Figure 11 (a) Retractable schematic diagram of integrated wearable thermoelectric hydrogel batteries for energy collection. The thermoelectric properties of integrated 14 p-n couples of thermoelectric cells were studied, and the voltage changes of the equipment attached to the deformed wrist were studied.[75] Reproduced from ref. 75 with permission from John Wiley and Sons, copyright 2022. (b) Integrated module representation diagram and schematic diagram. (c) The left diagram shows the power supply to the LEDs relying on the different temperatures of the contact water. The right diagram shows the voltage output of the integrated module collecting human waste heat. Reproduced with permission from ref. 75. Copyright 2018, Nature Publishing Group. (d) The temperature simulations of three types cogenerators. (e) The diagram of the proposed hybrid cogenerator for electricity and water. (f) The curves of current–voltage (I–V) and output power density–voltage (P–V) of different hybrid cogenerators under 1 kW m–2 illumination. Reproduced with permission from ref. 91. Copyright 2022, Royal Society of Chemistry.
Figure 11a[75] depicts a p-n battery integration module using\({Fe(\text{ClO}_{4})}_{2}\)/\({Fe(\text{ClO}_{4})}_{3}\) as the n-type ion couple and K3[Fe(CN)6]/K4[Fe (CN)6] as the p-type ion couple. The p-n to hydrogel electrolyte exhibited a voltage output of 29 mV, a current output of 8.5 Am–2, and an average maximum power density of 0.66 mW K–2 m–2 per p-n cell at ΔT of 10 K. By integrating the hydrogel electrolyte with graphite paper electrodes, a close-fitting portable thermal battery device was fabricated, achieving a voltage output of 0.16 V with 14 couples of p-n batteries (ΔT = 4.1 K). Figure 11b[8]illustrates the replacement of 50 thermoelectric hydrogels with integrated modules, as demonstrated by Xu et al. In this configuration, the hydrogels were utilized as batteries and interconnected in series using Cu wires. Figure 11c shows an aqueous electrolyte consisting of a mixture of urea (24 M), GdmCl (2.6 M), and 0.4 M of K3[Fe(CN)6]/K4[Fe(CN)6]. It was observed that the integrated module, when used with graphite as an electrode, exhibited an output voltage of 3.4 V and an output current of 1.2 mA at ΔT = 18 K, which was sufficient to power the LEDs. Furthermore, Zhou et al.[90] used PVA as the matrix to prepare n-type and p-type thermoelectric hydrogels with [Fe3+/Fe2+] and [Fe(CN)6]4–/[Fe(CN)6]3–as the respective redox couple. They connected, integrated, packaged, and attached 59 pairs of p-n hydrogels onto the human arm. At an ambient temperature of 5 °C, an output voltage of about 0.7 V and an output current of 2 µA was generated, with a corresponding maximum output power of 0.3 µW. Additionally, the integration of thermoelectric hydrogels has shown promising applications in the field of solar energy conversion. For this purpose, Miao et al. successfully developed a double-network hydrogel by combining AM with starch for the utilization of solar radiation to create a temperature gradient between the hot side of TEG and water, resulting in water evaporation and electricity generation (Figures 11d and 11e). The proposed design yields a high power density of up to 11.39 W m‒2 and achieves multi-level utilization of solar energy (Figure 11f).[91] In recent research, Ma et al. once again prepared an excellent toughness thermogalvanic hydrogel thermocell for human health monitoring, with a high tensile strength of 19MPa and a high thermal power of 6.5mV K-1.[100]