The electrochemical thermoelectric coefficient (S_EC = dV/dT, where V and T are the redox potential and temperature, respectively) is a significant material parameter, which enables thermal power generation with use of thermocell during heat cycles. Recently, the thermal power generation has been demonstrated using two kinds of Prussian blue analogues (PBAs) with different S_EC^[1]. To realize the thermocell with high efficiency, the S_EC values are systematically investigated for three PBAs, Na_xCo[Fe(CN)₆]_0.71 (NCF71), Na_xCo[Fe(CN)₆]_0.90 (NCF90), and Na_xMn[Fe(CN)₆]_0.83 (NMF83) against sodium concentration (x).

The figure shows the discharge curve (the black solid line) and S_EC against x (the orange closed circles) of NCF90. The discharge curve shows two plateaus regions (plateaus II and III) around 3.8 and 3.2 V. Plateau II is ascribed to the reaction ^[2]: Co³⁺[Fe³⁺(CN)₆]_0.6[Fe²⁺(CN)₆]_0.3+ 0.6Na⁺ → Na_0.6Co³⁺[Fe²⁺(CN)₆]_0.9. Plateau III is ascribed to the reaction^[2]: Na_0.6Co³⁺[Fe²⁺(CN)₆]_0.9 + Na⁺ → Na_1.6Co²⁺[Fe²⁺(CN)₆]_0.9. Thus the plateau II (the plateau III) on the discharge curve means reduction process of Fe³⁺ (Co³⁺)^[2]. On the other hand, S_EC distributes around 0.4 mV K^-1 (1.4 mV K^-1) in the plateau II (the plateau III). Therefore, it is suggested that S_EC significantly depend on the redox site. In the presentation, we also report S_EC of NCF71 and NMF83 and discuss the material and redox site dependences of S_EC by using ionic model (point charge model) approach.

[1] T. Shibata, Y. Fukuzumi, W. Kobayashi, Y. Moritomo, submitted.

[2] R. L. Magnússon, W. Kobayashi, M. Takachi Y. Moritomo, AIP adv. 7 045002 (2017).