2.3.3. Heat collection and management devices
In addition, thermoelectric materials are used as a heat source for different applications to create a temperature difference between the hot and the cold ends. In this regard, Ma et al.[92] combined AM with sodium alginate to form double-network hydrogels and added guanidinium hydrochloride to enhance their thermoelectric properties and improve their heat dissipation ability. In this study, a 30 mm computer CPU was selected as the subject, and its normal operating temperature was observed to reach 76.2 °C. Notably, after applying the hydrogel onto the surface of the CPU, its operating temperature decreased by 15.1 °C, reaching 61.1 °C, while maintaining a stable voltage output of approximately 43.5 mV. At the same time, Ma and his team reported a multifunctional superelastic graphene-based thermoelectric sponge that is also committed to thermal management. It is not only relatively stable in performance, but also can cool the working CPU by 8K.[101] In another approach, Hu and co-workers[32] employed K3[Fe(CN)6]/K4[Fe(CN)6] as redox couples in the electrolyte of thermochemical batteries and achieved evaporation and absorption of moisture through mutual regulation of Li+ and Br ions. The researchers inserted a Ti plate in the middle of the hydrogel and placed two Ti meshes, one connected to a mobile phone cell as the positive pole and the other as the negative pole of the thermochemical batteries. This configuration resulted in varying temperatures during battery operation with different discharge efficiencies. Moreover, the studies illustrate that thermoelectric materials have promising prospects in the heat management of electronic devices.
3. Conclusions and Perspectives
Given the current global energy crisis, developing thermoelectric technologies is undoubtedly a good choice. However, traditional solid-state materials are no longer suitable for certain applications, leading to the emergence of quasi-solid-state gels as a promising alternative. In recent years, the research progress of thermoelectric hydrogels has advanced rapidly, especially in the development of quasi-solid-state thermoelectric hydrogels. According to the current research status, these materials have the potential for a diverse range of applications. This review provides an overview of the working principles and commonly used redox couples of thermocells based on the thermogalvanic effect. It then focuses on recent advances in hydrogel network matrices, electrode materials, surface and compositional structure of these thermocells. Lastly, the review presents the more advanced results in this field.
Despite significant progress in present thermoelectric research, there remains considerable scope for the development of thermoelectric batteries.
Firstly, the optimization of electrolytes is worth discussing. When hydrogels are used as electrolytes, the selection of different matrices and catalysts allows for the tailoring of their mechanical properties, with their ionic conductivity, thermal conductivity, and \(S\). Currently, the [Fe(CN)6]4–/[Fe(CN)6]3–redox ion couple is the most intensively studied in terms of its thermoelectric effect. However, further exploration is needed to identify couples that exhibit a more prominent thermogalvanic effect. Additionally, we observed that the addition of reagents, such as NaCl and KCl, can enhance the efficiency of the oxidation-reduction process, thereby improving the overall\(S\). Furthermore, the hydrogels achieved good working efficiency at both low and high temperatures by adding alcohol reagents. These three options provide opportunities for further optimization of the quasi-solid-state hydrogel electrolyte.
Besides, while addressing energy problems, the design of thermoelectric devices should not be confined to the laboratory setting but should also prioritize practicality in their structural design. The paper folding structure, pleated structures, helical structures, integrated textile structures, and island bridge structures with stretchable electrodes presented in this review offer various possible applications. Depending on the specific practical needs and material properties, different structures can be chosen to address problems in multiple environments, thus, making energy conversion more efficient.
Moreover, electrode materials play a critical role in the composition of batteries and should not be overlooked. Presently, metal electrodes are most commonly used, followed by carbon-based material electrodes, while thin film electrodes are rarely employed in battery applications. From the viewpoint of thermoelectric development, it is important to investigate the impact of the physical properties of electrodes on the overall performance. This call for a particular focus on flexible electrode materials with good mechanical properties, toughness, and low cost.
In conclusion, when studying thermogalvanic or thermal diffusion effects, the following steps involve optimizing the \(S\), improving electrical conductivity, and reducing thermal conductivity. This can be achieved through effective cooperation among the electrolyte, structure design, and electrodes, along with the addition of suitable solvents. The rapid development of thermoelectric materials has demonstrated their enormous commercial potential, which can drive environmental and economic developments in the future.
Acknowledgement
This work was supported by the National Nature Science Foundation of China (51977079), Beijing Natural Science Foundation(3232054), the Key Laboratory of Icing and Anti/De-icing of CARDC (Grant No. IADL 20210401), the Central Guidance on Local Science and Technology Development Fund of Hebei Province (226Z1204G), the Top Young Innovative Talents of Colleges and Universities of Higher Learning Institutions of Hebei (BJ2021095), Youth Elite Scientists Sponsorship Program by Chinese Society for Electrical Engineering (CSEE-YESS-2017002), and the Fundamental Research Funds for the Central Universities (2020MS115)
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