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.