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Liu et al. Soft Sci 2024;4:44 https://dx.doi.org/10.20517/ss.2024.59 Page 15 of 21

tests and a solar-to-hydrogen efficiency of up to 0.4%. A large-area generator (112 square centimeters)
consisting of 36 units yielded an open circuit voltage of 4.4 volts and a power of 20.1 milliwatts, as well as
0.5 millimoles of hydrogen and 0.2 millimoles of oxygen after six hours of outdoor operation.

CONCLUSION AND OUTLOOK
In summary, thermal energy conversion technology based on the thermogalvanic effect is a relatively
cutting-edge form of energy conversion. In this field, various thermoelectric materials play an essential role.
However, thermoelectric materials have faced challenges in their widespread application in industry,
households, and other scenarios. The emerging hydrogel thermocell has the advantages of high
thermopower, low cost, low thermal conductivity, excellent flexibility, and scalability, which may be an ideal
choice for thermoelectric materials working at room temperature and expanding the application range of
thermoelectric materials. In recent years, they have made considerable progress by optimizing electrodes,
designing electrolytes, and improving device modules, but there are still some challenges.

Firstly, the underlying mechanism of hydrogel thermocells has yet to be fully understood. The associated
theoretical framework has yet to be completely established. There is an urgent need for effective theoretical
guidance to facilitate material optimization and design for practical applications. Consequently, the current
research should focus on deepening the understanding of its mechanism and promoting the development of
a comprehensive and systematic research framework.

Secondly, the efficiency of hydrogel thermocells should be improved. The three parameters of
thermopower, conductivity, and thermal conductivity within the thermocell are interdependent, presenting
an extreme challenge in achieving efficient improvement. For example, tuning the solvation structure of the
redox couple can increase the thermopower but simultaneously decrease the conductivity. Although
optimizing electrodes can accelerate reactions and enhance conductivity without lowering thermopower,
thermodynamic constraints impose limits on conversion efficiency. Future research should focus on the
synergistic optimization of electrolytes and electrodes to circumvent these issues. This approach aims to
decouple the three parameters, thereby improving the efficiency of a single thermocell.

Finally, given the high thermopower characteristic of thermocells, their application potential in flexible
sensor and refrigeration fields deserves further exploration. In the field of flexible sensor technology,
hydrogel thermocells have been combined with various sensor types, such as pressure, humidity, light, and
power sensors. These integrated systems are poised to find utility in various applications, such as electronic
skin, monitoring human activities, personalized healthcare, and facilitating advanced human-computer
interfaces.

DECLARATIONS
Authors’ contributions
Literature review, data collection, data analysis and interpretation, and manuscript drafting: Liu W, Fang Y
Conceptualization and content development: Lyu X, Peng X, Luo ZZ
Critical revision of the manuscript, supervision, and project administration: Luo ZZ, Zou Z

Availability of data and materials
Not applicable.