Page 69 - Read Online

Page 4 of 28 Choi et al. Energy Mater. 2025, 5, 500106 https://dx.doi.org/10.20517/energymater.2025.50

Figure 1. Overview of hydrogel-based ionic thermoelectric (i-TE) materials. Reproduced with permission [35-40] . Copyright 2024, Elsevier
Ltd.; Copyright 2025, Royal Society of Chemistry; Copyright 2023, Elsevier Ltd.; Copyright 2024, Elsevier Ltd.; Copyright 2022, Elsevier
Ltd.; Copyright 2023, Wiley-VCH GmbH.

is defined as :
[53]

where S represents the Seebeck coefficient, σ denotes the electrical conductivity, and κ signifies the thermal
conductivity [Figure 2A and B]. Z represents the TE material parameter, which quantifies the intrinsic
ability of a broad range of TE materials to convert thermal energy into electrical energy. T is the absolute
temperature at which the material operates, playing a critical role in determining TE efficiency. Since
Wang et al. confirmed that ZT applies to both e-TE and i-TE, it serves as a key indicator for assessing the
efficiency of TE materials in both systems [54,55] . Interestingly, the underlying relationships among key
parameters such as electrical/ionic conductivity, thermal conductivity, and Seebeck coefficient are
fundamentally different. In e-TE materials, increasing the carrier density is a common strategy to improve
electrical conductivity. However, this approach leads to a reduction in the Seebeck coefficient due to an
increased symmetry in the carrier energy distribution near the Fermi level. In addition, the thermal
conductivity consists of both electronic and lattice contributions, with the electronic thermal conductivity
increasing as the carrier density increases. As a result, optimizing the balance among these three parameters
is essential for improving the ZT value. In contrast, ionic conductivity in i-TE materials increases with ion