Figure 8 (a) Fabrication process illustration of the knitted
fabric integrated TEG.[70] (b) Schematic
illustration of the mechanism for thermoelectric material with wrinkle
structure.[66] Reproduced from ref. 66 with
permission from AIP Publishing, copyright 2019. (c) Schematic diagram
and photographs of the stretchable TEG using origami-like folding
deformation.[63] Copyright 2018, MDPI. (d)
llustration of the mechanism of connected p-n cell based on hydrogel
electrolytes.[75] Reproduced from ref. 75 with
permission from John Wiley and Sons, copyright 2022. (e) Schematic
illustration for fabrication of stretchable TEG with 3D helical
architecture.[68] Copyright 2018, American
Association for the Advancement of Science.
Surface structure engineering involves designing the surface
microstructure of an ion polymer matrix, creating a gap between the
electrode and the matrix.[60] The gap thus created
enhances the compressibility of the hydrogel, leading to improved
sensitivity and a broader range of response. Interestingly, external
mechanical forces can alter the contact area between the ionic polymer
matrix and the electrode.[61] At present, the
surface structure of hydrogels can be classified into several types,
including folding structure,[62,63] wrinkle
structure,[64–66] spiral
structure,[67,68] textile integration
structure,[69,70] and island bridge structure with
retractable electrodes,[71–75]as shown in Figure
8. Among them, the island bridge structure has gained widespread
popularity due to its ability to preserve the mechanical properties of
thermoelectric materials. It presents a simple construction process and
scalability, making it suitable for the development of electronic skin
applications. However, the performance of this structure relies heavily
on electrode materials, and in current research, copper tablets are
predominantly used as electrodes in the structure. The island bridge
structures have been found to increase the flexibility of hydrogels in
energy devices, although their operational lifespan can be somewhat
affected. Moreover, the paper folding structure in hydrogels can enhance
their tensile strain performance and is recognized as a powerful tool in
the pursuit of obtaining complex 3D configurations and unprecedented
performance through the graphic design of conventional materials. The
folding structure is known for its stability, generally forming at the
bonding interface of hydrogels through stretching or external
influences. In contrast, the spiral structure directly improves the
strain of thermoelectric hydrogels and reduces the influence of the
environment on their conductivity. Similarly, the integration of
thermoelectric generators (TEG) into textiles presents a favorable
structure due to their malleability, lightweight, comfort, and air
permeability, allowing for increased heat contact between thermoelectric
materials and heat sources in a 3D interlocking mode.
2.2.3. Impacts of
electrode materials
After discussing the influence of electrolytes as cells or capacitors,
it becomes evident that electrodes play a key role in determining the
performance of thermoelectric devices, considering their working
principle. Wang et al. combined gel with meso/microporous graphene
nanocomposites to prepare a flexible supercapacitor with excellent
electrochemical performance.[76] Herein, we mainly
focus on metal and carbon-based material electrodes, which are currently
the most widely used electrode types in the field of energy storage and
batteries.
Various metal-based electrodes, such as Ni,[77]Cu,[78,79] and W [80]electrodes, have been developed so far for thermal cells. As shown in
Figure 9a,[81] the Ni-based thermal cells show
similar performance to Pt foils, making them promising alternatives to
Pt or nanostructured carbon electrodes under alkaline conditions.
Additionally, Cu electrodes have also demonstrated advantages for
thermal batteries. Duan et al. (Figure 9b)[82]reported a basic Cu electrode with a power density of 0.06 W
m–2, which was effectively improved to 0.75 W
m–2 after modification to a 3D multi-structured Cu
electrode. Moreover, the 2D Au/Cu electrode was modified to a 3D Au/Cu
electrode configuration, resulting in a remarkable improvement in power
density by 1072%. These outcomes imply that optimizing the electrode
structure can effectively enhance the power density of the system.