2. Development and application of the thermogalvanic effect in the field
of electrochemistry
2.1. Working Principle of Redox Reaction Thermocouple
This
review delves into the exploration of thermocouples and their
utilization in generating thermal potential through the exploitation of
the thermogalvanic effect. The thermogalvanic effect finds typical
applications in thermoelectric cells, wherein redox couples such
as\([{Fe(CN)}_{6}^{4-}\)/\({Fe(CN)}_{6}^{3-}],[\text{Fe}^{2+}/\text{Fe}^{3+}]\)and \([I^{-}/I_{3}^{-}]\) are employed to convert heat
energy into electricity, as illustrated in Figure 2. Usually, these
redox couples, along with electrolytes, can reach a certain degree of
the Seebeck coefficient. The Seebeck coefficient of ionic thermoelectric
materials based on the thermogalvanic effect is determined by Eq. (1) as
follows:[23]
\(S=\frac{V}{T}=\frac{S}{\text{nF\ }}\) (1)
where \(V,\ S,\ n\), and \(F\) represent the open circuit operating
voltage of the ionic thermoelectric material, the partial molar entropy
difference of the redox couple, the number of electrons transferred
during the redox reaction, and the Faraday constant, respectively. When
considering the Seebeck coefficient, the energy conversion efficiency of
thermoelectric materials is often determined using the dimensionless
quality factor, defined by Eq. (2) as follows:[24]
\(ZT=\frac{S^{2}\text{σT}}{k}=\frac{S^{2}\text{σT}}{(k_{e}+k_{l})}\)(2)
where \(S\) represents the Seebeck coefficient, \(\sigma\)denotes the
conductivity of the material, T represents the absolute
temperature, and \(k\) represents the thermal conductivity.
Specifically, \(k_{e}\) denotes the electron thermal conductivity while\(k_{l}\) is the lattice thermal conductivity. Indeed, achieving a highZT value corresponds to a higher energy conversion efficiency
in thermoelectric materials. The difference in redox states
significantly influences the entropy of ion partial moles between
different valence states and determines its \(S\). Additionally,\(S^{2}\sigma\) can be interpreted as the thermoelectric power factor.
To attain high ZT values, it is crucial to have a higher\(S^{2}\sigma\) while minimizing the\(k\). This insight highlights a
direction for improving the thermogalvanic effect of ionic
thermoelectric materials.