Figure 2 (a) Composition and working principle of thermocouple based on redox reaction. (b) Thermocouple working equivalent circuit. Reproduced with permission from ref. 99. Copyright 2021, American Chemical Society
Thermoelectric devices can be categorized into p-type and n-type based on the value of the thermal potential, as illustrated in Figure 3. The p-type devices typically exhibit a negative thermal potential, inducing an oxidation reaction at the thermal end. Consequently, electrons flow from this end to the cold end through an external circuit, generating a reduction reaction at the cold end. In contrast, n-type thermoelectric devices operate in the opposite manner, with positive thermal potential.[25]Typical examples of p-type redox couples include [\({Fe(CN)}_{6}^{4-}\)/\({Fe(CN)}_{6}^{3-}\)], [\(\text{FeBr}_{4}^{2-}\)/\(\text{FeBr}_{4}^{-}\)], and [\(\text{FeCl}_{4}^{2-}\)/\(\text{FeCl}_{4}^{-}\)]. On the other hand, common n-type redox couples consist of [\({Co(byp)}_{3}^{3+}\)/\({Co(byp)}_{3}^{2+}\)], [\(\text{Fe}^{3+}\)/\(\text{Fe}^{2+}\)], [\(\text{Cu}^{2+}\)/\(\text{Cu}^{+}\)], [\(\text{Zn}^{2+}\)/\(\text{Zn}^{+}\)], and [\(I^{-}\)/\(I_{3}^{-}\)]. The effects and characteristics of different redox couples can vary significantly. Table 2 provide a simple comparison of commonly used redox couples with their respective properties.
Table 2 Comparison of different redox couples properties