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.