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Page 2 of 21 Liu et al. Soft Sci 2024;4:44 https://dx.doi.org/10.20517/ss.2024.59

INTRODUCTION
Low-grade heat energy from natural and anthropogenic sources (such as solar radiation, industrial plants,
[1-7]
automobiles, and the human body) is abundant but underutilized, leading to significant energy wastage .
Consequently, developing and effectively using this thermal energy has become a meaningful way to
promote sustainable development [8-12] . As a critical component of green energy, thermoelectric materials can
convert thermal energy into electricity [13-19] . They also play an essential role in improving energy efficiency
and reducing environmental burdens [20-23] . Although inorganic solid-state thermoelectric materials have
been the subject of extensive research [24-27] , their constituent elements are often rare and expensive. They
typically generate thermopower at the microvolt level, hindering their widespread commercialization .
[28]
Ionic thermoelectric materials, such as thermogalvanic hydrogels, are intriguing because their thermopower
is two to three orders of magnitude higher than their inorganic counterparts [21,29-31] .

Thermogalvanic hydrogel materials have significant prospects for low-grade thermal energy harvesting
because of their low cost, excellent scalability, flexibility, and high thermopower [32-39] . The high thermopower
implies the generation of high voltages with a slight temperature gradient, thereby simplifying device design
and integration [40-43] . Regarding thermogalvanic hydrogel preparation techniques, researchers can access
diverse methods. Hydrogels can be broadly categorized into physically and chemically crosslinked types
based on their crosslinking methods [44-46] . The physical crosslinking method mostly uses hydrogen bonds,
van der Waals forces, and other non-covalent interactions to keep the hydrogel network structure
stable [47-50] . Conversely, chemical crosslinking enhances the structural integrity of hydrogels by forming
covalent bonds [51-53] . Each of these crosslinking techniques possesses unique advantages and application
scenarios [54,55] . Through meticulous design and optimization, thermogalvanic hydrogels are poised to emerge
as pivotal components in thermal energy harvesting technologies [2,15,56,57] .

The principle of thermogalvanic hydrogels converting thermal energy into electrical energy differs from
traditional electronic materials [58-60] . The principle in thermogalvanic hydrogels is mainly based on the
thermogalvanic effect . This effect is based on the redox reaction of redox couples in ionic conductors at
[61]
the electrode interface under the thermal drive [40,62] . Owing to the disparity in reaction entropy between
anions and cations, the ions undergo oxidation reactions in the high-temperature region and reduction
reactions in the low-temperature region. The reaction generates a thermal voltage between the two
electrodes, typically reaching several millivolts per Kelvin.

In recent years, significant progress has been achieved in thermogalvanic hydrogels. For example, by adding
chaotropic cations and highly soluble amide derivatives to the redox couple electrolyte, the thermopower of
thermogalvanic hydrogels was as high as 6.5 mV·K -1[63] . Furthermore, efforts to improve the output
performance of electrodes by utilizing a large specific surface area to promote reaction kinetics have also
proven effective . In addition to the significant progress in performance, functional thermogalvanic
[64]
hydrogels with properties such as flexibility, stretchability, toughness, antifreeze and self-healing are crucial
for practical applications. Compared to traditional solid-state thermocells, hydrogel thermocells -
extensively investigated in recent years - are more competitive and practical, as illustrated in Figure 1.
However, thermogalvanic hydrogels still face challenges, including low energy conversion efficiency, low
power density and stability issues in integrated devices for practical applications. In recent years, some
excellent reviews have been made on thermogalvanic hydrogels. This paper aims to provide new insights to
guide future research toward more efficient single thermocells and higher-performance low-grade heat-
harvesting devices, among other directions.