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