Prussian blue analogues (PBA), which composed of cyano-bridged jungle-gym type framework and alkali metal ions, are promising material for electrode of batteries, because its framework is robust against electrochemical intercalation/deintercalation of alkali metal ions. In addition, the material consists of ubiquitous elements and is easily synthesized from aqueous solution. Thus, PBAs are promising electrode material for sodium ion secondary battery (SIB). We will introduce the overall features of PBAs as SIB electrode. Furthermore, PBAs are promising material for energy harvesting. We proposed a new type of battery (tertiary battery) that can be charged by the environmental heat using the difference in the thermal coefficient (α= dV/dT) of the redox potential (V) between the anode (α_anode) and cathode (α_cathode) materials. The tertiary battery can convert the environmental heat energy into electric energy during the thermal cycle, i.e., (i) heating to T_H, (ii) discharge at T_H, (iii) cooling to T_L, and (iv) discharge at T_L. In the (i) heating process, the V values of the anode and cathode change by α_anodeΔT and α_cathodeΔT, respectively. We expect a thermally induced change in the cell voltage (V_cell) as large as (α_cathode - α_anode)ΔT. The stored electric energy can be extracted by the (ii) discharge process at T_H. We fabricated several tertiary batteries made of PBA electrodes with different a and evaluated the thermal voltage (V_cell) and discharge capacity (Q_cell). For example, Na_xCo[Fe(CN)₆]_0.87/Na_xNi[Fe(CN)₆]_0.94 tertiary battery exhibits V_cell = 24 mV and Q_cell = 2.4 mAh/g per unit weight of total active material contains in the cathode and anode. We will correlate the observed Q_cell values with the electrode parameters of the tertiary batteries.

1. Y. Shimaura, T. Shibata, and Y. Moritomo, "Interrelation between discharge capacity and charge coefficient of redox potential in tertiary batteries made of transition metal hexacyanoferrate", Jpn. J. Appl. Phys. 61, 044004 (2022)
2. I. Nagai, Y. Shimaura, T. Shibata, and Y. Moritomo, "Performance of tertiary battery made of Prussian blue analogues", Appl, Phys. Express. 14, 094004 (2021).
3. Y. Moritomo, Y. Yoshida, D. Inoue, H. Iwaizumi, S. Kobayashi, S. Kawaguchi, and T. Shibata "Origin of the material dependence of the temperature coefficient of the redox potential in coordination polymers", J. Phys. Soc. Jpn. 90, 063801 (2021).
4. T. Shibata, H. Iwaizumi, Y. Fukuzumi, and Y. Moritomo, "Energy harvesting thermocell with use of phase transition” Sci. Res., 10 1813 (2020)
5. I. Takahara, T. Shibata, Y. Fukuzumi, and Y. Moritomo, " Improved thermal cyclability of tertiary battery made of Prussian blue analogues ", Chem. Select 4, 8558-8563 (2019)
6. Y. Fukuzumi, K. Amaha, W. Kobayashi, H. Niwa, and Y. Moritomo, “Prussian blue analogues as promising thermal power generation materials”, Energy Technology, 6 (2018) 1 – 7.
7. T. Shibata, Y. Fukuzumi, W. Kobayashi, and Y. Moritomo, “Thermal efficiency of a thermocell made of Prussian blue analogues”, Sci. Reps. Appl. Phys. Express. 8 (2018) 14784.
8. T. Shibata, Y. Fukuzumi, W. Kobayashi, and Y. Moritomo, Thermal power generation during heat cycle near room temperature, Appl. Phys. Express. 11, 018101 (2018).
9. M. Takachi, T. Matsuda, and Y. Moritomo, “Cobalt hexacyanoferrate as cathode material of Na⁺ secondary battery”, Appl. Phys. Express 6, 025802 (2013).
10. M. Takachi, T. Matsuda, and Y. Moritomo, “Redox reaction in Prussian blue analogues with fast Na+ intercalation”, Jpn. J. Appl. Phys. 52, 092202 (2013).