for quasi-solid thermocell sowing to its desirable characteristics such as biocompatibility, non-toxicity, non-corrosiveness, and excellent water solubility.[42–47] Along these lines, Wu et al. successfully developed a quasi-solid thermal cell that mimics the structure of layered fibrils and organized nanochannels found in natural muscles.[36] PVA was employed as a matrix in this system, facilitating the permeation of [\({Fe(CN)}_{6}^{4-}\)/\({Fe(CN)}_{6}^{3-}\)] redox couple into the hydrogel via solvent exchange. Moreover, it was observed that mechanical training strengthens the hydrogen bonding within the PVA chain; however, it weakens the interaction between the PVA chain and water molecules. Consequently, this released additional water molecules, thereby promoting ion conduction within the polymer networks. As demonstrated in Figures 6a and 6b, the quasi-solid heat cell showcased a tensile strength of 470%, along with toughness and fatigue thresholds of 17900 J m–2 and 2500 J m–2, respectively, surpassing those of natural muscles. In another attempt, Zhang et al.[93]introduced a novel redox couple as PVA-SO4/32– gel, displaying remarkable tensile properties. The gel demonstrated its strength by easily lifting a weight of 1 kg, as depicted in Figure 6c. Notably, the gel exhibited an impressive tensile strength of up to 833 kPa, with a tensile strain of approximately 220%. Furthermore, the combination of dimethyl sulfoxide (DMSO) and PVA has recently garnered attention. In this direction, Zhang et al.[94] capitalized on this combination to develop a thermogalvanic gel. As shown in Figure 6e, the presence of DMSO greatly enhanced the tensile strength of the gel at the expense of reduced tensile strain. However, the introduction of ethylene glycol (EG) helped in attaining the maximum Young’s modulus of up to 0.217 MPa.